Small Signal Pentode

[ChrisM]

Looking for a pentode (not EF86, want something else) for a preamp. It will be used in the V2 position and set up for low gain. The idea is since it is V2 it will be clipping and producing great harmonic tones but the gain of that stage will be low so it's not swamping the PI/output tubes.

Anyways what tube would you suggest checking out? The miniature pentode/triode packages kinda interest me but I don't know what to do with the triode part (don't really want to not use it).

Thanks!

[Phil_S]

I wonder if you can employ a typical power tube like EL84 or 6AQ5?

[ChrisM]

I was thinking more along the lines of a miniature 9 pin package.

I was also considering something octal (as I have little experience with them in preamps) but from my reading the 6SJ7 isn't ideal for what I am doing.

[Gibsonman63]

I wonder if you can employ a typical power tube like EL84 or 6AQ5?

Merlin B advises againt it, but I can't remember why. (I'll have to re-re-re-read that one again for it to sink in.)
Instead of typical power tubes, cascode is more common. (cascaded stages configured as a pentode)

[Cliff Schecht]

6AU6 is a great option if you aren't pushing it into heavy distortion. Also look at the 6SJ7/6SK7 8-pin pentodes. I'll bet dollars to donuts that there are equivalent 9-pin versions of both available.

[diagrammatiks]

pentode connected triode?

fun way to use a 12at7.

[Cliff Schecht]

Never heard of that one.. Details?

[David Root]

Try a 5879. Gibson famously used them in the '57 GA-40. Very nice tone and I dare say a bit different than EF86. I've never had a microphonic 5879, whereas microphonic EF86s are a dime a dozen it seems.

They have a unique pin-out so you are hardwiring the circuit for just that tube, no substitutions possible.

Also and important, did I say CHEAP?

[10thTx]

Try a 5879.

Fabulous tube!!! I built a number of amps using them in the preamp and overdrive sections.

On the Hoffman amp forum, do a search for:

HoSo56 and Tweed Overdrive Special for some examples of using a 5879 in the preamp position.

With respect, 10thtx

[ChrisM]

6AU6 is a great option if you aren't pushing it into heavy distortion. Also look at the 6SJ7/6SK7 8-pin pentodes. I'll bet dollars to donuts that there are equivalent 9-pin versions of both available.

Hmm would prefer a 9 pin tube but yeah I saw that. Have heard mixed things about it.

The 6SJ7 was what I was thinking about early on but again read some not so positive things about it. Bit worried about it and noise especially in a V2 clipping position.


The 5879 has me VERY intrigued. Thanks for the tip off it's now at the top of my list for candidates.

[da Geezer]

Try a 5879.

Fabulous tube!!! I built a number of amps using them in the preamp and overdrive sections.

On the Hoffman amp forum, do a search for:

HoSo56 and Tweed Overdrive Special for some examples of using a 5879 in the preamp position.

With respect, 10thtx

Yep....LOTS of info on the use of the 5879 pentode over @ the Hoffman forum. Absolutely my favorite!

G

[ChrisM]

Cool!
Looking like parallel 12AX7 V1, 5879 V2, 12AX7 LTP V3 with a pair of EL34s in cathode bias.

[10thTx]

http://www.el34world.com/Forum/index.php?topic=9636.0

http://www.el34world.com/Forum/index.php?topic=10999.0

http://www.el34world.com/Forum/index.php?topic=8387.0

Soundclip of the Tweed Overdrive Special

http://www.soundclick.com/player/single ... i&newref=1

And better playing and great tone of Geezer's HoSo56 w/5879

http://soundcloud.com/dageezer/snatch-i ... blues-demo

Some schematic examples attached.


With respect, 10thtx

[ChrisM]

Thanks you!
Will have to sign up to the Hoffman forum too.

[Cliff Schecht]

6AU6 is a great option if you aren't pushing it into heavy distortion. Also look at the 6SJ7/6SK7 8-pin pentodes. I'll bet dollars to donuts that there are equivalent 9-pin versions of both available.

Hmm would prefer a 9 pin tube but yeah I saw that. Have heard mixed things about it.

The 6SJ7 was what I was thinking about early on but again read some not so positive things about it. Bit worried about it and noise especially in a V2 clipping position.


The 5879 has me VERY intrigued. Thanks for the tip off it's now at the top of my list for candidates.

I've had good luck with the 6SJ7. Maybe I'm just lucky. My '54 Silvertone 1331 uses a 6SJ7 feeding a 6V6 and that amp is pure blooz, great overdrive is available from ONE stage (and really touch sensitive too) and the amp is very smooth in the high end. It's a perfect little slide amp and has enough mojoey rattle in the speaker/baffle to make Hound Dog Taylor happy. Part of the smoothness of the amp comes from the pentode stage being setup for MEGA gain (grid leak biased) which sacrifices bandwidth for more gain. I don't know the cutoff frequency at this time but I suspect it's in the audio range and knocking out some of the extreme highs.

I've also used 6SJ7 as the input stage to a 3 stage preamp design and while that amp sounded terrible with guitar, it was a superb harmonica amp (this was partly due to my using an old 12" Jensen PA speaker instead of a proper guitar amp speaker). If you connect the outside metal shield of the 6SJ7 to its cathode all of the noise and microphonics that these are "notorious" for disappear immediately. Seemed more stable than the EF86 that everyone is after.

Just a tad of research shows that the 5879 is as close as one can get to a 9-pin equivalent of the 6SJ7. If that's the case then I would recommend you use a REALLY large screen dropping resistor. I was using something like 470k, maybe even higher, and I noticed that when the screen is dropped really negative WRT the plate the clipping pentode has a lot of natural bloom, moreso that I've heard in any other amp. This was with a 6AU6 in V1 and V2 driving an 807 but IME the 6SJ7 is similarly as bloomy when pushed into full-on distortion. Study the original Champ and other super early SE 6V6 based amps. They tend to use a single 6SJ7 feeding a 6V6 and this IME is a really cool and unique flavor of distortion, a nice changeup from the typical AX7 distortion.