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Dear Pilovis,If you want to improve performances of this circuit, increasing output power and decreasing distortion at Hi-Fi level, this is the modified schematic.
The plate voltage of the PL504 has been increased at about 110Vdc (Caution, dangerous voltage!) and there is a global negative feedback to linearize bandwidth and decrease total distortion.
T.b.d resistor should be verified sperimentally to obtain about 90 Vdc on the 47 uF capacitor.
The two output transformers are Hammond 119DA
Hammond Mfg. - "Classic" 600 Ohm - Speaker Matching Transformer - Hammond P/N 119DA
Note: LM317 needs a generous heatsink.
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Top Level 600 ohm audio transformer :
TU020
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I've learned a lot from your amplifier schematics. I built my amp using an ECC88 and PL500, and I even shelled out some cash for the Hammond OT you recommended. Now I know that this OT isn’t really meant for a single-ended amp at all—it lacks an air gap and laminated windings, so it starts distorting like crazy above 1W. I just wanted to share my experience so maybe others will think twice before buying that type of transformer.
By the way, an old radio OT with a 5k primary sounds way better—with fuller bass and less distortion. Well my B+ is sitting at 95V...
The Hammond Classic 600 Ohm speaker matching transformer (119 DA Series) is intended to handle AC only. That's why it can be small and has no air gap. It is intended to "match" the high impedance secondary (600 Ohm) of output transformers in older tube equipment. By connecting this matching transformer you can connect modern 8 Ohm speakers to the equipment as high impedance speakers are nowadays hard to get. Taking this transformer apart and adding an air gap will not help, as it is too small to handle the required DC. You can only try to use it by connecting it in parafeed mode.
https://www.tubecad.com/2014/09/blog0308.htm will give you some more background on this concept.
This implies connecting the OPT via an electrolytic capacitor, which only passes AC. But the negative side is that you also need a high impedance choke in the anode lead.
https://www.tubecad.com/2014/09/blog0308.htm will give you some more background on this concept.
This implies connecting the OPT via an electrolytic capacitor, which only passes AC. But the negative side is that you also need a high impedance choke in the anode lead.
You could also consider to build a SRPP output stage with two EL/PL500 or 504's on top of each other. The output has an impedance of 800 Ohms and the small matching transformer will do fine as it can handle - according to the specs - 12 Watts. Disadvantage more than 300 V on the HT and you have to be sure to handle that safely. The sound quality of this type of amplifiers is very good.
Attachments
This SRPP is very promising 😉
$ 1 0.000005 10.20027730826997 50 5 43 5e-11
173 192 288 272 288 0 4 280
173 224 128 288 128 0 4 280
r 272 160 272 256 0 150
r 224 128 224 256 0 1000
w 224 256 272 256 0
r 256 320 256 384 0 150
209 320 320 320 384 4 0.00001 21.909705990137237 0.001 0 1
w 256 320 320 320 0
w 256 384 320 384 0
g 256 384 256 432 0 0
R 288 32 128 32 0 0 40 350 0 0 0.5
w 288 32 288 96 0
209 368 256 448 256 4 0.000047 211.5308433650985 0.001 0 1
w 272 160 368 160 0
w 368 160 368 256 0
w 320 384 448 384 0
T 448 256 592 384 0 4 0.16666666666666666 -0.045992645191479956 0.4939523570102481 0.999
r 592 256 592 384 0 8
R 112 288 16 288 0 1 50 40 0 0 0.5
r 112 288 192 288 0 1000
o 17 32 7 13315 0.0001 0.0001 0 1 9.246847328295344
o 1 64 0 4099 320 0.4 1 2 1 3
o 0 64 0 4099 320 0.4 2 2 0 3
https://www.falstad.com/circuit/circuitjs.html
$ 1 0.000005 10.20027730826997 50 5 43 5e-11
173 192 288 272 288 0 4 280
173 224 128 288 128 0 4 280
r 272 160 272 256 0 150
r 224 128 224 256 0 1000
w 224 256 272 256 0
r 256 320 256 384 0 150
209 320 320 320 384 4 0.00001 21.909705990137237 0.001 0 1
w 256 320 320 320 0
w 256 384 320 384 0
g 256 384 256 432 0 0
R 288 32 128 32 0 0 40 350 0 0 0.5
w 288 32 288 96 0
209 368 256 448 256 4 0.000047 211.5308433650985 0.001 0 1
w 272 160 368 160 0
w 368 160 368 256 0
w 320 384 448 384 0
T 448 256 592 384 0 4 0.16666666666666666 -0.045992645191479956 0.4939523570102481 0.999
r 592 256 592 384 0 8
R 112 288 16 288 0 1 50 40 0 0 0.5
r 112 288 192 288 0 1000
o 17 32 7 13315 0.0001 0.0001 0 1 9.246847328295344
o 1 64 0 4099 320 0.4 1 2 1 3
o 0 64 0 4099 320 0.4 2 2 0 3
https://www.falstad.com/circuit/circuitjs.html
For the curious, lurkers & others, some History of this Circuit.
William Hewlett's contribution to the circuit used in the HP 200 Series Oscillators
was the addition of an incandescent light bulb in the oscillator feedback circuit.
By the time I joined HP in 1965 the much advanced HP 200 CD was
by numbers the most sold instrument in the product line.
Dr Sinclair founded a company here, Sinclair Labs. He was very interested in
antennas. Now SincLabs, they are in Aurora, Ontario, just north of Toronto.
Attachments
Figure 2 allows the primaries to be wound in parallel and runs on single tube type B+ (no doubled B+ ). This could be useful for some power xfmrs pressed into output service. (reduced winding to winding capacitance for some such OTs. )