Jerry Preamp Mods

Jerry Preamp Mods

Postby claytushaywood » Tue Dec 13, 2011 4:07 pm

I'm about to get crackin on modding my silverface (non master volume) pro reverb amp. I was gonna do a combination of the mods waldo put together on his site. mostly post alembic stuff from the site. first of all, i was wondering if anyone has done these exact mods and had any sound clips or reviews of the sound change?

Now, the pro reverb does not have a mid control. rather it has a 6k fixed resistor for the mid tone. Does anyone have experience with this for jerry tones? Should I add the mid control pot? I will be keeping the amp and as it is silverface and im gonna be modding it anyways i'm not worried about value.

I've had some interesting tones develop from tube swaps in the amp. Things like using a 12ax7 in the phase inverter location and using a 12at7 in the normal channel preamp position for two different "channels". Or even removing v1 completely. yall have any suggestions for jerry tones? I'll take tube suggestions as well if you have some moderate to low priced favorites.

I'm also having a problem with my amp that I just began to notice. Not crazy annoying, but it definitely gets on my nerves. When I play notes in the low register, mostly on the low E string. I get this kind of interference type sound when i sustain a note. its like a background noise that develops after the note loses its initial attack and sustains on behind the note. not overbearing or anything. sounds like a distorted am radio sound when you're going in a tunnel behind the note. i also changed my pickups recently and shieled my guitar. id say its low noise and i didnt notice it right after i shieled the guitar. i noticed it after i played the amp super loud though.

i'm also not happy with the current sound that i get when overdriving the amp at high volumes. its kinda of fizzy and farty sounding distortion. is that mostly the speakers? will the mods help me with either of these problems?

thanks a bunch
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Re: Jerry Preamp Mods

Postby mijknahs » Tue Dec 13, 2011 4:37 pm

Speakers are one of the best upgrades to a stock amp. Much more noticable than upgrading the caps or tubes. Try to get some JBL K120s or E120s that are in good shape and have the original cones. I can't recommend any new speakers since I haven't tried any that compare.

Then after that, experiment with the preamp mods like cap and resistor upgrades. Those are not drastic changes but add reliability and consistancy.

Are you using the power tubes or an external power amp?
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Re: Jerry Preamp Mods

Postby claytushaywood » Tue Dec 13, 2011 4:44 pm

interested in info about the alembic preamp mods on waldo's page. and queries in OP
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Re: Jerry Preamp Mods

Postby zambiland » Wed Dec 14, 2011 12:05 pm

I would add the midrange knob. Around 15 years ago I found a blackface Bandmaster for $25 that became the vehicle for my amp experiments and I added the midrange (big success!), a presence control (another big success!) and a Torres effects loop mod kit (huge failure and much anger at Torres who sent me the wrong kit and refused to honor the implied warranty of fitness for a particular purpose and refused to help me in any way to get it to work right. I was so pissed, as it cost me the first channel of my amp. Avoid that guy like the plague).

The odd noise you are hearing sounds like bad tubes to me. It also could be bias problems. It could also be a rubbing voice coil in the speaker. It sounds like this amp is due for an overhaul anyway. New tubes, new filter caps (and maybe new 25/25 caps in the preamp section), new plate resistors, etc. should clean the amp up greatly. You might also want to convert the bias circuit to blackface specs and beef up the bias cap a bit. I did that on my Showman along with extra filter caps and it gave the amp a lot more headroom (always a good thing in Jerry-land!).
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Re: Jerry Preamp Mods

Postby tigerstrat » Wed Dec 14, 2011 12:53 pm

zambiland wrote:I would add the midrange knob. Around 15 years ago I found a blackface Bandmaster for $25 that became the vehicle for my amp experiments and I added the midrange (big success!), a presence control (another big success!) and a Torres effects loop mod kit (huge failure and much anger at Torres who sent me the wrong kit and refused to honor the implied warranty of fitness for a particular purpose and refused to help me in any way to get it to work right. I was so pissed, as it cost me the first channel of my amp. Avoid that guy like the plague).

The odd noise you are hearing sounds like bad tubes to me. It also could be bias problems. It could also be a rubbing voice coil in the speaker. It sounds like this amp is due for an overhaul anyway. New tubes, new filter caps (and maybe new 25/25 caps in the preamp section), new plate resistors, etc. should clean the amp up greatly. You might also want to convert the bias circuit to blackface specs and beef up the bias cap a bit. I did that on my Showman along with extra filter caps and it gave the amp a lot more headroom (always a good thing in Jerry-land!).


agree on adding the Mid pot.... and that the mystery noise could be a microphonic tube. Also about converting to BF bias circuit, although you didn't mention what year the Pro Reverb is, or if this has already been done.
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Re: Jerry Preamp Mods

Postby claytushaywood » Fri Dec 16, 2011 8:56 pm

Thanks! It was a microphonic preamp tube... though it didnt sound it that has in my previous experience with microphonic tubes.

Yeh the midrange and presence knob are definitely on my list. The amp has been blackfaced and does have new enough filter caps. What do you mean by extra filter caps?

I'm really hating the overdrive on this amp these days... i have jensen c12n's that i'm about to change. just wondering if those are the problem (they're like 10 year old ones i believe) or if something else could be responsible. power tubes arent too old, bias is set, preamp tubes are good now (i was switching em around- putting a 12ax7 in phase inverter and ecc81 in v2, pulling v1 and ended up blasting the amp and making a chinese tube go bad) Now i'm really liking the v1 pulled and an ecc81 in v2 and 12ax7 in PI- good and clean and punchy. what could be causing my hatred of the sound of this amp cranked? (Besides the stuff i just mentioned)

anyways- i hate the reverb right now, its really metallic and sharp, but i swear i used to love it? what could it be?
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Re: Jerry Preamp Mods

Postby tigerstrat » Sat Dec 17, 2011 10:33 am

I would rethink having the ecc81/12at7 as your 1st gain stage (V2); in my exp it's very important to have a high quality 7025/ecc83/12ax7 in that spot. If you want less gain in V2, try a 5751, not a 12at7. OTOH, I have found that a well-balanced 12ax7 can make a decent PI (V6). Go ahead and pull V1 if Normal channel is unused, and V5 can probably go too.
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Re: Jerry Preamp Mods

Postby zambiland » Sat Dec 17, 2011 11:23 am

Two things I would check. 1 is the speakers. You might not like the tone of those speakers as they break up.
The 2nds is the bias. It sounds like you might have the amp biased a little cold. You get more crossover distortion as the amp gets cranked, which sounds like harshness as the amp breaks up. Bias is not an absolute number but a range where you make tradeoffs for a warmer, louder, punchier tone for tube life. I tend to bias at about 75-80% of static dissipation. It's been a while though, these days I usually just guitar through an SMS Classic into the power amp section of an Eden WTX-500 into a JBL. It's pretty much my dream tone.

As far as extra filter caps, I mean adding some extra capacitance. I added 200mf of caps to my Showman, which basically acts like a reservoir of instant power for transients. Since I played bass through it a lot, it's a great mod for that.
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Re: Jerry Preamp Mods

Postby claytushaywood » Thu Feb 09, 2012 12:05 pm

zambiland wrote:Two things I would check. 1 is the speakers. You might not like the tone of those speakers as they break up.
The 2nds is the bias. It sounds like you might have the amp biased a little cold. You get more crossover distortion as the amp gets cranked, which sounds like harshness as the amp breaks up. Bias is not an absolute number but a range where you make tradeoffs for a warmer, louder, punchier tone for tube life. I tend to bias at about 75-80% of static dissipation. It's been a while though, these days I usually just guitar through an SMS Classic into the power amp section of an Eden WTX-500 into a JBL. It's pretty much my dream tone.

As far as extra filter caps, I mean adding some extra capacitance. I added 200mf of caps to my Showman, which basically acts like a reservoir of instant power for transients. Since I played bass through it a lot, it's a great mod for that.


Word up... I really am a huge fan of running this amp with my danelectro (super low output pickups), when it was biased hot with new tubes i could crank it to 7 in a band situation and get the nicest cleanish preamp with power tube crunch thing goin on, or somethin like that. I will get the SMS one day, though I'm just finishing up collecting parts for the alembic mods and I'm loving the options of the preamp out with the post alembic tap mods.

About the filter caps... So instead of 100uf/350V you're using 200uf caps? Does it add a lot of bass? is this a good idea for jerry mods? I was thinking I should do 100uf/450v and increase voltage as much as I could. what about the 22uf (or are they 25uf)- the other caps in the filter cap can? Should I try to increase those in either voltage or capacitance as well. I dont want to change the general voice too much, but waldo's page does mention something about putting the biggest caps you can in there.

You seem to know a lot about these mods... Some guy was telling me that adding a mid range control and a presence control was not as easy as thought. I figured it was just replacing the 6.8k resistor with a pot. Do you have any links with diagrams or schematics? I'm not very skilled with all this yet.

I have some other questions for you about these mods too. Got any opinions on the reverb? Jerry's verb never really sounds like a normal spring verb to me. I know there's stuff in waldos page about the reverb section, but what about a good tank or dwell and tone control additions?

Sorry for all the newb questions- you seem like you have some quality experience. I forgot about this post, regrettably, until now. Thanks for the help!

tigerstrat wrote:I would rethink having the ecc81/12at7 as your 1st gain stage (V2); in my exp it's very important to have a high quality 7025/ecc83/12ax7 in that spot. If you want less gain in V2, try a 5751, not a 12at7. OTOH, I have found that a well-balanced 12ax7 can make a decent PI (V6). Go ahead and pull V1 if Normal channel is unused, and V5 can probably go too.


I got matched 12ax7's from mullard for my phase inverter tube and it sounds GREAT! after retubing and adjusting bias (was in need) and using this balanced PI, the amp is gettin back to it's roots.

But man, Have you ever tried a 12at7 in a loud fender? It's not as much about being able to turn the amp up louder without splitting volumes, but that is nice, considering my affinity for having the power section running hard. it really more than anything makes the amp sound lighter and more airy i guess, it's not as tight and unforgiving, but it doesnt have a lot of sag or anything. It might be because my 12ax7's are mulalrd and tung sol reissues and the 12at7's i have...1 is a really old westinghouse and the other is an old rca.

Think I could be liking the brand or the american factor?
by the by, where's a reliable place to get NOS tubes? I dont have a ton of money, but I think some more options for NOS tubes in my preamp would be grand
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Re: Jerry Preamp Mods

Postby jeffm725 » Thu Feb 09, 2012 12:34 pm

.......................................................have you heard the one about the yellow dog?
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