Converting EL38 amp to 300b Amp

[Paul Barker]

10k px4 grid stoppers are suprising and 100ohm anode resistors. But ignore Markoni suggested methods is very brave.

[Paul Barker]

The 300b looks OK for 20 Watts class A1 push pull.



I looked at the px4, 8 watts. I’ll not do that.

[Paul Barker]

On the 10k grid stoppers Marconi showed with the PX4, Osram on the PX25 showed 5k which Osram termed Oscillation Stoppers. Osram also represented the capacitance in doted line the stopper is to prevent oscillation because of. Note also Osram have levelled thier circuit around the coupling transformer. It reminds me of when I built a cirquit around my Sowter tvc’s and one fried. I rang Sowter, and Peter said “did you ask me if you could put 100v potential?” I had to pay full price for a replacement.

[Paul Barker]

I like the way Marco explained the turns ratio of the OPT you might have wound yourself in those days.



Both manufacturers recommend “automatic bias”

I have seen somewhere but cant find it, the max grid resistance for PX25 like WE.state for the 300b 50 k ohm.

[Paul Barker]

I like the way Marco explained the turns ratio of the OPT you might have wound yourself in those days.



Both manufacturers recommend “automatic bias”

I have seen somewhere but cant find it, the max grid resistance for PX25 like WE.state for the 300b 50 k ohm.

Not only have the 50k rule been neglected but perhaps many circuits are shortening the life of the valves through paracitic oscillation neglected.

[Paul Barker]

The 300b looks OK for 20 Watts class A1 push pull.



I looked at the px4, 8 watts. I’ll not do that.

For Max searching out lowest PP distortion.

The operating point if the amp with 8k peak to peak transformers only stands if the loudspeakers are 8ohm. My esl’s go to 2 ohm at some frequencies. My Celstian are 4 ohm. So best I find an operating point for 4 ohm. I found drop b+ because the right extension dives into class b. So Ive positioned by anode volta the voltage swing equidistant in class A full potential. The distortion looks pretty good to me. Glad to see the 3rd almost vanishing. I dont like that 3rd in pp. probably why I prefer SE Nd stop blaiming the second of SE, within reason.

[Paul Barker]

That means in my case I have to check 3rd h at every stage, which is fortunately only the driver and the output stage. So the driver must operate in it’s sweet spot and not run into its dubious zone, so in PP that exceeding the bare minimum to keep driving the 3rd will get nasty. In short, more head room than you would ordinary think.

[Paul Barker]

With a bias of 63v the PX4 or 6b4g on my investigations so far are optimal. 0.1 3rd h.

6J5 poor but mainly because the choke load Id be using was wound for el34.

I am happy I can use 6b4g. 10y wasnt quite as good and anyway wouldnt match the choke.

So My linup looks like 27 to 6j5 concertina to centre tapped inductive loaded 6b4g driving 300b C 0.1 R 250k because I’ll use auto bias on the 300b’s.

I’ll send b+ to the choke via a choke, as Stephie.

[Nick]

Not only have the 50k rule been neglected but perhaps many circuits are shortening the life of the valves through paracitic oscillation neglected.

I think you are tilting at windmills there. The minimum grid resistance will be in the case of fixed bias, and why would any oscillation due to Miller capacitance shorten the life of the valve?

[Paul Barker]

Not only have the 50k rule been neglected but perhaps many circuits are shortening the life of the valves through paracitic oscillation neglected.

I think you are tilting at windmills there. The minimum grid resistance will be in the case of fixed bias, and why would any oscillation due to Miller capacitance shorten the life of the valve?

I know minimum is related to fixed bias. Mentioned it because its not shown on all data sheets.

I always thought the valve working away oscillating was wearing out.

[Paul Barker]

So the 3rd harmonic of the 6b4g is 0.11

[Paul Barker]

The 2nd h at the 27 operating point is 1.5%.

So thats the line up per channel one 27 one 6J5 two 6b4g pp two 300b pp.(I have another Nightingale top plate punched so that may take all the signal stuff, but I dont know where its plinth is. But could get a plinth made. Though it may turn up.)
Have two identical power transformers so can monoblock, but if signal section all fits in one chassis supplies are seperate whether in two chassis or one.

[Nick]

I always thought the valve working away oscillating was wearing out.

You are passing a constant current through that valve at idle such that its dissipating watts of power . Why would you imagine a bit of extra RF would "wear it out". Do you think the cathode is going to run out of electrons?

[Paul Barker]

[Max N]

Thanks for wading in, all input is useful
I don't actually have much to add except that I think a good cap can sound better than a bad transformer - apologies for stating the obvious.....

Yes. I would add to that, even if you have a decent transformer, it might not be the right place for it or the topology doesn't suit. Reason I have the one cap even though I like transformer coupling/iron loading. It's in the one place the system is single ended. There may be a point in that ....

Apologies to Paul for dragging this up again in his thread.
I’ve been thinking about this and I have to admit that when I asked the question I was expecting the answer to be ‘yes, the capacitor part of a good bifilar is better than a conventional cap’.
But I realise now that was just a preconception or bias on my part.
I think I still believe that a good transformer in the right topology is better than a good cap, but that is still just a bias.
I hope one day to explore this through further measurements and listening…and explore more fully Nick’s parafeed suggestion.