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 ?
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 ?
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
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
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
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
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.
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.
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.
Eli Duttman said:
Hello Eli,
that's the usual *global* negative feedback arrangement, I use this all the time. I think Cerrem was referring to connecting the nfb to the TAIL resistor
An externally hosted image should be here but it was not working when we last tested it.
For a hifi application look no further than the schematic for the old Quad II - feedback is applied to the cathodes of the LTP paraphase connected driver stage. Be aware that you can do this using 12AX7 as well. (I've done it in a cheap 6BQ5 PP amplifier that never made it to market.)
I think applying local feedback via the secondaries to the cathodes of the output tubes can be great or really blah sounding depending on opt.. I have had great results with some output transformers (Citation II opts notably) and not others - and I don't really know why. (Haven't investigated, Acrosound don't sound very good using this connection.)
To do this with the normal 4, 8, 16 ohm transformer you ground the 4 ohm tap which is actually the electrical center of the transformer and take the local feedback from 0 ohm tap and the 16 ohm tap at the other. Global feedback if used is taken from the 16 ohm tap, but is electrically 1/2 of the total output winding so you would calculate the feedback based on the ratio present at a conventional 4 ohm tap - in this case the resistance would be 1/2 the value as used on a 16 ohm tap.
The actual reduction in gain can be quite significant, remember that this now increases the required drive voltage. Say for example it originally took 60Vpk to drive the output tubes to full power adding the output which at full power might be 20Vpk (1/2 the total across the secondary - in this case 50W on the 16 ohm tap) so the drive requirement is now 80Vpk, not 60Vpk for the same power level.
Note I generally found the direct connection to provide too much feedback for good sound with some transformers and padded down by as much as 6dB at the cathodes.
My 4 chassis 120W monoblocks used Citation II opts and cathode feedback. Still one of my best sounding commercial designs.
I think applying local feedback via the secondaries to the cathodes of the output tubes can be great or really blah sounding depending on opt.. I have had great results with some output transformers (Citation II opts notably) and not others - and I don't really know why. (Haven't investigated, Acrosound don't sound very good using this connection.)
To do this with the normal 4, 8, 16 ohm transformer you ground the 4 ohm tap which is actually the electrical center of the transformer and take the local feedback from 0 ohm tap and the 16 ohm tap at the other. Global feedback if used is taken from the 16 ohm tap, but is electrically 1/2 of the total output winding so you would calculate the feedback based on the ratio present at a conventional 4 ohm tap - in this case the resistance would be 1/2 the value as used on a 16 ohm tap.
The actual reduction in gain can be quite significant, remember that this now increases the required drive voltage. Say for example it originally took 60Vpk to drive the output tubes to full power adding the output which at full power might be 20Vpk (1/2 the total across the secondary - in this case 50W on the 16 ohm tap) so the drive requirement is now 80Vpk, not 60Vpk for the same power level.
Note I generally found the direct connection to provide too much feedback for good sound with some transformers and padded down by as much as 6dB at the cathodes.
My 4 chassis 120W monoblocks used Citation II opts and cathode feedback. Still one of my best sounding commercial designs.
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