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Old 12th November 2012, 09:17 AM   #11
DF96 is offline DF96  England
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9-pin RF pentodes were never very popular in the US, although common over here. You could try EF80/6BX6 with some resistor changes. That may have more gain, so other adjustments might be needed too.
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Old 12th November 2012, 09:48 AM   #12
SY is offline SY  United States
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Although the title says "RCA," I seriously doubt the provenance. The "designer" has made some... ummmm... creative choices. I'd suspect a Chinese origin.
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Old 12th November 2012, 09:55 AM   #13
AJT is offline AJT  Philippines
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no SY, i saw that from a webpage many years ago by an english speaking guy....it does not use global negative feedback, the pentodes are run at less than 1ma current and i suspect that the gain is rather very high......
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Old 12th November 2012, 10:15 AM   #14
SY is offline SY  United States
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But ultimate origin? Not RCA, I think. The input stage is a poor version of the Williamson (there must be almost 0.3mA going though the cathodyne!), the feedback arrangement is relatively ineffective, and RCA engineers were smart enough to understand feedback from the OPT secondary.

RCA's actual pentode driver amp was quite an excellent and innovative design.
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Old 12th November 2012, 10:25 AM   #15
DF96 is offline DF96  England
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It is RCA. It appears in RC-19 receiving tube manual: circuit 19-11 on page 361. It is described as "High-Fidelity Audio Amplifier, Class AB1, Output 10 Watts". The verson linked to is slightly redrawn, but clearly the same circuit. No GNFB. Same component values. My guess is that RCA published it in different places, not clear with they would redraw it as printing plates were expensive.
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Old 12th November 2012, 10:25 AM   #16
pancaQe is offline pancaQe  United States
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Can anyone suggest a simple, quality 6V6 push pull amp design/schematic. At this point it looks like I'm gonna have to back track anyway. I'm looking at schematics using 6SN7. Any suggestions?
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Old 12th November 2012, 10:56 AM   #17
SY is offline SY  United States
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Well, Dave, you've thoroughly disillusioned me!
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Old 12th November 2012, 11:00 AM   #18
DF96 is offline DF96  England
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I must admit I was surprised, but I guess even RCA employed some designers who were not as good as others. Or maybe it was a Friday afternoon design?
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Old 12th November 2012, 11:10 AM   #19
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That is a Japanese-drawn circuit. I think it came from Asano's famous book of the 1970s in which he give circuits and photos for dozen of tube amps that he built. Ultimately he built over 100 amps.

DF96 is correct in that it is the late 50s RCA tube manual suggested design.

Many of those tube manual designs strike me as a bit odd looking at them with modern eyes!
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Old 12th November 2012, 11:33 AM   #20
DF96 is offline DF96  England
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My guess is that it is a low-power version of an originally higher power amp, where the extra driver stage might have a purpose. There is a 35W circuit, with some similarities, on the next page of the RCA manual. A 100-200W PA amplifier somewhere might be the original?
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