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FDP Forum / Amp Mods, Repairs, and Projects / progressive volume loss, Princeton Reverb II

Stratopastor

Stockton-on-Tees

now even awesomer
Mar 6th, 2017 05:27 AM   Edit   Profile   Print Topic   Search Topic

I haven't gigged like this for a while (covers band reunion! kind-of an enjoyable car crash...). Noticed a progressive loss of volume during a power-chord type song. Maste volume was on 8. What would induce a fade like that? Power tubes? I've had a pair of modern-production Tung-Sol 6V6s in there since 2010. The amp plays happily at low volume for long periods...

any ideas? thanks in advance.

champster
Contributing Member
********

Oceanside, CA

Mar 6th, 2017 09:37 AM   Edit   Profile   Print Topic   Search Topic

I had one that I bought new with the EV speaker. The PRII was the most unreliable amp ever. What didn't go wrong?

littleuch
Contributing Member
**********
*****

Florida

Mar 6th, 2017 10:05 AM   Edit   Profile   Print Topic   Search Topic

There's a guy who has a dedicated website to the PRII....oh, wait. :-p

I gigged my PRII pretty hard for 10 years without a problem. One preamp tube (newly installed) went poof one time.

Honestly, I would suspect power tubes first, but you know more about this stuff than I do.



guitarcapo

U.S.A.

Mar 6th, 2017 12:59 PM   Edit   Profile   Print Topic   Search Topic

It's a pretty simple amp design. Pretty similar to a basic PR but it has a master volume and a funky pedal design.

I'm guessing it MIGHT be a bias issue related to the pedal...which was powered by a tap into the bias supply.

SCHEMATIC

Stratopastor

Stockton-on-Tees

now even awesomer
Mar 8th, 2017 07:43 AM   Edit   Profile   Print Topic   Search Topic

Larry, haha, thanks for that. I'll check that guy out. :-)

Guitarcapo, thanks, I'm not using the pedal but I will check the bias.

I'm not scared of soak-testing it into an attenuator and tracking it down for myself.... I just thought, given that it's a design with a lot in common with other Fenders, maybe someone was able to say 'oh yeah, I know what that is...'

pdf64

UK

Mar 11th, 2017 12:52 PM   Edit   Profile   Print Topic   Search Topic

Firstly, the PRII doesn't look to have much to limit screen grid current when overdriven, so to avoid over-stressing the 6V6, I suggest to add 470 ohm 1 watt screen grid resistors if it's to be used cranked.

I find that volume fade under heavy power amp overdrive is a common thing, especially with a single speaker, even more so with smaller / non ventilated magnets.
Check how hot the speaker magnet is getting at the end of the heavy song.
Careful, it may be too hot to touch for long!
If that's the case, the rig's power output may be getting reduced (due to voice coil resistance increasing, leading to greater loss).
Consider trying a beefier speaker (eg with a bigger, ventilated magnet), or an speaker cab (of the correct impedance) with 2 or more speaker drive units, eg 2x10, 2x12, so the power gets shared.

(This message was last edited by pdf64 at 10:51 AM, Mar 12th, 2017)

Stratopastor

Stockton-on-Tees

now even awesomer
Mar 16th, 2017 04:00 AM   Edit   Profile   Print Topic   Search Topic

Thanks Peter... I'll look into the speaker-fade idea.

I fitted my PRII with screen-grid resistors some years ago on the advice of Unquiet, and wrote it up for the PRII website. Considering the larger amps in the Rivera-era range DID have screen-grid resistors, it's a mystery why they left them off the PRII, Champ II, Bassman 20 and Super Champ...

cheers

Andrew

Stratopastor

Stockton-on-Tees

now even awesomer
Mar 18th, 2017 09:03 AM   Edit   Profile   Print Topic   Search Topic

Turns out the output valves were biased way hotter than I thought... my own fault. I'd love to tell you it was through a misapplication of Maxwell's Equations or the Duckworth-Lewis method, but it was actually a mishandling of... Ohm's Law. Just keep walking, there's nothing to see here.

(This message was last edited by Stratopastor at 11:04 AM, Mar 18th, 2017)

FDP Forum / Amp Mods, Repairs, and Projects / progressive volume loss, Princeton Reverb II




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