4 x EL84 vs. 2 x EL34

[Gingertube]

This cathode biased EL34 pair at B+ of 380V and Raa of 3K4 gave me 32Watts RMS continuous sine wave - measured.
The PI got 22K grid stops (not shown on schematic) to stop the thing taking off at volume settings above about two thirds max.
Cheers,
Ian

[Zozobra]

This cathode biased EL34 pair at B+ of 380V and Raa of 3K4 gave me 32Watts RMS continuous sine wave - measured.
The PI got 22K grid stops (not shown on schematic) to stop the thing taking off at volume settings above about two thirds max.
Cheers,
Ian

I'm interested in building something like this for my next project as I've only built class ab1 amps trying to squeeze the most power out of the tubes possible so something like this would be nice to try.

What should I be looking for power supply wise for the B+? I'm guessing something like 290vac in the region of 300mA would be a good start. Does that sound reasonable?

[Gingertube]

From Datasheet
max signal anode current ( 2 tubes) 190mA
max sognal screen current (2 tubes) 45 mA
A bit for the preamp 10mA
250mA will be fine.

I used a voltage doubler with a 150V AC secondary (so 290-0-290 or 300-0-300 would be fine): Here is the rest of the schematic for that amp, shows the power supply.
Cheers,
Ian

[pdf64]

Bear in mind that the B+ winding current may be higher than the B+ dc, depending on the rectification and smoothing arrangement used.
Pete

[Zozobra]

Thanks guys. I'll be using a custom wound toroidal PT (Aktualno?ci - TOROIDY.PL Transformatory Toroidalne Producent, Audio, Separacyjne, Trójfazowe, 230/110V, 110/230V, Na zamówienie) as I can source them cheaply, so specification isn't so much of an issue. Shipping will probably be the major cost! I'm going to specify 0-290VAC@370mA (~380VDC@260mA) for the B+ and 3.9A for the heaters.

I'll be looking to use one of Merlin's universal OTs (if he has any left in stock...) which has a 3.25k primary so I may have to do a little bit of fudging.

I'm undecided on the preamp as of yet. I'm thinking of something along the lines of a superlead bright channel and a single stage clean, similar to the hot cat 30 using a similar PI connection.

[soundguruman]

You should run 2X EL34 in triode.
18 watts, or there about.
To overdrive EL34s is to get the real sound.
Preamp distortion is an imitation of what you really want.

LOUD is not where it's at. GO for tone, and lower volume instead.

[Chuck H]

I'm no genius with plots. How does one figure the right OT pri Z for triode operation??? I know it's said to be much lower than what you want for pentode operation and the lower efficiency of the higher pri Z is at least part of the power reduction when a typically pentode operated amp is run triode. I've never tried idealizing the OT for triode operation though and can't help but wonder how much difference it would make in the tone and feel of the amp compared to what we're use to with this sort of circuit.

[big_teee]

You should run 2X EL34 in triode.
18 watts, or there about.
To overdrive EL34s is to get the real sound.
Preamp distortion is an imitation of what you really want.

LOUD is not where it's at. GO for tone, and lower volume instead.

How do I temporarily run my 2204 Clone in Triode mode.
I would like to try it?

[soundguruman]

How do I temporarily run my 2204 Clone in Triode mode.
I would like to try it?

It's easy as pie are round, dude.

You just hook pin 4 thru the screen grid resistor, to pin 3.
(I just use a sort wire, from pin 6 to pin 3)
In other words, the screen is connected to the plate, thru the 1K screen grid resistor.
Do the same, on both halves of the output transformer...

You are no longer hooking the screens to the screen grid power supply...that's disconnected now.

No change in bias....unless you wanna bias it hotter, recommended.

No change in preamp voltage...it stays about the same as always.

Less current to output tubes, overall...the amp runs cooler. Good excuse to heat up the bias setting...

Sound Quality: sounds almost exactly the same as before. Just not as loud.
You can CRANK that puppy, and get a better overdrive, at a lower volume.

Now this may help show how...
marshall jcm 900 power.doc
apologize that this is WORD file, hope you can c it...

See the power hi/lo switch in JCM 900?
Changes the amp from pentode to triode.

It just moves the screen resistor(s) from power supply, and connects them to PLATE, instead.
However for EL34, use 1K screen resistor, not 470.

AND since you are doing 2204...
marshall JCM800_2204.pdf

I would like you to change the cathode bias of V1B.
I would like you to change 680n cap to 10uF. + of cap faces cathode.
I would like you to change 2200 ohm bypass resistor to 1500 ohm.

Now you are ready to ROCK.

* If you are testing this amp in your house, and the neighbors call the police...
It means the amp is working correctly.