6AU6 pentodes

[TGRANT]

I have a bunch of 6AU6 pentodes, and for the fun of it I cobbled together a three stage single ended amp using the 6AU6 as the final tube connected to the output transformer. I’m having a heck of a time getting it to work - I just can’t find the correct bias point.

Has anyone ever used this tube as the power or output tube in a three stage amp? There are a few schematics out there that show the 6AU6 as a driver or preamp, but none that show it as the output tube. I’m wondering if it’s not suited for this application. The first two stages use 12au7’s for no other reason than that’s what I have on hand. The first two stages work great, but when I add in the 6AU6 pentode the wave form goes to heck.

I'm using the 6AU6 becasue I have 8 of them and want to play.

[korneluk]

Is the output connected as a pentode or triode strapped?

here are the recommended op points:

Class A Amplifier (Triode Connected)
Plate Voltage ................................. 250 V
Grid No. 1 Voltage ............................ -4 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 330 Ù
Amplification Factor .......................... 36
Plate Resistance (approx) ..................... 7.5K Ù
Transconductance .............................. 4800 µ
Plate Current ................................. 12.2 mA

In pentode mode, the plate resistance is very high. See here:

http://www.nj7p.org/Tube4.php?tube=6AU6

[coldcathode]

I found Triode Strapped Curve for 6AU6

See attached. The posting limitations will make this hard to read.

Not going to get much power out of it but if you are just playing around try this

Vp = 250
Io = 12mA
Bias = -4V (333R Rk for "self bias" use 3 x 1K 2W parallel)
OPT primary 10K to (whatever speaker hope its got like 98dB SPL! LOL)
Pout = about 0.35Watts
THD = about 9%

Might be able to push 1 watt in Push Pull??

[coldcathode]

Might make a decent headphone amp?

[korneluk]

Originally posted by coldcathode
Not going to get much power out of it but if you are just playing around try this

Vp = 250
Io = 12mA
Bias = -4V (333R Rk for "self bias" use 3 x 1K 2W parallel)
OPT primary 10K to (whatever speaker hope its got like 98dB SPL! LOL)

Yup, same op points I listed above.

[kenpeter]

Four or Eight 6AU6 in parallel might get it done.
With a heap of sockets, more filaments to heat,
and extra wiring to keep the noise out of...

Or one in Triodlington with a bipolar power transistor
of fairly low beta, Darlington or Sziklai configured.

Abusing the Bipolar to boost the plate or cathode
current of the 6AU6. Abusing the Triode strapped
screen to grant both components of the pair "Mu".
And to provide a runaway control for the bipolar.

You can do the same with a MOSFET and a small
resistor across the gate to drain. But a nuisance
to bias this pair such that the Triode always cuts
off first. Which is only an issue if you care about
AB. Works just fine as-is for class A purposes.

If you want to drive G1 of the 6AU6 into A2, then
perhaps the Sziklai version makes better sense?
Darlington version might be confused by the grid
leak, and any leakage error would be multiplied.

[Theodore]

Originally posted by TGRANT
single ended amp using the 6AU6 as the final tube connected to the output transformer.

What plate supply voltage and what transformer impedance are you using?

With a small signal pentode like this, you'll need a much higher impedance load than with a power pentode. For a 250V plate supply, and 150V screen, you're looking at something like 18k. (And about 1 watt out) It could be done with a lower impedance, but you'll get even less power out of it.

[TGRANT]

What plate supply voltage and what transformer impedance are you using?

The plate supply is about 300V. The transformer is about 6K with an 8 ohm speaker.

Actually I think you have all answered by question. It sounds like the tube isn’t designed to develop any kind of power, and I think I’m overdriving it with the first two stages. I’ll try the suggestions though and see if I can get it to work.

I think I might scrap the 6au6 and try playing with a 6v6 or something like that.

I’m new to tubes, and my goal is to play and experiment and learn with what I have, hence the 6au6.

I was using a 1K grid resistor and had abouut 10mA plate current, so I think the bias was wrong afterall.

[Theodore]

I wouldn't necessarily write off the 6AU6, just don't use it with that output transformer. It's true though, that you won't get more than about 1.5 watts in any single-ended setup and probably won't get more than about 4 watts in push pull.

You're better off using it as an input stage. I'm building an amp right now with just that: http://prairieboxes.blogspot.com/200...kazu-12pp.html

[coldcathode]

I’m new to tubes, and my goal is to play and experiment and learn with what I have, hence the 6au6.

TGRANT,

I probably have a couple dozen 6AU6's maybe more. They were pretty common in old car radios I think.

I suggest playing around with what you have.

If your a "newbie" and on a limited budget (like myself) the 6V6 is not a bad choice.