I'm modifying a Webster-Chicago (later Webcor) PA amp - Model W830 from 1939.
Actually, that's taking too much credit. The amp sounds great as is. All I did was change to 3-prong power, replace all the electrolytics, a burnt resistor and a cracked coupling cap.
It has two input channels, so I wired a 1/4" jack paralleling both of them and use the 2 volumes to mix the 6K7 and the 6J7 in the input sockets. Schematic shows 2 x 6J7, but the previous owner put a 6K7 in the first spot and with the lower gain it's actually kinda cool.
I'm looking to add a master volume, but the spot that I thought would work (and be easy) makes it start to sound fizzy as soon as I turn it down a bit. It does work to lower the volume, but it sounds like I'm just not driving the next stage enough.
The configuration is:
6K7 - pentode channel 1 input (sparkplug-type wire to the grid)
6J7 - pentode channel 2 input (sparkplug-type wire to the grid)
6N7 - dual-triode used as mixer
6N7 - dual-triode, cathodes and plates strapped together driving a transformer to the output tubes
2x 6L6 - PP output
I thought I could change the fixed 500k resistor between the mixer 6N7 and the driver 6N7 (right after the single tone control and cap) with a 500k pot.
That way the mixer plates still see the same 500k and I would just be tapping a voltage to drive the driver.
Apparently wrong-headed thinking.
This is my first post, so I hope the attachment of the schematic works.
I'd appreciate any guidance or suggestions.
Thank you in advance for taking the time to read this.
atrain5600
Actually, that's taking too much credit. The amp sounds great as is. All I did was change to 3-prong power, replace all the electrolytics, a burnt resistor and a cracked coupling cap.
It has two input channels, so I wired a 1/4" jack paralleling both of them and use the 2 volumes to mix the 6K7 and the 6J7 in the input sockets. Schematic shows 2 x 6J7, but the previous owner put a 6K7 in the first spot and with the lower gain it's actually kinda cool.
I'm looking to add a master volume, but the spot that I thought would work (and be easy) makes it start to sound fizzy as soon as I turn it down a bit. It does work to lower the volume, but it sounds like I'm just not driving the next stage enough.
The configuration is:
6K7 - pentode channel 1 input (sparkplug-type wire to the grid)
6J7 - pentode channel 2 input (sparkplug-type wire to the grid)
6N7 - dual-triode used as mixer
6N7 - dual-triode, cathodes and plates strapped together driving a transformer to the output tubes
2x 6L6 - PP output
I thought I could change the fixed 500k resistor between the mixer 6N7 and the driver 6N7 (right after the single tone control and cap) with a 500k pot.
That way the mixer plates still see the same 500k and I would just be tapping a voltage to drive the driver.
Apparently wrong-headed thinking.
This is my first post, so I hope the attachment of the schematic works.
I'd appreciate any guidance or suggestions.
Thank you in advance for taking the time to read this.
atrain5600
mater volume addition
By the way, the stain was on the schematic when I opened the chassis.
That's not me being sloppy (this time).
By the way, the stain was on the schematic when I opened the chassis.
That's not me being sloppy (this time).
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