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HI. I recently got an old Gibsonette amp up and running. The speaker was toast so I replaced the Field Coil section with a 1K 10 watt resistor and ran it into a modern magnet speaker. Everything seems to be working great but when I went to check the voltages and current I noticed that one 6V6 tube was running @ 33.3mA and the other @ 38mA. These tubes are running in parallel and are both biased by one 120 Ohm resistor. Is there any way to get these evenly biased?
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Use a 240 ohm resistor (and bypass cap) for each cathode.
What's the plate voltage - those currents seem quite high?My band:- http://www.youtube.com/user/RedwingBand -
The plate voltage read at 286 volts. According to the 6V6GT spec sheet I was looking at, and perhaps I am looking at it wrong as I am somewhat new to amp building & repairing, the plate current should be 49.5 and the plate voltage should be around 250.Comment
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I did try a 180 Ohm to get the current up. Perhaps I was going the wrong direction but again I was going by a 6V6 chart I downloaded. Nevertheless the current went up on both tubes and the inconsistency remained the same, just a higher current.
Here's the schematic. I have touched it up a little and modified it according to the field coil speaker replacement:
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Sorry for so many posts. I made a mistake in my first post. The tubes are currently biased by a 220 Ohm resistor not a 120 Ohm.....Comment
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You would have to have perfectly matched tubes and even though they probably are a matched set they can still be off a tad. FWIW you would never notice any difference in a couple ma's here or there and as time goes on they will drift even further apart and it can also happen after 20 minutes or so.KBComment
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Thanks everyone for your replies. I thought about that but then I thought isn't that the whole idea of biasing tubes? to keep them as equal as possible? It seems that could be achieved if they were not in parallel and had separate bias resistors (and caps).Comment
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OK........One thing I was doing wrong was looking at the MAX plate current for the 6V6 tube which states 49.5mA. So 70% of that is 34.65. I think the 220 Ohm resistor in this case is a pretty good fit (Tube 1= 33.5mA, Tube 2=38mA) but I am still wondering...........Is there any workaround in getting the plate current even besides buying a brand new pair of already matched tubes? Thanks in advance
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From your schematic, the 6V6 pair are operating in push-pull, rather than parallel.
In case you were tempted, the amp won't operate correctly with seperate cathode resistors for each 6V6, as I suggested in post#2.
Like AmpKat says, 5mA discrepancy is not a problem.
Tubes are rarely matched closer than 2mA.
Even then, after a few hours use, a matched pair might be 5mA apart.
Can you hear a problem?
If not then best to put this concern out of your mind.My band:- http://www.youtube.com/user/RedwingBandComment
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Thanks. You are right. I just read about 15 minutes ago that although they may appear to be in parallel it is push-pull. I thought 5mA was kind of off more than it should be but again I am somewhat new to a lot of this. It actually sounds very good and like an amp should sound. The Gibsonette is very basic so no fireworks here but it does produce a full properly functioning amp sound so with that, and the info you guys gave me, I'm good with it. Thanks again everyone for your time
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