Anyone built an EL84 triode strapped Single Ended amp?

[walkman]

Wondering if anyone else has done this and what plate voltage they used.

I was running mine little amp at round 250v and found that I liked the tone so much better when triode strapped. The OT is 7k at 16ohm so works out great for 8ohm triode strapped .

[cbarrow7625]

What exactly do you mean by triode strapped? I have done a 12au7 cascode-type push-pull output stage and absolutely love it. Way better than the sound of the straight 12au7 PP in the amp preciously.

Not sure if we're talking about the same thing. Got a schematic?

[walkman]

From Wikipedia...

"A pentode (or, less commonly, a tetrode) can have its screen grid (grid 2) connected to the anode (plate) and the resulting "triode-strapped" (or "triode-connected") device has characteristics very similar to a triode (lower anode resistance, lower noise, more drive voltage required). "

[mooreamps]

Wondering if anyone else has done this and what plate voltage they used.

I was running mine little amp at round 250v and found that I liked the tone so much better when triode strapped. The OT is 7k at 16ohm so works out great for 8ohm triode strapped .

I did... But it was a stereo amp for a surface computer, not for a guitar amp....
I wired the power tubes for triode because the pentode tone does not sound good at full volume when playing music videos...

The plate volts is around 300 volts and the OT is 7K at 8 ohms....

Given just how good this sounds, I am currently working on bringing up my 813 project to drive speakers for my desktop computer at home....


-g

[Chuck H]

Forgive my inability to site the associated math... But IIRC a triode operated pentode should be "ideally" matched to a significantly lower primary impedance. 7k would seem typical of normal pentode operation. Which is fine if your going to have a switch and can't reasonably change the priZ. But if your dedicated triode I think the ideal priZ would be much lower to optimize.

[Gingertube]

Chuck etc.
The definitive 6BQ5 / EL84 Application Report. You are right, 3K5 Primary Z for Triode Mode.
http://www.retrovox.com.au/STC6BQ5.pdf
Cheers,
Ian

[walkman]

Yes I can agree that for me the EL84 triode strapped gives a much smoother sound. Overdrive sound is much browner in sound than you get with the regular pentode. There is much less woofieness, bark and fizz that is typically found with the pentode mode. Sounds particularly good with 2x12 closed back box.

[Wombaticus]

Forgive my inability to site the associated math... But IIRC a triode operated pentode should be "ideally" matched to a significantly lower primary impedance. 7k would seem typical of normal pentode operation. Which is fine if your going to have a switch and can't reasonably change the priZ. But if your dedicated triode I think the ideal priZ would be much lower to optimize.

On the other hand, triodes aren't nearly as fussy about the load impedance they drive...

[Chuck H]

Yup. That's one reason the triode/pentode switch works so well as a volume reduction. The less than ideal primary impedance reduces efficiency even further. I have a book here written by J.L. Hood where he discusses a particular tube circuit where triode operation changes the ideal primary impedance on the order of 1/4 that of pentode operation. Since I've never built a dedicated triode output amp I've wondered what the difference in tone would be if I idealized rather than run the pentodes as triodes into the standard pentode intended OT.

[mooreamps]

....and is not the effective primary impedance dependent on the loads being driven from the secondary winding ???



-g

[Chuck H]

Yes. Are not speakers just as expensive as output transformers?