Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on October 19, 2023, 12:13:57 pm
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There was a pretty good video released today on the PSIONIC AUDIO (https://www.youtube.com/watch?v=lLMkFsw7bzE&t=176s) YT channel, installing a LaMar MV on a Bassman, that planted the seed to try it on my UL Pro Reverb rebuild since the stock pre-PI MV is underwhelming.
The stock UL is wired with only a bias balance control, though I have already converted this one to balance + adjustable bias.
Some questions arise in that, in the stock UL circuit, the resistors feeding the "hum balanced" bias to the power tube grids, are not equal value as in a standard AB763 (220k). These are 68K & 47K.
The questions I have are:
Are they different values from each other because of the UL factor?
Can a dual-ganged pot (w/equal values) be installed in their place?
If an equal-value dual pot will work, what value?
NOTE: the schematic section below is from a Twin but same circuit & component values
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> Are they different values from each other because of the UL factor?
No.
Later Fender LongTails have offset plate resistors to balance the unbalance. Here the designer choose to offset the grid resistors instead. Why? Maybe good dope.
The design is already "off label" for a quad of fix-bias 6L6. 100K grid resistor max, here the lower pair share about 73k, so 140k per grid. But Fender learned to bias his big amps cold so they would not runaway with large grid resistors.
The idea of "balancing" a high-power cold-bias guitar amp is probably folly. If point-oh percent distortion appeals to you, use transistors. Tube-dude don't sweat 20% unbalance.
If you can find dual-50K, do that. Else 100K but watch the bias.
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Thanks for the insights & direction.
These later 70s amps seem to deviate from the tried & true just enough to be confusing.
The later 70s schematics I was studying seem to drop the value of the PI plate resistors but make them equal & also drop the bias grid feed resistors value considerably, sometimes equal value(early MV), sometimes not (UL).