Some Mods for Marshall Class 5

[NJDeVriese]

Thanks for the info! I really appreciate it. I do have one question... And it's probably a stupid one. What does it mean to jump capacitor c13? I have to be honest... I haven't done much tinkering other than pedals so I figured what the heck. And I DO know how to safely drain the filter caps so don't think I'm gonna kill myself LOL. Just alittle help for a newbie would be appreciated.

Thanks,
Nik

[blamo]

Originally Posted by NJDeVriese

Thanks for the info! I really appreciate it. I do have one question... And it's probably a stupid one. What does it mean to jump capacitor c13? I have to be honest... I haven't done much tinkering other than pedals so I figured what the heck. And I DO know how to safely drain the filter caps so don't think I'm gonna kill myself LOL. Just alittle help for a newbie would be appreciated.

Thanks,
Nik

bypass it with a wire.

you can either remove it and replace it with a wire or you can just solder a wire to each end.

[NJDeVriese]

Which of the mods listed actually pertains to the tone stack? This thing COULD sound great if it wasn't so darn bassy and unusable in that aspect. Thanks

[bitsleftover]

It's kind of a combined effort. Most of these changes either tighten the bass or add a little sparkle and when taken out of context they only make a slight difference, when done together they improve the tone a lot.
My advice would be to take the plunge. Make the changes as per the schematic and see if you like it. I think you will. Once the PCB board is out it won't take much longer to do them all anyway and you can always put it back to stock if you don't like it.

[bitsleftover]

One thing I didn't do was the output board mod.
Can anyone explain what it achieves please.
Before I installed the VVR mod I removed the output cable from the board, soldered a stereo plug to it and put it in the speaker out or headphones jack as an attenuator.
Does it just remove the need to do this?
Cheers

[NJDeVriese]

I would have the luck of posting the 666th reply Anyway, can anybody help me out with positioning the new coupling caps with the outside foil pointed in the right position.

[bitsleftover]

Hi
I need help.
Ive done all the mods apart from the Cathode Bias Resistor (R27) and the output board wire.
Also I have fitted the Hall Amplification VVR board and applied it to the Power section only. The B+ voltages seem to work and adjust fine.
I fitted a master volume (as Halls recommend) and followed Lyles advice a few posts back by removing R24 and R26 and installing a 1meg pot as a voltage divider in their place.
All other mods are as per the schematic.
The problem I have now is this.
When I hit a chord with the amp turned up loud, ie, Vol 10 Master 10 and VVR 10 it sounds great for a second or so, but as the chord begins to decay slightly I get the horrible loud pulsing Cha-cha-cha-cha-cha-cha which overpowers the guitar sound. It sounds kind of like a very distorted pulsed crackle at about one per second or sometimes very fast ie 10 per second.
The longer the amp has been powered up the better it gets and after an hour or so it sometimes stops altogether.
It seems to start at anywhere above 8 on the vvr control and anywhere above 7-8 on the master volume. By which I mean if either one is on full the other can only go up to about 7or8 and seems to be very interactive between the 2.
I could understand it if it was the other way around ie having to turn the Master Vol down a bit when the VVR was turned down but with the vvr maxed it should be as if it isn’t even fitted.
What am I doing wrong?
Am I over-overdriving my power amp? This is not a musical type of distortion in any way, more a kind of “something is F*@#&d” distortion!
With R24 and R26 replaced by my 1meg pot, when I have it up to 10 am I over-overdriving my power amp? would I be better re installing R24 and R26 and fitting the pot between these resistors so that the Gain level of that stage will never exceed the value you have set with your mods?
Or do I have another problem?
Wierdly enough, I also did the headphone trick, (the stereo jack plug on the speaker cable plugged into the headphone socket) and in headphone mode the problem disappears so its good for quiet practice.
What can I do? Please help someone. As it is Im getting the piss taken out of me by the guys for messing with forces I dont fully understand and destroying my new amp!
Please don’t just say turn it down. I get that enough from my wife
Any other advive is welcome.
Cheers

[X–Ray]

Originally Posted by bitsleftover

Hi
I need help.
Ive done all the mods apart from the Cathode Bias Resistor (R27) and the output board wire.
Also I have fitted the Hall Amplification VVR board and applied it to the Power section only. The B+ voltages seem to work and adjust fine.
I fitted a master volume (as Halls recommend) and followed Lyles advice a few posts back by removing R24 and R26 and installing a 1meg pot as a voltage divider in their place.
All other mods are as per the schematic.
The problem I have now is this.
When I hit a chord with the amp turned up loud, ie, Vol 10 Master 10 and VVR 10 it sounds great for a second or so, but as the chord begins to decay slightly I get the horrible loud pulsing Cha-cha-cha-cha-cha-cha which overpowers the guitar sound. It sounds kind of like a very distorted pulsed crackle at about one per second or sometimes very fast ie 10 per second.
The longer the amp has been powered up the better it gets and after an hour or so it sometimes stops altogether.
It seems to start at anywhere above 8 on the vvr control and anywhere above 7-8 on the master volume. By which I mean if either one is on full the other can only go up to about 7or8 and seems to be very interactive between the 2.
I could understand it if it was the other way around ie having to turn the Master Vol down a bit when the VVR was turned down but with the vvr maxed it should be as if it isn’t even fitted.
What am I doing wrong?
Am I over-overdriving my power amp? This is not a musical type of distortion in any way, more a kind of “something is F*@#&d” distortion!
With R24 and R26 replaced by my 1meg pot, when I have it up to 10 am I over-overdriving my power amp? would I be better re installing R24 and R26 and fitting the pot between these resistors so that the Gain level of that stage will never exceed the value you have set with your mods?
Or do I have another problem?
Wierdly enough, I also did the headphone trick, (the stereo jack plug on the speaker cable plugged into the headphone socket) and in headphone mode the problem disappears so its good for quiet practice.
What can I do? Please help someone. As it is Im getting the piss taken out of me by the guys for messing with forces I dont fully understand and destroying my new amp!
Please don’t just say turn it down. I get that enough from my wife
Any other advive is welcome.
Cheers

Originally Posted by bitsleftover
(in a color a bit easier to read than black ;~)

Hi
I need help.
Ive done all the mods apart from the Cathode Bias Resistor (R27) and the output board wire.
Also I have fitted the Hall Amplification VVR board and applied it to the Power section only. The B+ voltages seem to work and adjust fine.
I fitted a master volume (as Halls recommend) and followed Lyles advice a few posts back by removing R24 and R26 and installing a 1meg pot as a voltage divider in their place.
All other mods are as per the schematic.
The problem I have now is this.
When I hit a chord with the amp turned up loud, ie, Vol 10 Master 10 and VVR 10 it sounds great for a second or so, but as the chord begins to decay slightly I get the horrible loud pulsing Cha-cha-cha-cha-cha-cha which overpowers the guitar sound. It sounds kind of like a very distorted pulsed crackle at about one per second or sometimes very fast ie 10 per second.
The longer the amp has been powered up the better it gets and after an hour or so it sometimes stops altogether.
It seems to start at anywhere above 8 on the vvr control and anywhere above 7-8 on the master volume. By which I mean if either one is on full the other can only go up to about 7or8 and seems to be very interactive between the 2.
I could understand it if it was the other way around ie having to turn the Master Vol down a bit when the VVR was turned down but with the vvr maxed it should be as if it isn’t even fitted.
What am I doing wrong?
Am I over-overdriving my power amp? This is not a musical type of distortion in any way, more a kind of “something is F*@#&d” distortion!
With R24 and R26 replaced by my 1meg pot, when I have it up to 10 am I over-overdriving my power amp? would I be better re installing R24 and R26 and fitting the pot between these resistors so that the Gain level of that stage will never exceed the value you have set with your mods?
Or do I have another problem?
Wierdly enough, I also did the headphone trick, (the stereo jack plug on the speaker cable plugged into the headphone socket) and in headphone mode the problem disappears so its good for quiet practice.
What can I do? Please help someone. As it is Im getting the piss taken out of me by the guys for messing with forces I dont fully understand and destroying my new amp!
________________________________________________

[glpease]

Originally Posted by X–Ray

Originally Posted by bitsleftover
(in a color a bit easier to read than black ;~)

Hi
I need help.
Ive done all the mods apart from the Cathode Bias Resistor (R27) and the output board wire.
Also I have fitted the Hall Amplification VVR board and applied it to the Power section only. The B+ voltages seem to work and adjust fine.
I fitted a master volume (as Halls recommend) and followed Lyles advice a few posts back by removing R24 and R26 and installing a 1meg pot as a voltage divider in their place.
All other mods are as per the schematic.
The problem I have now is this.
When I hit a chord with the amp turned up loud, ie, Vol 10 Master 10 and VVR 10 it sounds great for a second or so, but as the chord begins to decay slightly I get the horrible loud pulsing Cha-cha-cha-cha-cha-cha which overpowers the guitar sound. It sounds kind of like a very distorted pulsed crackle at about one per second or sometimes very fast ie 10 per second.
The longer the amp has been powered up the better it gets and after an hour or so it sometimes stops altogether.
It seems to start at anywhere above 8 on the vvr control and anywhere above 7-8 on the master volume. By which I mean if either one is on full the other can only go up to about 7or8 and seems to be very interactive between the 2.
I could understand it if it was the other way around ie having to turn the Master Vol down a bit when the VVR was turned down but with the vvr maxed it should be as if it isn’t even fitted.
What am I doing wrong?
Am I over-overdriving my power amp? This is not a musical type of distortion in any way, more a kind of “something is F*@#&d” distortion!
With R24 and R26 replaced by my 1meg pot, when I have it up to 10 am I over-overdriving my power amp? would I be better re installing R24 and R26 and fitting the pot between these resistors so that the Gain level of that stage will never exceed the value you have set with your mods?
Or do I have another problem?
Wierdly enough, I also did the headphone trick, (the stereo jack plug on the speaker cable plugged into the headphone socket) and in headphone mode the problem disappears so its good for quiet practice.
What can I do? Please help someone. As it is Im getting the piss taken out of me by the guys for messing with forces I dont fully understand and destroying my new amp!
________________________________________________

Have you put a scope on the B+ to see what's going on there? If you haven't access to a scope, you might be able to see the pulsing with a fast reading DVM—an analogue meter would be better, in this case. Seeing what the B+ is doing would offer a clue.

Does the oscillation rate change with control settings? Does it speed up or slow down as you reduce the MV setting? It could be common motorboating, because of an error in decoupling the B+ for the preamp valves.

You should be running the regulated B+ to the PT, and a separate, non-regulated HT line to R4. Is there a diode on the unregulated B+ out? (I don't have a schematic for the Hall board, so I'm shooting in the dark.)

If I recall correctly, Lyle's mods for R24/R26 set them at 470k/1M, which reduces the signal at V3's grid by about 0.7. If you're going to use a 1M MV pot, you should leave R24 in the circuit, as running the full tilt signal out of V2 into the EL84 is overkill. You can put R24 back in the circuit easily enough by wiring it in series with the top lug of the pot. That would be the first thing I'd try, just to get another data point. It will likely change the symptoms, but probably won't cure it. I think power supply decoupling is the more likely suspect.

[bitsleftover]

X-Ray - Thanks for that. I dont know how I managed to get a black on black but I was late for work and had to leave it as it was.

Gplease - Thanks for the advice. The VVR side of it all checked out fine, I hadnt thought of using an analogue meter so had to dust off the old AVO but all was OK
It turned out to be the Master Vol in the end.
Much as I hate to admit it I had cocked the whole thing up and attached the Master Vol output to the wrong side of R25 in effect removing that from the circuit aswell as R24 and R26. Totally over loading V3.
Its strange how its possible to make a mistake, check it, re check it and check it again a dozen times without seeing anything wrong, then go back to it another day and think "what the hell have I done there?! That looks wrong"

While I had the board pulled I changed the Bias resistor and installed the Output Board wire for the headphone Mod so thats everything done and working and sounding like really nice little amp. it now does everything I wanted it to do. I can play loud enough to jam with the lads. I can play quiet enough to practice while the kids are in bed and i can get a great sound out of my Les Paul and my Strat.
Im one happy little Rocker!
Thanks Lyle and thanks to the rest of you who share your sucesses and failures.
I really appreciate the help from everyone on mylespaul over the past couple of weeks. Im stunned by the way people give up their time to help out other like minded people. Its restored my faith in human nature.

[X–Ray]

Originally Posted by bitsleftover

<snip>

Its strange how its possible to make a mistake, check it, re check it and check it again a dozen times without seeing anything wrong, then go back to it another day and think "what the hell have I done there?! That looks wrong"

<snip>

Im one happy little Rocker!

<snip>

Isn't that the truth?

Great to read a happy ending

;~)

[flarocks]

Just as a follow up to the seriously cool info this thread has provided, I have been using my Class 5 head for a couple of months now with all the mods Lyle suggested (done by me!) and I can say that this amp kicks serious ass now!!!!! I play in a ten piece R&B band with three horn players and a violent drummer and the amp keeps up with all of it thru a 1x12 with the volume on 6. It is like a mini plexi, but a real mini plexi...or at least that is my opinion. Seriously, if you haven't finished all the mods or are still on the fence, go for it and don't look back. And to Lyle, you are generous, smart and giving of your time for making this mod possible for common folk like myself. It is a shot out of the park dude!!!!!!!! THANK YOU!

[X–Ray]

Originally Posted by flarocks

<snip>

I have been using

<snip>

all the mods Lyle suggested

<snip>

if you haven't finished all the mods

<snip>

I would love to make these mods to my C5 and have tried to reach Lyle a couple of times to no avail

Problem is, while I know how to operate an amp, I am amp tech dependent

That is, I am not a gear head and try as I might, I have followed this thread in attempting to get a definitive and concise understanding of exactly what parts at what values are needed so I can have my amp tech make the mods but I get brain freeze instead

What I need, if someone were generous to do this, is a kind of, "Class5 Mods For Dummies" which I can then give to my tech because he will not read this thread and I am not sharp enough to explain it, much less really understand all the twists and turns I read a long the way - although, it is probably pretty easy for you folks to comprehend...

Any assistance would greatly appreciated

Thanks!

:~)

[Double D]

Hey everybody! First time poster here, thanks to all and especially to Lyle! I’ve been following this thread for a couple months, studying everyone’s notes and photos, taking my own notes, licking my chops.

Shortly before I found you guys I bought the Limited Edition C5, which must be mkII because the tubes are mounted on a separate PCB facing down. The components seem to match as I’m going through the schematic and mods, but the circuit board is definitely laid out differently.

For example, I had to mount the volume pot’s bright cap on the solder side where the two non-grounded traces meet C9 and C13. I did this because I finally realized my board has no LK19, and though the components seem to match, the LKs apparently will not due to the different layout.

Also, R12 for the “marshall emphasis cap” does not seem to be on the main board, so it must be located on the tube PCB, which I haven’t yet removed. Same for R25.

The OT matches the position of the mkI photos and could probably use the 90 degree rotation (though I’ve been mostly using HBs).

I couldn’t really find any info about the limited edition differences when searching google and this thread, so hopefully the info above helps somebody else.

I'll post more mkII info as I go through. Here is a pic of my board and the mods I've done so far. The C10 jumper is actually the one leg of the volume bright cap (mounted on other side).

Don't mind my R24 Master Volume control wires. I have some questions about that which I'll save for another post.

[bilbo_moria]

Thanks to Lyle and all for this very interesting discussion.

I'm currently implementing mods proposed by Lyle. Still too much boomy bass for my taste so I'm going to "bright up" a little more my Class 5.

A question about replacing C13 by a jumper and removing R9. What I understand is that R9 is there to bias the AC coupled CF (and C13 to avoid scratchy volume pot). If R9 is removed, how is the CF biased ? I've done the mod, it works but it's a sudden idea ... too late for now, I will measure voltages tomorrow ...

Any ideas, gentlemen, am I wrong ?

Thanks to all !

EDIT : btw, I made samples before mods and, of course, I plan to make samples after ...
Stratocaster, Marshall 2045 2x12 closed back.
Clean : http://youtu.be/brsrkzO-0I8
Crunch : http://youtu.be/rcscZnrqh0U

[LesterNtele]

When I played my buddy's C5, I did not like it at all. Very woofy and boomy with that nasty sizzle on top.
Here's what I tried:
Changed V1 tube to a 12AT7 to reduce some gain and allow me to set it on 10.
Turned the vol to 10, treble to 10, mids to 2 and bass to 2.
Plug a Timmy right in front of the input. Turned the gain up a little, cut the bass way down. Voila! Now the amp sounded great. Nice strong crunch without the boominess, yet still maintaining a tighter low end, and quite a nice top end that lost some sizzle, yet is still fairly bright.

[Double D]

Hey everybody, could someone please help me with the Master Volume mod on this amp? I understand there are some different approaches.

The first one I found from Lyle was post #385. This is where he mentions R24/26 junction is the "fixed master volume circuit" and a better place to adjust than R50/51, which is the voltage divider.

I wired a 500K audio taper pot for R24 with no success. Must R26 be changed to 220K? Lyle's schematic has R26 as 470K.

Later I found posts #208/209 about a 1M pot. I should mention that I have the mkII version, but these components match as far as I can tell.

What I'd like the MV to do (if possible) is span from full power to even less than the .1w attenuator volume (for 4am shredding). Is this possible? Can anybody help me figure this out? Anybody with pot value / taper recommendations?

Hope someone's still out there. Thanks!

[X–Ray]

Originally Posted by Double D

Hey everybody, could someone please help me with the Master Volume mod on this amp? I understand there are some different approaches.

The first one I found from Lyle was post #385. This is where he mentions R24/26 junction is the "fixed master volume circuit" and a better place to adjust than R50/51, which is the voltage divider.

I wired a 500K audio taper pot for R24 with no success. Must R26 be changed to 220K? Lyle's schematic has R26 as 470K.

Later I found posts #208/209 about a 1M pot. I should mention that I have the mkII version, but these components match as far as I can tell.

What I'd like the MV to do (if possible) is span from full power to even less than the .1w attenuator volume (for 4am shredding). Is this possible? Can anybody help me figure this out? Anybody with pot value / taper recommendations?

Hope someone's still out there. Thanks!

Hope your luck is better than mine in getting a response, haha

;~)

[forklord]

just got the parts and some new tubes in the mail, havent bothered picking at it yet cuz even after i do the mod i'll still be without a gain or master volume circuit....i've been looking at some attenuators but not sure what would work well with these mods....theres a little sucker cheap on ebay right now and the reyes audio one. what do you guys think? I tried to get ahold of that guy at hall amplification with his VVR mod but to no avail...

is an attenuator what i even need? or some sort of power scaling device? there are a few terms being thrown around and not too sure what the difference is.

[glpease]

Originally Posted by Double D

Hey everybody, could someone please help me with the Master Volume mod on this amp? I understand there are some different approaches.

The first one I found from Lyle was post #385. This is where he mentions R24/26 junction is the "fixed master volume circuit" and a better place to adjust than R50/51, which is the voltage divider.

I wired a 500K audio taper pot for R24 with no success. Must R26 be changed to 220K? Lyle's schematic has R26 as 470K.

Later I found posts #208/209 about a 1M pot. I should mention that I have the mkII version, but these components match as far as I can tell.

What I'd like the MV to do (if possible) is span from full power to even less than the .1w attenuator volume (for 4am shredding). Is this possible? Can anybody help me figure this out? Anybody with pot value / taper recommendations?

Hope someone's still out there. Thanks!

Leave R24 alone. You can put a 500k MV pot at R26, and the easiest way in the second generation version is to remove R26, and wire the two connections, instead, to pins one and three of the MV pot. The wiper of the pot then has to be connected to the grid stopper of V3. The easiest way to do this is to find the wire on the interconnect between the main circuit board and the sub-board the valves are mounted on, cut it, and splice in there. Or, you can do some trace cutting on the main board. The idea is that you want to break the connection between the R24/R26 junction that feeds V3. The wiper of the pot will then connect there, and the pot's two outer terminals will connect where R26 was.

I'm not sure a MV will really be much use with the stock circuit topology, though driving the preamp hard and reducing the amplitude of the signal to the EL84 will certainly help get rid of some muddiness and piercing highs.

One change I highly recommend is changing C11 to at least a 47µF/350V cap. I saw a lot of signal on the B+ with the stock 10µF, and replacing this cap cleaned it up, and did a lot to tighten things up. Changing C13 to 47µF wouldn't be a bad idea, either.

There's a potential for oscillation at the input side. I removed R20 and put a 1M directly across the input jack, removed all the links between the input and the connector between the boards, and used mini shielded cable to connect to the grid of V1a, which improved things a lot.

Also, a 1M across the volume control, and changing R7 to 68k reduced hiss at full volume a lot, by my amp currently is so radically different from the stock one, I'm not sure the increased HF response would be welcome in a more or less stock circuit...

[bilbo_moria]

Originally Posted by glpease

One change I highly recommend is changing C11 to at least a 47µF/350V cap. I saw a lot of signal on the B+ with the stock 10µF, and replacing this cap cleaned it up, and did a lot to tighten things up. Changing C13 to 47µF wouldn't be a bad idea, either.

You mean C3, right ?

[glpease]

Originally Posted by bilbo_moria

You mean C3, right ?

Actually, I mean C11, the smoothing capacitor after R2. WIth a hot signal into V1, I scoped the B+ and found just enough of the signal present to be of concern. Possibly a bad cap, but either way, since I was replacing it, and had a 47µF handy, problem gone.

EDIT: Oh! And, no I don't mean C13, but C15! Failing eyeballs...

Putting a 47µF at C13 would be silly. Sorry for that!

[Double D]

Thanks very much for your reply, GLPEASE. I read through many of your earlier posts while I was planning the mods out. Your suggestion for the MV circuit makes sense based on my intermediate knowledge of electronics. I will try this, but I still have uncertainty: When you say leave R24 alone, do you mean leave it as 100k per Lyle's schematic or what it was (which I've forgotten)? 470K?

Your other suggestions sound interesting as well. I should clear up that I've done the majority of Lyle's mods already. And, as a twice veteran of BillM's Blues Jr. mods and general tweeker, I'm curious to know more about yours.

On the volume control circuit, I've already done Lyle's suggestion of the 120pf cap across the 2 non-grounded lugs (which seems a bit too bright when I'm in full power with the volume set low), I jumped C13 with one of the legs of the 120pf bright cap, and switched R7 to 470K. Could you expand on the benefits of your values and putting a 47uf cap at C13 rather than jumping? Does the 1M across the volume you suggest go in parallel with the 120pf bright cap and work in conjunction with your R7 value of 68K?

Your recommendation for upping C11 from 10uf to 47uf, does this relate to "power stiffening" (in the parlance of BillM)? Tighter response, eh? Is your C13 suggestion needed in conjunction with this?

Finally, I believe I see what you mean about cleaning up the path of the input signal. Did you leave the circuit-mounted jack or switch to a switchcraft-style open jack? You wired the 1M resistor right from the tip lug to the (grounded) ring lug? Did you use shielded GUITAR? cable to send the input signal to right before the cable route to valve board? Did you solder its shield to grounds? (I believe R19 is on the valve board in my mkII version, so the input signal would still get to it after the board-to-board cable route).

Really cool suggestions, GLPEASE, thanks again. I guess most urgent for me is finding out what value that R24 is for the MV mod so I can get jamming on the amp while I'm pondering these other options.

Rock on!

[glpease]

Originally Posted by Double D

Thanks very much for your reply, GLPEASE. I read through many of your earlier posts while I was planning the mods out. Your suggestion for the MV circuit makes sense based on my intermediate knowledge of electronics. I will try this, but I still have uncertainty: When you say leave R24 alone, do you mean leave it as 100k per Lyle's schematic or what it was (which I've forgotten)? 470K?

The overall maximum level will be determined by the divider formed by R24/R26, so what you do with R24 will depend on the value used for R26, and whether or not you want a little or a lot of attenuation at full rotation. You could actually do away with R24, with the realization that at full volume, you might be overdriving the EL84 to the point of nastiness. Lyle's 100k is a good value. I'm running directly from the output of the tone stack, so attenuation isn't necessary. I'd personally play with the values a little until I found a combination that was pleasing at full volume. If you use Lyle's 100k and a 1M pot, you should be in the ballpark. The nice thing about having too much signal is that you can always turn it down.

Your other suggestions sound interesting as well. I should clear up that I've done the majority of Lyle's mods already. And, as a twice veteran of BillM's Blues Jr. mods and general tweeker, I'm curious to know more about yours.

On the volume control circuit, I've already done Lyle's suggestion of the 120pf cap across the 2 non-grounded lugs (which seems a bit too bright when I'm in full power with the volume set low), I jumped C13 with one of the legs of the 120pf bright cap, and switched R7 to 470K. Could you expand on the benefits of your values and putting a 47uf cap at C13 rather than jumping? Does the 1M across the volume you suggest go in parallel with the 120pf bright cap and work in conjunction with your R7 value of 68K?

Your recommendation for upping C11 from 10uf to 47uf, does this relate to "power stiffening" (in the parlance of BillM)? Tighter response, eh? Is your C13 suggestion needed in conjunction with this?

Finally, I believe I see what you mean about cleaning up the path of the input signal. Did you leave the circuit-mounted jack or switch to a switchcraft-style open jack? You wired the 1M resistor right from the tip lug to the (grounded) ring lug? Did you use shielded GUITAR? cable to send the input signal to right before the cable route to valve board? Did you solder its shield to grounds? (I believe R19 is on the valve board in my mkII version, so the input signal would still get to it after the board-to-board cable route).

Really cool suggestions, GLPEASE, thanks again. I guess most urgent for me is finding out what value that R24 is for the MV mod so I can get jamming on the amp while I'm pondering these other options.

Rock on!

The topology of mine, at this point, is V1a operating as a gain stage, V1b not used, V2 as a typical Marshall gain/CF stage (820 Rk, bypassed by 0.68µF, the way Marshall did it for a couple years - a little mid boost, and a nice driving tone), then the tone stack (33k slope resistor), the output running directly to the MV, then the EL84. The rewiring of V2 is pretty straight forward on the second version of the C5, since it can all be done on the valve sub-board, with a mess-o-parts pulled out of the main board. Moving the tone stack was a little trickier, but a little shielded cable, and you're golden.

I'm still using the PC mounted input jack (though I replaced it with a new one, since the stock one had a faulty ground switch on the tip). The 1M resistor is just soldered on the board directly underneath the jack, tip to ground. I removed a few of the "links", and simply used a piece of shielded mini-cable to go directly from the tip to the interconnect plug at the other end of the CB. Ideally, actually, it should go directly to the sub-board, but I didn't feel like doing it, and the tendency to oscillation is minimized now, if not completely eliminated. Only connect the shield to ground at one end - in this case, the jack end.

There's a hell of a lot of EMF floating around all those links, and the CB designer didn't do a very good job of separating high/low impedance traces, so there are lots of places where coupling might be a problem if you juice the gain. I'm seeing leakage signal on V1b's anode with no signal at all on the grid. I'm not sure whether this is electron spill in the valve itself, or crosstalk - since I'm not using it at the moment, I just ignored it for the time being. Next time I open the amp, I'll do some more investigating, at least change V1 to see if it goes away. (I seem to recall a bad series of Phillips, I think, ECC83s that were notorious for crosstalk between the anodes.)

Power supply stiffening - more or less. I'm running the first triode pretty warm, so it doesn't hurt.

I don't have my notes in front of me, so I'll have to respond to the other bit, about R7 and so on, later.

Cheers,
Greg

[glpease]

Originally Posted by Double D

On the volume control circuit, I've already done Lyle's suggestion of the 120pf cap across the 2 non-grounded lugs (which seems a bit too bright when I'm in full power with the volume set low), I jumped C13 with one of the legs of the 120pf bright cap, and switched R7 to 470K. Could you expand on the benefits of your values and putting a 47uf cap at C13 rather than jumping? Does the 1M across the volume you suggest go in parallel with the 120pf bright cap and work in conjunction with your R7 value of 68K?

See my corrected comment about C13 above. I meant C15. I blame my fuzzy eyeballs for misreading the tiny blue print on the schematic.

Changing R7 to 470k will reduce the amplitude of the signal to the CF a little, and that's probably not a bad thing. Since it's a CF, you don't have to worry about HF loss from capacitance, but might generate a little more hiss. Personally, I'd probably reduce it, not make it bigger.

The 1M across the volume pot (across the outer terminals) turns it, more or less, into a 500k pot, which will also tame a little his, with very little effect on the gain of the first stage.

The only reason I don't like C13 jumpered is that you'll get DC on the volume pot, and it can make it scratchy. IF I were going to put a DC coupled CF after the first gain stage, I'd put the volume pot after the tone control, but that's some trace cutting, rebiasing and careful wiring to get right, for very little result. I don't really see the problem with the AC coupled CF that some people do. Maybe I'm missing something.

Cheers,
Greg

[Double D]

Greg, thanks for all that great info. Sounds like you know this circuit cold and you've created quite a little beast of your own.

I dug into the MV circuit last night and started with a 1M guitar pot I had lying around and the 100K suggestion for R24. Aside from me accidentally wiring the pot reverse (full attenuation at normal 10, wide open at normal 1), I'm finally getting the MV effect. It definitely adds some cool flexibility to be able to drive your preamp tubes and EL84 separately.

With this 1M / 100K combo on R26/24, however, I find that all the low MV action happens in the tiny mechanical space between 0 and 1 (or 10 and 9 with my current accidental reverse wiring). Also the full power seems to fill out at around midway on the 1M MV pot. I didn't really notice any EL84 harshness past that full volume point on the dial (like you mentioned might happen) but I wasn't able to really crank the amp last night .

Any ideas how I might spread that 0-1 MV action out over more turning distance and have the wide open setting be farther up the dial? Different taper / values? I'm not exactly sure how the voltage divider works between those 2 resistance values, but I also assume a guitar pot is not the ideal style pot for an amp.

I should also mention that I've added the Hammond 158M choke and rotated the OT 90 degrees on mine, really happy with the results from that. I currently don't seem to have too much problem with hiss, but I've added your fixes to my notes anyway, thank you very much.

Also, I'm definitely going to do your input pathway fix. It's a really good call, that stock signal path does look messy and fragile in the mkII version.

Thanks again,
-Dirk

[Iron Broadsword]

Anybody got any clips of this beast after the mods they've done?

[blamo]

nope.

no such clips exist.

[Garrincha]

I think I've recorded a demo bit with the C5 after I did the mods. When I get home tonight I'll see if I can dig that one out. It's nothing special and was meant to test drive a new strat more than the amp mods...

[glpease]

Originally Posted by Double D

Greg, thanks for all that great info. Sounds like you know this circuit cold and you've created quite a little beast of your own.

I dug into the MV circuit last night and started with a 1M guitar pot I had lying around and the 100K suggestion for R24. Aside from me accidentally wiring the pot reverse (full attenuation at normal 10, wide open at normal 1), I'm finally getting the MV effect. It definitely adds some cool flexibility to be able to drive your preamp tubes and EL84 separately.

With this 1M / 100K combo on R26/24, however, I find that all the low MV action happens in the tiny mechanical space between 0 and 1 (or 10 and 9 with my current accidental reverse wiring). Also the full power seems to fill out at around midway on the 1M MV pot. I didn't really notice any EL84 harshness past that full volume point on the dial (like you mentioned might happen) but I wasn't able to really crank the amp last night .

Any ideas how I might spread that 0-1 MV action out over more turning distance and have the wide open setting be farther up the dial? Different taper / values? I'm not exactly sure how the voltage divider works between those 2 resistance values, but I also assume a guitar pot is not the ideal style pot for an amp.

I should also mention that I've added the Hammond 158M choke and rotated the OT 90 degrees on mine, really happy with the results from that. I currently don't seem to have too much problem with hiss, but I've added your fixes to my notes anyway, thank you very much.

Also, I'm definitely going to do your input pathway fix. It's a really good call, that stock signal path does look messy and fragile in the mkII version.

Thanks again,
-Dirk

Reverse the leads to the pot! It's a log taper, so when you wire it in backwards, you're getting exactly what it's supposed to do that way. As for guitar/other pots, the only thing you have to consider is the taper. (Of course, it's possible it's a linear taper, in which case it'll be the same backwards as forwards. Some people like linears in guitars, some log or audio taper. If you google "log taper pot" you'll find some very good graphs and charts on the things.)

One of these days, I need to make some recordings of the thing. The only thing I can say is that whenever I turn it on, I simply don't want to stop playing it, which is my only real test for an amp or a guitar. If I can just noodle around for hours without thinking of the passage of time, it's a good one. This amp now passes the test. Before, for me, it was more fiddling, and less playing, trying to find those magic places on the knobs where I just fell in love with the tone. Now, I've turned the dials all over the place, and loved it, and I've jacked it up to a couple different cabinets and loved it. I spent FAR too much time on this thing, but in the end, it was worth every minute to me. And, with the MV, I can get some pretty nice crunchy tones at fairly low volumes. It doesn't really sing until the EL is working, but I can suffer through that if it means not finding myself face to face with a 12 gauge after waking SWMBO at three in the morning.

I almost want to get another one...