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Old 17th February 2011, 09:13 PM   #21
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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.
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Old 17th February 2011, 09:23 PM   #22
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Seems the scan was just a part of the page.
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File Type: pdf Untitled.pdf (134.0 KB, 142 views)
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Old 10th February 2013, 06:06 AM   #23
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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.
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Old 11th February 2013, 01:51 AM   #24
sulky is offline sulky  United States
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Originally Posted by VintageAmp View Post
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?
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Old 11th February 2013, 03:20 AM   #25
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I tend to stick with the original values. With matched devices, the lower emitter works Ok Better damping factor is the benefit.
Your outline is totally correct. There were at least 3 versions of the 400a
JS
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