Any low voltage pentode circuits out there?

[Esppse]

I am currently doing a Triple Valvecaster that I have modded quite a bit and it's coming out nicely.

At this point, it's basically a low voltage mid gain tube preamp, using 12AU7s.

I'd like to add a last pentode stage to get something like EL34 distortion, but want to do it at low voltage.

Is there a Pentode tube that will work in a circuit like the valvecaster? Would EF86 work?

[merlinb]

A lot of experimentation is needed for starved tube projects, but at least its easy enough to breadboard. There are lots of DIY low voltage attempts to be found if you google, but results are very mixed, to say the least.

[amptramp]

There were a number of car radio tubes designed to work at 30 volts maximum on the plate.  The 12CN5 comes to mind and you might be able to find one at a surplus store or in a collector's stash somewhere.  There is data available for the 6AU6 in almost any operating voltage.  The EF86 may give you what you want but it may not sound like an EL34 and since EL34's are usually used in push-pull, you may not be able to get the even harmonic cancellation of push-pull devices.

[printer2]

The 6AK5 has 120V as a typical operating voltage in a datasheet. The Chinese equivalent has been used (although in triode I think) in headphone amplifiers that operate at 28V.

[mozz]

I vote for the 6au6, i have a few hundred looking for a home.
Here is the website with a list of car automobile tubes, some pentodes there.
https://www.junkbox.com/electronics/lowvoltagetubes.shtml

[PRR]

12CN5 comes to mind and you might be able to find one

4 buck everyday at Tube Depot.
https://www.tubedepot.com/products/12cn5
That's in Memphis USA, for shipping and customs planning. I know there are similar tube mongers on the Canadian side.

[Esppse]

Ah thanks for all the suggestions guys, I haven't heard of any of these before today. I'll toy with a simple isolated circuit like the Valvecaster and substitute the pentode in, and maybe play around with the resistor values to see if I can get it to sound somewhat nice at 9v-12v. If only 12AX7s are 4 bucks each!

[amptramp]

Note that a lot of the pentodes in the car radio tube family are remote cutoff, designed for automatic volume control.  This will introduce non-linearity if the signal levels are large.  I have a Hammond Solovox that uses 6SK7 tubes to control the volume, but to avoid even-order harmonic generation, they use two of them in push-pull.

[merlinb]

Note that a lot of the pentodes in the car radio tube family are remote cutoff, designed for automatic volume control.  This will introduce non-linearity if the signal levels are large. 

I hardly think that matters in a Valvecaster, it doesn't even get close to linear to begin with!

[printer2]

I did a P-P amp running on 35V with 6AU6's and a pair of 25L6's once. I think I eventually eliminated the screen resistors and tied them to the supply. The fastest and simplest way of finding the sweet spot of the tube would be using a pot for the plate, screen and cathode resistors. I have about 50 of the 6AU6, won them with a couple of buck minimum bid. I came to the conclusion that the 6AK5 might be better suited at lower voltages and picked up a half dozen Chinese 6J1 (a buck a piece at the time) but never got around to playing with them. The circuit was on a breadboard that sat come up.