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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 20 Apr 2011, 15:53 
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Joined: 01 Feb 2011, 20:33
Posts: 21
Location: USA
I got an old RCA RS-199B amp, the one with a 6FQ7 and 2 6BQ5. It's completely original, so I started by making a complete schematic and measuring voltages at relevant points. Had to try it out, of course, and using my preamp of 2 12AX7 or 12AU7 it sounds wonderful. But, one of the power tubes glows a little. If I switch them, that position still glows. If I put 2 new tubes, the same one in the same spot glows. The other socket position never glows. They share a big cathode resistor, can I split that up and bias them separately?
I'm ready to replace the PS caps and coupling caps---could the old beat ones be contributing to this?
Again it sounds great, but I can't stand that glowing plate, is it killing the tube? And by glow I mean with low light and my hand shading it there is a faint orange glow on one side of the tube. You can't see it in normal light. Am I nuts or is there a simple fix for this issue?


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PostPosted: 20 Apr 2011, 17:22 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, A bit unusual, but there is a possible explanation or two. I am not familiar with that amp, but from your description it has a push- pull output. The normal reason a tube glows is that the small amount of gas inside it is being energized (much like a neon bulb). The higher the voltage the more likely it is to occur. So the question is really why one socket and the tube placed in it is apparently operating at a higher voltage than the other. Or if you would rather why the other one is lower. My initial thought is that the glowing one is correct and the other lower one has some issue :? . A few factors can affect this. First guess is that the one not glowing is drawing more current. This would cause the anode voltage to be lower as there is some resistance in the output transformer wire. Why would it draw more current has several possibilities. In no particular order, there is something upsetting the bias on the grid of that tube, perhaps a leaky coupling cap from the preceeding stage, a bad "grid leak" resistor (the one from the grid to ground) that has gone high in value, higher voltage on the screen of the tube.

If you have the schematic and can post it, I may be able to give additional thoughts. BTW, have you measured the anode voltages? Screens? Grids?

Good listening
Bruce

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PostPosted: 21 Apr 2011, 07:45 
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Joined: 18 Jun 2010, 12:46
Posts: 284
Location: Northern VA
This should be the schematic. SE, with global feedback.


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