[rfeenstra]

Interesting, Russ! I learn something every day.

[rfeenstra]

Been using this amp every day for a long time now.  Sounded great till today.  One of the channels started humming loudly.  Turned out to be one of the 6U8 matched pair driver tubes.  Looks like I'll have to order another matched pair.  Russ, you were right about them not lasting as long as one would wish.  For now, back to solid state!

[PhilcoJohn]

Any how looks like a complicated radio to restore. But at the end it would be a great radio to listen too.

[rfeenstra]

Yes, it is an excellent sounding amplifier. I have a matched pair of 6U8's now but have not reconnected the amp. Still using the Legacy Audio solid state amps

[morzh]

It's that nice, almost sensual expectation when you hold the new tubes in hands and imagine how they will start glowing, how you hear the sound....
I went through it when having restored the Eicos and gotten the tubes from Jim.

Enjoy.

[rfeenstra]

Youzzah, morzh, that's getting deep! Love it.

[MrFixr55]

The hot 7591s give "that warm tube sound". No expert, but I would see what the current is with the bias voltage set per the Tube Spec Sheets, then verify cathode current. Most of these amps have a Cathode Bias C- supply instead of "self bias" (cathode resistor) A popular hack for many amps of this vintage is to put 10 Ohm (correction to original post) 1/4 Watt resistors between cathode and ground for each output tube. These will act as fuses and (hopefully) open instead of the output transformer. BTW, smart move to replace the selenium rectifier and bias circuit resistors. If the C- supply quits, the tubes and output transformer take a major current overload.

I believe that the 7591 was designed by Westinghouse and was one of the last Power Pentodes designed, and was supposed to have better fidelity and less distortion than 6L6, 6V6, 5881, etc.

Very important to note that there is a major difference in pinouts (base diagram) between 7591s and the 6L6, 6V6, 5881, etc.), particularly with cathode / suppressor and grid pin assignments, so it is not a direct swap between these and 6L6s, etc.

OOps!! Edited this to recommend 1/4W resistors instead of 1/2 W Units

[RGK20m3]

I highly recommend the Dave Gillespie mods; I adapted his circuit ideas to my Eico ST70. Basically you regulate the bias and screen voltages proportionate to the B+, and eliminates the common cathode resistor set up (which didn’t apply to the ST70). Guy knows a lot!

[rfeenstra]

It is my intention to install the the mods that Dave recommends. I just haven't gotten that far. At the moment, I have set the amp aside as I have a nasty habit of forgetting to turn it OFF and finding it ON a couple of days later! Not good.

I have the resistors for the cathodes and can easily build the rest of the Dave's biasing system. I just need to get a "round tuit." There are a lot of things I need one of those for!


I was surprised to see this thread revisited.