Pentode Amp Local Feedback Schemes

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Cressy Snr
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#1 Pentode Amp Local Feedback Schemes

Post by Cressy Snr »

Not that I'm about to try it because I don't want to fook up the sound of the Barbarik, but I wonder how this would perform compared to the plate-to-grid feedback around the output stage I am currently using.

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Saw it in the RCA tube manual RC-20 where it is used in combination with a global feedback loop,
in the "High Fidelity Amplifier - 50W." Looks interesting.
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#2 Re: Pentode Amp Local Feedback Schemes

Post by Mike H »

Great in theory, getting the impedances right might be a bit of a prob. Depends what the divider ratio is. Personally I would consider DC blocking caps for each Rfb, that would help a lot.
 
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#3 Re: Pentode Amp Local Feedback Schemes

Post by Cressy Snr »

Yes, I was thinking it would have been prudent to use caps in the feedback line, which is adding components compared to the scheme I have at the moment. Nevertheless it is another viable feedback solution worth looking at for a balanced PP pentode amplifier.
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Nick
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#4 Re: Pentode Amp Local Feedback Schemes

Post by Nick »

is that not positive feedback?
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#5 Re: Pentode Amp Local Feedback Schemes

Post by IslandPink »

I'm still trying to get my head round whether it's positive or negative feedback, but it does have the disadvantage that the feedback ratio is not constant in the low bass because you're the other side of the coupling cap and its time constant , so you might get phase shift down there affecting the bass rendition.

Edit : I believe it is positive feedback .
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#6 Re: Pentode Amp Local Feedback Schemes

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#7 Re: Pentode Amp Local Feedback Schemes

Post by Mike H »

Nick wrote:is that not positive feedback?
Pretty sure not, having traced the signal path a few times; if V1 grid is inverting input, its cathode is non-inverting input; V2 inverts the signal that's used for feedback, so no. Also I've seen transistor pre-amp circuits wired liked this for NFB as well.
 
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#8 Re: Pentode Amp Local Feedback Schemes

Post by Nick »

Mike H wrote:
Nick wrote:is that not positive feedback?
Pretty sure not, having traced the signal path a few times; if V1 grid is inverting input, its cathode is non-inverting input; V2 inverts the signal that's used for feedback, so no. Also I've seen transistor pre-amp circuits wired liked this for NFB as well.
You are right, tha cathode and anode are in phase.
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#9 Re: Pentode Amp Local Feedback Schemes

Post by IslandPink »

..and the coupling cap ?
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#10 Re: Pentode Amp Local Feedback Schemes

Post by Lynn Olson »

The Barbarik schematic shows the cathode resistors for the 12SJ7's as 620 ohms. That would imply the feedback resistor for the output tubes needs to be in the 6~12K range, which will load down the output tubes a bit, although not as much as the speaker load. The feedback resistor will also need a coupling cap; otherwise 30~60 mA will flow through it.

One of the clever things about the original Barbarik is the high value of the 450K (150K +150K +150K) feedback resistor. Only 0.4 mA flows through it, so a coupling cap isn't necessary.
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#11 Re: Pentode Amp Local Feedback Schemes

Post by Cressy Snr »

The Barbarik uses around 10% current feedback, which over the years seems to have become the level that gives the best results, in an amp that uses plate-to-grid feedback. I know Gary's works slightly different with the direct coupling, but the overall principle is the same and both seem to give fantastic sound with pentode push-pull amps.

Whether the plate-to-cathode feedback scheme would be better I'm not sure, but it is worth discussing.
I wouldn't want to change my own amp though. Once you hit something as good as this is, the motivation to alter
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#12 Pentode Amp Local Feedback Schemes

Post by AndressoureMM »

Hi,

Im considering building K8077 subwoofer kit but would like to get some feedback from unhappy users. Is it difficult to assemble ?
Id also like to have some feedback about the configuration in which youre using it. What is your main amp and speaker set ? How good/bad does the subwoofer kit fits into that ?
And finally, most important question : how has the sound changed by adding the kit to your music system ?

Thx for your feedback.
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#13 Re: Pentode Amp Local Feedback Schemes

Post by Cressy Snr »

Que?
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#14 Re: Pentode Amp Local Feedback Schemes

Post by pre65 »

AndressoureMM wrote:Hi,

Im considering building K8077 subwoofer kit
As usual, Google is your friend Steve.

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