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rongon 26th June 2020 09:07 PM

"El Cheapo" with EL86-triodes?
Hey Eli, or whoever might be interested...

Thinking of this amp, the ever-popular "El Cheapo" using PP 12AQ5/6AQ5 (6V6oids) with a 12AT7 LTP driver:

Has anyone made an El Cheapo with PP EL86-triode? That would allow the use of more commonly attainable OPTs with 3k to 5k p-p primary impedance, which wouldn't work so well with 6V6oids. EL86 won't stand as high plate voltages as 6V6, but 275V plate-cathode should be safe for them wired triode. What's the usual resulting B+ from an N77U using a voltage-doubler? I figure about 300V, which would be pretty close to perfect.

Also, I've read many times in many places that it's easier to make a high-performing OPT with a lower stepdown ratio. Perhaps that opens up the door to Edcor iron?

EDCOR - CXPP Series Output Transformers

Those are just Edcor's 25W and 30W offerings. They also have 50W rated OPTs in similar impedances.

What I don't know is what kind of primary inductance they have.

Eli Duttman 27th June 2020 12:16 AM

Voltage doubling a N-77U gets you a bit more than 300 V. 2.6X the AC mains voltage is a reasonable approximation. Resort to boosting "iron" was needed to get the B+ up to where a 6V6oid is "happy".

For a "6" WPC triode wired 6V6oid "El Cheapo", Edcor's GXPP15-8K should be quite satisfactory. I'm guessing that 0.68 μF. as the high pass cap. value will be safe.

The 'T7 triode sounds very good with an IB of 3 mA. and roughly 210 V. on the plate. To use 275 V. B+ requires reducing the LTP load resistor value and unacceptable loss of gain. You might solve that problem by adding DC coupled ZVN0545A source followers between the LTP and the "finals". Something has to be done to raise the net load impedance "seen" by the 'T7 triodes.

rongon 27th June 2020 12:44 AM

Ah, yes. The driver tube is key, of course.

Thanks for the estimate of B+ from the N-77U. I get about 120VAC mains here. If the N-77U makes +310V rectified, then with cathode bias of something like -30V for the EL86s, that would put their plate-cathode voltage at +280V. It's a 12W tube (actually 12W plate + 2W screen), so 45mA each should be within reason.

If the 12AT7s have 175V on their plates, about -2V grid bias, 3mA plate current each, do you think that would work out well enough?

The reason I'm going here is that I have some Russian 6P43P-E I'd like to use, and I have a pair of 25W-rated 6.6k OPTs as well.

Eli Duttman 27th June 2020 03:32 AM

1 Attachment(s)
I'm thinking a topology other than "El Cheapo" might be "best". Perhaps something along the lines of the near Class "B" 6Y6 amp I worked on with Jeff Yourison could be adapted to your tubes.

Full pentode mode with regulated g2 B+ allows reasonable liberty to be taken with the anode B+ voltage limit. As long as you are relentless in keeping anode dissipation within published limits, things tend to work out well. Heat, not mere volts, is the tube destroyer.

BTW, Antek's AS-1T230 toroidal power trafo looks like a very nice fit for this project.

Ketje 27th June 2020 01:44 PM

If you replace the choke by a mosfet capacitance multiplyer the voltage gets lower and then use the EL86.

rongon 27th June 2020 10:36 PM

I figure Russian 6P43P-E with rated max screen voltage of 250V can live with 270V plate-cathode if wired triode.

If there's 300VDC for the B+ to the 12AT7 LTP, then there should be enough swing to drive the output tubes.


Eli Duttman 27th June 2020 11:14 PM

The problem is adequately loading the 'T7 LTP. Roughly 210 V. on the plate and an IB of 3 mA. are in order. The RP of the 'T7 is approx. 15 K. Even with ZVN0545A buffering, the plate load resistors should be 33 K. The voltage drop for 3 mA. in 33 K is 99 V. Things are being cut too fine for my liking. However, it could work out OK.

Ketje 29th June 2020 12:38 PM

2 Attachment(s)
The EL86 can be used with minor changes.
Replacing the choke with a 150Ω resistor (or a choke with same DCresistance) gets the power down to 280V.Minus drop cathode resistor and DCresistance OPT gives 250V on the tube, is the max for a PL84 (=EL86 other heater).
With 275V supply for the ECC81 the anode gets 135V, enough room to give the output tube 26V.
Only have to find a transformer 6,3V at 4A for the heaters.

artosalo 30th June 2020 07:58 AM


If there's 300VDC for the B+ to the 12AT7 LTP, then there should be enough swing to drive the output tubes.
Take note that EL86 as triode has much lower gain than EL84 (as triode).
To drive full, EL86 requires 17 Vrms while EL84 only 7,5 Vrms.

Eli Duttman 30th June 2020 09:16 AM


Originally Posted by artosalo (
Take note that EL86 as triode has much lower gain than EL84 (as triode).
To drive full, EL86 requires 17 Vrms while EL84 only 7,5 Vrms.

This point pretty well stops things, in their tracks. :( Reduced B+ = reduced net 'T7 anode load impedance and consequent reduced gain.

More and more, voltage amplifier into "concertina" phase splitter looks right for PP EL86 "finals". A nice MOSFET has an advantage over a triode, in the "concertina" phase splitter role, when B+ volts are scarce. The FET can swing closer to the B+ rail than a triode can.

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