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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
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Old 19th July 2018, 01:54 AM   #1
rongon is offline rongon  United States
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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
Default Sim of RCA Tube Manual 50 Watt Amplifier - results and questions

I did a simulation of this circuit...

Click the image to open in full size.

...except I substituted 6BL8 for the 7199 input stage/phase splitter, 6AC7 instead of 6CB6 push-pull driver, and 6L6GC instead of 7027A.

I used a model of a Dyna A470 OPT (the Stereo 70 one).

Result? It's very sensitive. It takes only about 240mV peak to get 47W continuous output into 4 ohms. Otherwise it looks excellent.

To reduce that sensitivity... What would be the problem with using a 6SN7 in the first stage and phase splitter, to reduce the gain? The feedback network would need to be adjusted, of course, but the rest could remain the same. The triode section of 6BL8 isn't that much different from a 6SN7 triode (mu = 20, although 6SN7 gm is lower).

Come to think of it -- 6N6P has mu of about 20 and gm at > 5mA/V. That's very close to the triode section of a 6BL8.

I would rather it got to full power with about 1V to 1.5V peak input.
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I've attached the .asc in case anybody wants to check my work.
--
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File Type: asc PP RCA-style_6BL8_6AC7_6L6GC DynaA470 (+450V)_00.asc (10.9 KB, 10 views)

Last edited by rongon; 19th July 2018 at 02:19 AM.
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Old 19th July 2018, 02:07 AM   #2
PRR is offline PRR  United States
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You don't need a sim to know it is hi-gain. 820/10 points to gain like 82. 20-25 is more typical.

You could use that pot on the front-- it's not there for decoration.
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Old 19th July 2018, 02:17 AM   #3
rongon is offline rongon  United States
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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
Yes, but of course the pot would always be used way down at like 7 or 8 o'clock. I suppose these were designed to be used with their input pots to set sensitivity, driven by an external preamp with its own volume control (a phono preamp or tape head preamp). Of course I'd want to drive it with a DAC or more modern phono preamp whose level matches that of a typical CD player's. Seems a shame to just throw gain away like that. Common problem with using vintage designs with digital sources. SY was always going on about that.
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Old 19th July 2018, 02:47 AM   #4
PRR is offline PRR  United States
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> Common problem with using vintage designs with digital sources. SY was always going on about that.

Words to ponder.
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Old 19th July 2018, 03:39 AM   #5
smoking-amp is offline smoking-amp  United States
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Change R6 to 39 Ohms. Maybe drop R5 by 30 Ohms to keep same pentode current.

Changing tubes will just affect the internal N Fdbk loop gain, not the final gain.

Last edited by smoking-amp; 19th July 2018 at 03:45 AM.
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Old 19th July 2018, 04:33 AM   #6
rongon is offline rongon  United States
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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
Yes, I'm seeing what you mean. Getting everything biased up correctly with a 6N6P in place of the 7199 increased harmonic distortion a whole lot. I guess that's because there's not as much gain to apply to the NFB loop. This amp is a tricky one for me.

I suppose it's one to build as designed and see how it works out. Wavebourn suggested 6F1P in place of the 7199, which is close to a 6BL8. I have a sleeve of 6AC7, which are electrically equivalent to 6CB6A (except without the shielding, as Wavebourn pointed out).

Over the years, SY, Wavebourn, and a few others here have brought up this design as an example of an excellent one. Who am I to try to mess with it?
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Old 19th July 2018, 08:15 AM   #7
TonyTecson is offline TonyTecson  Philippines
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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
you can try the 6bh11 in place of the 6cb6a...perhaps you mean 6f12p in place of the 7199?
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Old 19th July 2018, 10:24 AM   #8
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by rongon View Post
...except I substituted 6BL8 for the 7199 input stage/phase splitter, 6AC7 instead of 6CB6 push-pull driver, and 6L6GC instead of 7027A.

I used a model of a Dyna A470 OPT (the Stereo 70 one).
Just an idea

Some more substitutions, UL = no screen stabiliser, and you end up with only 4 tubes and less sensitive input.
Mona
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Old 19th July 2018, 11:08 AM   #9
TonyTecson is offline TonyTecson  Philippines
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Sim of RCA Tube Manual 50 Watt Amplifier - results and questions
i think that the whole point of the RCA design is the G2 regulation and the use of a high quality OPT, aside from topology.....i am not sure if the original OPT used in that amp is still available today....otherwise it may b hard to catch the essence of the amp as designed in those days...
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