The near non existent 6RA8 tube

maxhifi said:
Exactly. The easiest way to do this is to pull the output tubes and use an external AC voltage source to feed a small voltage into the 8 ohm tap of the output transformer, then measure the voltage on the primary of the transformer, and use this to calculate the turns ratio of the transformer. Square this number and multiply it by 8, and that's your approximate primary impedance.
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Yeah, that's beyond my ability and equipment presently.
 
RS Steve said:
Yeah, I have a tech that might have the equipment to pull that off. I will ask him.
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It actually isn't that big of a deal, but if reading it puts you off it isn't a bad idea to ask a tech. You can even use the filament supply in the amp as the AC voltage source, then all you need is a multimeter, but better let a tech figure this out if you aren't confident.
 
I use a variac and an isolation transformer. Iso trans from plate to plate, and adjust the variac to get about 1 vac across the full secondary winding. Note the exact voltage, and the exact voltage plate to plate.

plate to plate volts / speaker side volts = turns ratio
turns ratio squared = impedance ratio
impedance ratio * load = primary impedance

if the full secondary is rated for 16 ohms, then thats your load.

the exact voltages you use aren't critical but use all of the decimals your meter has. Makes the result more accurate. I usually aim for 1vac because most meters can read that with a pretty good level of accuracy without needing to switch to the millivolt scale.
 
gadget73 said:
I use a variac and an isolation transformer. Iso trans from plate to plate, and adjust the variac to get about 1 vac across the full secondary winding. Note the exact voltage, and the exact voltage plate to plate.

plate to plate volts / speaker side volts = turns ratio
turns ratio squared = impedance ratio
impedance ratio * load = primary impedance

if the full secondary is rated for 16 ohms, then thats your load.

the exact voltages you use aren't critical but use all of the decimals your meter has. Makes the result more accurate. I usually aim for 1vac because most meters can read that with a pretty good level of accuracy without needing to switch to the millivolt scale.
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I'm reading the service manual saying that it's got 16db negative feedback, dampening factor of 20 for 16 ohm load if this helps.
 
the voltage ratio is going to be somewhere between 20 and 30:1, primary to 8 Ohm tap. Plug the two plate leads into the wall. measure the exact voltage. measure secondary voltage. Done. Nothing but a sacrificial extension cord and a volt meter needed.
cheers,
Douglas
 
gadget73 said:
and in the same bad "can't find them" sort of way. The late tube era "hail Mary" designs are interesting but annoying just because of such limited supply.
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I know this too well. There's a ELL-80 pair in my Fisher Custom Electra console which is just that. Difficult to find tube, and expensive. Those last gasp tubes, especially German and Japanese oddities and rarities tend expensive.
 
have a pair of those in my Philharmonic chassis, and a spare set. There are options for that one at least. ELL80 to 2x EL95 adapters exist.
 
The used 6RA8 quad I ordered are shipped and supposed to show up next week, keeping my figures crossed that they are usable.
 
A TV vertical output triode is likely a close fit, something like a 6S4. It even has the correct pinout!! So a plug-in replacement! Specs for transconductance are somewhat lower, so crank up the volume control another 23 degrees.
 
Tonight I installed the new JJ cans, now the amp is restored completely. I will install the new tubes I received as a quad since they seem to be fairly matched according to the seller testing. I will keep the other original trio as spares. Tomorrow I'll fire it up and get it biased correctly, then give it a listen. Any ideas on how to adjust the hum pots without using a distortion analyzer? I'd like to verify they are adjusted correctly before buttoning the amp up.

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