Scott vintage radio

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

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!
 
The Amplifier was finished months ago.
I sent you a message and never heard back.
In that time ive bought a house and moved.
So its ready for ya as soon as I find the box its in.
I have a friend who is the husband of a deceased audiophile friend who makes regularly scheduled trips to your neck of the woods.
She's there now so it will be a while before she's heading that way again but I was thinking Monica will handle the amp with much more care than anyone in the business of shipping things would...
Otherwise I can pack for apocalypse and send it...
 
trainbufftony said:
The Amplifier was finished months ago.
I sent you a message and never heard back.
In that time ive bought a house and moved.
So its ready for ya as soon as I find the box its in.
I have a friend who is the husband of a deceased audiophile friend who makes regularly scheduled trips to your neck of the woods.
She's there now so it will be a while before she's heading that way again but I was thinking Monica will handle the amp with much more care than anyone in the business of shipping things would...
Otherwise I can pack for apocalypse and send it...
Click to expand...
You probably know this, but pull the tubes and bubble wrap them separately, then pack them in a way where they won't move. I use sharpie to number the tubes and corresponding sockets so an end user/buyer can just pop them in the right way in about a minute. Really cuts down on glass breakage or anything internal to the tube loosening up. I'm batting 100 on that method. On open-chassis amps, I stuff them in the back and add more bubble wrap until everything is tight. Then pad and box up around the whole chassis.
 
Sorry I didn’t respond.

I am game with however ypu think is the best way. When you find it, I am on board with waiting until Monica is back in the area.
I understand that you are done with the amplifier does that mean the tuner as well? Are both done? Or.. I am not familiar with how these things work.

Congrats on your new house! Thanks your efforts.
 
xtiaan2000 said:
Sorry I didn’t respond.

I am game with however ypu think is the best way. When you find it, I am on board with waiting until Monica is back in the area.
I understand that you are done with the amplifier does that mean the tuner as well? Are both done? Or.. I am not familiar with how these things work.

Congrats on your new house! Thanks your efforts.
Click to expand...
I finished the amp, didn't touch the tuna.
When I moved in tried to get ahold of ya and got no response so I stopped until I heard from ya.
Also, I was moving.
Id like to get the amplifier to you, then proceed with fixing up the tuner.
The amplifier is the real gem here, the tuner can be made functional but FM radio will be your limitation.
Its a format that has been abandoned 10 years ago by almost anyone with any skin in the game.
But it will make a nicely matched set...
 
trainbufftony said:
The Amplifier was finished months ago.
I sent you a message and never heard back.
In that time ive bought a house and moved.
So its ready for ya as soon as I find the box its in.
I have a friend who is the husband of a deceased audiophile friend who makes regularly scheduled trips to your neck of the woods.
She's there now so it will be a while before she's heading that way again but I was thinking Monica will handle the amp with much more care than anyone in the business of shipping things would...
Otherwise I can pack for apocalypse and send it...
Click to expand...
It sounds like you have been busy!
Moving, ugh!
 
You must log in or register to reply here.

Share this page

IH8MUD Forum | Toyota, Lexus & Off-Road Community
Community platform by XenForo® © 2010-2026 XenForo Ltd.

© 2000-2026 Tie Rod Media LLC

XenForo add-ons & extensions by xenBaker
Back
Top Bottom