Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: silverfox on July 03, 2026, 07:07:58 pm
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I've been away for years now. Okay, Someone gave me a Fischer 500-S stereo receiver that is in great not working shape. It is missing tubes and the strings that connect the tuning capacitors are broken, as well the AM ferrite antenna. The guts look fantastic as does the Iron. So, I figured I would use the Iron to build a stereo amp. I know, ebay says the sale value of this model receiver is, $2000. Not really. The vintage HiFi market just ain't there anymore.
Here is a link to the schematic: https://www.fisherconsoles.com/non%20co ... 9%20sm.pdf
I've started some research to determine what potential tubes would work with this amp. And, I've run the design question through AI to come up with potential output tubes that will match the Iron in question. The phase inverter would likely be based on a Marshal 30-40 watt power amp section in mono, or the Marshall 20/20. I would like to use either 6L6 or EL34 tubes but they won't work well with these output transformers unless I parallel the OT's and just go mono. I have EL84's, however, not sure those can be made to work with the OT's I have.
The output transformers are in the 7k range and plugging an 8 ohm speaker into the 16 ohm tap would bring the reflected impedance down to, 3.3k. Which should work for EL34's. Perhaps two class A amps, however, I'm more familiar with AB output circuits and don't know what the difference, tone wise, Class A, vs AB.
I had thought the EL84 would be a good candidate, but they are not designed for this work load as suggested by the chatbot, prior to my informing it I would build the power supply and won't be using anything but the Iron. The EL84M would work, however like the original 7189 tubes they are quite expensive. As well, I have the all of those tubes. If there is interest, I can post the AI research, it's short, and reached the same information, as well, several of the same sites I used are included as references' in the AI output.
In the past, I built a stereo power amp based on the Sunn Sceptre. I still use it with my Fractal Audio AX8, into a 2-12 cabinet with Sheffield Speakers. Sounds absolutely fantastic.
Thank you for any assistance the forum may be in this project.
Best regards,
Silverfox.
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It's too bad you can't make it work as is, as it's already a stereo amp with dedicated inputs.
6V6's should work with the OT's as is, depending on what you use for a preamp you'll probably be in excess of 400V B+.
Again, depending on what you use for your preamp, you will likely have to knock down your heater voltages.
4xEl34/6L6 might be a thing, but it could draw too much from the PT.
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Again, depending on what you use for your preamp
I'm just building a power amp. Thanks for the info.
Funny that there are two filament supplies, and what appears to be ground lift on the power tube filaments.
Regards.
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Funny that there are two filament supplies, and what appears to be ground lift on the power tube filaments.
Kinda different. Looks like some sort of filtering set up to me.