Pentode tube help?

[BackwoodsJoe]

I'm looking for a low gain, low noise, high current pentode for a preamp. THX, joe

[Merlinb]

How low gain and how high current? A small power pentode might do the job, e.g., EL84, EL85, 6K6, 6V6 etc.
Otherwise an EF80 / 6BX6 might be worth a try, though I don't know how noisy it might be.

[BackwoodsJoe]

Merlinb,
Thanks for the reply. What I'm looking for is a pentode with enough current to push a tone stack like a black face princeton uses. I'd like to try that classic circuit with a pentode inplace of the usuall 1st position triode.

[Merlinb]

Merlinb,
Thanks for the reply. What I'm looking for is a pentode with enough current to push a tone stack like a black face princeton uses. I'd like to try that classic circuit with a pentode inplace of the usuall 1st position triode.

Well even a classic EF86 or 6AU6 can do that, provided you run it with a low load (33k say) and a healthy screen voltage.

[morcey2]

Another option is a pentode/triode tube like the 6U8 or 6GH8. Run the pentode as normal coupled to the triode running as a cathode follower to drive the tone stack.

(putting on asbestos underwear)

Another other option is using a high-voltage mosfet as a source follower after the pentode. If you run the mosfet from a higher voltage than the pentode and bias it correctly, you won't have to worry about nasty solid-state clipping. You won't get the cool compression that you can get from a DC-coupled CF being overdriven though.

(removing asbestos underwear)

Matt.

[zaphod_phil]

Another option is a pentode/triode tube like the 6U8 or 6GH8. Run the pentode as normal coupled to the triode running as a cathode follower to drive the tone stack.

+1

[BackwoodsJoe]

morcey2, I'll see if I can score a 6U8 or 6GH8. There's so many pentodes, but all you hear about is EF86. Like I said I don't have to have a lot of gain, but I need something fairly quiet with good tone.
THX, joe

[davent]

Hello,
This 6U8 preamp was posted on one of the sister forums in their early days by a kid (15 -16 years old) named Kerry who seems to have dissappeared. I beleive others around here and at ax84 had some input in it's design. There were some video clips kicking around but not sure where to find them.

This has been a problem for some time now but there is still no way to add an attachment in theTube Talk forum, other forums have been fine.

Stuck the schematic over here in another thread.

http://www.18watt.com/modules.php?name= ... 786#194786

Edit: Found the thread at Sewatt with the link to the clips. http://www.sewatt.com/node/15377

Take care
dave

[zaphod_phil]

Yeah, I was involved in some of the design discussion about that preamp, which mainly took place on AX84. It would work nicely with an 18W PI/power stage. However, what Matt was recommending was simply strapping the triode side of the tube to the pentode, as a cathode follower, to drive the tone stack. tkdrew built an amp using a 6GH8A this way, but without a TMB tone stack. Its sounded pretty good and you can find the details on ppwatt.com.

Here are some notes about Kerry's 6U8 preamp from AX84, for anyone who's interested:

http://www.ax84.com/bbs/dm.php?thread=309556

"According to Iain, Zoe had their Blues preamp working with an ECF82/6U8 using Rk = 2k2, Rp = 220k, Rs = 820k. Zoe also reported getting good results with alot of compression, with Rs=470k. The Vox AC10 uses 330k, so you can try various values around the 330k to 820k range and see what sounds best to you. Also try 5k6/120k (used by Kerry in his VF-10's preamp) for the triode section, and tweak from there." http://www.ax84.com/bbs/dm.php?id=331178

Yeah, 330k - 820k seems to be the useful zone, depending on the cathode resistor. 470k with a 3k3 cathode and 220k plate looks the nicest, but obviously you need to tune by ear from there, cos there might be sweeter tone somewhere nearby in real life. http://www.ax84.com/bbs/dm.php?id=331179

I might also like to try this 2-stage preamp in "reverse" - ie with the triode section in front, so that the pentode gets driven into distortion.

[dotfret]

Lookit, I've been keeping this idea to myself for so long it is festering, so here it is -

My idea is to use a MCTube type device to replace V1 of the 18 watt, as some sort of plug-in panel which can drop into a normal style head.

Now, one serious but mostly ignored contender for a novel sound from such a device is the ECF804, which is a Brimar design (even if most of them carry H&K labels). These are still cheap because no-one has designed a circuit for them yet, and 150V max on the pentode requires thought. So that's one idea ...

Now, those "divided by 13" people have been using the 5879 pentode in their amps for a while -

http://www.dividedby13.com/jrt915.html

I don't know where to look, but I do know that there are circuits to be found "out there", because Gabi told me he found some a couple of years ago. And that's another idea ...

[dotfret]

Then again, I was trying to put the question into an 18 watt context. A 6SJ7 would probably answer the question, treat it like a low gain ef86 and ignore the fact that it looks a bit historic - a great design, cheap, and very resistant to microphony compared to EF86. Use a 6J5 as a buffer triode if you want to go back in time ....