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In Reply to: RE: Fisher 400 receiver conversion posted by used-hifi on July 23, 2017 at 16:27:43
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.
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".
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!
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.
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