Pulling out the "evil" distortion circuit on a 1976 V-Ampeg

[NoFi]

Hi, as you may know, some V series Ampegs from the late seventies have a distortion circuit which has a reputation for being useless and ugly sounding.
Well, IT IS. The more you push it, the more clipping it adds, but the overall volume drops... So i'd like to pull it out which would free a pot hole for a master volume, a triode for a gain stage and some space for various mods.

I would already have started to take away some of the components if i had not read that warning on the Ampeg schematic : Do not operate amplifier with either optoisolator P101 or P102 removed from circuit as damage will result to remaining device.
The optoisolators seem involved in the footswitching of the distortion circuit... here is the warning (7) :




So basically how can i remove the distortion circuit without damaging anything ?
Why would removing the optos cause any damage ?


Distortion circuit :


Complete Schematic :
http://www.ampegv4.com/images/VT22_Schem2.jpg

The distortion circuit seems to be some sort of feedback loop with clipping diodes, and it has a dual ganged "distortion"' pot.

I'm also open to any mod suggestions on what to do after the distortion thing is taken out. The goal is to have something different on channel one (Ampegs are quite versatile amps with the almost parametric mids, so i'm not sure where to go). Maybe a could keep the optoisolators to switch that second triode gain stage in and out which would give me an interesting boost feature ?
By fitting an opto in parallel with a cathode resistor arrangement maybe ?

Thanks.
Greg

[Rob Mercure]

NoFi,

P 101 shorts around R103 increasing EQ'd positive feedback from the second stage to the first stage's cathode. P 102 allows this same signal - varied by R 103B to hit the same point after having been clipped by the "anti-parallel" diode pair. If you remove either of the optos the "other" opto gets slammed with a higher signal - if you remove both there's not one left to damage. If you carefully read the reference there is no "dire warning" <grin> of damage to the rest of the amp, just the optos.

Generally I like the the master volume V-4 that I've got as well as any master volume model and unless you really want a "distortion machine" with no need for pedals I'd recommend the older Ampeg circuit (see the other Ampeg post you responded to recently) - and perhaps the PCB would still be close enough to the earlier model to make modification less "painful." In any case the earlier model provides a better "starting point" for modifications.

Rob

[NoFi]

Rob, thanks a lot !
You are totally right on the warning, it applies to the optos only. I must have missed some subtleties of the English language. Good news.

So if i understand correctly, applying some positive feedback increases the gain, and the optos allow to "kill" the positive feedbacks paths to switch off the distortion ?
Do you think i can try a 12ax7 in there, will it support the higher gain of the 1-2-3 section ?
I'm not a gain fanatic, but i use single coil guitars so i could use an extra gain stage. I've been thinking maybe i can remove the diodes, keep only one positive feedback path, solder a trimpot instead of the distortion control, and then install a master volume where the distortion pot was.
I really want a master volume.

Thanks for stating on the other thread that the master volume in your amp was installed instead of R21 or R22 (i'm guessing it's probably R21?). I looks similar to the way the master volume pedal Ampeg sold back in the days works. However, i'm surprised it's just a variable resistance between the signal and ground. I might be missing something but wouldn't a voltage divider (1 meg marshall style) work better here as a master volume ?

Interestingly, with the distortion knob turned down, the first channel sounds very close to channel 2. And the first stage of Channel 2 seems identical to the older ampeg circuit.

While we are at it, what's the use of the 6k11 in the "classic" v4 schematic ?
It looks like it could be some sort of cathode follower to reinforce the signal before the midrange tonestack ?

I never had the chance to compare an amp with the regular circuit to this one. This circuit with the distortion knob has no 6k11 to drive the EQ, so from a theoretical perspective, what sound differences are we to expect between the two versions ?

Your amp with the master volume being from 77, I don't know if the two circuit variations "cohabited" in the late seventies, or if the distortion knob is something they attempted in 76 and quickly stopped.

Sorry, that's a lot of questions.

[Rob Mercure]

NF,

Couldn't hurt to sub in a 12AX7 and see what happens. Hmm, your version really doesn't have a "master" volume does it? If that's what you want you could always sub in a pot where the amp I referred to uses one but it is indeed a "voltage divider" - a potentiometer which is by definition a voltage divider - which replaces a fixed resistor with the wiper connected to R23. Replacing R 37 with a potentiometer would suffice.

When you turn down the distortion knob there is little or no signal fedback so it's not suprising that the two channels sound similar - I suppose the P/M/A engineers designed it that way.

The 6K11 provides three gain stages to feed the variable mid control which utilizes a tapped inductor in a feedback loop. This is similar to the way the treble control cap is wired in the classic Baxandall tone controls which were placed in a feedback loop - the Ampeg amp uses the "James" variation of the Bax controls which are "loss only" while the Bax stage were true "cut/boost" type - although the only reason that I can think that three stages were used instead of one was for noise reasons. But the result is to add a mid "hump/notch" that can be "moved" in frequency as the midpoint in the James controls is fixed. When combined with the "Ultra Hi" switch and the individual channel sensitivity controls the result is an extremely flexible design that is, IHMO, sorely under appreciated and under valued - and if the circuits were p-to-p or eyelet it would be a modifier's dream platform. Just think what you could do by chaining one channel to the other with the sensitivity switch to set levels and the UH switch to bound treble response (or to be rewired on one channel to bound bass response)! Ah well.

Don't get too hung up on the circuit variations. I'm sure about my chassis date by the various component date codes which preclude manufacturer before 1977. But as I've never seen a schematic for my amp with the master volume drawn in I consider the model a "transition" one even though I've seen several of these. Just because the schemo for your amp has a 1976 date on it doesn't mean that the amp was produced during that year - or the following one. You might consider checking out the codes on the pots, caps, trannies, and other components - while this doesn't give you an assembly date (or does it, I'll have to see if there is a final checkout stamp/sticker on mine - can't remember which manufacturers included this and which ones didn't) what it does is give you a "no earlier than" date. That is, if you've got 1977 components it couldn't have been made earlier but the parts may have set in a bin for a year or two before being used. This is why you have to check as many parts as have date codes to get a good idea. (The classic confuser are Fenders that have CTS pots with 1967 date codes. It seems that Fender bought several years worth of these in 1967 and I've seen silver face amps made in 1971 with 1967 made pots - much to the chagrin of the owner who thought he had a 1967 amp. But the tranny date codes told another story.). So my theory is that the circuit was designed in 1976 but put into production later.

Hope that covered all of it. Ray Ivers, for one, is much more up on the nuances of the "real" Ampeg circuits.

Rob

[NoFi]

Thanks again. The pots in the amp and the original rola speakers are dated 76, but as you mention, maybe they had some stock.

I understand the master volume now, if R23 is connected to the wiper.
The 10k R39 in mine could certainly be replaced in a similar fashion.
I tried the external master volume control (that goes in the preamp out) with a 25k pot and it works quite well !

I also tried a 12ax7 in V1, the action of the distortion knob is more noticeable. On the first 75% of the pot's turn, the volume goes up. On the last third of the turn, the diode clipping starts, the sound is more and more distorted (not very musical) but the overall volume fades.

So i finally took away the diodes... and now the distortion knob only adds some volume. Quite an amount of volume actually with the 12ax7.

I could not try the amp loud but i will after some sleep. Lets see how a distorted ampeg sounds.

[NoFi]

Well it works well with the 12ax7 in V1 and the diodes d1 and d2 pulled out !
I dont know why they didn't do it that way right from the start. The knob still adds distortion except it goes from subtle ovrdrive to dirty crunch to nasty overdrive without killing the overall volume.

Maybe i could still tweak the distortion sound a bit, what could i do to smooth the highs ?
Where exactly is the equalization on the signal feedback, is it that 0.1uF cap and the R4 100k resistor, I guess 0.1uF is sufficient to let enough bass through ? But maybe C3 0.01 looks a bit small ?

Well it's already an improvement anyway...

[Enzo]

Because in those days "fuzztones" were more of a novelty. No one was going for a subtle addition of some edge. COMpanies were adding distortion to their products without understanding it. Everett Hull, founder of AMpeg was a jazz cat, and he hated the rock and roll. Slap some distortion of some sort in there, that'll sell those rock n rollers... He thought amps should be clean and hi fidelity. Kinda like an Ampeg with the distortion turned off.

[Steve Conner]

I remember hearing that when Ampeg were developing the SVT, they had to make sure Everett Hull never found out about it until it was too late.

[NoFi]

I remember hearing that when Ampeg were developing the SVT, they had to make sure Everett Hull never found out about it until it was too late.

Hehe , well i'm glad they did it. I really got the Ampegs for the cleans, but it cant hurt to be able to push it.

Like it is suggested on the above "notes" (#6) from the schematic, i will surely try 6550's or KT88's in the amp as soon as i have the opportunity to get some.
Is a rebias the only thing needed ?
(I've already installed an adjustable bias on the amp).