6U8A Triode/Pentode

[jon]

Was wondering if anyone had a nice circuit for these. I've got a bunch.

[gingertube]

The pentode is a small signal pentode and the triode is low mu and reasonably beefy.

Usual use would be at the front end of the power amp, pentode amp into the triode as a concertina splitter.

Lots of early Ampegs had this arrangement. 6U8A is the same as a ECF82.

With minor component value tweaks it can be used where ever you see a 7199, 6AN8, 6BL8 (ECF80).

Go to Schematic Heaven and look at some 'B' Series Ampeg Circuits.

Cheers,
Ian

Edited to change 6BM8 to read 6BL8 which is what I meant.

[jon]

I was originally thinking, pentode to triode in a cf configuration.

[gingertube]

That should be OK.
The pentode is sharp cutoff which is ideal for audio (as distinct from remote cutoff pentodes which have more of a compression effect). To correct what I said above, the triode section is medium mu with up to about 20mA capability. Good for CF outut.

See also 6BR8 which is identical.

You could also chase down this link - maybe something of value in it. "Blues Preamp".
http://www.ax84.com/bbs/dm.php?thread=378865

Cheers,
Ian

[overtone]

isn't the ECF82 is used in V3A normal input and V3B "vibrato oscillator" of the old Vox AC10 circuit?

Best, tony

[jon]

isn't the ECF82 is used in V3A normal input and V3B "vibrato oscillator" of the old Vox AC10 circuit?

Best, tony

Yes it is

[katopan]

Quite a while ago over at Wattkins a preamp design was worked up using the 6U8. Sounds really good.

The pentode harmonics seem to disappear if you run the pentode first into the triode (as a gain stage). But using the triode to drive the pentode did the trick. Worked better with a lower than normal preamp supply voltage.

http://www.wattkins.com/node/18048
http://www.wattkins.com/node/17970

[niversen]

i have been thinking about tearing my 18 watt lite II down when i get it back from a buddy of mine and using a 6u8a in the preamp. my thoughts are doing an input like cliff chappel's october with a dark and bright channel with the two triodes of a 12ax7 and then have those go into the pentode of the 6u8a. then use the triode section as a catodyne PI. after that just tweak the 18 watt power amp section to taste. oh yea, i'll probably do a solid state recto. i'm thinking just a gain knob for each "channel", a master volume, and a tone control. for the tone control i'll just reverse wire a vox style cut control.

[roberto]

Can't see that forum.

Something like this?
http://www.mrdwab.com/john/Blues-pre.jpg
http://www.mrdwab.com/john/50watt-blues.jpg

[gingertube]

Can't see that forum?
Did you mean the Wattkins or the AX84.
The Blues Preamp on AX84 is identical to the preamp schematic you posted except using a 7199 in lieu of the 6U8.
Thanks for posting it - saves the guys here having to search the other forum.
Cheers,
Ian

[davent]

Unfortunately you have to be a member and sign in to see any of the wattkins sites but if you search around for a Savage Croaker you might find a schematic. Here's a link to the Pushy Croaker, developed by the same people, which i'm pretty sure has the same preamp setup.

http://ompldr.org/vM2Y4eg/pushycroaker_1.6.pdf

dave

[jon]

you know if you took that design and inserted a fender/marshall style tone stack after the triode then put a volume control after the pentode you would have a quazi DR.Z prescription ES