Vox AC30 Custom Classic Common Issues

Lyle Caldwell

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Go to tubedepot.com. While the same parts cost less from Mouser, it's much more difficult to know what to use at Mouser, whereas Tube Depot lists only things useful for amps.

There are three kinds of resistors you'll want:

Ceramic for high wattage resistors (screens on a Marshall output tube for instance).

Metal oxide for 1-2W resistors in the power supply.

.5W or 1W carbon film resistors in the signal path. Most schematics say "all resistors .5W unless noted" but look for where they specify 1W, 5W, 10W, etc.

Caps are trickier. Most pf value caps should be silver micas. Most caps under 1uf should be film caps, whether Mallory, Sozo, or Orange Drops. Orange Drops are more typically used in Fenders, Mallorys and Sozo "mustard caps" more often in Marshalls and Voxes. Sozo has a new line of blue caps designed for Fenders - I haven't used them yet.

Electrolytic caps used for filter purposes have a polarity and a connection style - either radial (leads on one side of the cap) or axial (leads on each side of the cap). Choose the connection style that fits your board.
 

nedhogan

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I have a niggly question; Regarding V1, each side now uses a 220k plate resistor but each side shares a single 1.5k cathode resistor whereas the old ones had one each. Do you know if this affects the biasing adversly from the originals? I know changing the value of this affects the bias but does sharing?
 

lespaul01

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I clipped the bright cap on my CC2 and love how it sounds. This amp takes pedals great too. My amp has never sounded better since putting in a Nice Metal Base Amperex GZ34, 4 Amperex Sittard Holland 7189(industrial El84) Box Getters and 3 Amperex 12AX7. Although I find that a nice low noise 7025A in V1 makes this baby dead quiet.

This amp is great for tube rolling and it has seen at least 100 old stock tubes in the last 6 months.

Thanks for the mods and your time.
 

Lyle Caldwell

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I have a niggly question; Regarding V1, each side now uses a 220k plate resistor but each side shares a single 1.5k cathode resistor whereas the old ones had one each. Do you know if this affects the biasing adversly from the originals? I know changing the value of this affects the bias but does sharing?
The JMI era AC30s all had shared cathodes here except the non-Top Boost Treble amps.
 

Thundergod

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Those 220K resistors... are they any 220K resistors or is there any other value one should take on account? (Ohms? Watts? Sorry, complete amp ignorant here).
 

nedhogan

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Those 220K resistors... are they any 220K resistors or is there any other value one should take on account? (Ohms? Watts? Sorry, complete amp ignorant here).
1 watt or 1/2 watt. I put vintage 1 watt carbon comp ones in. Like I said previously though, they just to say fit in the circuit board holes. 1/2 watt ones would be just as good, just more stray in resistance due to heat and a shorter root-mean-time to failure. The stock resistors are 1/2 watt though. I just like over engineering!
They make it sound a bit more gainy sooner on the volume and subtly changes the tone for the better IMO. I think the gain goes a bit too far in the wrong direction at full volume but has a great tone to it half way to two thirds. Really sounds far better at gigs now.

Just be carefull, the screws on the board are made of lead, you need the perfect fit screw driver and alot of luck, then you need to unclip all the connecting buses then be CAREFULL, remove the 12ax7's that are a pain to get back in again.
 

nedhogan

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I feel like a thread hog but, I've been thinking about putting a post volume knob on the TB channel and installing a channel switch rather than the blender switch.
My question is.. What size pot do you suggest and would it need a treble bleed cap and a coupling cap or could I just plonk it in? I think it could go in just before the opamp?

Also thanks for all your replies your very helpfull, great thread!
 

Lyle Caldwell

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Those 220K resistors... are they any 220K resistors or is there any other value one should take on account? (Ohms? Watts? Sorry, complete amp ignorant here).
220K = 220 thousand ohms.

I think I said earlier in the thread, you can use 1/2W or higher. I usually use 1Ws in amps.

Not to rain on Ned's parade, but I loathe carbon composite resistors.
 

Lyle Caldwell

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I feel like a thread hog but, I've been thinking about putting a post volume knob on the TB channel and installing a channel switch rather than the blender switch.
My question is.. What size pot do you suggest and would it need a treble bleed cap and a coupling cap or could I just plonk it in? I think it could go in just before the opamp?

Also thanks for all your replies your very helpfull, great thread!
I'm not sure how useful a master for the TB channel would be but you could do one easily enough. There's no DC voltage present after the tone stack, though the CC does have a coupling cap there to protect the opamp if a cap in the tone stack should fail and pass DC. Anyway, R16 (150K) and R17 (270K) already form a voltage divider - replace them with a 500K audio taper pot and you're fine. No need for a treble bleed cap here.

A channel switch is easy to implement. Just keep the inputs linked and wire a switch that either grounds out the "upstream" side of R15 or the "upstream" side of R17. Hmm, use an on-off-on switch so the middle switch position would be both channels.
 

nedhogan

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Interesting. I think it would be usefull so that I could switch quickly between songs without messing up the levels. After I replaced the reverb tank, the normal ch sounds really surfy with the master right up. So the TB is rediculously loud if I switch quickly. I'll probs use an a/b/y pedal.

I wouldnt want the TB to go any louder really, so would it work well if I left the 270k and replaced the 150k with a 150k pot or would that mess up the tone?
Would I need a double gang 150k pot? To keep the total Q the same or something! i.e. increasing the 270k as I lower the 150k to ground.

In regard to carbon comps... Just followed the crowd! So what, metal film or carbon film ones then?
 

Lyle Caldwell

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Interesting. I think it would be usefull so that I could switch quickly between songs without messing up the levels. After I replaced the reverb tank, the normal ch sounds really surfy with the master right up. So the TB is rediculously loud if I switch quickly. I'll probs use an a/b/y pedal.

I wouldnt want the TB to go any louder really, so would it work well if I left the 270k and replaced the 150k with a 150k pot or would that mess up the tone?
Would I need a double gang 150k pot? To keep the total Q the same or something! i.e. increasing the 270k as I lower the 150k to ground.

In regard to carbon comps... Just followed the crowd! So what, metal film or carbon film ones then?
If you just want to adjust the balance between the two channels at two fixed preferred settings then you don't need a pot at all. To decrease the level of the Top Boost channel just increase the value of R17 a bit. As it increases in value the volume of the TB channel will go down in the mix. Stock is 270K. Odds are you won't want to go higher than a 470K. A 330K will probably do you nicely.

See here for my thoughts on resistors:

Psionic Audio - Articles - "Mojo" Parts in Guitars, Effects, & Amplifiers Part 1: Resistors
 

nedhogan

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On checking my carbon composite plate resistors in my amp, found them to have drifted from 220k to 245k. That seems like a heck of a lot of drift!! The thing that made me feel like posting is that I replaced them with carbon film, expecting it to sound even better but, it sounded dull like it sounded un-modded. Instantly replaced them agian with the old drifted carbon comps and was happy again.

Told myself off for worrying that they had drifted too far out. AC's had drifty carbon comps in them in the 60s and sounded great... just the occational fire. ah ah

I disagree with you that carbon comps sound no different to modern resistors.
They are more un-relieable but sound great! Either that or Im used to hearing 240k resistors where there should be 220k

Did notice though, Different solders and different quality soldering make a big difference. I used silver solder and found it to unfortuntly, really cut the bass. The shody soldering I had done the first time round sounded best.
 

Lyle Caldwell

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Ned,

Not sure what you are saying.

You describe the un-modded sound as dull, then the modded sound as dull, then the old resistors (the previously dull un-modded sound) and you're happy.

Work on a LOT more amps before forming opinions on resistor types.
 

bluedot

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Hi guys,

I'm having troubles :(. I decided to mod my VOX AC30 CCH with the mods in the first post by Lyle Caldwell and I was hoping I could get some troubleshooting advice. I can't currently post pictures of the work I've done, so I will let you know what problems I am having, and I will take pictures later this afternoon and post them. Thanks so much in advance for any help.

Here are the mods I did:

"1 - R6 and R7 are the plate resistors for V1 (Top Boost and Normal channels). In the Custom Classic they are 100K resistors. Change these to 220K resistors. CHECK.

2 - there is a 120pf bright cap (C13) across the Top Boost volume knob.
REMOVED INSTEAD OF 68 pf SILVER MICA.

3 - Vox went with a 56pf treble cap in the tone stack (C16). Changing this to the "vintage" 47pf value very slightly reduces the treble.
CHECK.

4 - they have made the Normal channel fairly useless in tone (way too dark or way too bright) and limited its max gain. Jumper the 330K resistor ahead of the Normal volume pot (R49), change the 220pf bright cap (C12) to a 100pf cap, and then put a 68pf cap across lugs 1 and 2 of the Normal volume pot. This will allow the Normal channel to have more gain and go from a warmer but not too dark sound to a brighter but not too bright sound.
68 pf cap across lugs one and two of normal volume pot - check. I don't know what a jumper is, so I did not perform this mod on R49. "I also did the trem speed mod: Replace R37 with 100k 1/2 watt metal film resistor. Replace R38 with 27k 1/2 watt metal film resistor.

Here are the problems I'm having: my volume output is so low I have to turn the volume channels up all the way to get a moderate level of sound out of my speakers. Volume works on each independent channel - normal, top boost, and the two blended - but they are all very quiet. The tremolo doesn't work at all now, and I am left wondering if the resistors are directional.

I will post pictures this afternoon - I just wanted to post this now as I'm eager to get my amp working!

Thanks guys,

Jason
 

b-squared

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Just to throw a plug for Lyle...he modded my CC2X; I drove to Memphis to drop it off at his house.

When I drove down to pick it up, he had it sitting next to a '67 that he'd just gone through for a guy--it looked awesome.

Anyway...he made me turn my back, and he proceeded to jam on the '67--great, GREAT tone.

He then plugged into mine, and tweaked the knobs a bit...and got the EXACT same tone. He then showed me how to switch from a '67 6-input model to the 4-input '64 model (think Jimmy Page tone) with the flip of a micro-switch.

Very cool...The Edge tones, and JP tones.

I loved the amp--had to eventually give it up because it was just too darn heavy for me to move around without a roadie...with the road case it was well over 100 pounds.

I sold it to another MLP member, and as far as I know he's still happy with it. :D

Lyle is dependable, and his turnaround is better than most, I think.

BB
 

Lyle Caldwell

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1) Resistors are not directional

2) Jumpering a component means to solder a piece of wire across the component, effectively taking the component out of the circuit.

Without detailed photos this is hard to troubleshoot, though I suspect your problem is in the trem circuit. Make sure you don't have any solder globs that are touching other connections there.
 

Lyle Caldwell

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Thanks man! Next time we meet, remind me not to have the flu. That was a surreal day.

Just to throw a plug for Lyle...he modded my CC2X; I drove to Memphis to drop it off at his house.

When I drove down to pick it up, he had it sitting next to a '67 that he'd just gone through for a guy--it looked awesome.

Anyway...he made me turn my back, and he proceeded to jam on the '67--great, GREAT tone.

He then plugged into mine, and tweaked the knobs a bit...and got the EXACT same tone. He then showed me how to switch from a '67 6-input model to the 4-input '64 model (think Jimmy Page tone) with the flip of a micro-switch.

Very cool...The Edge tones, and JP tones.

I loved the amp--had to eventually give it up because it was just too darn heavy for me to move around without a roadie...with the road case it was well over 100 pounds.

I sold it to another MLP member, and as far as I know he's still happy with it. :D

Lyle is dependable, and his turnaround is better than most, I think.

BB
 

b-squared

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I remember you being sick, but didn't mention it. :D

Hope all's well with you!

BB
 


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