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Old 6th December 2006, 06:16 PM   #1
Jaap is offline Jaap  Netherlands
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Default How to accomplish local feedback in pp amp ?

I want to make a transformer coupled pentode push pull amp with a regulated screen supply. I am thinking of using 12E1/S11E12 or perhaps 807 or KT66/6550 tubes. Driver could be a 5842 or E810F triode.
I don't want to use global feedback so it should be local feedback around the power tube. I am aware of the Tabor amp of Gary Pimm but my amp has an interstage transformer and no coupling cap.
I am not an EE can anybody help me realise this idea of local feedback ?
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Old 6th December 2006, 06:33 PM   #2
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Resistors plate-to-grid, maybe 470K-1M (plus a DC blocking capacitor, of course). May take some experimentation...

Also, you can use the output transformer secondary for cathode feedback if it has 4-8-16 taps (or better, 8-16-32). But it's a few dBs at best - won't have much effect.
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Old 6th December 2006, 06:37 PM   #3
Jaap is offline Jaap  Netherlands
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Default how big

should that capacitor be ?
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Old 6th December 2006, 06:42 PM   #4
cerrem is offline cerrem  United States
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Your best bet for output stage local feedback is to apply the feedback from secondary of output transformer to the Phase Inverter tube.....
This has many benefits....

If you use a Long Tailed Phase Inverter...applying feedback from a low impedance source, such as the secondary, provides "bootstraping" to the tail resistor....
This "bootstraping" technique allows better coupling of each side of the Phase Inverter...since it effectively makes the tail resistor appear larger to AC signal than it actually is....

Also, this type of feedback helps AC "BALANCE" the Phase inverter and Output stage....

Chris
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Old 6th December 2006, 06:50 PM   #5
Giaime is offline Giaime  Italy
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Hello Chris,

how to connect the secondary of the OPT to a Long Tail? I did not understand. You mean taking feedback to the grids of the Long Tail, or how?

Thank you in advance!
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Old 6th December 2006, 08:03 PM   #6
Svein_B is offline Svein_B  Norway
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Quote:
Originally posted by cerrem

If you use a Long Tailed Phase Inverter...applying feedback from a low impedance source, such as the secondary, provides "bootstraping" to the tail resistor....

Chris
Interesting idea. I have never seen this.
Is this described in any available litterature?

I can [almost] understand that this will help balance the phase inverter. As I see it, it would however not give any signal attenuation as conventional feedback does.

SveinB.
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Old 6th December 2006, 08:41 PM   #7
jane is offline jane  Norway
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Quote:
Originally posted by Svein_B
Interesting idea. I have never seen this.
Is this described in any available litterature?
Take a look at old Fender/Marshall’s amps. They have been using this trick for decades.

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Old 6th December 2006, 10:07 PM   #8
Geoff H is offline Geoff H  Australia
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An unbypassed common cathode resistor provides local feedback whilst balancing the tubes. (Layfayette 6BQ5s) Smaller individual resistors in series with the cathodes provides local feed back for the tubes independantly.

Feedback arround more than one stage is not local feedback. The phase inverter normally has heaps of local feed back. Cathode res= Plate res. Unity gain.

Geoff
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Old 6th December 2006, 11:13 PM   #9
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Quote:
how to connect the secondary of the OPT to a Long Tail?
Giaime,

Take a look at the "El Cheapo" schematic. An appealing point about the topology is that the only caps. inside the NFB loop are those that couple the splitter/driver to the PP "finals". Phase shift problems are minimized.
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Old 7th December 2006, 06:14 AM   #10
Svein_B is offline Svein_B  Norway
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As I understood cerrem, he suggested connecting the feedback to the LTP tail resistor.
Following jane's hint , I found a couple of descriptions of the Fender/Marshall circuit:
http://www.aikenamps.com/LongTailPair.htm
http://www.pentodepress.com/summary.html

... not sure how relevant this would be for hi-fi.

SveinB.
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