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Old 1st July 2009, 04:51 PM   #11
jackand08 is offline jackand08  United Kingdom
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Thanks for all of the replies and the help. Yes, the schematic was originally from the RCA manual as explained here. I just have a few more questions:

>Is the power amplifier circuit in the image that I posted just a redrawn version of the RCA circuit or does is it different (the RCA circuit diagram seems to be drawn a bit more clearer to me)?

>Will the amount of feedback and the plate load affect the amplifier in any way?
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Old 3rd July 2009, 12:17 AM   #12
msholden is offline msholden  United States
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The schematic is a redraw but I made no changes to my knowledge. I found that schematic hard to read and drew it to my liking.

If you wish step 14 has a better resolution and more detail: http://www.instructables.com/id/6V6-...Tube-Mono-Amp/

This is the article that inspired me to try the circuit from the RCA receiving tub manual:

Composed by J. B. Calvert
Created 30 October 2001
Last revised 2 November 2001

He did a great article on the function of the amp. I took his word about the performance and he was rite.

This amp suits me just fine. There are probably beater amps out there. My goal was to see what 10 watts will sound like with a speaker with an spl of 100 dB or better. I am officially now a low watt high efficiency fan.

Cheers to all.
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Old 2nd September 2012, 08:18 AM   #13
Nafty is offline Nafty  Israel
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I like this schematic particularly. It must perform very realistic timbers and a very wide sound stage. Kind regards, Nafty
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Old 2nd September 2012, 11:35 AM   #14
Bone is offline Bone  United Kingdom
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Those 6AU6s are redundant; you can drive 6V6s directly from the 12AU6
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Old 2nd September 2012, 02:12 PM   #15
msholden is offline msholden  United States
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Default 6v6 Power Amp

I am not a tube designer, I am a project builder. I am into CAD and just wanted to redraw it.
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Old 2nd September 2012, 02:55 PM   #16
DF96 is offline DF96  England
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When redrawing it is considered good manners to give attribution to the original source, to avoid any accusations of plagiarism.

That circuit seems to use a strange bias method for the EF94/6AU6, but I suppose RCA knew what they were doing.
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Old 2nd September 2012, 07:31 PM   #17
Nafty is offline Nafty  Israel
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In fact, they knew exactly what they ware doing. Ain't strange bias arrangement. There is a well calculated bootstrap arrangement. Kind regards, Nafty
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Old 2nd September 2012, 08:35 PM   #18
DF96 is offline DF96  England
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The EF94 grid sits at 3/4 of the cathode voltage, which is unusual. I assume they did it in order to have a higher cathode resistor so the feedback resistor could be higher too so it doesn't load the output too much. There is a bootstrap, true, but I suspect that is accidental as it is not really necessary in a stage driven by an ECC82.
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Old 3rd September 2012, 08:10 PM   #19
Nafty is offline Nafty  Israel
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I don't know about "accidental".
To much arrangements for a accidental bootstrap.
I think (IMHO) that they stablish a timbre compromise greater than a performance compromise. we may simply open the plate resistors of the output valves and try it without. Timbre matters always (IMHO). Kind regards. Nafy
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Old 3rd September 2012, 08:51 PM   #20
DF96 is offline DF96  England
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The resistor which is bootstrapped is not normally present, as the normal cathode bias arrangement does not need a resistor from cathode to grid. They needed this resistor to establish the correct bias, as the cathode resistor was too large to do this in the simple way. This extra k-g resistor is bootstrapped but I don't think it would matter if it were not, hence my suggestion that the bootstrap is accidental. I don't mean they don't realise it is bootstrapped; I mean that the boostrap is unnecessary but happens to arise from a circuit arrangement which they need for other reasons. I have no idea at all what this will do to 'timbre'.
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