Scott vintage radio

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I let it play late into the night last night and I began another list of things to look at.
Noise in the phase switch.
One dirty tube socket
Power tubes biased a little too hot.
Music was just the radio for that clip and it will probably get taken down for copyright violation....
 
I got it on the bench today to try and dial back the bias current because the power tubes were running ever so slightly red plates in a completely dark room. Here are my findings:

The bias is set via 2x 18 ohm resistors which get jumpered out in operation. I set the bias according to procedures but the resistors have drifted from their original value as carbon comps tend to do. I had the output stage biased hot because the resistors that are used as a reference won't hold value.
I took them out and replaced them with 10 ohm film resistors which won't drift. The 10ohm value allows you to read the output stage current as voltage without having to divide by the value of the resistor. Makes things a bit more fool proof.

While I was desoldering the cathode pin in order to install some bias sense resistors I broke off the pin to one cathode. Rats.
I have been down this path before and rather than install a new socket that doesn't match, I pulled an unused pin from another socket and slipped it into that hole. Tragedy averted.

20251007_170208.webp

Next step is to put tubes back in, fire it up and set the bias once more. Only this time I expect it will stay put.
 
trainbufftony said:
I got it on the bench today to try and dial back the bias current because the power tubes were running ever so slightly red plates in a completely dark room. Here are my findings:

The bias is set via 2x 18 ohm resistors which get jumpered out in operation. I set the bias according to procedures but the resistors have drifted from their original value as carbon comps tend to do. I had the output stage biased hot because the resistors that are used as a reference won't hold value.
I took them out and replaced them with 10 ohm film resistors which won't drift. The 10ohm value allows you to read the output stage current as voltage without having to divide by the value of the resistor. Makes things a bit more fool proof.

While I was desoldering the cathode pin in order to install some bias sense resistors I broke off the pin to one cathode. Rats.
I have been down this path before and rather than install a new socket that doesn't match, I pulled an unused pin from another socket and slipped it into that hole. Tragedy averted.

View attachment 4006800
Next step is to put tubes back in, fire it up and set the bias once more. Only this time I expect it will stay put.
Click to expand...
Man, I love a good Dale 1% resistor. Quality stuff.

Questions:
Is this a Class A or Class AB amp? I'm assuming Class AB since there's a pair of power tubes.
In Class AB I'm most familiar with fixed bias amps. This is cathode biased?
Won't lowering the resistance from the cathode to ground make the bias hotter?
 
Fixed bias.
Cathode resistor gets jumpered out in stock form. So Cathode is ground.
I put 10ohms between each Cathode and ground in order to measure and dial in the adjustable fixed bias.
 
trainbufftony said:
Fixed bias.
Cathode resistor gets jumpered out in stock form. So Cathode is ground.
I put 10ohms between each Cathode and ground in order to measure and dial in the adjustable fixed bias.
Click to expand...
I typically do that with 1ohm resistors. I mean, 10 makes the math easy as well I suppose!
 
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