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In Reply to: RE: Fisher 400 receiver conversion posted by used-hifi on July 23, 2017 at 16:27:43
triode strap them, go cathode bias and run them conservative to make up for the high plate voltage.
jj 6v6 might better tolerate the voltage.
have you already recapped the unit?
you could also change the power transformer and dump the voltage doubler but you'd have to be sure to get the correct voltages to all the other stages in the amplifier. that will take a bit of work!
I just went through an HK TA-224. It is the one of few rebuilds I didn't change a thing from original. Just bought some nice panasonic caps and replaced all of the electrolytic caps.
As complex/busy as these receivers are I think it is wise to keep the original intent/design in mind and work toward a restoration - rather than modify.
the first tube amp I owned and went through was a Fisher 400. I still have the 12ax7s. I wish I would have kept it. Having built a few SETs and push pull amps - I really appreciate the old receivers now in all of their "original" glory.
Follow Ups:
You should be advising him to swap it out for the HK you worked on. 400V is well over the rating for a 6V6. He'll end up with a butchered receiver that won't perform as well as one worth less (and that assumes he can find a quad of 6V6s that will last longer than 20 minutes). Seriously, it's just a bad idea.
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Buy Chinese. Bury freedom.
Edits: 07/25/17
yes but next time when you and a few others tell him not to do it he'll have a great experience to work from.you are right, it is bad advice.
Edits: 07/25/17
LOL typical engineer with zero vision only think and do by the book only as if everything has been thought of and done with nothing to learn so lets just do whatever is by the book
guys I have laugh you so called experts out there
Thanks for the laughs
now onto real work!
I
I am quickly coming to the conclusion that you have NOOOO idea what engineering is all about, let alone a niche application there-of like tube circuit design. Good luck...and remember the voltages you are contemplating are quite lethal.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I think its more appropriate in that you Sir have NOOOO idea who i am, what i know, or what experimentation(thinking outside the normal small box) that most people believe in.
now with that said I could have converted to 6v6 easy with a few changes but nOOOOO you cant do that because of some book says so? BS!
Start thinking for your self as I do!
It was once said that small kids could think (imagination) and act on those thoughts, its only later in life that your brainwashed into thinking only such things "can't" be done because someone in some engineering book says so.
Fortunately I was NOT brainwashed :)
Lawrence
So sorry, I only know you by what you write. And the conclusions that can be drawn from that are not going to change because you claim to be able to 'think outside the box'.
Matter of fact, I do believe that such language has only been used by folks who can't. Those who can just label the process Engineering.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Your moniker says it all :)
Good luck be well
Some of us have decades of experience either working on vintage gear or designing gear front scratch; you seem to have neither.
Your receiver is a horrendous choice to retrofit for 6V6's, this is the unanimous advice that you've gotten from all who have replied.
You admit that this is for background music, so why do you really care anyway?
Only small-minded individuals find it necessary to resort to personal insults over trivia like this. It's ironic that you would refer to anyone else in those same terms. You have my blessing to do whatever you want to the damn thing. You'll learn the hard way what everyone else already knows.
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Buy Chinese. Bury freedom.
Why bother to heed any advice offered here by the experienced members?
guesser - in your experience is it likely an alignment would be needed if the caps are changed in the AM/FM sections?
what is the likely failure mode if these caps are bad? is it possible the AM/FM sections could take out the output transformers or create a short?
what is the likelihood the alignment for the AM/FM section isn't wrong with the factory caps that are potentially bad?
would you replace all caps regardless of type or just the paper caps?
in the case of this HK224 here I have found that my FM section is a bit flaky, volume levels of received signal fluctuate. there seems to be minimal frequency drift however... I'll probably change those caps out soon. for now I did the coupling caps from/to the concertina phase splitter and put the receiver in service.
there is always the ongoing battle of "recap everything" vs "recap what is bad". I've found the old timers generally prefer to only fix what is broken... and everyone that is roughly 50 years old and younger tend to lean toward "change everything."
You never want to replace caps in RF circuits unless they are truly defective. Mica and Ceramic caps, which are commonly used in RF circuits, rarely go bad.Especially in point to point wiring, just moving caps can alter the RF alignment.
Edits: 07/25/17
I miss-communicated there but appreciate what you said.
I meant the paper/wax capacitors in the FM/AM sections. I know the mylar and ceramic are fine.
Does anyone happen to have a spare fisher 19kc oscillator coil for the MPX unit on this fisher receiver?
Thanks
That's such a difficult job, rebuilding a tube-type tuner. Even 40 years ago, when these units didn't yet need such extensive maintenance and replacement of parts, most owners couldn't afford to have a front end refurbished. I've seen things fail in tuners (both tube and solid-state) that you just wouldn't think would ever go bad. I still remember a KLH that wouldn't tune properly... after many, many hours of testing, it turned out to have a shorted turn in the discriminator transformer. The customer was happy... me, not so much.
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Buy Chinese. Bury freedom.
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.
its in good used fully working condition...
email me privately please
lawrenceharasim@gmail.com
Hmmm... I did a 500 receiver for my brother in law some years ago and a little out of greed and mostly for reliability and sound I replaced the Mullard 12ax7s with new issue tubes (Sovtec 12ax7LPS) in the phono and MPX sections. I have tried to use them in differing applications since but seem to end up pulling them and putting something else in.
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