For the cheap seats with larger amps and not wanting to replace the OPT with an UL type, this mod of putting the cathodes through a balanced winding on the OPT is interesting. 0-4-16 Ohm taps, or a 25VCT winding, etc. The connection should buck, or reduce the gain. It also works with single ended pentode/beam amps. It's desirable to use a tube that has plenty of cathode though it even made an AA-5 plastic table radio with a 50L6 sound a bit better.
This isn't UL but it cleaned up the old sound reinforcement amplifiers!
This amp also
This amp also too
This amp also as well because 150 watts.
How to hook up an a/b test switch for cathode feedback vs. normal beam tube operation, but can only report on my subjective impression as to the above links, that the amps sounded better.
More drive to the output grids is needed for the scheme. Sometimes the voltage amp or driver tubes need higher plate voltage. It may be worthwhile to add an adjustment in some cases to set the best operating point for linearity it is done before the power stage and inside the 'global' feedback loop.
This isn't UL but it cleaned up the old sound reinforcement amplifiers!
This amp also
This amp also too
This amp also as well because 150 watts.
How to hook up an a/b test switch for cathode feedback vs. normal beam tube operation, but can only report on my subjective impression as to the above links, that the amps sounded better.
More drive to the output grids is needed for the scheme. Sometimes the voltage amp or driver tubes need higher plate voltage. It may be worthwhile to add an adjustment in some cases to set the best operating point for linearity it is done before the power stage and inside the 'global' feedback loop.
Was your Tubelab SPP clone using PL84 with ultralinear wiring of the output transformer?
What was the B+ you used for that?
I'm curious, because I was thinking of trying that using 6P43P-E outputs with a 250V B+ and cathode bias, so probably 230V plate/screen to cathode for the output tubes.
yes, they are, so now i amp making power traffo with filament taps for 6.3 and 10 volts to accommodate the regular 6bq5's i just wanted to make use of these fine TV tubes...i am making a ppp amp...
https://frank.pocnet.net/sheets/127/1/10BQ5.pdf
https://frank.pocnet.net/sheets/127/1/10BQ5.pdf
I described my experiences here, and I have learnt quite a lot as part of that and since ...
SPP with PL84
I had a custom OPT from Toroidy with UL taps so I wanted to build it UL. I probably could have had an easier time if I had plumped for pentode, and used a zener or similar to regulate the screen supply.
@Francois G is trying his own solution soon and that will interesting to see how he gets on.
If I rework it then I will use 70V 200VA toroids that I have lying around. I would use a doubler for the B+ (aiming for 190v) and a quadrupler for a negative rail to provide higher B+ for the headroom of the driver/splitter,
Man, all this recent discussion has me looking again at building a silly "V8" engine shaped amplifier. Might do triode connected 6P43P-E, two banks with two pairs per side on a 90 degree slant. 250 volts B+, toroidal outputs, cathode bias. At the very least Maybe I'll mock up a chassis and transformers so I can get some plans drawn up. 18-20 watts per channel should be plenty.
What I built was in the LTSpice model. So 216V B+, cathode bias 470R G2 stopper. Then 275V B+ supply for the driver/splitter. I used it every day for 6 months no issues. It is some time ago but I think I was using around 225V B+ to plate and G2 and a lower value stopper when I had some red plating, so it is quite marginal.
The 6CW5 looks like it is a little tougher. It has values documented in the GE tube data for class AB operation with 250V on the anode, whereas the UL 84 only provides cases in the Philips data sheet for up to 200V.
As yet said, UL84, EL86 and PL84 are the same tubes, with the exception of their heaters. And it can safely be assumed that this also relates to any tube of the xCW5 family. The different UL84 data result from it's intention as the AF power tube in hot chassis radios that may be operated at 110 VDC or 220 VDC power grids.
Best regards!
I have a design that uses a vacuum or solid state diff front end with folded cascode output for direct drive to the output stage, With some magic mojo, it could also incorporate feedback for triode emulation (coupling caps help). I also might set things up for auto-bias, as the current sources feeding the folded cascode could be trimmed to set the bias in the output stage without directly affecting the gain path too much. I did have an amp set up to try this a few years ago using 6JB5/6JC5 outputs, but I may change the setup to use the Russki equivalent of the 6V6, as those tubes are cheaper and easier to come by (as are the sockets, too).
Quick note regarding the 6CW5 and 6P43P-E.
I built a 6CW5 amplifier a while back, and it's great. Pentode connected, regulated screen voltage at about 175 volts with regards to the cathode, plate voltage about 270 with regards to cathode, garter bias, and toroidal outputs. Love it, thread here-
https://www.diyaudio.com/community/threads/6n2p-6cw5-design-with-toroidal-outputs.327541/
Yesterday as a quick function check, I swapped out the 6CW5, and installed a set of 6P43P-E. Immediately found some issues.
For reference, the 6CW5 is dead silent at idle, and during warm up. Global negative feedback without any compensation or extra stability measures used. AC filament wiring.
With the 6P43P-E installed there is immediate hum a second or so after power on, and a noticeable increase in heat. About twenty seconds in an obvious LFO (motorboating) sound is heard. I powered it down and let it cool. After a moment I reinstalled the original 6CW5 tubes and all was normal again.
Clearly in pentode mode the 6P43P-E need some additional measures taken to be stable in the same circuit. I did not try disconnecting the feedback but might another time. Gonna try triode mode and see how they behave.
I built a 6CW5 amplifier a while back, and it's great. Pentode connected, regulated screen voltage at about 175 volts with regards to the cathode, plate voltage about 270 with regards to cathode, garter bias, and toroidal outputs. Love it, thread here-
https://www.diyaudio.com/community/threads/6n2p-6cw5-design-with-toroidal-outputs.327541/
Yesterday as a quick function check, I swapped out the 6CW5, and installed a set of 6P43P-E. Immediately found some issues.
For reference, the 6CW5 is dead silent at idle, and during warm up. Global negative feedback without any compensation or extra stability measures used. AC filament wiring.
With the 6P43P-E installed there is immediate hum a second or so after power on, and a noticeable increase in heat. About twenty seconds in an obvious LFO (motorboating) sound is heard. I powered it down and let it cool. After a moment I reinstalled the original 6CW5 tubes and all was normal again.
Clearly in pentode mode the 6P43P-E need some additional measures taken to be stable in the same circuit. I did not try disconnecting the feedback but might another time. Gonna try triode mode and see how they behave.
I did try using 6CW5 in my "No Light District" amplifier a few years back. I started out with Yugoslavian 6CW5s, but they red-plated. I was probably trying out too many new concepts all at once in that design, a sure way to invoke the rath of the circuit gods... I got some better 6CW5s from Stan at ESRC, but still couldn't get a decent bias point. I should revisit that design and try to make it work right. I think what killed it was combining slippery low-impedance cathode bias with garter biasing - it was a positive feedback sort of situation that caused the bias to lose control.