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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: RE: fisher tube rectified integrated EL84 amps posted by rockdoc on December 8, 2016 at 07:05:24:
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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Follow Ups
- IMO&E it is a good idea to use 7189s / EL84Ms to their capacity - Timbo in Oz 20:19:14 12/10/16 (0)
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