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In Reply to: RE: fisher tube rectified integrated EL84 amps posted by rockdoc on December 07, 2016 at 13:42:01
According to the service manual linked below, the early Fisher X101 used 3 12AX7's and 3 7025's. So, essentially 6 12AX7's along with the 7189 outputs and the 5AR4 rectifier. While the X100 uses 4 12AX7's and 2 12DW7's (7247)in the preamp section. The 7247 is a dual triode with each bottle being half a 12AX7 and half a 12AU7 inside. You can search here for posts about the benefits of this. I have no idea if the transformers are the same between the two models or not. I have never heard one of the early x-101's but I do have the X-100...
good luck
--Matt
"When you think everything is someone else's fault, you will suffer a lot."
--Dalai Lama
Follow Ups:
I wish they would have used a 7247 for the phase-splitter driver for the output section.I have a 101 that I bought for a dollar at a hamfest in 1989 and I left it stock. I have rebuilt a few but mine has all 12AX7s.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I have 2 X101 amps waiting for restoration, and just became aware of the X-100. Thanks!
IF you really want to do them up, think about a rethink rather than a 'restore'.Don't waste money on NoS 7189s or A's, the EL84M is a fine neutral tube.
In particular think about using a low-gain high GM tube to drive the EL84Ms, not a high gain low-gm type like the 12AX7/7025.
Why? You will get lower distortion, and can use less loop NFB.
You do need to have heater current in reserve.
But, IF you drop the GZ34 rectifier and go with SR SS diodes, maybe even Schottkys, you can use its 5V heater rail inveigled to 6.3V, to heat the low mu High GM valve.
You can also get much higher storage in the PSU because you no longer have any serious PIV issues. Modern 400 to 500V / 100 degree electros have a lot more uf than the Fishers had as OEM.
Even PP film caps can exceed the original values and still fit. If there's no choke-filter in the PSU think about adding one if it will fit. CLCC or CLCCC.
If there's enough heater current with that extra rail you could use a 6CG7/6FQ7 - with a FET CCS under it - in perfectly balanced LTP mode.
Much nicer sound, higher headroom, far tighter bass.
If they have an RIAA section, find Eli's fet assisted version - here at AA - of the 'RCA twin 12AX7 stage' and make those mods too.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 12/10/16 12/10/16
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