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Hello, I would like your opinion on a conversion from 7868 to 6v6, I do not like the sound of the 7868 plus i have several 6v6 that will last me a long time, please look at the schematic below
comments,suggestions are very welcome
Thanks
Lawrence
Follow Ups:
Maybe, a new set is all you need? Sure would be a lot easier.Well, good luck --- whatever you decide to do.
8^)
Edits: 07/26/17 07/26/17
I have not responded because I have been busy doing a sympathetic restoration, plus one channel was down
now does anyone want to trade me some 7868 valves, I have a bunch of tubes including gm70 matched pairs I will be selling off and LOTs of 6v6
I don't have money to allocate to this project so need to sell or trade stuff.
Now I will say after pulling the 100uf cap that feeds the finals I put in very hi voltage 1200v "DC link" type, its no longer grey and limp but becoming a dynamic tube amp its better then the other fisher I had.
I plan on squeezing some russian ssg type in a few coupling areas,this 400 looks ruff on the surface but inside is like bran new! I also installed a 25watt variable resistor for biasing the tubes.
Now if the amp didnt come alive, then I would have made it work for 6v6 bu t the 7868 tube is OK
and get something you might like better, or take the $$ and buy some new parts and do a proper build.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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.
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.
And in any case your belief that it is the 7868s is most unlikley to be true.
Restore it properly, with Jim McShane's 7868s.
You WILL be surprised. Bet you.
Rebuild the PSU, replace all the capacitors and resistors and test all the tube sockets.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Why not sell it and use the money to build a nice little 6v6 from scratch? All kinds of schematics available. You would have all new parts to boot.
True very true but its all there not much to do but small stuff...plus this has a tuner a TUBE TUNER :)
But not the best tuner made and with the state of "over the air" broadcasting the point is moot. It wouldn't take much to find a better tuner in it's stead. If you do like the tuner that much you could take the outputs into another system, bypassing the amp section.
Honestly you could make someone else very happy that wants to restore it rather than hack it up.
Edits: 07/25/17
I am afraid I am not a big fan of receivers; too many parts cramed into too small a space. I prefer separate units for each.
Well, there are indeed a lot of parts crammed in into the space, but when done right, like those old Fishers, they can really charm.
True, but so can seperates when done right. Maybe even more so.
Tuner VS Receiver - it's an almost meaningless argument considering the poor audio quality of most FM broadcasts. I switched to a solid-state tuner (Luxman) for this purpose a long time ago, and have never regretted it. We have one classical station here that's decent, but it's low power and plagued with multipath. Really, I don't see why any audiophile would be concerned about the quality of a tuner unless they have a truly exceptional situation at their location.
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Buy Chinese. Bury freedom.
Tuners can't reject multipath.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
I would like to think the stations I work for have good quality FM analog sound. We process pretty lightly. Light compression, limiting, clipping and composite clipping. Not winning any loudness wars.
I think you have to live in a major city to get really good fm. I don't, so it isn't on my list of sources at the moment.
Really good FM is now HD simulcast which further compresses the signal. About 12 years ago I went to great lengths to resore a Fisher FM100 and have it aligned. In less than a year both WXPN and WRTI, the only stations worth the trouble went digital. Man was I pissed.
I knew it was too good to last .........
Yes ....Very helpful thanks
Based on voltage ratings, transconductance and filament requirements, the best sub for those is probably the 7355. Unfortunately, they're rare and expensive now. You're lucky the 7868 is available as a reissue, and I see on his Website that Jim McShane supplies hand-matched quads for only $120. Forget the 6V6, it's not even close to what you need for 25W/channel. The Fisher output transfomers are the wrong impedance, and the tube itself won't survive that voltage or dissipation.
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Buy Chinese. Bury freedom.
The Question in my original post
What needs to be done to make 6v6 tubes work? and yes there will be less output, but output is not everything now is it?
I cannot find a calculator to calculate grid bias so i can set it up on paper before i go and change what needs to be changed, i need your help with this.
6v6 needs good voltage swing/drive and the 12ax7 should be sufficient.
I see no reason why not to convert
You'd have to drop the B+, kind of a lot. You don't want to go with cathode bias because then you won't get that extra bit of oomph from class B (this also prevents you from just slapping in some big 50 watt resistor to shed 150 volts), and the power supply is a voltage doubler, so you can't exactly just switch to a choke input filter. Then again, if you could lower the voltage produced by the power supply, now all the small signal tubes are going to be off. It's a can of worms for sure!
If the space and filament current are available, EL34's might be an option that's a little more workable. Depending on your 400, you might have 6.4K OT's or 10K OT's.
I am even less impressed with el34 tubes then 7591 variety!
I want 6v6!
Sorry, you can't have 6V6s. However, adding some filament power will allow you to use 6L6s. The filament winding already present will easily feed 4X 6L6 heaters. Add a 1 A./6.3 VAC filament trafo and you can easily energize 2X 6BL8s, etc. A pentode voltage amplifier, instead of a 'X7 section, will give you the extra gain needed to drive 6L6s.
You can entertain the thought of triode wired "finals", for extra "finesse".
Eli D.
I was also thinking 6L6 but the extra fill tfr will be a tight fit on that chassis. I think if it were me, I'd sell it and start with an amp designed for 6V6, the Bell 3030 comes to mind.
Thanks keep the information rolling :)
from what I gather 6v6 tubes will handle 400v easy but not the screen and I can drop voltage with a pot same with the neg bias supply.?
"I would like your opinion on a conversion from 7868 to 6v6"
Why bother to ask? It seems that you intend to do this regardless of the consequences.
--------------------------
Buy Chinese. Bury freedom.
Yes so please if you can help and know how to calculate please help me out
besides 8k primary for 7868 valve will be fine for the 6v6
remember 7591 is 7868 in an octal socket for low distortion 8k primary should be used as for the 6v6 no issues
come on guys lets put some ingenuity in this little project think!
Edits: 07/24/17 07/24/17
A 6V6 sounds good when installed in a properly designed 6V6 amplifier.
Just slapping a 6V6 in any old amplifier and moving some voltages lower does not mean you will get the classic 6V6 sound.
On thing in particular here is the OPT impedance. That alone will have a significant effect on sonics.
Tweak the sound by "rolling" small signal tubes.
Also, heed Jim's warning about modern 7591s/7868s being intolerant of grid to ground resistance liberties. IMO, a good way to deal with the matter is "borrowing" from Fisher's X-100 and use the 12DW7/7247, instead of 12AX7s.
Eli D.
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