ECF82 / 6U8 substitutes EF86 + PI triode?

[Mr. Lime]

Having some parts left over I was thinking about a single channel low power amp with capabilities for great cleans and distortion tones. I would like to do some things different compared to my other amps so the circuit should employ a cathodyne PI feeding an EL84 PA and a pentode preamp stage. There aren't many designs around which follow those lines but cloning alone sin't satisfying for me.

I would borrowed some ideas from the HoSo56 circuit (preamp) and some from the 5E3 Tweedle Dee mod (PA) as both do sound good to me and aren't too difficult.
Before I start and draw a specific schematic I would like to know if an ECF82 (pentode & 12AT7 triode in one bottle) would fit my bill and how the pentode could be compared to the more common EF86?

The 12AT7 could be set up as a split load PI while the pentode serves as an input stage. In between a 12AX7 could feed a tone stack + recovery gain stage.

Has anyone experiences with this tube or could give some advice?
They aren't expensive at all so maybe interesting for other builders as well?!

Thanks a lot!

[tubeswell]

FWIW The ECF82 triode is more like a cross between a 12AU7 and a 12AY7. highish transconductance (8.5mS), low plate resistance (5k) and u=40.

Recommends a 20k plate resistor here

https://frank.pocnet.net/sheets/129/6/6U8.pdf

Tried a 6U8 in front of a 20W amp a few years back. See schematic. I don't think I kept the amp in this incarnation for very long tho.

[Mr. Lime]

Thanks for replying!

So the ECF82 would be a nice candidate for a cathodyne PI?
Cool that you already tried out something similar!

I roughly drew a schematic to show in which direction the amp could go.
Not sure about a global feedback loop as many EL84 amps come without it. Depends on the headroom at the end..

So you recommend 20k for plate and tail R on the cathodyne instead of 47k?

Thanks a lot!

[tubeswell]

...
So you recommend 20k for plate and tail R on the cathodyne instead of 47k?

Not me. The datasheets. Its a higher-current tube than a 12AX7. More current = more voltage across a given resistance, so with a higher current tube, you don't actually need 47k load resistors to get a decent voltage drop across the load. If you can keep the load lower, it improves impedance bridging (especially in a cathodyne that is driving lower input impedance on output tube grids)

[Mr. Lime]

There seem to be more pentode/triode combinations out there that can be used for guitar amps.
Another commercial example is the Madamp G3 Blues using a 6F1P.
Here's the datasheet:
http://www.mif.pg.gda.pl/homepages/fran ... 6/6F1P.pdf

Funny I found that schematic today and it's pretty much what I was thinking about to do!

Which of those two tubes are to prefer in this application using the triode as PI and the pentode as input stage?

[gingertube]

See also 6BL8/ECF80
https://www.radiomuseum.org/tubes/tube_6bl8.html
Good for Power Amp Section like this:
https://www.ozvalveamps.org/playmaster/playmast_116.jpg
Cheers,
Ian

[Mr. Lime]

Thank you!

This one‘s also very interesting.
What do you guys think about the pentode? Do they sound better overdriven at the end of the preamp circuit before the PI or as input stage?

[oldmacman]

Check out how the Standel 50L15 / S-80 uses one of these: gain stage + cathodyne PI.

standel_s80_schematic.jpg

[Mr. Lime]

Pretty nice as well!

There's a try on a 6BM8 from Geezer but I'm not able to open the sch. file and see what it looks like..
Thread here: http://el34world.com/Forum/index.php?to ... 40#msg1540

[sluckey]

how do you make it look so tidy?

Search for a free program called ExpressSCH.

[Mr. Lime]

Thanks, ExpressSCH did the trick!

Well I understand that the 6BM8 is more kind of power amp pentode/triode and would suite as substitute for EL84s.

Here's some more info about tubes with pentode and triode in a single envelope:
http://el34world.com/Forum/index.php?to ... zAbucRbilw

6JZ8 12DZ Medium-Mu Triode + Beam Power Pentode
6LU8 12DZ Medium-Mu Triode + Beam Power Pentode
6MF8 12DZ Hi-Mu Triode + Beam Power Pentode
6MY8 12DZ Medium-Mu Triode + Pentode
12JZ8 12DZ Except for filament voltage, identical to 6JZ8

I'm still not sure which pentode/triode valve would fit the bill best as alternative to a EF86 + ECC83.

Lots of tubes can be found here:
https://frank.pocnet.net/other/Compactr ... rvey.html
Thanks a lot!