Figuring out negative feedback

[Jeremy1283]

why don't you try 27k-16ohm (that NFB setup was most likely in Ed's amp back in 1977) or 47k -8ohm

Why do you think eds amp had this? I am really confused on nfb

According to amp archives, this site etc, the usual stock NFB was 47k-16 or 47k-8 for 1968 lead amps. 27k is from the JTM era. In fact 27k-8 is almost the same NFB (mathematically) as 47k-16 but the OT tap is different, hence it should sound slightly different (more open or brighter maybe)

Gotcha so eds amp was later changed to 100k 4 ohms. Hmmm
I have 47k on 4 ohms.

[MikeO]

just curious... has anyone tried NO feedback on a 12,000 series build?

[jnew]

Yep. Lots of hair and gain with added noise. Not necessarily in a bad way. May loosen things up too much for some people.

[MikeO]

I was trying out the resonance pot mod and something weird was happening. I couldn't get a clean tone, even with the volume on 2. It turns out my impedance selector crapped out so I had no feedback. It sounds closer to 'the sound' to me now.
I've heard a few stories from a few long time amp guys here in LA who claim EVH had no feedback ¯\_(ツ)_/¯
Who knows. It's not really the sound I was looking for, just happened by accident.

My amp is a 1980 JMP 2203 converted to a 1959 (someone drilled out the other holes before me)
V1a: 820r/250uf
V1b: .68uf/820r
V2a: 820r/.68uf

Without the variac, it's a little harsh at the moment. I added a 220uf to V2a and it smooths out the highs but the bass is a tad looser. I replaced that with a 25uf/25v and it seems like a good compromise.

[jnew]

EVH had no NFB, is what I think you meant. Hmm, I can't say this would be completely out of the question. I may have to revisit that. Definitely had something "HOT" about it, for lack of better words.

[mightymike]

A 27k NFB in Series with 1M Pot would cover a lot of range

[MikeO]

EVH had no NFB, is what I think you meant. Hmm, I can't say this would be completely out of the question. I may have to revisit that. Definitely had something "HOT" about it, for lack of better words.

yes, no NFB. I figured you guys knew what I meant.

Anyway, after reading my post again... what I said doesn't make sense. My impedance selector is fine. No matter where I place the purple NFB wire, the sound does not change.

First I installed a Lar/Mar style PPIMV (250k) and everything was fine. Then I started experimenting with bypass caps on V2 and everything seemed fine. Then I installed a resonance pot (1meg with .0047uf, NFB is 47k on 8ohm tap) and then I noticed I was getting breakup no matter where my volumes were, AC/DC overdrive as low as 2 and full 'metal' on 10. I removed the resonance pot and put it back to 47k/8ohm but the sound didn't change. Presence control did nothing. It's obviously something simple but since I'm using the original PCB, I'm annoyed Oh well, that's part of the fun right?

A 27k NFB in Series with 1M Pot would cover a lot of range

Yeah that's what I plan on using... eventually. Good call

[jnew]

Double check the solder connections on the NFB resistor.

[MikeO]

Double check the solder connections on the NFB resistor.

That was the first thing I checked. I just took it to my rehearsal space so I could crank it. It's in the bright channel only. It's either the volume pot or a component. I'll find it tonight. The Normal channel works.. well, normal (clean at lower levels)

[rgorke]

you have a LarMar PPIMV installed? If so, that is why your presence doesn't work. I had one for the longest time and thought it was me that I wasn't hearing a change by using the presence. Someone can explain better than me. the PPIMV's kill the presence control.

[jnew]

He had the Lar/Mar but mentioned that he removed it and it's still not working right. It's interesting that it doesn't work on the lead channel but is fine on the normal channel.

[MikeO]

you have a LarMar PPIMV installed? If so, that is why your presence doesn't work. I had one for the longest time and thought it was me that I wasn't hearing a change by using the presence. Someone can explain better than me. the PPIMV's kill the presence control.

It shouldn't. I've installed at least 3-4 of them in older Marshalls for Jeff from Smashing Pumpkins. We had to turn down the presence a bit on the last tour. We switched to heritage G12H speakers (in black Orange 412 cabs). After they broke in, we turned the presence up from 5 to 6. Haha. G12H can be harsh at first but they sit in a mix so nicely with a loud two guitar band.

[MikeO]

He had the Lar/Mar but mentioned that he removed it and it's still not working right. It's interesting that it doesn't work on the lead channel but is fine on the normal channel.

Oh I meant that I was getting very early breakup on the lead channel but not on the normal channel. I'm working on that amp now.

[jnew]

DOH! Nevermind.

[scottbaierl]

So, I have taken a major big boy step and removed the PPIMV that was in my amp since I got it. I now have a killer attenuator and the PPIMV has been dimed for the last month or more. When I took the MV out and put in the 47k NFB resistor, howling is gone. Not sure why but don't really care.

Also, I may start another thread on this but the amp does sound better without the MV in it. Maybe I am hearing the upped NFB but there is more clarity. The MV was in the indicator light hole and the indicator had a new home to the left of the switches. I am thinking of just putting a dummy switch in there to pretend to be a polarity switch.

If anyone has a busted on/off/standby switch they don't want, let me know.

I haven't had much time to play it but I am liking how it sounds now.

Thanks for the input and the NFB info that vhjunkie posted must be a sticky!!

I had a similar problem a while back... I was getting howl when I turned the PPIMV up. I moved the negative feedback over to the 4 ohm tap and it went away. Moved it back to the 8 and it reappeared. I found that when I dialed the presence all the way up, the howl also went away. It turned out to be a bad ground on the speaker jack.