6U8A for Marshall kind of Preamp

[Mr. Lime]

I'm having some 6U8A tubes laying around and wonder what I should do with them.

As one half is a pentode similar to an EF86 and the other half is a low power triode, I'm thinking about a Marshall'ish preamp with the U68A as cathode follower and previous gain stage. with one to two triodes placed in front of it.
I have read that those pentodes sound best when driven hard, that's why I would use it rather in the back of the preamp instead of an input gain stage..

The well known HoSo56 circuit goes the other route..

What do you guys think?
Has someone tried something like this before?


Thanks!

[statorvane]

Some of the guys at SEWATT have been tinkering with 6U8As in the preamp of a "Thrifty" amp in the "Tone Per Buck Club" forum for a few years. Several threads worth checking out there.

[10thTx]

I have used 5879 tubes in numerous amps in a 3rd or 4th gain stage including the following: Tweed Overdrive Special, D'Mars, the double "C", T "lite", Carolina Overdrive Special and a few others.

The schematics for these are on the Hoffman amp forum (also called EL34 World).

With respect, 10thtx

[10thTx]

Here is a DRAFT unproven idea for you to ponder. I have NOT built this but simply drew this up for you to give some thought on how one might approach this idea.

I am NOT familiar with 6U8A. So, you'd obviously have to adapt the schematic to that topology. However, it is an example of the type of stuff I have done with pentodes in 3rd or 4th gain stages. There is an attached ExpressSCH schematic that is easily editable.

You can easily do a "search" on the Hoffman amp forum for the other builds that are proven.

With respect, 10thtx

[MakerDP]

I have used this tube feeding a self-split power section and a 5E3 tone circuit. The way I used it, I would say It's just OK. I used the designs over at the SEWatt/PPWatt/etc. forums and their design is based on the "these values work for the EF86 so they will work just fine for this pentode" mindset, which is entirely short-sighted but I won't go into that history.... Somewhere on my desk I have a better design I napkined with the input from the AX84 (RIP) guys... way different values based on drawing up proper load-lines and such to get the best performance out of the pentode. I'll see if I can dig it out. I never got around to trying out my design.

Pentodes usually sound better following a triode in a preamp. A great design to try, IMO would be something like a parallel 12AX7 stage feeding the pentode side and then use the triode as a C.F. before the TS, or the JTM/Plexi pre using the 6U8A in place of the 2nd preamp tube (pentode feeding triode CF as above.)

[Mr. Lime]

Thanks a lot for the impressions!

I see SEWatt is a good place for unconventional circuits and people seem to come over the same ideas again and again.
The D'Mars schematic I've already seen but I'm curious about any new approach!

Concerning the poweramp, I have an SE KT88 amp for modding ready or I could do another small amp with EL84 self split PA.
Self split I tried with a TW kind of preamp but I think a higher gain preamp might word better with this poweramp..

The Texas Raptor is pretty similar to what I had in mind. Not sure if the CF after the pentode is really needed but on the other hand the 6U8A triode is rather low powered and might not serve as a good tone stack /Fx Loop recovery stage or something like a fake PI where negative feedback from the OT could be added to.

Looking forward for your further ideas!
Thanks

[MakerDP]

Well you mentioned a "Marshall kind of Preamp" and the imperfectly designed C.F. is definitely part of that sound, but yeah you are free to approach this with a blank slate for sure.

My view of self-split is that it leans towards a single-ended sound/vibe but with the benefits of noise-cancelling push-pull and falls somewhere in between the two power-wise.

You mention the DMars... I actually built one of those with the 5879 and tube eff loop... great sounding extremely versatile amp! That thing is a beast and a wonderful design. I tweaked on the 5879 resistor values a bit but can't remember exactly what I did or whether or not it improved the sound or not... lol. 10thTx was a huge help in getting my mind made up on which variant to build. He's a very patient and gracious guy... not to mentioned talented in his design work.

[Kagliostro]

As to have some idea, give a look to Sunn schematics

No, sorry, I confused the 6U8A tube with the 6AN8 tube

Sorry

A rapid search give this results

http://www.solorb.com/elect/musiccirc/6U8Amp/

https://music-electronics-forum.com/att ... 1245899017

http://www.solorb.com/elect/musiccirc/6U8AmpV3/

http://www.mrdwab.com/john/Blues-pre.jpg + http://www.mrdwab.com/john/50watt-blues.jpg



Franco

[SoulFetish]

I love the 6U8A tube, and I think it's a really useful tube. I designed a 1 tube transformerless spring reverb circuit to retrofit a silvertone 1474.
But I drew out, and compiled some vector curves for the triode and pentode (with various screen voltages) in Illustrator to draw out some load lines.
I put them together in a PDF if anyone wants them:

6U8A Pentode•Triode Curves_pt-1.pdf

6U8A Pentode•Triode Curves_pt-2.pdf

[shane]

Thanks for those SoulFetish, very useful although the triode curves dont seem to show up when I look at the file. Would love to see that reverb circuit if your willing!

[MakerDP]

Pt 1 is the pentode, Pt 2 is the triode.

[shane]

Oh yes I realise that but when I open it on my phone the triode curves dont show up. Maybe it's an issue with my phone

[Kagliostro]

Problem solved ?

K

[SoulFetish]

Thanks for those SoulFetish, very useful although the triode curves dont seem to show up when I look at the file. Would love to see that reverb circuit if your willing!

Yeah, that's no problem. I'll dig it up and post it when I can.

[caps8419]

What Vg2 does the first pentode graph depict? I see the rest after it have Vg2 labeled but that one does not. Thanks!