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In Reply to: RE: Fisher 400 receiver conversion posted by rage on July 25, 2017 at 08:20:23
Nope nothing has been done or "up graded" as they say
Guys this is not a pristine fisher 400 by any means its a good working unit that I want to put 6v6 valves in, besides its not going to be my "main" amplifier for gosh sakes...mostly for background music, I just like the tone of 6v6 NOT Triod strapped! Think grommies here the little gem amp is wonderful :)
No one is answering my question, please be specific on what exactly needs to be done to make the 6v6 tube function in this receiver.
you need to replace the output tube sockets with octal sockets.
you can find the pinouts on TDSL for 6v6.
you'll need to take your screen voltage down stream in the power supply - you should probably put around 200 volts to the screens.
cathode bias will be easier than messing with the negative supply.
you need to calculate what current to run the output tubes at and select the appropriate cathode resistor(s).
try and run them ~ 75% of there rated max for dissipation to make up for the higher plate voltage.
I'd probably put a few CL80s on the primary of the power tranformer, might drop your voltage some. (especially with the doubler)
you should probably recap the power supply before you change the output tubes and verify proper operation before changing output tubes.
if your outputs really are 8k impedance then it won't be a perfect match however it worked fine in the dynaco ST70. (cathode bias and 8k output TX)
I think it will work fine and if you operate the 6v6 conservatively you should be OK. I don't recall the B+ in the Fisher 400, think its close to 400V ... 360V would be better, not sure if the CL80s on the primary will get you that low.
try and keep the original coupling caps in place in the FM section initially to avoid having to do an alignment.
look up the fisher doctor, he had a great manual on overhauling the fisher 400/500.
get the selenium rectifier out of there right away as I think they are a high failure rate.
Anyone have a nice receiver that uses 6v6 or the 7408 tubes want to trade?
what condition is your fisher in? I'd consider trade for HK TA-224.
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