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Altec 1569A repairs

C

CVElectronic

Member
I've been handed a 1959 Altec 1569A by a customer, and I'm getting into it to see what needs to be done, after cleaning it. (It was ungodly filthy -- looked like it had been stored in a stable.) SN is in the 800s.

The only non-standard thing it needs is a replacement tube socket, which I've ordered. One of the 5U4 sockets has a large chip out of it and on a PT HV pin.

In looking at the circuit, there are a couple of things that I don't like on paper. 1) The 6CA7 screens being fed via the first reservoir capacitor through a single resistor, and 2) One bias control for all four output tubes. I understand that these were commercial amps, designed to be simple, rugged, and easy to service, but some of it is maybe too simple.

In particular, has anyone come up with a better 6CA7 screen supply filtering solution that they like? I've read some of the threads on this amp. There are discussions of it, but rather than reinvent the wheel, I thought I'd ask first.

It had three Sylvania 6CA7s in it, all of which test good, but only three :-( I am thinking of a quartet of EH 6CA7s for it.
 
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Guess you could use individual 1k screen resistors. Or build a second node off the 1st filter . I would use a small resistance 5w , maybe 100 ohm? That supplies the 1k screens . Or maybe 470 ohm screeens with the 100 ohm 5w and a 20-40uf cap.

But I think you are over thinking it.
You don't want the screens to sag too much or you'll induce compression.

I highly recommend the eh 6ca7's , pretty robust for a modern tube.
 
There are different ways to do a screen supply. Obviously Altec did it in the least parts way. You should check what voltage the screen is supposed to be at vs. what it is in the amp. You might want to adjust the voltage if it needs it. This would be an opportunity to add an RC circuit to the screen supply. If you really want to get into it some builders use a regulated screen supply either SS or tube. An easy implementation of a "regulated or fixed" screen supply is with a zener diode of suitable wattage. This will set the screen voltage at a fixed value to the B+. But for the most part i just use a suitable dropping resistor and a film cap.
 
I think what the OP thinks is a bias control might really be a balance control. :dunno:
 
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cademan said:
I think what you OP think is a bias control might really a balance control. :dunno:
Click to expand...

If you have a control pot with a negative voltage on one side, ground on the other, and a wiper connected to bias-feed resistors that moves between ground and the negative voltage, I would call it a bias control.
 
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