Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: howardmoon on April 06, 2022, 10:36:15 am

Title: Supertwin bias question
Post by: howardmoon on April 06, 2022, 10:36:15 am: Hi, I'm reading these voltages on my 6L6GC-STR output tubes-
pin 3: 451V
pin 4: 451V should this not be more like 400V?
pin 5: -59V should this not be more like -37V?

Looking at the schematic, I'm seeing some differences in my amp. The output balance trimpot wiper has a 2.7k resistor going to ground. But my amp has two 27k in parallel going to ground, I didn't leave out a decimal point there-red, violet, orange from the center lug of the pot to the chassis. So I have the equivalent of a 14.5k resistor where it should be 2.7k?

Also the schematic has a 47ohm resistor between the first bias cap and the diode leading to the 60v secondary winding and it has been replaced with a (clearly different looking) 270 ohm resistor in my amp.

So this is all messing with my head! Help appreciated!
Title: Re: Supertwin bias question
Post by: sluckey on April 06, 2022, 10:54:25 am: Quote from: howardmoon on April 06, 2022, 10:36:15 am
Hi, I'm reading these voltages on my 6L6GC-STR output tubes-
pin 3: 451V
pin 4: 451V should this not be more like 400V?
pin 5: -59V should this not be more like -37V?
Voltage on pin 4 should be almost identical to pin 3. Pin 5 looks good. Tubes would burn up with only -37V.

Quote
Looking at the schematic, I'm seeing some differences in my amp. The output balance trimpot wiper has a 2.7k resistor going to ground. But my amp has two 27k in parallel going to ground, I didn't leave out a decimal point there-red, violet, orange from the center lug of the pot to the chassis. So I have the equivalent of a 14.5k resistor where it should be 2.7k?
Someone was attempting to increase the negative bias voltage.

Quote
Also the schematic has a 47ohm resistor between the first bias cap and the diode leading to the 60v secondary winding and it has been replaced with a (clearly different looking) 270 ohm resistor in my amp.
Someone was attempting to decrease the negative bias voltage.

So this is all messing with my head! Help appreciated!
[/quote]
Title: Re: Supertwin bias question
Post by: howardmoon on April 06, 2022, 11:04:20 am: Thanks! Possibly the same person doing both mods? Anyway I can see I still have a lot of homework to do before I understand this stuff.