Some Mods for the Vox Nighttrain

Lyle Caldwell

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Had a client send in his Night Train. Pretty good amp in stock form, but too bright and harsh. Not really pleasant to play, was never really happy with the sound.

The good news is it really is a pretty good amp in stock form, so not much was needed to make him happy.

Here are the tweaks (huge JPG so I'm linking):

http://psionicaudio.net/images/Vox_NT15_mods.jpg

From the client: "Just had band rehearsal last night, the Night Train sounds killer! I ended up skipping the pedal board and plugged straight in, which I would have never done before! Thanks so much!"

I'm still not a big fan of the MV implemented in this amp, but it would be too expensive to mod that section. So the amp will clean up quite a lot as the MV is turned down. At real stage volumes, not much of an issue.

And if you are careful, you can do these mods without removing the board or anything from the chassis.
 

Alligatorbling

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lyle you are a wealth of modding info....

btw, what does the other side of your face look like? you should make another avatar, but of the other side of your face.
 

tazzboy

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Can't wait to see what he comes up with for the Vox AC 100 Classic Players Head.
 

SJJMcGhee

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Man, sure you've had the pleasure of using the ac30 cc2 and the c2, which would you say is the better amp, purely in terms of things such as reliability? Looking at one, for a giigng amp, don't want it dying on me :fingersx:

Cheers man.
 

Lyle Caldwell

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They're the same in terms of reliability. Build quality is the same, components are the same, etc. CC has more features, C2 has better cosmetics (apparently Vox was having issues getting EU approved fireproof grillcloth when they came out with the CC, and the panels can be anywhere from pink to purple, though there are a lot that have the "right" red).
 

SJJMcGhee

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They're the same in terms of reliability. Build quality is the same, components are the same, etc. CC has more features, C2 has better cosmetics (apparently Vox was having issues getting EU approved fireproof grillcloth when they came out with the CC, and the panels can be anywhere from pink to purple, though there are a lot that have the "right" red).
Cheers man, much appreciated on the info, cc2 is usually cheaper used, probably go for one of them, thanks alot man :wave:
 

Ed B

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lyle you are a wealth of modding info....

btw, what does the other side of your face look like? you should make another avatar, but of the other side of your face.
He's got a Tyson tattoo. :laugh2:
 

Lyle Caldwell

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OK...

First, C19 after the 10K R21 rolls off highs above 16K. That's pretty useless, but as we are removing R21 because it forms a voltage divider with R20 (we don't want to pad the input and if we want to reduce gain there are better places to do it) we are removing C19 and jumpering R21.

With that done, we then increase R22 from 27K to 33K. You can actually use anything between 33K to 68K here.

The next stage changed is the tone stack. What Vox is doing here with the "Bright/Thick" switch is going between the tone stack (which is indeed bright!) and a tonestack bypass.

When the tonestack is bypassed, the signal goes through C28 which in conjunction with the volume pot (we're ignoring the slight impact of R35 because we're jumpering it - read on for that) creates a highpass filter at 32Hz so the amp is full range with gobs and gobs of gain.

But when the tonestack is engaged in stock form, the stock 150K R35 before the volume pot makes the 47pF C27 have a much bigger impact on the brightness - it's frankly unpleasant. Way too much glass, actually hurts to listen to. Jumpering R35 both takes away the nasty highs and lets the amp have more gain overall.

Before we leave the tonestack, we're changing C29 from 250pF to 500pF (or 470pF if that's what you have).

With 250pF, the midrange is slightly scooped centered at 800Hz and there is emphasis starting at about 3K.

With 500pF, the midrange is slightly scooped centered at 500Hz but there is emphasis starting at about 1K.

I find the second setup to be more musically useful - the amp has more "attitude" and sits in a mix better. If your idea of good tone is Dream Theater, keep the 250pF cap.

Next we add the "Marshall emphasis" circuit to give the amp a little more oomph - just a 470K resistor in parallel with a 500pF (or 470pF) cap before the grid of V2. This puts a highpass filter in at 678Hz (500pF) or 721Hz (470pF). There are still plenty of lows, but the amp gets more aggressive and brighter without being treble-dominant.

After V3b the stock .0022uF C22 and 150K R27 create a highpass filter centered at 482Hz. This is too much in conjunction with the emphasis we added in the previous step, but the previous step is the better place to add this because pF value caps come in a wider range of values than larger caps. Anyway, we want to lower the highpass frequency here and while we're at it, use a higher quality cap than stock. But we also want to put in a voltage divider here so the gain doesn't get splatty. So we use a .022uF followed by a total impedance of 197K is used, which sets the highpass at 37Hz. So C22 becomes .022uF and we add a 47K resistor inline with the new C22 which forms a voltage divider in conjunction with the 150K R27. And don't worry, even with this voltage divider, there is still tons of gain available. If you want a little less gain, use a larger resistor than 47K up to 150K.

Then after V4a we change C10 to .022uf mostly because we're upgrading the coupling caps and a high quality .022uf cap takes the same board space as the stock low quality .1uf cap.

Then for some of the same reasons we change the .1uf C16 and C26 to .022uF. And we change the 470K R33 and R34 to 220K. This both changes the highpass from 3Hz to 33Hz but also changes the time constant in the output section, so the nature of the overdrive is a little less harsh. Changing the 3.3K R5 and R6 to 10K also helps.

We're not messing with C15 and C30 because there is no significant voltage across them, so they don't effect the tone aside from setting additional highpass filters, but as they are set at 8Hz they are "transparent."

Last, we upgrade the 470R R7 and R8 to 1Ks to offer the EL84s a little bit more protection and draw a little less current. I used 5Ws but 1W is fine.
 

tab

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Hey Lyle -
I've been down the C5 path with most of the modifications suggested in that thread so I'm excited to venture down this path with your suggestions on the Night Train.

It seems like 50% of these amps have a 60 cycle hum issue - some people have swapped tubes to remedy the issue and others can't seem to get rid of it regardless of what they try (tubes, power conditioners, cables). Mine has a slight hum, nothing bad, but a friends has a noticeably louder hum. Could any of the suggested modifications you have listed for this amp help that situation?

I'm also assuming that 1W Carbon Film Resistors and 500v Silver Mica caps will work fine?
 

tab

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I had a chance to rip into my amp today and everything I got done went well.
I removed the PCB which made it fairly easy to install and remove the parts.

I'm used to seeing hot glue on circuit boards but this amp has a yellowish glue that was used. I ended up scraping it off in a few places to identify the parts. To save others a little time if they dive into this I snapped a photo and labeled the components.

http://i859.photobucket.com/albums/ab154/fastback6nine/NT15H_mod.jpg

I ordered a few incorrect parts so this will have to wait to get finished until the new parts come in.

One item I actually have the parts for but am unsure where to place them is for the "Marshall emphasis" circuit. Lyle is there any chance you could point a finger to help me along? I see where it's called out on the schematic you modified but am not sure where the trace is on the PCB.
 

wjdunn3

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I bought one of these recently also and was contemplating doing these mods. I've reading the Lyle's write-up trying to understand all the parts. TAB, since you have been through the parts, would mind detailing the parts list...because I was not sure exactly what kind of caps to get and voltage values, or what wattage ratings on all the resistors.
 

tab

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Lyle did a great job of outlining the replacement parts above :applause: but I'll share what I grabbed.

C19 - Removed Part
R21 - Jumpered contacts on the back of the PCB
R22 - 33k 1W Carbon Film Resistor
R29 - 500pf 500v Silver Mica Capacitor
R35 - Jumpered contacts on the back of the PCB

C22 - .022uf 630v Mallory Cap with 47K 1W resistor in parallel
C10, C16, C26 - .022uf 630v Mallory Capacitors

R33, R34
- 220k 1W Carbon Film Resistors
R5, R6 - 10k 1W Carbon Film Resistors
R7, R8 - 1K 1W Carbon Film Resistors

Marshall Emphasis Circuit - 470k 1W Carbon Film Resistor with 500pf 500v Silver Mica Capacitor in paralell

The .022uf Mallory caps were the parts I ended up being short, so those are on order. The Marshall Emphasis Circuit is one thing I'm waiting for more info on. I don't know where to place the parts. I hope Lyle will have time to respond - and there's local amp repair places I can visit if not.

I grabbed all of my parts from tubedepot.com
Their site is easy to navigate and shipping is fair for such small and light part orders.

I count 15 total parts that are needed.
When you trim the legs off of the caps\pots as you fit them into place you can use two of them for the needed jumpers.

Had I not goofed on the .022uf parts and only had one complete order shipped, parts + shipping would have been $12.00.
 

David Corrales

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Lyle did a great job of outlining the replacement parts above :applause: but I'll share what I grabbed.

C19 - Removed Part
R21 - Jumpered contacts on the back of the PCB
R22 - 33k 1W Carbon Film Resistor
R29 - 500pf 500v Silver Mica Capacitor
R35 - Jumpered contacts on the back of the PCB

C22 - .022uf 630v Mallory Cap with 47K 1W resistor in parallel
C10, C16, C26 - .022uf 630v Mallory Capacitors

R33, R34
- 220k 1W Carbon Film Resistors
R5, R6 - 10k 1W Carbon Film Resistors
R7, R8 - 1K 1W Carbon Film Resistors

Marshall Emphasis Circuit - 470k 1W Carbon Film Resistor with 500pf 500v Silver Mica Capacitor in paralell

The .022uf Mallory caps were the parts I ended up being short, so those are on order. The Marshall Emphasis Circuit is one thing I'm waiting for more info on. I don't know where to place the parts. I hope Lyle will have time to respond - and there's local amp repair places I can visit if not.

I grabbed all of my parts from tubedepot.com
Their site is easy to navigate and shipping is fair for such small and light part orders.

I count 15 total parts that are needed.
When you trim the legs off of the caps\pots as you fit them into place you can use two of them for the needed jumpers.

Had I not goofed on the .022uf parts and only had one complete order shipped, parts + shipping would have been $12.00.
I'm about to try these mods as well... I've tried everything with the NT. Even bought an EQ pedal but it just won't gel with me.
 

tab

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The suggested changes Lyle posted for the Marshall Class 5 ended up working out great for me - I love that amp now!

The NT15H isn't a bad amp, but it was a little harsh or bright for my tastes. I believe this will be another good change for the better once everything is installed and the head is landed back on top of the 1960AV.
 

zepsongs

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I keep reading a lot of people claiming the Vox NT is bright, just what settings and what speakers is everyone using with these heads?

I run mine through an Avatar 2x12 loaded with Eminence Delta 12s. I keep the gain at 9 o'clock, the treb & bass between 12 & 1 and adjust the mids as needed (currently maxed).

I bought it to replace my blown Tweaker. I figured for the money if I I wasn't happy with it at least it would make a good backup or loaner. So far I have been very impressed with little beast, handle pedals like a champ. About the only thing I could see doing to it would be to replace the stock tubes for a bit more headroom.
 

jordans0nly

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None of this stuff makes sense to me, lol.

Do those Avatar cabs come pre loaded? I'd love to get a 2x12.
 

tab

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I've got everything in place other than the emphasis cap\resistor.
Excited to get that in place and fire this thing up...
 


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