Coaxing the JCM out of a Peavey VTM 60..

[azrael]

I recenty acquired a Peavey VTM 60. Sounds great, got it for a great price. Only 300 for a fullstack!
I retubed it with JJ6L6GCs.

Now, I'm looking to get a little closer to that tone in my head...I knew it would be necessary, but as a budding EE student, I was willing to take up the challenge.

I need it punchier, fatter, thicker midrange, with less fizz.

I've been doing some research, and have a small list of mods that may help. Any input is welcome, whether its on how effective these may be, or if it's a bad idea, or if you think there's something else I can do to get that tone.

- remove the bright cap, C1
- replace R2 with 220K or 330K
- replace R5 with a choke
- replace R8 with 50k pot and 10k limiting resistor

a couple of questions:
what value should the choke be?
What should the wattage rating on the bias pot and resistor be?

[tedmich]

I recenty acquired a Peavey VTM 60. Sounds great, got it for a great price. Only 300 for a fullstack!
I retubed it with JJ6L6GCs.

Now, I'm looking to get a little closer to that tone in my head...I knew it would be necessary, but as a budding EE student, I was willing to take up the challenge.

I need it punchier, fatter, thicker midrange, with less fizz.

I've been doing some research, and have a small list of mods that may help. Any input is welcome, whether its on how effective these may be, or if it's a bad idea, or if you think there's something else I can do to get that tone.

- remove the bright cap, C1
- replace R2 with 220K or 330K
- replace R5 with a choke
- replace R8 with 50k pot and 10k limiting resistor

a couple of questions:
what value should the choke be?
What should the wattage rating on the bias pot and resistor be?

I'd skip the choke, but thats just me...

The bias pot needs to take -56 v and about 35-50 ma for each tube I think.

Check to see if the DIP switches can get you closer to the sound you want, the manual (available at Peavey) might help! Systematically exploring settings is skipped by many who just "dime" everything but the VTM is very tweakable.

For mods I would think you'd want to play with the bypass and coupling caps to get more mids and less fizz, although modding the tone stack to give ranges you like better is a possibility too. Much of the VTMs DIP switch tone changes are on the cathode bypass caps (or their series resistors) and changes there can be seen as analogous to other PV amps such as the XXX, Bravo, Rockmaster or even JSX. Bigger bypass caps give boosts in lower freq. etc. see Merlins excellent treatment here:
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

The hot ticket for VTM->JCM is using EL34's in some peoples minds (see FJA VTM mod), and the JSX schematic (at schematic heaven) should be a good guide as to how to do this.

search "keystring VTM" here too!

also: http://acapella.harmony-central.com/....php?t=2282795

[azrael]

Yes, I've been to that VTM thread on HCAF. That's where I got most of these ideas.

Why would you skip the choke mod? A user reported more note clarity and definition.

I have messed with the DIP switches, they're pretty cool. Definitely tweakable.


When you say the bypass caps, you mean the ones connected to the DIP switch? Wouldn't that just change the frequency that the DIP switches modify? I am not that familiar with the VTM's schematic, but the switches seem to change frequency ranges rather than boost or cut anything, based on the manual.

[tedmich]

Yes, I've been to that VTM thread on HCAF. That's where I got most of these ideas.

Why would you skip the choke mod? A user reported more note clarity and definition.

I have messed with the DIP switches, they're pretty cool. Definitely tweakable.


When you say the bypass caps, you mean the ones connected to the DIP switch? Wouldn't that just change the frequency that the DIP switches modify? I am not that familiar with the VTM's schematic, but the switches seem to change frequency ranges rather than boost or cut anything, based on the manual.

IME changing the tubes, coupling caps or bypass caps have a much bigger impact than PS mods*, and there is a bit of hype surrounding PT/OT swaps and choke additions IMO. But chokes are fairly cheap and some do report nice improvements so have at it!

Do check Merlins chapter out, you can get boosts with cathode bypass caps, albeit not sharp notch effects like a tone stack, but you can get added bass gain, hopefully while avoiding blocking dist. As an example the PV Rockmaster is said to thicken up nicely by going from 0.47->1.0uF on its 3rd gain stage bypass cap and changing the coupling cap feeding it from 0.001->0.0022uF.

Did you PM for the French guy's VTM mods mentioned in the HC link?

* ditto for good tubes and a high quality cab!

[Enzo]

I'd put the choke at the bottom of my list too. I'd think there were a lot more things that would have more effect on tone that that.

Your bias adjust won't work. Making R8 variable is only half the story. R8 and R9 work together as a voltage divider. They start with whatever is across C7, probably not much more than -56v, and divide it down. SInce R9 is only 470 ohms, try calculating what the bias voltage would be starting with 56v at C7. Try it with R8 at 50k, then again with R8 at 10k. Not much difference. The difference is 55.5 and 53.5v, two volts. Do the calculations anyway to make sure you understand.


You have to increase R9. I use something like 3k. I have 3k resistors, but 2.7k and 3.3k are standard value resistors near that. 3k is not critical at all, I just find it works well for me. Remember, voltage division is about the ratios, not the resistance itself.

Now redo the calculations with R9 at 3k for R8 at 50k and again at 10k. A lot more range now.

[azrael]

That explains the comments at the HCAF page.

People who did the bias mod were saying that it was still too cold.

I finally got that PDF of mods. MAN, there's a lot of stuff going on here. I really wish he would say what each couple of mods did. Only a few of them mention the reasoning.
I have separated the PDF file to the relevant parts, since it was too big to upload the whole thing.


What are all these trace cuts and jumpers supposed to do?
What are C1, C6, C12, and C14? Why does he replace them?
Which caps are the coupling caps you mentioned?
I also noticed this mod on the HCAF page: "Screen resistor - increase to 470 ohm". What does that do?
I haven't gotten the chance to read that Merlin chapter yet, I'm kinda in the middle of finals at school right now, haha.
Feel free to point out any useful mods in that file, if you can be arsed to, that is.

[tedmich]

That explains the comments at the HCAF page.

People who did the bias mod were saying that it was still too cold.

I finally got that PDF of mods. MAN, there's a lot of stuff going on here. I really wish he would say what each couple of mods did. Only a few of them mention the reasoning.
I have separated the PDF file to the relevant parts, since it was too big to upload the whole thing.


What are all these trace cuts and jumpers supposed to do?
What are C1, C6, C12, and C14? Why does he replace them?
Which caps are the coupling caps you mentioned?
I also noticed this mod on the HCAF page: "Screen resistor - increase to 470 ohm". What does that do?
I haven't gotten the chance to read that Merlin chapter yet, I'm kinda in the middle of finals at school right now, haha.
Feel free to point out any useful mods in that file, if you can be arsed to, that is.

Hmm from a cursory examination the mods included do NOT seem to be the most well thought out. C1/C6/C14/C12 are the PS filter caps going up the gain stages, they are fed B+ from the C1/C2 and C3/C4 series filters (50uF each) and its likely pretty clean already (other designs are happy with 47/47/22/22). Bumping to 47uF won't hurt and MAY settle noise and help bass but not much. Some techs replace any filter cap 10+ yrs old and 47uf can be found that are the same size as older 22uf caps but if it ain't broke...

The coupling caps "couple" the preamp gain stages, they are usually small values 0.001-0.047uf and high voltage (400-600v), in the VTM they are mainly the 0.022uf/400v caps; C3/10/8/9. Some swear by replacing these with big film/foil polypropylene caps (Sprague Orange drops) but changing values has a greater tone impact (often for the worse, creating oscillation or blocking distortion).

The 6L6 screen resistors might give a harsher more compressed sound going from 100->1k, the more conservative 470ohm switch could also degrade the tone but you are welcome to try it. These resistors hold the screen grid to a lower potential than the plates, and many do use 1k for 6L6 and EL34s.

Changing the plate (anode) resistors from 100k->330k is a bit extreme, I'd try 150k first, Merlin's chapter will clarify its role in the overall gain stage.

Not sure on the cut traces maybe for the bias mod, I try to never cut traces and its usually not necessary.

[azrael]

I just finished up finals, not sure if I'll modify the amp again before my next gig, unless I get some more money...
Should have some time this week to read that chapter. I've got the PDF on my iPhone so I'll read it at work.

I changed the anode resistor to 150K, didn't work well enough, and now I'm pretty happy with it at 220K. Really helped remove the fizz. It's pretty close to where I would like it, with a little bit more midrange punch, I'd be happy. Right now, I just have the mids knob all the way.


Would replacing all the 1/4W resistors with metal film ones be a worthwhile task? Would it lower noise?

Right now, noise is my main issue with the amp, I'm fairly satisfied with the sound.

[azrael]

Just snipped the bright cap fellas. It's gettin there! Still not satisfied with the noise that's present...

[azrael]

I was looking closer at the schematic for the vtm60......
And comparing it to the JCM800 preamp schematic...

Has anyone tried replacing the treble peaking circuit at the low gain input? A lot of JCM800 users replace it with a 68K resistor, like the Marshall Super Leads.

[azrael]

Just if anyone is checking this out still, I went ahead and added the bias control. Thanks for the tip on R9, I was able to get it to 28mA with a 50K pot in series with a 10k resistor. May have to try a bigger one.
I had to do sone trace cuts to add a point where I could read the bias from, but it was totally worth it. I used the method which calls for a 1 ohm resistor off Pin 8. Worked like a charm!

Here's some other stuff I did:
replaced first treble peaking circuit with 68k ala old Super Leads.
Put a 10k resistor on the mid control's wiper to add mids.
Changed slope resistor in tonestack to 47K to shift tonestack range a bit and make it meatier.


Sounds brilliant, there's no fizz and it's really thick and meaty.

[azrael]

Here's a clip of it so far:
netmusicians.org - the guitar gear mp3 database

I've done a few more mods, but not recorded it since then.

[jrfrond]

I'll go the other way here. I agree with the switch to EL34's, but also, power supply mods. Part of the JCM sound is a fairly-stiff power supply, and I would bump up the filtering AND add a choke in place of the resistor. Other tweaks can be made to tailor the overall tonality and gain structure, but much of ANY amp's tone originates in the power supply.

[azrael]

What value of choke would you recommend?
And you're saying bump up the filter cap values to 47uF? That's a pretty big change, no?

Here's a newer clip:
netmusicians.org - the guitar gear mp3 database
got a bit of mic clipping unfortunately..

[Guitarist]

On the clean input move that "treble peaking" circuit up-stream towards the the dirty channels first gain stage and out of the Low path. Be sure to nix that treble bleed on the Pre pot. Your guitar has to now move through a "second" Volume pot so a buffer/Screamer might be handy here at lower settings.

That relay mod might do all this anyways.