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In Reply to: RE: Schematics (1 of 2) posted by Koonw on June 14, 2008 at 02:54:11:
The old 'rca otl amplifier' is very similar to the 1954 'Dickie & Makowsky scheme' I had used for the otl guitar amplifier that I have posted here some months ago.
It has about 40 dB of negative feedback (as a matter of fact the final stage made of 12AT7, 6SN7 and 3X-6080 has unitary gain because the output signal returns directly to the cathode).
I have noticed that so much negative feedback results in weird effects in case of hard clipping.
This is the main reason why I have tried to trade in more gain (from the very same tubes) in exchange for a lower negative feedback.
In my proposed scheme (I will be glad to send better jpeg files at the e-mail address of your choice) the open loop gain (@ 8 Ohm)is about 116 (23*1.8*14*6.5mA/V*4*8) whereas the closed loop gain is about 16.
To me this give the best sonic results with my speakers (ESB 7/08, four way Italian floor speakers manufactured in 1989).
Coming to your question the plate resistor of the first triode from the left is 27K (with a current of 2.2 mA) and its cathode resistor (i.e. NFB resistor) is 10K.
You should see here the input phase splitter of the 'modified Futterman schematic'.
There is also a second phase splitter (see the second triode from the left, 33K plate and cathode resistors, 2.5 mA) but its function (in tandem with the following 6SN7 tube differential amplifier) is only to boost the differential drive of the totem pole (4X) 6080 tubes (originating from the 27K plate resistor) to up to 135 V p-p.
The reason why many OTL schematics with 6SN7 differential drive and 6080 tubes exhibit a 18K plate resistor lies in this basic fact: there is no way to have 135 v P-P in the load with a different plate resistor.
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Follow Ups
- RE: Schematics (1 of 2) - cellai@space.it 13:28:26 06/14/08 (2)
- RE: Schematics (1 of 2) - koon 00:02:53 06/16/08 (1)
- RE: Schematics (1 of 2) - cellai@space.it 01:41:00 06/16/08 (0)
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