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Hello all,
I picked up a Stromberg Carlson AU-58 with plans to convert it to a guitar amp. 2x 6L6 PA amp from 50's.
I settled on building the Fender 5A6 circuit as the Stromberg had all the right tubes besides the phase inverter.
I ended up more or less gutting the amp, and building the 5A6 circuit fresh.
I separated the shared power tube cathode bias resistor and bypass cap to two separate 220R, 8UF each. Bias is healthy, around 85% or so.
I changed the tone control to a more typical Fender TMB tone stack with values from a 5F6. I added 470ohm screen resistors on the power tubes.
So both plates of the 6SJ7's go into a separate volume control, which both go into the tone stack and then directly into the 6N7 phase inverter.
The amp sounds fine and is putting out about 25W but lacks gain, sounds a bit weak. Doesn't break up until it's dimed and even then, not much at all.
I have heard of similar complaints from Fender's that use a 6SJ7 like the 5C1 champ. So maybe it's just how the amp is.
I know most amps have an extra gain stage after a TMB tone stack, and I'm sure that would help.
So I'm trying to think of what can be done. How do I get this thing to break up and get some of that glassy pentode sound?
My first thought was cascading both 6SJ7's. No one will ever use the low gain channel of this amp.
Good idea or not? Better to go 6SJ7 > TMB > 6SJ7 > 6N7? // OR 6SJ7 > 6SJ7 > TMB > 6N7?
Also thinking about modifying the 6SJ7 circuit for more amplification. I know these tubes are known for microphonics when driving them too hard.
Currently in channel 1 if have 100mV going into the grid, I get 760mV on the plate, so voltage gain of... 7.6? Doesn't seem like much...
I have preamp plate supply voltage of 276V. With current circuit I'm getting 117V at the plates, 60V at the screens, 2.5V at cathode.
Should be able to coax way more out of this thing. Load lines and anything to do with design is new to me. I guess I am just looking for some input/guidance to make sure I'm going in the right direction before diving in and experimenting.
http://www.valvewizard.co.uk/pentode.html
I picked up a Stromberg Carlson AU-58 with plans to convert it to a guitar amp. 2x 6L6 PA amp from 50's.
I settled on building the Fender 5A6 circuit as the Stromberg had all the right tubes besides the phase inverter.
I ended up more or less gutting the amp, and building the 5A6 circuit fresh.
I separated the shared power tube cathode bias resistor and bypass cap to two separate 220R, 8UF each. Bias is healthy, around 85% or so.
I changed the tone control to a more typical Fender TMB tone stack with values from a 5F6. I added 470ohm screen resistors on the power tubes.
So both plates of the 6SJ7's go into a separate volume control, which both go into the tone stack and then directly into the 6N7 phase inverter.
The amp sounds fine and is putting out about 25W but lacks gain, sounds a bit weak. Doesn't break up until it's dimed and even then, not much at all.
I have heard of similar complaints from Fender's that use a 6SJ7 like the 5C1 champ. So maybe it's just how the amp is.
I know most amps have an extra gain stage after a TMB tone stack, and I'm sure that would help.
So I'm trying to think of what can be done. How do I get this thing to break up and get some of that glassy pentode sound?
My first thought was cascading both 6SJ7's. No one will ever use the low gain channel of this amp.
Good idea or not? Better to go 6SJ7 > TMB > 6SJ7 > 6N7? // OR 6SJ7 > 6SJ7 > TMB > 6N7?
Also thinking about modifying the 6SJ7 circuit for more amplification. I know these tubes are known for microphonics when driving them too hard.
Currently in channel 1 if have 100mV going into the grid, I get 760mV on the plate, so voltage gain of... 7.6? Doesn't seem like much...
I have preamp plate supply voltage of 276V. With current circuit I'm getting 117V at the plates, 60V at the screens, 2.5V at cathode.
Should be able to coax way more out of this thing. Load lines and anything to do with design is new to me. I guess I am just looking for some input/guidance to make sure I'm going in the right direction before diving in and experimenting.
http://www.valvewizard.co.uk/pentode.html
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