I got some papers with my newly purchased classic 400A, and here are the "goodies" scanned. One page at a time..
Rolv-Karsten
Seems the scan was just a part of the page.
The drawing just4.pdf is actually the early E-line bias technique. For the 400a: measure across an emitter resistor and shoot for 45-48C with thermal probe. Early units have .33 emitters and later ones have .67ohm. This will alter the voltage drop obtained in the bias setting. Also, the early units used the RCA parts and later units used the FT317 / 417 combo. They are actually different schematics.
Hey VintageAmp, thanks for the response... Looks like, this post adds good interesting info every few years
Anyway, thanks for the added info. I would have sent you the amp for repair if I had known how much work I was in for when I worked on this one myself...Back when I was fixing my amp I found quite a few different old 400a schematics that changed during the production. As you indicated, the earliest had the .33 ohm resistors on the transistor stages, later they moved to the .68, and I have two different 400a schematics both with the RCA 1A15 / 1A16 but showing the different resistor values along with a few other value changes around the schematics. Later the combined 400a/4000 schematic shows the .68 / 1 ohm values and the FT317 / 417 pairing.
I can't remember which values I finally used, would need to find my old notes. Since threshold kept upping that emitter resistor value over time I think they may have found more stable or safer operation with the increase, or it may just be due to component supply changes at the time. Just curious, do you tend to keep with the original values in an amp or opt for changing resistor values when you rebuild?
Did you ever track down a suitable substitute for these that matched the TO39 case of the originals?
Thanks,
Eric M.
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