I recently came a cross a junk Fisher 500C, well beyond repair.

The transformers seem to be in good shape so I was wondering
if they might work well in a new design.

Being that the 500C used 7868's, an obsolete and expensive tube,
the question is, for which currently available tube might the output
transformers work well?

I'm not looking for maximum power but for a really clear, sweet
musical sound so another thing comes to mind; How well could
they work in a single ended application?

TIA for your help.

________________ Marc Stager
email

In article , wrote:

I recently came a cross a junk Fisher 500C, well beyond repair.

The transformers seem to be in good shape so I was wondering
if they might work well in a new design.

Being that the 500C used 7868's, an obsolete and expensive tube,
the question is, for which currently available tube might the output
transformers work well?

I'm not looking for maximum power but for a really clear, sweet
musical sound so another thing comes to mind; How well could
they work in a single ended application?


Forget Single Ended, try Triode connected EL34's in push pull. If you
must have the extra second order does of second order distortion SE
provides, you can get it with the push pull circuit by adjusting the
driver to maximize the second order distortion.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

Hi John ,

Forget Single Ended, try Triode connected EL34's in push pull.

I wouln't go for SE with that trannies also . Basicly to do so you have
to build PP and just use one 1/2 .....
But I had a quick look at the 7868 data and I think it's closser to the
6L6 than EL34 .....
Maybe the power tranny isn't build to deliver 3A heater current ....

Best regards ,

Ronald .

wrote...

Being that the 500C used 7868's, an obsolete
and expensive tube, the question is, for which
currently available tube might the output
transformers work well?

The 7591A! The EH version of the tube is real
nice and it's virtually identical to the 7868. The
only real difference was the basing.

I'm not looking for maximum power but for a
really clear, sweet musical sound so another
thing comes to mind; How well could they
work in a single ended application?

Not good, stick with the 7591As.

BTW, I have them if you need any (shameless
commercial plug).

Jim McShane
Need Tubes? Got a H-K Citation (Pre) Amp?
Check http://pages.prodigy.net/jimmcshane
Repro knobs for Citation gear in stock!

Hi:

By actual measurement, the 500C's OPTs are ~ 6600 ohm P-P to 4, 9 (yes 9)
and 16 ohm sec. Since the 500 used 7591s in the output stage in std fixed
bias, pentode mode, obviously, 7591s would be a good choice as would its
novar based variant the 7868. The 6600 ohm value should also be a good
match to the 6L6/5881/807 in certain pentode modes as well as at least a
couple of modes of EL34.

Although single ended operation is probably best avoided as has already been
pointed out, I can recall at least one published SE design that seemed to
work well. This was the famous "Single Ended Glory" (for under $100) amp by
Eric Barbour or John Atwood (forget who at the moment). It was published in
Glass Audio quite a few years ago and used a #45 driven by a 6SN7. Since
this was a intended as a low cost amp the author suggested scrounging up old
OPTs from junked receivers. He specifically mentioned PP OPTs from things
like Fisher and Sansui recvrs. The stated reason a PP OPT could be used was
that the 45 operates at such low power, the standing bias current was not
enough to saturate the core even w/o typical air gap. The amp was capable
of 2W . I never built this amp so can't verify if saturation was an issue
but it did get rave reviews from those who did build it.

The one big glitch with this design is that the 45 is anything but cheap
these days having been discovered by the true believers (and much to the
horror of antique radio collectors).

--
Steve

You might be able to try a SE parafeed amp.

But I'd agree with Ronald, John & Jim - you'd be better off with a PP
design. A friend of mine put EL34s in his 500C and was happy with the
results.

- jon

From:
Newsgroups: rec.audio.tubes
Date: Mon, 03 Nov 2003 20:16:01 GMT
Subject: Uses for Fisher 500C Transformers?

I recently came a cross a junk Fisher 500C, well beyond repair.

The transformers seem to be in good shape so I was wondering
if they might work well in a new design.

Being that the 500C used 7868's, an obsolete and expensive tube,
the question is, for which currently available tube might the output
transformers work well?

I'm not looking for maximum power but for a really clear, sweet
musical sound so another thing comes to mind; How well could
they work in a single ended application?

TIA for your help.

________________ Marc Stager
email

Forget Single Ended, try Triode connected EL34's in push pull.

I wouln't go for SE with that trannies also . Basicly to do so you have
to build PP and just use one 1/2 .....
But I had a quick look at the 7868 data and I think it's closser to the
6L6 than EL34 .....
Maybe the power tranny isn't build to deliver 3A heater current ....

In that case, build an 807 PP amp. NOS 807s are just like a 6L6 with a top
cap, are dirt cheap and sound great. Heater current is only 0.9A per tube. I
am using a class A, no NFB amp, with plate voltage 400V and plate diss. 22W
per tube. My output trannies were originally meant for 6V6s but sound waaay
better with the 807s. I got the schematic off the net, at
http://hollowstate.netfirms.com/DIY_files/proto.htm hopefully they won't
mind me posting this link. You could substitute EF86 for EF37, and 12AU7 for
6SN7 if you wish.

Cheers
Henry

In that case, build an 807 PP amp. NOS 807s are just like a 6L6 with
a top cap, are dirt cheap and sound great. Heater current is only 0.9A
per tube. I am using a class A, no NFB amp, with plate voltage 400V
and plate diss. 22W per tube. My output trannies were originally meant
for 6V6s but sound waaay better with the 807s. I got the schematic off
the net, at http://hollowstate.netfirms.com/DIY_files/proto.htm
hopefully they won't mind me posting this link. You could substitute
EF86 for EF37, and 12AU7 for 6SN7 if you wish.

Cheers
Henry

Henry, that 777 looks interesting.

I have about a dozen nice old 6L6's on hand. There is a consensus here
that these would be fine with the Fisher 500C output trannies, as would
EL34's. Ronald prefers the 6L6 to the EL34, and Jim McShane suggests
using 7591A, being that it is closest to the original 7868 for which
those transformers were originally used.

If, as you say, "807s are just like a 6L6 with a top cap, I should be
able drop the 6L6's right in instead of the 807's, correct? (Changing
the wiring to the socket, of course) What if I were to get 7591A's -
would the circuit need any radical changes? Rather than compromise the
sound perhaps a direct substitution would be the best way to go.

Since I only have a single power transformer, I'd have to use it for
both sides. Two questions: (1) Can I simply draw power from both
channels from it without any changes in the basic layout? I would
probably use bigger PS caps after the choke, bypassed with polys. And
(2), I've had good results using Hexfreds instead of rectifier tubes.
Any reason not to go that route? I know about the soft start advantages
of the tube but solid state rectifiers are used on some very decent
amplifiers... Marantz 8B and 9, Citation II (Hexfred mod works great on
mine), and McIntosh 40 and 75 watt tube amps, to name a few.

Lastly, you said something (else) very interesting, "... meant for 6V6s
but sound waaay better with the 807s" If 807 are equivalent to the 6L6...
I have a pair Langevin 138M amplifiers which use 6V6's. They sound
fine with the 6V6's but the tubes are getting old. I'll do a listening
test in any case but are you saying that I can use 6L6's as direct,
drop in subs without ill effect, and even improve on the sound?

Thanks you and everyone who responded.

________________ Marc Stager

If, as you say, "807s are just like a 6L6 with a top cap, I should be
able drop the 6L6's right in instead of the 807's, correct? (Changing
the wiring to the socket, of course) What if I were to get 7591A's -
would the circuit need any radical changes? Rather than compromise the
sound perhaps a direct substitution would be the best way to go.

Since I only have a single power transformer, I'd have to use it for
both sides. Two questions: (1) Can I simply draw power from both
channels from it without any changes in the basic layout? I would
probably use bigger PS caps after the choke, bypassed with polys. And
(2), I've had good results using Hexfreds instead of rectifier tubes.
Any reason not to go that route? I know about the soft start advantages
of the tube but solid state rectifiers are used on some very decent
amplifiers... Marantz 8B and 9, Citation II (Hexfred mod works great on
mine), and McIntosh 40 and 75 watt tube amps, to name a few.

Lastly, you said something (else) very interesting, "... meant for 6V6s
but sound waaay better with the 807s" If 807 are equivalent to the 6L6...
I have a pair Langevin 138M amplifiers which use 6V6's. They sound
fine with the 6V6's but the tubes are getting old. I'll do a listening
test in any case but are you saying that I can use 6L6's as direct,
drop in subs without ill effect, and even improve on the sound?

Thanks you and everyone who responded.

________________ Marc Stager

Answers! 1) Yes no problems. This is what I have done. You might need to
adjust the resistors to get the correct voltages, I am running the same
circuit but with lower voltages. About 100V on the EF37A, 290V on the plate
of the 6SN7 and 400V on the 807s. I have also gone to cathode bias on the
output tubes because its easier. I use an induvidual cathode resistor and
cap for each tube. I would use what you've got- maybe get the 6L6s tested
beforehand. They are really good tubes for audio IMO. Why spend money if you
have them lying around; you could even recycle the 500C chassis and put it
in a new rackmount box if you wnated.

2) I have gone for a choke input, with two 560uF/450V caps in series, giving
me ~280uF. I have also used SS diode rectifiers, I tried UF4007 but fried
them, so I am now using plain old IN5404. I would prefer a tube rectifier
for aesthetics, but 240mA is a lot to ask of any tube rectifier. Go with
your hexfreds if you can!

As for putting the 6L6 in your 6V6 amp; if it is cathode biased, it will
work fine temporarily but you might want to try your speakers one tap up (ie
from 8 ohm tap to 16 ohm tap). Long term you might want to decrease the
cathode resistor value, the 6L6 will barely be getting warm and can handle
more current! If it is fixed bias, I would seek more expert advice.