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Post subject: '74 bandmaster reverb question
Posted: Fri Jun 08, 2012 11:08 am
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I was hoping you could help me with a question I have re: 6v6's in this amp. Would it be possible to try a couple of 6v6's in place of the 6L6's? I'm thinking the JJ brand since I've heard these are more than capable of handling higher voltages. If so, would I just follow the same procedure for biasing the tubes at approx. -52v at the middle lug of the bias pot? Oh and one more thing, I remember reading that a voltage of between -48 and -54 volts is acceptable but which direction would bias the tubes hotter?
Oh and I forgot to mention, I've already converted it from a hum balance pot to a bias pot.
Thanks in advance,
Abe

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Post subject: Re: '74 bandmaster reverb question
Posted: Fri Jun 08, 2012 12:36 pm
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You can use the JJ 6V6 or GT 6V6 S, as both are rated high enough for this amp. As far as bias, the negative voltage at the middle lug of the bias pot really means nothing, as different tubes will draw different current with the same setting of the bias pot. For example, a 6V6 might red-plate at the same voltage setting that is correct for a 6L6. To get an accurate bias setting, at idle, measure the Plate current (or Cathode current) and Plate voltage at one of the tubes, multiply those two numbers to get the Plate wattage dissipated at idle for that tube. The bias pot will adjust both tubes, and you can check both to see if matched, but one adjustment affects both tubes. Typically, set the bias between 50% and 70% of the tubes max Plate wattage rating. Using bias probes makes it easier, but not necessary.

The closer that the negative voltage at the middle lug of the bias pot is to zero, the hotter the bias (more current) through the output tubes. The farther away from zero the voltage is, the lower the current. Example, -48 volts will give higher current than -54 volts.

If you don't have bias probes, I can tell you how to check the idle current/wattage with just a VOM if you're interested. :idea:

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Post subject: Re: '74 bandmaster reverb question
Posted: Sun Jun 10, 2012 7:16 pm
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Thanks Shimmilou for a thorough response. I do have a bias probe so i'll go ahead and measure the plate voltage and run the calculations based on the current readings I get off the probe and make my adjustments accordingly. I lso wanna say thanks to retro for all the help he's provided off line. :wink:

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Last edited by socal323 on Sun Jun 10, 2012 7:29 pm, edited 1 time in total.

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Post subject: Re: '74 bandmaster reverb question
Posted: Sun Jun 10, 2012 7:22 pm
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Aw shucks......

:oops:

I'm always happy to help, socal.

8)

Arjay

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