High voltage pseudo-triode

[revintage]

This works in Spice, but will it work for real?

www.revintage.se/EL34.pdf

[EC8010]

No. Have you looked at Vhk? Or Va(max.) for that matter. SPICE is nice but bears little relation to reality where valves are concerned.

[revintage]

Vamax is OK (if we use Rk 1k and Rs 82k) and Vhk is no problem with double heater windings. Pls come up with something about the amplifying circuit instead . Also, if you are certain the circuit won´t work also explain.

[anatech]

Hi Lars,
With signal I think your screen grid rating has been exceeded and also your Va may be in trouble. Plate dissipation is only 25W here.

I also think your 1M grid resistance on TVT2 is too high, especially with those high plate voltages.

What are you trying to achieve exactly?

-Chris

[revintage]

Originally posted by anatech
Hi Lars,
With signal I think your screen grid rating has been exceeded and also your Va may be in trouble. Plate dissipation is only 25W here.

I also think your 1M grid resistance on TVT2 is too high, especially with those high plate voltages.

What are you trying to achieve exactly?

-Chris

Don´t worry about grid R etc., this is just a raw-model.

The circuit will have to be adjusted so ratings is not exceeded.

It is the principle I am interested in!!!

The idea was to be able to use EL34 as triode in a bridged ESL DD drive amp.

[EC8010]

The circuit is near enough an SRPP. The problem is that you will exceed a number of ratings. Simply having two heater windings won't solve the upper Vhk problem. I assume you want to swing significant voltage on the upper valve. That means Vhk will be exceeded, and that's why I say it won't work. Sorry.

[anatech]

Hi Lars,
I believe the screen is only rated for about 450V. I'd say that if it worked, it would be on the hairy edge. A short from one of your ESL's would probably cause fatal havoc with the amp and speaker.

Have you considered driving the panels differentially between the plates? You could then regulate your screens and reduce the risk of a catastrophe.

-Chris

[revintage]

Originally posted by anatech
Hi Lars,
I believe the screen is only rated for about 450V. I'd say that if it worked, it would be on the hairy edge. A short from one of your ESL's would probably cause fatal havoc with the amp and speaker.

Have you considered driving the panels differentially between the plates? You could then regulate your screens and reduce the risk of a catastrophe.

-Chris

Hi again,

Forgot to tell you. The idea was, that this was only half of a bridged amp with both sides of the stators connected by capacitors. This would give around 1100V rms on the stators.

Here is the bridged mode:

www.revintage.se/EL342.pdf

Have made an alternative connection of the screens to get a DC of ca 400V on the lower EL34 (that is AC-connected as a triode). Do not know which one is the better.

Have used the AC connected triode-mode in a preamp with EF86 and it works there.

About Vhk: Why wouldn´t two separate heater-windings with one side attached to each of the upper cathodes help the problem?

[corne]

Hi Lars,

R3 causes the Vg2 to be lower than the anode voltage, this restricts the maximum output voltage swing.
The maximum possible signal swing (sine wave) is less than 2*Vg2.

The maximum Vg2 voltage specification is often determined by the max power dissipation of g2 under penthode operation. In penthode mode g2 power dissipation increases drastically if Va < Vg2. But in triode mode Vg2=Va so max power g2 dissipation is reached at a higher Vg2.
The question is if Vg2 breakdown (arcing) to any other structures is reached before max power dissipation of g2. I never checked this for the EL34 but in triode mode (Va=Vg3=Vg2) Vg2 can go to Vg2_0 which is rated at 800V. But pushing it over 1kV might be too much.

Looking at the abuse NOS EL34s can take before failure you might be able to get this circuit working, but I wouldn't bet my NOS EL34s on it.

A solution could be running the bottom tube as penthode. You'll also have to modifying the upper tube circuit so it determines the bias voltage. Supply a fixed voltage (750V) via a high value resistor to g1 and connect g1 via a capacitor to the anode of the bottom tube.


A direct drive ESL design can be found on http://ultranalog.com/


Corne

[SY]

About Vhk: Why wouldn´t two separate heater-windings with one side attached to each of the upper cathodes help the problem?

Because the upper tube's cathode must swing more than Vhk(max). If the heater voltage reference is fixed, you'll exceed the Vhk rating. If you tie that reference to the signal, you have added all sorts of stray reactances and the circuit will be difficult to stabilize (not to mention having bandwidth problems).