Re-Tubing '59 Bassman LTD
Hello Skip,
I purchased a used '59 Bassman LTD amp. The pre-amp tubes are 12AX7's. I noted that in the Bassman Chronicles, Submission #10, "Resistor Change", that Mark Overman recommended upgrading the resistors in Positions #R28 and #R29 from 1/2 Watt to a full Watt due to the low gain tubes being used. I got the impression that he was most concerned because the phase inverter tube (V3?) he was using was a 12AU7, which is only about 20% of the gain value of the original 12AX7.
Question #1: Do you think any of the higher gain tubes, such as 12AT7 or 12AY7 could be safely used in this position (or all 3 positions) without having to do any resistor changes? I'd like to at least experiment with the sounds of these different tubes, if possible, but don't want to risk damage.
Secondly, I have a JJ GZ34S Rectifier tube. Can you tell me how this "compares" with the seemingly more popular 5AR4? (I guess we're talking "Sag value?)
Thirdly, When biasing an amp, are you actually biasing the Rectifier Tube and the Power Tubes, altogether, as one unit?
Thanks so much!
Rick
Skip - Thanks for ANSWERING! :)
Your information, opinions, and insights are duly noted... and VERY much appreciated!
Thanks again!
Rick
Hello Rick- Thanks for writing.
I am a pretty much a vintage guy, but I've said all along that the RI Bassman is a good value and I have serviced quite a few. I have noticed that they often have open plate-load resistors, which is VERY rare in original vintage Bassman amps, even when running low-gain preamp tubes. They BOTH use 1/2 watt resistors...... I would certainly use 1 watt resistors in the RI in the event that you need to replace them. Personally, I think that the old Allen-Bradley 1/2 watt resistors were a little better quality than those used in the RI amps, which may be part of the problem. You SHOULD be able to use any 12AX7, 12AT7, 12AU7, etc. without damaging the plate-load resistors, but it's obviously a little risky in the RI.
GZ-34 is just the "Euro" designation for the 5AR4. I'll pass on commenting about the reliability of new-production rectifier tubes and their sag value, though!! May favorites are vintage 5U4s and 5R4s.
"Bias" refers to power tubes alone, but it can change when a different rectifier is used.
Skip Simmons