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FDP Forum / Amp Mods, Repairs, and Projects / '73 Twin Reverb Odd Filter Cap Mod

SoK66

USA

We had the hit but Van got the money
May 20th, 2019 05:04 PM   Edit   Profile  

Sorry, haven't posted in some time but have a question maybe our more astute amp mavens can answer. Player and guitar teacher at the local college called, refered by a guy to whom I'd sold an 18 Watt TMB build some years ago. He said he'd had his '73 Twin Reverb worked on by a local tech and it sounded awful. Not sure yet why he didn't want to return it, my hunch is the guy did exactly what he'd been asked to do. Anyway, here's the changes made to the amp:

1) 5751 in V2
2) 12AU7 in V3
3) 3rd and 4th filter cap upped to 47uf 500v from spec 20uf 500v
4) Biased at 28% dissipation
5) Changed the two 3-inline rectifier diodes to one on each rail.
6) Replaced all the .1 600v "brown blobs" with .1 600v green Russian caps. I think the brand is Kuznek. I know this isn't a "mod", per se, I've just never heard of the brand. All of the brown blobs test within spec.

I'm in process of undoing the mods, waiting on some 2ouf 500v F&T filter caps. In the interim I replaced V2 with a 12AX7, added a small trim pot to the bias circuit and brought up the bias to 59%. Amp sounds much better, but the bass is rather boomy. Figuring this may be the extra capacitance in the preamp filter. Whatever, can anyone explain what the guy had been trying to accomplish with these changes? He reduced the gain in the preamp & reverb circuits with the tube changes, but increased the capacitance...WTH?


rfrakes331K
Contributing Member
*******

IL USA

RonHalen Jokingly He Says
May 20th, 2019 06:50 PM   Edit   Profile  

S, seems like I remember reading about adding more capacitance to the power supply so it would be more resilient. Was the extra capacitance in the Power supply?

willie

Too Near Atlanta GA

Amp Tech Emeritus
May 20th, 2019 08:58 PM   Edit   Profile  

Odd changes, but it appears that the intent was to reduce gain in the Vibrato channel while increasing bass response, as well as attempting to reduce reverb gain. No idea why the removal of the brown blobs was done...they are not nearly as bad as some claim they are. The rectifier change was likely okay. The 5751 is a great tube for V1 btw....I can't see the wisdom of a 12AU7 in V3. I"ve seen the filter caps bumped up in capacitance but I don't see the point in a Twin.

The output tube bias is obviously anemic..usually should see about 32 ma total current draw per tube. I think you're on the right track to restoring the amp to be as it should be. I'd also look carefully for any other obscure changes that might have been made.

willie


SoK66

USA

We had the hit but Van got the money
May 21st, 2019 06:33 AM   Edit   Profile  

I've never seen or hear of these Russian coupling caps. Only info I could find was they were NOS Russian military paper in oil. I'm going to test the blobs for leakage and if they're ok I'll put them back in. And yes, I'll be sleuthing for other mods as well. Forgot how great these can sound with a little work.

Leftee
Contributing Member
**********
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**********
**********

VA

May 21st, 2019 07:10 AM   Edit   Profile  

NOS paper in oil caps just sounds like a bad idea.

pdf64

UK

May 21st, 2019 09:48 AM   Edit   Profile  

Yes, look to be PIO Soviet era manufacture, see link.
Maybe they're fine but I suggest to check them all for value and dc leakage. PIO would seem to be a fundamentally less reliable technology than regular plastic film caps, as they have obvious failure mechanisms.
Old PIOs are fine for guitar tone controls, maybe not so much in high voltage amps, I'd be very wary of them feeding the power tube control grids.

Regarding the bass response, bear in mind SFs are more bassy than BFs, due to the 10nF coupling cap to the phase splitter.
And there's more bass than a BF at a given bass setting, due to the 30% audio taper spec on the pot (rather than 10% audio taper of BFs).

External link

Elmo66
Contributing Member
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Midwest USA

May 21st, 2019 01:24 PM   Edit   Profile  

The soviet paper in oil caps are highly reguarded by the vintage hi-fi repair folk for use as coupling caps. Probably overkill for a guitar amp..

SoK66

USA

We had the hit but Van got the money
May 21st, 2019 05:03 PM   Edit   Profile  

Thanks for the tip on the Russian caps. Looked to be a very odd change. I'm going to test all the brown blobs and put them back in if they're not leaking DC.

SoK66

USA

We had the hit but Van got the money
May 21st, 2019 05:03 PM   Edit   Profile  

Thanks for the tip on the Russian caps. Looked to be a very odd change. I'm going to test all the brown blobs and put them back in if they're not leaking DC.

Bubbalou
Contributing Member
**********
*

USA

THE LOW END OF UPPER TEJAS
May 23rd, 2019 06:03 AM   Edit   Profile  

I would leave the The caps as is 47uf

FDP Forum / Amp Mods, Repairs, and Projects / '73 Twin Reverb Odd Filter Cap Mod




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