Home |
Search |
Today's Posts |
|
#1
Posted to rec.audio.tubes
|
|||
|
|||
NFB windings, was there a US style and UK style?
I have a couple of output transformers with separate feedback windings, and was hoping that one might be a suitable replacement for the failed transformer in the RCA 82 C4 Monitor Amplifier. Schematic here http://www.waltzingbear.com/Schematics/RCA/BA-4C.htm
However it uses a high impedance winding (1/6th of primary anode-anode), whereas the transformers I have both use low impedance windings, about 1:100.. I have a schematic for a British circuit by P J Baxandall - last on this page http://mike.wepoco.com/Home/retro-ge...rld-amplifiers which places the feedback winding in series with the input pentode cathode, rather than the high impedance potential divider used in the RCA circuit. Baxandall claimed to be using a transformer design patented by the BBC (Mayo, Tanner, Ellis). I've seen the same arrangement used in a Marconi push-pull amplifier. I reckon I'm going to have to adopt the Baxandall arrangement, but would be interested to learn what others think of these circuits. Presumably it was amplifiers like this that inspired the rather quirky Quad 2 extra feedback to the output valves. http://www.drtube.com/schematics/quad/quad-22.gif Michael |
#2
Posted to rec.audio.tubes
|
|||
|
|||
NFB windings, was there a US style and UK style?
On May 18, 7:54*pm, mike s wrote:
I have a couple of output transformers with separate feedback windings, and was hoping that one might be a suitable replacement for the failed transformer in the *RCA 82 C4 Monitor Amplifier. *Schematic herehttp://www.waltzingbear.com/Schematics/RCA/BA-4C.htm However it uses a high impedance winding *(1/6th of primary anode-anode), whereas the transformers I have both use low impedance windings, about 1:100. *I have a schematic for a British circuit by P J Baxandall *- last on this page *http://mike.wepoco.com/Home/retro-ge...-wire...*which places the feedback winding in series with the input pentode cathode, rather than the high impedance potential divider used in the RCA circuit. *Baxandall claimed to be using a transformer design patented by the BBC (Mayo, Tanner, Ellis). *I've seen the same arrangement used in a Marconi push-pull amplifier. I reckon I'm going to have to adopt the Baxandall arrangement, *but would be interested to learn what others think of these circuits. *Presumably it was amplifiers like this that inspired the rather quirky Quad 2 extra feedback to the output valves.http://www.drtube.com/schematics/quad/quad-22..gif Michael You won't easily find any "drop in" new replacement OPT which could be used in the the circuits by RCA, Peter Baxandall, or Peter Walker of Quad. All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. You say you "have a couple of OPT" without saying just exactly what their specification might be. To know what you are doing, you need to know every single bit of information which describes each tranny fully. You say your OPT in your RCA monitot amp has failed, so we assume it has an open primary or shorted turns. The Hammond 1650P OPT with 6k6 to 4, 8, 16 ohms will be OK with 6L6, 807, 1622 etc, but you will need to addapt the Hammond OPT to the RCA circuit you already seem to have. The RCA circuit has NFB loop with R24, 24k, and R8, 2k7, and we don't know how much voltage is generated at the NFB winding across OPT terminals 1 to 3. The lazy dumb ****wits at RCA ommitted to provide us with a more clearly drawn schematic with all the working signal voltages for all electrodes and transformer windings. My guess is that OPT terminal 3 produces a NFB voltage of about 50Vrms to be able to supply a high enough FB voltage at V3 6SN7 cathode, maybe 5Vrms, so that the amount of NFB is at least 12dB. A normal OPT with no FB winding and just speaker secondaries could be used in the RCA circuit but you'd have to re-arrange the FB network to V3 cathode with R24 being a lower value. This will affect the way V3 cathode is biased; if R24 is made smaller it reduces the total value of Rk. Fso if R24 = 4k7, the R8 may need to be increased to 3k3 to give the equivalent of what is in the schematic, ie, 2k7 // 24k for the dc cathode current, ie, about 2k2. I'd never ever try Baxandall's circuit, and I'm no great fan of Quad, or ANY circuit which employs a "paraphase" input pair or uses the output of one triode to drive another as done on the RCA circuit. It is always better to employ a LONG TAIL PAIR as in many of my amplifier schematics which you may inspect at my website at http://www.turneraudio.com.au I've always used NFB applied to a cathode of SE triode input stage ahead of an LTP driver stage and this is better because the distortion of the input tube is included in the FB and thus reduced along with all following stages in the loop. The input tube works at low signal levels so second order products are minimal. The Williamson, Leak and Radford circuits emboby such principles as I do, but to make the amp unconditionally stable regardless of load reactance will challenge your abilities sorely unless you have a full understanding of what you are doing. Patrick Turner. |
#3
Posted to rec.audio.tubes
|
|||
|
|||
NFB windings, was there a US style and UK style?
On 05/18/11 17:52, Patrick Turner so wittily quipped:
All these old circuits have serious shortcomings which are only made worse when someone tries to use an unsuitable OPT at home and they do not understand how NOT to build an oscillator while trying to build an amplifier. ew, bad memories |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Change your life style now...... | Tech | |||
New Orleans-style funk | Car Audio | |||
moderating rec.audio.low-end style | Audio Opinions | |||
OT? Old vhs-style cassettes | Pro Audio | |||
Old style rotary attenuators | Pro Audio |