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RE: What capacitors to buy to refurbish old RCA amp?

Posted by maxhifi (A) on February 10, 2016 at 12:24:45

In Reply to: RE: What capacitors to buy to refurbish old RCA amp? posted by Michael Samra on February 10, 2016 at 11:59:58:

The feedback configuration threw me for a loop at first too (no pun intended!) Especially the little trick they used to bias the first 6SL7 section properly. DC current flows through the feedback resistor, which would upset the bias of the first 6SL7 section - but rather than let that happen, they put the grid a bit positive with a voltage divider, while C2 makes sure it's still referenced to ground AC wise.

I think that from a conservative design standpoint, a second 5R4 would be a good idea.. but I have to say that in reality, it's really not necessary. At idle the amplifier draws 210mA, which approaches but does not exceed the 250mA limit of the tube. And I have used these amplifiers a LOT, I mean in the 20,000 -30,000 hour range since I got them.. and the USED GE 5R4GA's which came with the amp (and who knows how long they were used before I got them) are still good! And pretty hard use too, I used these to rock the house! When you have it really cranked the 0D3s dim a bit during transients.

I haven't noticed that the driver is inadequate... I suppose a more robust driver MAY be able to push the 807s into AB2, but that would probably expose power supply weakness. One possible area of concern is balance in the phase inverter, I do want to investigate that once all the capacitors are replaced. I will definitely make sure that the driver doesn't clip before the output stage does, but something tells me the 1950s powerhouse of engineering which was RCA wouldn't have let something out the door which was designed that badly!

Which model of RCA do you have? Are they the rack mount style ones with built in tube tester? I'd love to get my hands on a pair of those some day!

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