Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on August 06, 2020, 09:25:40 pm
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So I found the old post here about adding a bias pot to a 6G6-A and used it to put one in my early version (has the rectifier wires clipped). It works and I dropped a pair of STR387s into it. With the 50KL (all in the pack I got seem to measure 58K at one end) pot turned all of the way down I'm still getting 475V/31ma on the tubes - I can't get above 31ma. Rather than a lot of trial and error can someone help answer these two questions: 1 - I have to lower the 47K I have in conjunction with the 50K pot, correct? And any idea how much I have to lower it to get a bit more play but not go overboard (33K, maybe)?
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Is this circuit you are talking about ? It help us if you post schematic of your bias mod
47K
higher value = more negative voltage = colder bias
Lower value = less negative voltage = hot bias
You can work on the 1 watt resistor from the Power transformer
higher value = less negative volt
lower value = more .....
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https://el34world.com/Forum/index.php?topic=14674.0 (https://el34world.com/Forum/index.php?topic=14674.0)
It looks the similar to the one you posted but the post is specifically about the 6G6-A. No tube rectifier in this one - has those leads trimmed off and chassis hole plugged.
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All bias circuit work same with or without tube rectifier.
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The 6G6-A has a dedicated bias winding and no resistor before the diode. That's what I was referring to.
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The 6G6-A has a dedicated bias winding and no resistor before the diode. That's what I was referring to.
I now understand better what you mean.
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So I found the old post here about adding a bias pot to a 6G6-A and used it to put one in my early version (has the rectifier wires clipped). It works and I dropped a pair of STR387s into it. With the 50KL (all in the pack I got seem to measure 58K at one end) pot turned all of the way down I'm still getting 475V/31ma on the tubes - I can't get above 31ma. Rather than a lot of trial and error can someone help answer these two questions: 1 - I have to lower the 47K I have in conjunction with the 50K pot, correct? And any idea how much I have to lower it to get a bit more play but not go overboard (33K, maybe)?
So far I gather that you have a 50K pot in series with a 47K resistor, the AC bias voltage is taken from a bias tap on the HT winding or a completely separate winding, and there is no resistor between the winding and the diode.
If the resistor between the capacitors is still 10K like the original 6G6-A, then changing the 47K to 20K should give you a broad enough range for any 6L6GC or equivalent.
If the resistor between the capacitors is 15K as in Doug's drawing, then 33K would probably be a good replacement for the 47K.
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I changed it to the 15k like in the drawing. I'll try the 33k. I just wanted to not keep unsoldering and trial and error the resistor, if I could help it. It's safe now but on the cold side and reached the end of the pot. Sounds good but has the bass channel blocking distortion sound that sounds like it's not uncommon for these amps.