Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zfiles1701 on October 30, 2010, 11:15:39 am
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From what I understand the power tube tremelo will affect the amp if used with a VVR. When the VVR scales the amp down enough, the bias will be affected (will be scaling the whole amp). It will affect the sound. But, will it pose a threat to the amp in any other way? Burn up the power tubes? Smoke the amp? If it only makes it sound like crap until you switch the tremelo out, that's OK. My best friend from highschool will have to learn how to use it. If it poses a threat to the amp I'll have to figure something out. Don't want tremelo in when scaling for distortion anyway. Thanks
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No problemo if your VVR circuit also tracks bias.
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If the bias voltage does not change, and you reduce the plate and/or screen voltage, the tube passes less current. No risk of melt-down with that scenario.
The only likely problem is that as you turn down for less power, if the absolute value of bias voltage doesn't drop also, the tube is biased more towards cutoff. For example, if you start with 450v plate and screen with -40v bias, and then reduce plate/screen to 275v, the -40v of bias will probably result in only a couple-mA's of idle current. The tube is nearly-off.
For this case, you want the bias to track the supply voltage drop, and as the supply goes from 450v to 275v, the bias should go from -40v to -25v. There is a special version of power scaling that does bias tracking; maybe Dana Hall has a VVR that does it, but I'm not familiar with his products.
With smaller bias voltage, the only change in trem will be stronger trem (possibly too much). You would probably want to leave the trem tube supply voltage untouched. Or you could scale the entire amp to have a basically consistent sound at all levels. My concern is whether the trem tube will reliably oscillate if the supply voltage is dropped too much.