Fisher X-101-ST amplifier

[dave1138]

My latest project is a recent e-bay purchase, a Fisher X-101-ST, one of Fishers first stereo amps. This was the earlier model that did not have separate tone controls for each channel. The 5AR4 rectifier had cracked and the power switch was shot as many Fisher switches are. All but 2 of the other tubes tested good so after a continuity check of the transformers passed, I began the re-capping. I restuffed the e-caps and replaced all the other original caps. After cleaning the controls and tube sockets, adding a new cord, a CL-80 inrush current limiter, and bypassing the power switch I connected an audio source and plugged it in for the first time. As soon as the sound came up I immediately unplugged it since one of the 7189s was red plating. I had read on the Fisher Forum at Audiokarma that this amp used the filaments of the 2 phono pre-amp tubes for cathode bias. Not the best design and that configuration runs the 7189s at their maximum design limits. Some more researched revealed a mod to use a resistor and bypass cap for better biasing. After implementing this mod, I reconnected the speakers and audio source and tried it again. Nice clean sound and no discernable hum. That mod takes the 2 phono pre-amp tubes out of commission.
There's just enough room on the chassis to add a filament transformer to power those 12AX7s so that will be next.


The brass front panel has some discolored spots that I thought clean up but I discovered that they are in the metal itself.
My long term fix will be to scan the panel and attempt to reproduce the graphics on decals and cut a new piece of brass to put he decals on.

[TC Chris]

Let us know how the new filament transformer works.

Chris Campbell

[William]

That baby has some nice heavy-duty transformers. It should sound great when you get it done.

Bill

[walyfd]

Gorgeous
Love the brass faced units. I need to get after my X202

[electra225]

Dave, my understanding is that 7189's are heavy-duty 6BQ5's. Or is it the other way around? Anyways, I have a cheap little GE stereo that uses SE 7189's. Pretty exotic hardware for a mundane stereo. 6BQ5's are typically driven pretty hard. Would they provide an extra margin of safety or am I all wet......

[dave1138]

The 7189 is a higher voltage version of the 6BQ5 so a 7189 is a good sub for the 6BQ5 but not the reverse.
The 7189A has a slightly different pinout so they might not be 100% compatible without some wiring mods.

Code: Select all

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=7189

Code: Select all

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=7189a

[Firedome]

That's right, a 7189 has higher plate rating/power than a 6BQ5, and it's important to know that a 7189a is different than a 7189 and are not always plug-n-play. RETMA's # should have been more different imo.

[hermitcrab]

Yea, that rectifier tube has seen better days ...

[dave1138]

After putting a JJ 5AR4 in the fisher Statesman that I worked on, I read numerous accounts of that brand failing catastrophically.
So I replaced it with a NOS 5AR4 from Matsushita. I found a mod that puts diodes in series with the rectifier plates that will allow for
the not so durable new production 5AR4s to be used without flashing over. So I did that to the X-101-ST and selected a Genelex Gold Lion rectifier tube as a replacement.

[TC Chris]

Tell us more about the 5AR4 issue and the diode solution. I take it there's an acceptably low voltage drop from the additional diodes?

Chris Campbell

[dave1138]

I read about the mod on a Dynaco forum:
'https://dynacotubeaudio.forumotion.com/ ... -diode-mod

[dave1138]

Update:
I was not satisfied with the bias mod that I did and decided that adding a filament transformer for the phono section's tubes was not going to be that
easy. The sound was good and the 7189s no longer red-plated but I wanted to use the phono input. Last year, I had restored another Fisher amp with the filament bias setup (an X-100-B) There's a good post on the Fisher forum at Audiokarma
that explains the setup and offers a modification that fixes some of the shortcomings of that bias configuration:
https://audiokarma.org/forums/threads/i ... em.904119/
Since the 7189s weren't a matched set, I bought a matched set of Sovtek EL84Ms. After installing the Zener diodes I checked the phono section, everything was good, so I moved to the living room to play some records. In 2022, I replaced my JBL Lancer 77s with a set of Klipsch Cornwall IVs and have been amazed how good they sound with tube amps. The little X-101-ST exceeded my expectations, so I am making it my Primary amp for now.
This will give the 500-B a rest for a while. I also have a KX-200 that still needs a bit more work.