FDP Forum / progressive volume loss, Princeton Reverb II/ 8 messages in thread.

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Stratopastor



Stockton-on-Tees

now even awesomer
Mar 6th, 2017 05:27 AM        

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... <br /> <br /> any ideas? thanks in advance.



champster

Contributing Member
********

Oceanside, CA

Mar 6th, 2017 09:37 AM        

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        

There's a guy who has a dedicated website to the PRII....oh, wait. :-p<br /> <br /> I gigged my PRII pretty hard for 10 years without a problem. One preamp tube (newly installed) went poof one time. <br /> <br /> Honestly, I would suspect power tubes first, but you know more about this stuff than I do.<br /> <br />



guitarcapo



U.S.A.

Mar 6th, 2017 12:59 PM        

It's a pretty simple amp design. Pretty similar to a basic PR but it has a master volume and a funky pedal design. <br /> <br /> I'm guessing it MIGHT be a bias issue related to the pedal...which was powered by a tap into the bias supply.



Stratopastor



Stockton-on-Tees

now even awesomer
Mar 8th, 2017 07:43 AM        

Larry, haha, thanks for that. I'll check that guy out. :-)<br /> <br /> Guitarcapo, thanks, I'm not using the pedal but I will check the bias. <br /> <br /> 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        

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.<br /> <br /> 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.<br /> Check how hot the speaker magnet is getting at the end of the heavy song.<br /> Careful, it may be too hot to touch for long!<br /> If that's the case, the rig's power output may be getting reduced (due to voice coil resistance increasing, leading to greater loss).<br /> 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.



Stratopastor



Stockton-on-Tees

now even awesomer
Mar 16th, 2017 04:00 AM        

Thanks Peter... I'll look into the speaker-fade idea. <br /> <br /> 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...<br /> <br /> cheers<br /> <br /> Andrew



Stratopastor



Stockton-on-Tees

now even awesomer
Mar 18th, 2017 09:03 AM        

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.



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