6W 2xEL519 Half-hybrid Class-A OTL

[revintage]

I have been playing with the idea for a CF EL519 OTL with CCS for a long time but the problem has been high Zout when only using two tubes.

My analysis showed that the best workingpoint for the EL519 is in "enhanced triode" mode with Ua 55V with an added 100V to screen.

So I tried with a White-follower variation by modulating the CCS(Iq=800mA) with signal from the top of the EL519s.

This seems to do the trick as it is now possible get a Zout of 2.5ohm, on par with a typical 2A3 SE.

I get an indication of Pout ca 6W with in the ballpark of 1% THD into 8ohms.

No overall feedback and an evenly falling overtone footprint!

This will be the prototype for two 5xEL519 monoblocks.

 

[revintage]

Waddaya say, MJK and kenpeter?

Looks like some kind of triode-antitriode CF, doesn´t it?

Also have a servoassisted 40W 5xEL519 w/o output and coupling cap drawn up.

 

[revintage]

35W 5xEL519 OTL

This is the HP DC version:

An externally hosted image should be here but it was not working when we last tested it.

 

[Michael Koster]

Waddaya say, MJK and kenpeter?

Looks like some kind of triode-antitriode CF, doesn´t it?

Also have a servoassisted 40W 5xEL519 w/o output and coupling cap drawn up.

Yes, I was going to say there must be a servo to get rid of the cap.

It's of course a balanced inverse signal current mirror -- anti-triodally
it preserves the harmonic signature of a triode CF

My first two concerns are PSRR and whether the servo can compensate
for beta drift. You depend on constant beta to multiply the current of
the DN2540 at a constant ratio, both DC current and likewise signal current.

Perhaps the servo could DC modulate the DN2540?

I'll be really interested in how this works out IRL

Cheers,

Michael

 

[revintage]

Yepp, PSRR is lousy. Will have to depend on large expensive electrolytics.

Have simmed the PSU and there is no problem to achieve well below 1mV RMS at the output.

Hopefully the Beta problem is insignificant.

I have a few 5pin NJL3281 so I will try to get a better temp comp. As it is now it goes from 0,4A/tube to 0,7A/tube from 25 to 150degrees C. With enough heatsinks and Class-A I don´t think it will be a problem.

 

[Michael Koster]

The beta drift would only be an issue for the DC. You may have seen
through your spice modeling that unlike in conventional push-pull,
distortion does not change much if the mirroring signal current swing
is smaller or greater than the mirrored signal current swing, i.e the
signal balance does not have to be perfect to minimize distortion
because there is no distortion cancellation taking place.

2X output power, 1/2 the output impedance. Not a bad deal at all I
think. The overload behavior is even reasonable if circuit operating
margins are decent.

Cheers,

Michael

 

[revintage]

About Zout I actually used trial&error to find the value of R14 to get below 2,5ohms. In this case it is 2ohms.

Have done a few more sims and think I will settle for 4xEL519 mnoblocks at 1,6A as this will still give me a Pout over 25W/8ohm. It will also put a lot less stress on the BJTs.

Also i might try Slagle bifilar 1:1 ITs, that I have lying around for another project, instead of the coupling cap to eliminate the last one......

 

[Michael Koster]

About Zout I actually used trial&error to find the value of R14 to get below 2,5ohms. In this case it is 2ohms.

Have done a few more sims and think I will settle for 4xEL519 mnoblocks at 1,6A as this will still give me a Pout over 25W/8ohm. It will also put a lot less stress on the BJTs.

Also i might try Slagle bifilar 1:1 ITs, that I have lying around for another project, instead of the coupling cap to eliminate the last one......

Ah yes, the Zout reaches a minima when the signal currents are exactly matched and the load is shared 50/50 between the tube(s)
and the mirroring device(s), BJTs in your case.

Now I see how the servo works. I guess it would connect to the
bottom of the Slagleformer.

Waitaminute... Half-Hybrid? Isn't that a double oxymoron or something?
A hybrid is already half-and-half Is a half-hybrid only a quarter mix? ;-)

 

[revintage]

A hybrid is already half-and-half. Is a half-hybrid only a quarter mix? ;-)

😀 😎 😉 !

The major contributing factor to the sonic footprint will be tubes. So, YES, a quarter mix is a good description.

Ah yes, the Zout reaches a minima when the signal currents are exactly matched and the load is shared 50/50 between the tube(s)

Seems to be wrong😕!

You can adjust Zout down to .1ohm if you want but then you will not get full outswing even if you have adjusted B+ to make Ua 55V and Us 100V as in the original circuit.

So there must be a point where min Zout and max outswing meet. Yet to be determined!

I just choose R14 to get the same Zout as a typical 300B SE.

 

[revintage]

This way it will be even less "half-hybrid"😎 !

 

[Michael Koster]

😀 😎 😉 !

The major contributing factor to the sonic footprint will be tubes. So, YES, a quarter mix is a good description.



Seems to be wrong😕!

You can adjust Zout down to .1ohm if you want but then you will not get full outswing even if you have adjusted B+ to make Ua 55V and Us 100V as in the original circuit.

So there must be a point where min Zout and max outswing meet. Yet to be determined!

I just choose R14 to get the same Zout as a typical 300B SE.

Cool. all my recent designs are only 1/2 hybrid (at most...)

You're right, I guess the current swing reaches a maxima when the
mirror and mirrored are in balance, which is also max Po, but that's
not the lowest Zo.

Thanks,

Michael

 

[revintage]

Having problem with the temp sims I am just trying a MOSFET-version. Any suggestions of MOSFETs? Have simmed with 1x IRFP240 at 48W Pd, I thought of using 2 IRFP040 instead.