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Sempati 30th June 2012 06:55 PM

EL41 PP triode strapped
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After a lot of studying I tried to build my first push-pull amplifier, making use of an ECC40 as phase inverter and EL41 as output tubes, and a UNITRAN 9U14 output transformer.

I "designed"my own schematic, according to the "copy and paste" method, and using a conventional EZ81 power supply, using a 10H 200mA choke.

The result: output tubes become VERY hot and a lot of hum, bad sound.
Obviously I did something wrong.

Can anyone help me out and check this schematic?


es345 30th June 2012 07:36 PM


you need to change the separate cathode resistors of the EL41 to 300 Ohm. In the data sheet of the EL41 you will find one 150 Ohm Resistor used for both tubes.

Hans- Georg

Kay Pirinha 1st July 2012 10:08 AM

Hi Harm,

why do you use these EL41's? Do you have plenty of them? Remember that they have become very scarce and rare these days!

Best regards!

v4lve lover 1st July 2012 06:12 PM

they are not that scarce.
I beleive a certain datasheet lists them as an EL84 equivalent whilst they are not.

the EL84 however does share the bias setting of -7v Vg1 for class A the autobias resistor for 36 and respectively 44Ma will mean the EL41 will Bias significantly Hotter in a circuit designed for a 84

I have always wondered if it weren't possible to Bias output tubes whit a decent wattage zener diode . just like led bias. in this configuration matching tubes for Idle current draw would be easy.

have a nice day.


tricomp 1st July 2012 06:56 PM


To me your circuit looks somewhat strange:

Cathode-resistors of the EL-41 are 150 Ohm, each according to the EL41 data-sheet available here:
That should put your EL's in a safe operating zone.

Coupling cap's from the Phase-Inverter are 10nF. I would suggest 470nf ~ 680nF unless you don't want any bass.

Phase-Inverter Cathode & Anode resistors are way too big. 22K ~ 47K seems more appropriate.

With the rather low anode-supply you have for the Phase-Inverter you'll find it hard to set the operating point suitable for the required output (~13,5VRMS). Go with a self-biased, AC-coupled Phase-Inverter, it's much easier to control if you're no pro (like me :) )

Triode-coupled EL41? OK, that'll give you at the max. 4W output. Fine, if you have the speakers to match it. Connecting the Screen-grids through a 1KOhm resistor directly to the B+ (300V) supply will give you 3 times as much output.

No NFB? It may still sound OK. Hard to say without trying. To lower the Zout and thereby shaping-up bass performance besides reducing distortion I would go with NFB. It's easy to implement: Put a 100R resistor in series with RK(I) ground-end and connect a 10K trimpot between the junction and the hot 8-Ohm speaker terminal. The cold speaker terminal should be connected to ground.
If you experience self-oscillation, reverse the EL41 anode transformer connections.
Adjust the pot for best performance and suitable amplifier sensitivity. Applying more NFB = less sensitivity.



es345 1st July 2012 09:17 PM

Hi Harm,

here is an old schematic with PP El41. You can easily modify that to triode mode.

Hans- Georg

Kay Pirinha 2nd July 2012 05:09 PM

Well, in Germany we have to pay more than 20 Euro for one EL41. That equals about 25 bucks. Quite a lot for a not so scarce tube, isn't it?

Thus I'd prefer more common, and better, tubes, e.g. EL84s, for a new project - unless I have large quantities of them handy. EL41s are very sought-after by people who restore old radios.

Best regards!

DF96 2nd July 2012 05:21 PM


Originally Posted by tricomp
Coupling cap's from the Phase-Inverter are 10nF. I would suggest 470nf ~ 680nF unless you don't want any bass.

10nF and 680k give an LF rolloff at 23Hz. Possibly below the OPT or speaker LF rolloff so not too bad.

EL41 are fairly rare, although as most of them are used in European radios rather than guitar amps they don't attract silly prices. However, nobody is ever likely to make any more so they will get scarcer with time. I believe they are electrically equivalent to E80L (different base).

tricomp 2nd July 2012 09:57 PM


Originally Posted by DF96 (
10nF and 680k give an LF rolloff at 23Hz. Possibly below the OPT or speaker LF rolloff so not too bad.

I dont dispute your result but I just did a small AB2 PP Compactron-amp with a cheap Hammond 125E OPT and even with that measly thing full output (15W) was maintained down to 15Hz with a 220nF coupling cap. from P/I into a 470K gridresistor. Since the 125E is made for guitar it comes as no surprise that it starts dropping-off around 5KHz.
If what you need to get down there is a few more nF's I say it's not the place to save :)



davorin 3rd July 2012 03:59 AM

El41 is very close with pentode section of ECL86 (6GW8).

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