El34's and 800V on the plates

[Bernardduur]

Hey all!

In my search / look into the world of amps I noticed that some manufacturers produce an amp that works by putting approx 800V on the plates of El34's. This'll give these amps an power of 90W per pair.

Some examples: Dynacord B1001, Super Ciry DEA200, etc......

What are the advantages or dis-advantages of these designs? Are they better or worse then for example a 6550 driven amp? How safe is it? Can I use > 800V on the plates (by for example older amps (designed for 220V use) and the raise of the AC voltage to approx 230)?

Thanks!

[Steve Conner]

NOS EL34s could take it, but I'm not sure about new production ones.

One problem with the high voltage is that it forces a cold bias setting in order not to exceed the tubes' plate dissipation. This makes the amp run closer to Class-B, and the result is more odd harmonics leading to a thinner, harder tone.

Personally, I would put 6550s in there instead, they don't draw any more heater current than EL34s, and they are specced for use at 100W/pair. You can bias them hotter if the capacity of your PT allows.

[Bernardduur]

Thanks!

I was thinking about something like that. Then would I run the 6550's on a lower voltage / bias em higher or should I keep the 800V's?

(as most of these amps use 2 diode bridges I could shut one down to get to 1/2 the main voltage)

[Steve Conner]

If you ran at a lower voltage, then you could bias EL34s hotter too, so it would defeat the purpose of changing to 6550s. Halving the voltage would cut the power to 20W, unless you also halved the load impedance, then you get 45W.

You should probably keep the 800V, and expect a mighty 100+ watts output, especially on today's higher line voltages. You could fit a quarter-power switch that shuts down one of the rectifiers.

[mooreamps]

NOS EL34s could take it, but I'm not sure about new production ones.

One problem with the high voltage is that it forces a cold bias setting in order not to exceed the tubes' plate dissipation. This makes the amp run closer to Class-B, and the result is more odd harmonics leading to a thinner, harder tone.

I am not sure why you would say that. I like to run the EL-34's in a class A amp with a plate voltage around 600V, with a bias of approx -18 volts. That's hardly in the range of class B, or even class A/B.

-g

[Bruce / Mission Amps]

I am not sure why you would say that. I like to run the EL-34's in a class A amp with a plate voltage around 600V, with a bias of approx -18 volts. That's hardly in the range of class B, or even class A/B.

-g

What is the individual power tube dissipation at idle when set up like that?

[Steve Conner]

I'd say about 1.21 jiggawatts. I didn't know you could get flux capacitors in a 600V rating, either.

[mooreamps]

I'd say about 1.21 jiggawatts. I didn't know you could get flux capacitors in a 600V rating, either.

Variable ; from 2 to 24 watts.

-g

[6267]

With 360v on the screen, 600v anode, and -16 at the grid, the phillips plate curves say around 200ma (past max DC cathode current & Pd!) with a dissipation of course of .200*600=120w per tube, a figure surprisingly close to 1.2 jiggawatts. I'm assuming mooreamps has a fancy variable screen supply though, the cathode current seems to relax quite a bit even at 250v (still busts Pd)!

[ahamay79]

Try the new E-H 6CA7 tube. I put them in Traynor YBA-1A and they ran fine.

Dave

[6267]

Do you know the screen voltage on that traynor? I was having bad luck with those with 600v anodes and something like 400 on the screen, it was a class B industrial PA. The screens would pulse with the music visually, and they didn't seem happy. The tubes still "work" though. I should mention these were right around the time EH originally released them, so they may have been flukes.

[ahamay79]

The schematic shows 525 vdc to the screens, 537 vdc to the plates.

[6267]

Wow, EH must have really improved production on those. The amp I put them in was pretty mean to the Sylvania fat bottles it came with, and even their screens seemed stressed. I think I must have gotten a bad example, because they were drawing vastly different current within a few days, and the screens seemed to be poorly aligned (probably related to the current draw problem). Good to hear that they're good enough, 6CA7 is one of my favorite tubes for power output.

[defaced]

I very well may be reading the data sheet wrong, but it looks like the JJ E34L (6CA7) are good to 800v. http://www.jj-electronic.com/pdf/E34L.pdf

[mooreamps]

With 360v on the screen, 600v anode, and -16 at the grid, the phillips plate curves say around 200ma (past max DC cathode current & Pd!) with a dissipation of course of .200*600=120w per tube, a figure surprisingly close to 1.2 jiggawatts. (still busts Pd)!

In my 36 watt amp, Max plate current for the EL34 is 40 mills , and 60 mills for the KT-88, and no I don't show how I do it. Too many Jolly Jokers on the internet mouthing off at things they don't want to understand.

-g