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Post by cedarchoper58 on Mar 1, 2021 19:16:58 GMT -5
im running 488V on the plates on my 67 Black face pro reverb with 123V ac on the wall and some one told me i could put a 5R4GB rectifier in instead of my 5AR4/GZ34 and it would lower the plates down to around 435V. i looked at my bias chart and would have to rebias from 36ma up to around 45ma. Would this make the tubes run just as hot or hard and would my heater voltages go to low? Also what would it do to the sound? My attenutator has a 105V ac tap i could plug my amp into. what would this do sound wise and would the heaters go low (the heaters go low out of tolerence on my Marshall when i use the tap) and i would need to rebias for that? i coould just leave it as i have looked in my records and i have been running 480V on the plates for decades with no issues but it does get hot when i play for long periods. i do crank it to 7 and push a tube screamer into it thks
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pdf64
Wholenote
Posts: 558
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Post by pdf64 on Mar 2, 2021 10:15:53 GMT -5
Firstly, I suggest to try not to overly obsessed about idle anode dissipation. It’s just a limit that we should avoid exceeding, because doing so will significantly reduce valve life and if very high, cause rapid, perhaps catastrophic failure. It doesn’t, per se, affect how the amp works / sounds / responds. The important parameter that changes when the bias trimmer is tweaked is anode current, that’s what sets the stage gain and linearity. Higher current results in more gain and better linearity but shorter valve life, lower current results in lower gain, lower linearity and longer valve life. With a HT voltage up around 480-490V, I’d expect your amp to work and sound fine, and have a good valve life, with anode currents in the range 15mA up to 39mA. See what difference you can hear over that range. Swapping to a 5R4 wont affect the heater voltage, just the HT. Whereas the 105V outlet on your attenuator will reduce both HT and heater voltages. In neither case is a bias adjustment necessary; with lower HT voltage, idle anode voltage, current and so dissipation will naturally reduce, but that’s not a big deal, the amp will still work fine. And your amp’s power output will be reduced, so you may need a few dB less attenuation for the same sound pressure level from the cab (as you mentioned one, I’m assuming that you use an attenuator?).
My view is that you would be best using a BrownBox or similar, to reduce your mains voltage down to 117V (or whatever provides a heater voltage of exactly 6.3V). High mains voltage is something of a curse for vintage amp users.
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Post by SoK66 on Apr 6, 2021 7:51:33 GMT -5
I second Peter's recommendation of a "buck" transformer (line voltage attenuator) to reduce the line voltage into the range that existed when the '60s amps were made. 117vac seems to be the sweet spot for tube rectified Fenders of that era. The guide I use, whatever line voltage gets the heaters closest to 6.3vac is the ticket. A number of commercial products out there (Tone Preserver, Brown Box) or you can even make one, here's a link: www.geofex.com/Article_Folders/vintvolt/vintvolt.htmSome guys will even employ a variac.
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