Peavey Classic 30 Presence Mod Question

[dhuber]

I just used the Steve Ahola mods on my 2 month old Peavey classic 30 H. I have a pretty good understanding of electronics theory however I've never studied tube theory. So far I've only modded the circuitry between V1A and the tone stack using OD & silver mica caps. C1 390pf, C2 .002uf, C4 680 pf, C7 390pf, C9 1000pf, C11 390pf, C14 250pf, R5 390K, R12 2.3M. I'm pretty happy with this setup. I original had C9 .001uf and R13 8.2 K, but all the other mods really increased the bass to were my treble had to be set to full and I really had no mid range control.

I did not take out C8 or C10 on the instrument input. Having a capacitor between between me and 300V DC just gives me a fuzzy feeling of safety. Again I don't know much about tubes maybe it's impossible to short plate to grid?

I also have not modded the Presence circuitry yet because my lack of theory of what's happening and this is where my question is. It looks to me like Peavey is taking a portion of output from the output transformer sending it back to V3A via R56 and R57. This feedback must mix with the instrument signal at V3A or effect the bias of V3A depending on the frequency of the feed back signal. Most of the mod suggestions refer to increasing the value of R56 and R57. Increasing R56 looks like it would cut lower frequencies from coming back to V3A. Increasing R57 looks like it would reduce the amount of feedback. If feedback is reduced would not that decrease the amount of Presence? Can someone explain how this feedback is working? Thanks!

[Chuck H]

It looks to me like Peavey is taking a portion of output from the output transformer sending it back to V3A via R56 and R57.

Yes, But the other components, C53, C54 and R70 also effect the loop. Not so much for C36. But I cannot tell from the schematic what the values are for C53 and C54 (.1P10)? It would help me to help you if I knew those values. Could you measure them?

This feedback must mix with the instrument signal at V3A or effect the bias of V3A depending on the frequency of the feed back signal.

Not so much the bias because the caps are blocking DC from ground through the circuit.

Increasing R56 looks like it would cut lower frequencies from coming back to V3A. Increasing R57 looks like it would reduce the amount of feedback.

Yes

If feedback is reduced would not that decrease the amount of Presence?

Not really. By increasing R57 you increase overall resistance in the loop. This increases presence along with any other frequencies in the loop. "Presence" as it has always applied to guitar amp controls, means decreasing the amount of top end being fed back through the feedback loop. C37 is actually decreasing presence by bypassing top end over R56 so more top end is being fed back.

Those tend to be bright amps. But if you want to experiment with more presence, try lifting one end of that cap.

What would you like to change about the feedback charateristics of that amp? More presence, less presence, tighter bass, top end rolloff, do you want a bigger and more brash tone?

Chuck

[tubeswell]

The caps in those PV C30/DB stock NFB loops are all there for tone-shaping. You could pull them all out and just use a simple resistor with a NFB cut-out switch, and it would have the most impact

I have tried both variants of Steve's presence mod and the one that includes replacement of R70 with a 10k pot works better, although neither one alters the tone much, and I think this is because the fixed-bias output stage in that amp is set-up for cleaner sounds anyway. I went back to the stock configuration (as far as I can recall last time I looked in there, with one or two possible exceptions). Making that amp cathode-biased has the most effect on its tone (apart from Steve's tone stack mod - which is so good I don't understand why PV haven't cottoned onto it ages ago and make it stock - for the minute changes that are involved).

[daz]

I'm with you on this tubeswell.
1)-i too found the presence mods don't do a lot, and what they DO do isn't particularly good.
2)-cathode bias is indeed the best of the mods i did to mine. It really lets the amp breathe more naturally an sound and feel more organic. Actually i seems to be finding this with most any amp lately ! I'm starting to wonder why any amp even uses fixed bias anymore. I have yet to try both in any amp and find fixed sounding better in any way at all.

[dhuber]

Yes, But the other components, C53, C54 and R70 also effect the loop. Not so much for C36. But I cannot tell from the schematic what the values are for C53 and C54 (.1P10)? It would help me to help you if I knew those values. Could you measure them?

Chuck

I'm going for a slightly brighter sound. I had C9 at 1000 pf yesterday but I put it back to .001 uf today. I've owned a stock Peavey Classic 100W Head for 5 years now and I really like the clean channel on it. It looks like some people are going for a fender sound I just want a Peavey classic 100 sound.

I have the clean preamp sounding fairly decent and the over drive channel sounding great! I thought I would try to get the presence control to sound like my Classic 100 today. I took a 10 K pot and put it in place of R70. I took a bread board with a 220K and 330K resistors and put it in place of R56. I hear no difference between the 220K or 330K resistor so I left R56 at 220K. However with the 10K pot at R70 the amp sounded just like my C100. As the value of R70 dropped the treble increased. Once R70 dropped below 1000 ohms the amp got noisy. I then took 3 3300 ohm resistors and tapped them at 33% and 66% to ground via a toggle switch. The 3 3300 ohm resistors are not working like planned. I hear no difference by throwing the toggle switch.

I did a boo boo and had to remove the resistor string which involved cutting some hot glue I used to reinforce the resistors. I did put some stress on C53 and C54 while removing the hot glue. I hope I did not damage the capacitors. The only way I could test the value is take them out of the circuit and take them to work and put them on a capacitor tester. I'm assuming they are .1pf, that does seem awe-full small, is there such a thing? Anyway I've ohmed out the entire feedback section and have not found any faults. I'm ready to throw this back together and play. I'll sleep on it and maybe something will come to mind why the 6600 or 3300 ohms for R70 is not changing the presence like the pot was. Thanks for the replies.

I guess one other question I had today was: Does a feedback circuit cut the gain of the amp if the power output crosses above a certain threshold?

Thanks for the replies. I really appreciate the work Steve Ahola has done with the classic 30 it was a great starting point.

[dhuber]

I'll sleep on it and maybe something will come to mind why the 6600 or 3300 ohms for R70 is not changing the presence like the pot was.

I changed resistor values again but I went a little to far. I have my toggle switch set to select 9330, 800, or 330 ohms for R70 presence control. My previous statement about noise under 1000 ohms was wrong. 9330 is basically no presence. 800 is equivalent to turning the treble another half turn up on the dial and adding some jangle and a very slight break up. 330 is brighter yet. I don't hear any noise with any of these low resistances for R70. I know when I had a pot in and I lowered the resistance too much the amp produced noise. I think that's why Peavey uses a 150 resistor in series with this circuit in the 50 and 100 models. I'm thinking the magic number is some where between 1000 and 1500 for R70. My amps back together now but next time it's apart I will go with 2000, 1500, and 1000 on my to toggle switch for R70.

The most important mod on this amp I've not mentioned is a external master volume in the effects loop. I really like the tone stack and overdrive capacitor changes. The presence control is nice. I also put in a stand by power toggle switch.

[tubeswell]

I think that's why Peavey uses a 150 resistor in series with this circuit in the 50 and 100 models.

FWIW when I did the revised mod (i.e. the one that included replacing the R70), I included the 150R and did everything as per the Ahola schem. It worked without any problems.

[Enzo]

Um... 1000pf is the same thing as .001uf.

C53,54 are 0.1uf, not 0.1pf. A half inch piece of wire near the circuit would have 0.1pf of capacitance.

The feedback circuit in this and all tube amps is negative feedback, it has no thresholds, it is simply the output signal fed back to an earlier stage out of phase. The net effect of it is it cancels some of the incoming signal, essentially reducing the gain of the power amp stage. It is done to reduce unwanted distortion. Adding capacitors makes the process frequency selective. In other words, if you sent back 10% of the overall signal but had a cap bypassing a resistor so it sent back 30% of the high freqs, then that would act as a rolloff. That would REDUCE the presence. ( I made up the percentage numbers for illustration) SO if I removed C37, I would expect the amp to get brighter.

It is an active control, so it will respond to loud play different from quiter play. In other words when the amp is cranked you will want to find a different setting from the one you use in the bedroom, most likely .

REeducing the amount of feedback increases the presence. Disconnect the feedback line from the output jack and see what it sounds like. Brighter, louder, more raw.

The feedback is always negative, meaning out of phase. If it were positive, you would have plain old vanilla feedback and the amp would sit there going "WOOOOO" until you got sick of hearing it. It is often abbreviated NFB for negative feedback.