Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on January 06, 2021, 08:54:10 am
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3 tube amps all with 12ax7's & 1 P bass . 2 amps are great- warm and aggressive when desired. 1 amp is mainly aggressive can't seem to warm up.
Is this purely the preamp section that dictates this or possibly power tube bias?
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Hey pbman1953,
it really depends what you mean by "warm" and what you mean by "aggressive" :icon_biggrin:
If by aggressive you mean you can't get a clean tone at low volume, then yes the way the preamp tubes are biased will impact how much distortion you're getting right off the bat, but aggressive is not necessarily only about distortion but also about high frequency content as we tend to associate more treble with more aggressive (at least I do) and less "warm" (I don't), but then the coupling caps along with the tone-stack should come under scrutiny
If by "not warm" and "aggressive" you mean the transition between clean(ish) tone and distortion is abrupt and not very "tube-like", then the negative feedback plays a role in there but that's the power section.
Or if by those terms you mean, not "touch-sensitive" or "too stiff" then the power supply is most likely the culprit, its filtering and the plate voltages across the amp.
As for the classic unsatisfactory conclusion: it probably is all of the above :icon_biggrin:
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Th amp in question is one that I've been given help with before. A Sunn/Fender 300T that I've recently replaced 3 large filter caps. I did rebias according to how Sluckey instructed me in the past.
Don't get me wrong, the amp is amazing and after the cap install it works likes its a new amp. I'm trying to tweak it. I swapped out some preamp tubes which seemed to help. I have a 12ax7 that has lost is marking but it has really long grey plates. It's printing was yellow and there's printing of "ARN" and "USA"
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IMO, any amp with SIX 6550s is not designed to sound warm. It's purpose is to destroy ear drums. If you are chasing warm sounding bass amps then get an Ampeg B15. But don't expect it from that 300T.
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IMO, any amp with SIX 6550s is not designed to sound warm. It's purpose is to destroy ear drums.
I used to own a Sunn 300T, best bass tube amp I've ever owned or played and it's true that I mostly used it for the purpose Sluckey mentions. I've played a lot of shows (unmic'd) and never got over 2 on the master volume and I was using exclusively the distortion channel (which, if I remember the schematic, uses opamp's too).
What do you mostly use the amp for?
If you rarely get a 6550 sextet cooking, I guess you'll always miss some of the harmonic content (another thing people call "warmth" I guess), so that's that.
I've never had the faith nor the budget for tube rolling but if you have noticed a difference in the past I would go in that direction for the preamp, but my opinion (which is just that) is more or less Sluckey's, that this amp was not designed to played at a volume that makes tube rolling relevant anymore.
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You both do have a point, the amp is a beast. And it's personality is controlled by a volume control
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man, what a glorious amp though....
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Would anyone know if you can use less then all 3 pairs of tubes in the 300T. I know that defeats the need for 300 watts. I'd like to go back to Kt120's, but with 3 pairs I think the they are too taxing on the PT