Small Signal Pentode

[Phil_S]

I've got a small pile of 6AU6. They are cheap as dirt. Got them as part of an eBay lot that had other things I wanted. I tried building a Vox AC15-ish circuit and attempted to use the 6AU6 instead of EF86. That project was generally a failure, so I really don't if the tube is a good pick or not. I'm willing to sell several for a very reasonable price. PM me if interested.

Phil

[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.

Wanna make sure this gets read, it's at the bottom of the last page and I NEED ATTENTION!!

Just kidding .

Phil: I've got the same situation, I find these all of the time as gimmes and don't even really want them anymore. The only thing that really made me smile was the amount of bloom that my first amp had, undoubtedly due to the really large screen and plate resistors I was using which allows the load line to shift pretty dramatically as the amp sags.

[10thTx]

I've used 6AU6 and 5654 pentodes also. Those two are interchangeable if you're not exceeding the 5654 voltage. I have A/B'd those in an amp with 5879 & thought they sounded decent but not quite as good as the 5879.

With respect, 10thtx

[diagrammatiks]

Never heard of that one.. Details?

http://www.freewebs.com/valvewizard/cascode.html

[jon]

Matchless Clubman

[Cliff Schecht]

Never heard of that one.. Details?

http://www.freewebs.com/valvewizard/cascode.html

That was my first thought. Cascode adds gain and sort of linearizes the bottom triode by acting as a high impedance current source (i.e. bottom plate voltage is now relatively independent of plate current). Pretty neat!

[tubeswell]

Another option might be to use an EF86 but just set it up for lower gain with (~200V HT) a smaller plate resistor (~100k) and lower screen voltage (~60V or so) with -1V bias. (This is suggested in Merlin's 1st book BTW).

[Structo]

I had a friend that had a Matchless Clubman.
He never could find a decent 6SH7 after the original wore out.
I think he bought four or five and they all hummed.

[Cliff Schecht]

With tubes like the 6S*7 series that come in glass and metal envelopes, I almost always go for the metal can types. There's no audible difference other than the glass versions seem to be noisier and more susceptible to hum without the full shielding.

Also what Tubeswell suggested is a great point. Earlier amps try to squeeze too much out of the EF86 gain-wise which can encourage ringing and microphonics, especially when placed in a combo amp in proximity to the speaker. The easiest solution to this problem is to drop the gain.

[Markusv]

Chris
6GH8A

Med Mu triode and small pentode

Very versatile from what I hear

Markus V

[ChrisM]

Thinking about this more...

The idea with this amp is this.
3 preamp tubes

V1 .....???
V2 single 12AX7 reverb
V3 12AX7 LTPI

V4/V5 EL34 in cathode bias


Just thinking here if I want to use a pentode in V1 and that will push the PI and output tubes.
OR
Use a triode/pentode in V1 and have the triode pushing the pentode which would push the PI and power amp.

I'm not looking for monster gain, more great clean tones with some very harmonic lively distortion.


Basically I am trying to decide what type tube to use for V1.

[rdjones]

6AN8 ala Sunn and Univox.
7199.
The 6u8 family.

These are 3 different pinouts, with the 6AN8 being the most common in guitar amps, the 7199 being the (arguably) best spec'ed for audio, the 6U8, etc. with the most variants.

I'd lean toward the 6AN8 only because I have a few available.

rd

[tweedeluxe]

I've used the 6SJ7 (and 12SJ7) and the earlier 6J7 (w/grid cap) in several guitar amps, vox and tweed type circuits and even higher gain stuff. Always had good results.

[ChrisM]

Thinking about this more...

The idea with this amp is this.
3 preamp tubes

V1 .....???
V2 single 12AX7 reverb
V3 12AX7 LTPI

V4/V5 EL34 in cathode bias


Just thinking here if I want to use a pentode in V1 and that will push the PI and output tubes.
OR
Use a triode/pentode in V1 and have the triode pushing the pentode which would push the PI and power amp.

I'm not looking for monster gain, more great clean tones with some very harmonic lively distortion.


Basically I am trying to decide what type tube to use for V1.

Input?

[rdjones]

Chris
6GH8A

Med Mu triode and small pentode

Very versatile from what I hear

Markus V

Yes, 6GH8A is one of the tubes in what I call "the 6U8 family".
Pin base 9AE, here's a link to the tubes with this pinout.

http://www.nj7p.org/Tube4.php?base=9AE

Lots to choose from if you go with this pinout.

RedDog Steve