Amazing Original 500C Sound

365nut

Member
After success with my first 500C restoration thanks to help from knowledgeable people on this forum, I pulled out the 2nd of 3 of these I've had in storage for many years. It's serial No. is in the 55K range. A heavy layer of tar and nicotine covered the chassis. So, my first order of business was to remove that, as I HATE a dirty set. Except for the output tubes every tube in the set is an original Fisher. The 7591s were missing so I put in a set of EHs. No under chassis components have been replaced--all are original. I then powered it up gradually with a Sencore PR 57 variable isolation transformer. Running at 90 VAC, and an MP3 player into the Aux input, this baby sounds perceptibly better than the one I just restored. No hum. Absolutely no distortion and maximum volume on both channels. (I haven't yet done any cleaning of pots or switches yet, either). Were it not for the need to make recommended improvements to prolong tube life and provide safety, it seems a shame to tinker with this 500C. Really mind blowing how good it sounds. So Avery's engineers knew what they were doing--without a doubt--and they must have used good components, or there would have been some failure after 55 years. Another vote here for the great 500C.
 
I don't have a 500C but I have two 400's, one of which was a pristine original that I used for many years as is. I liked it so much I bought a second one mostly original except for a small amount of electrical work. I’ve since restored both, though both always worked well since buying them second hand, nice smooth sound.
 
The German Engineer's (Avery's "Dutchmen", as he called them) that designed them used what in the day were considered to be top drawer parts. Time and atmospheric contaminent's proved them wrong in some cases. FRAKO branded axial caps blow out their rubber seals and leak, both electrically and physically. The large can caps are hit or miss with regard to drying out and leaking electrically. I've seen both, those that are as wet and fresh as the day they were made and those that were so dried out, would make a Prune look like a beach ball. ERO-FOL mylar caps are generally regarded as the best parts in there. Rarely go bad, but they have to be checked anyway. SIEMENS brand selenium rectifier gets replaced due to low voltage output which can burn out the output tubes.

Best option is to go thru it with a meter and check all the voltages, and then check caps with a ohm meter or an ESR meter.
If you are going to use either EH tubes or TUNG-SOL ReIssue 7591's, replace the grid return resistors with 220K vs. the original 330K and the .047uf output coupling caps with .068uf 630v. This will ensure that the original R/C timing constant is as close to original as can be made, and allow the tubes to run better. Cathode (10ohm) and screen stabilizer(100ohm)resistors will also keep the tubes healthy.
 
I truly value your advice and will heed it--but was amazed at the sound, considering the age of all the components--so I thought it worth a post.
 
I've had a 500c collecting dust for years knowing it was a desirable piece of equipment. I finally decided I should give it a listen after threads like this and Larry's unbelievable knowledge. It's currently at the shop getting a full makeover. Can't wait to get it back and glowing along side my mc240.

I'll be back with photos of the beauty queen when she's in action singing again.

Good luck with your resto...
 
I'm just a Parrot with a memory like a bear trap(most of the time). Dave's the one with the unbelievable knowledge.
 
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