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Danko 21st October 2005 09:27 AM

EL84-PP - Why just 5W?
2 Attachment(s)

I have built the attached circuit. I think, that's very simple, common.

I have seen many many EL84 push-pull schematics, and all of them was working from about 300V supply. Almost all of them could deliver about 10W.

But my circuit delivers only 5W undistorted. Why?
What did I wrong? :)

Oh, another question! :-)
The amplifier's frequency-response starts to fall at about 15kHz. Why? If I apply a feedback from the output trafo, to the LTP, then will this falling at 15kHz go to a higher frequency?

reinhard 21st October 2005 09:37 AM

triode amp?

your schematic shows the El 84 triode connected, therefore
5 Watt is ok.

Connect the screen grids to the B+ and measure again. You will get someting between 10 and 17 Watts.


tubetvr 21st October 2005 09:39 AM

Your amplifier gives exactly the output power that is expected by EL84 push-pull in triode mode, see here

For higher power you need to connect the tubes as pentodes or as Ultralinear, in class AB pentode you can get up to 17W at 300V see here

Note that output power in datasheets are given as the output power from the tubes, the losses in the output transformer are not considered so in reality you would expect a bit less after the output transformer.

Regarding the fall at higher frequency it is probably due to the quality of the output transformer, feedback will improve the frequency response but too much feedback increase the risk of oscillation.

Regards Hans

Danko 21st October 2005 10:47 AM


I connected the G2 to the B+. Now I get about 7-8W for a 15ohm dummy load.
Do I have to change something on the output-tube's cathode-resistors?

andyjevans 21st October 2005 11:41 AM

How does it sound? You may well prefer the sound in triode if your speakers and room allow you to use this.

Danko 21st October 2005 11:49 AM

The volume is OK, but sometimes I think that the 2x5W is not enough. I think, 2x10W would be better, This is why I have choosen this schematic :)

I will google a little bit, to find out, how to connect the pentodes in pentode "mode" properly.

Danko 21st October 2005 01:32 PM

I searched the g00000gle, and I modified my circuit to this:

After I powered up, the output tubes were entirelly glowing! :-))

Theese tubes are my test-tubes, they are some old EL84 tubes. I tested the circuit with my /almost/ brand new 6P14P tubes, they were also glowing.

I tried to modify the resistors, but there was no change :(

Yvesm 21st October 2005 02:03 PM

Duh ?

300V is a bit near the limits for EL84 screens, but . . :xeye:
Use, say, 1K in each screen, check your cathode resistors as well as the bypass cap !

What is the cathode voltage ?

James D. 21st October 2005 02:11 PM

How loud is loud?

To get a percieved doubling in loudness you need to increase the acoustic power by 10 times... Given typical loudspeaker compression that means up to 15 times the electrical power or 75 Watts.

Going from 5W to 10W output is not really noticeble - particularly when the harder clipping of ultralinear or pentode mode is taken into account...

In other words you are not going to make the sound louder by changing this circuit - and you stand every chance of making it sound worse...

Still it's a good learning experience - so don't stop, just don't expect too much.

good luck


Danko 21st October 2005 02:21 PM

I played a little, with the grid-resistors. I tried 470R, 2k2, 22k...
With 22k the grid voltage was about 190-200V, and the plate was glowing only a little bit. But still glowing ...

The current state is: The screen-grid resistors are 22k, the B+ is 315V(loaded) and on the 150 Ohm cathode resistor, there's 21V.
On the screen, there's 160V.
And the speaker is hummming.

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