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.
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.
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
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
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??
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??
Attachments
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.
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.
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.
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.
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/2008/11/teaser-lama-kazu-12pp.html
You're better off using it as an input stage. I'm building an amp right now with just that: http://prairieboxes.blogspot.com/2008/11/teaser-lama-kazu-12pp.html
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.
TGRANT,
send me an email, I have a few "oddball" *V6's I will send you to play around with along with some more 6AU6's. Just need you to cover the shipping (couple $$).
My wife wants me to get rid of "some of damn light bulbs" from the basement.
Anyone interested in a couple 1616 H.W. Rectifiers?
5500V 800mA?
I have some really oddball stuff.
send me an email, I have a few "oddball" *V6's I will send you to play around with along with some more 6AU6's. Just need you to cover the shipping (couple $$).
My wife wants me to get rid of "some of damn light bulbs" from the basement.
Anyone interested in a couple 1616 H.W. Rectifiers?
5500V 800mA?
I have some really oddball stuff.
And this is the triodlington setup I tried to describe.
Sort of...
I ran into some issues with the transistor, mostly
cause I limited myself to those with spice models.
That stuck me with waaay to much HFE. +30 times
whatever the 6AU6 would conduct all by itself...
Sorta like 31 6AU6 in parallel, but not exactly.
Transformer primary impedance ends up about 533
ohms this way, which is lower than I was trying for.
Really needed a spice model for a horizontal sweep
or dynamic focus transistor with HFE around 10....
That would be a much better match for the 6AU6.
-------------------------------------------------
I also cheated and ran the 6AU6 in UL, rather than
as a triode. Mostly cause I needed to get both the
voltage gain and impedance up. Without completely
killing the feedback loop that allows the transistor
to share "Mu", and work as a voltage amplifier.
But the transistor needs as much governance and
control from the tube as it can get. Strapping as a
Triode would be preferable to UL, if the impedance
would just cooperate. Which again was a problem
than HFE was just too high in this example.
I might play more with this circuit sim at 300V and
see if I can trick it into working at a bit more
reasonable impedance. Or with a more suitable
bipolar.
------------------------------------------------------
The 6AU6 model was R McLean's.
The MJE13003 was a Motorola model.
I used a Tetrode symbol because the
spice model uses only 4 connections,
and complains when there's a fifth.
We all know there is a supressor.
----------------------------------------------------
2VRMS in gives 20VPP out, into 8 ohms...
Whatever Wattses is, I forget the rule?
Sort of...
I ran into some issues with the transistor, mostly
cause I limited myself to those with spice models.
That stuck me with waaay to much HFE. +30 times
whatever the 6AU6 would conduct all by itself...
Sorta like 31 6AU6 in parallel, but not exactly.
Transformer primary impedance ends up about 533
ohms this way, which is lower than I was trying for.
Really needed a spice model for a horizontal sweep
or dynamic focus transistor with HFE around 10....
That would be a much better match for the 6AU6.
-------------------------------------------------
I also cheated and ran the 6AU6 in UL, rather than
as a triode. Mostly cause I needed to get both the
voltage gain and impedance up. Without completely
killing the feedback loop that allows the transistor
to share "Mu", and work as a voltage amplifier.
But the transistor needs as much governance and
control from the tube as it can get. Strapping as a
Triode would be preferable to UL, if the impedance
would just cooperate. Which again was a problem
than HFE was just too high in this example.
I might play more with this circuit sim at 300V and
see if I can trick it into working at a bit more
reasonable impedance. Or with a more suitable
bipolar.
------------------------------------------------------
The 6AU6 model was R McLean's.
The MJE13003 was a Motorola model.
I used a Tetrode symbol because the
spice model uses only 4 connections,
and complains when there's a fifth.
We all know there is a supressor.
----------------------------------------------------
2VRMS in gives 20VPP out, into 8 ohms...
Whatever Wattses is, I forget the rule?
Attachments
I just checked with 6K primary impedance.
Change the emitter resistance to 150ohms,
else you have pointlessly high idle current...
16VPP@47K in gets you ~16VPP@8ohms out.
Neither the transistor nor tube is cooking
itself anywhere near full potential. Thats
like 4 Watts out. Er something like that...
6K OPT needs to handle 60mA bias current.
With the resistor change mentioned above.
-----------------------------------------------------
6AU6 sounds nice in pre-amp duty too.
Have one in my (1942?) Voice of Music,
and I wouldn't change a thing!
Change the emitter resistance to 150ohms,
else you have pointlessly high idle current...
16VPP@47K in gets you ~16VPP@8ohms out.
Neither the transistor nor tube is cooking
itself anywhere near full potential. Thats
like 4 Watts out. Er something like that...
6K OPT needs to handle 60mA bias current.
With the resistor change mentioned above.
-----------------------------------------------------
6AU6 sounds nice in pre-amp duty too.
Have one in my (1942?) Voice of Music,
and I wouldn't change a thing!
Some low, constant Beta, HV transistors for Ken's setup:
FJP5027 NPN 800V 50W hfe 15-30 or 10-20
FJP5321 NPN 500V 100W hfe 15-40
BUT11A NPN 450V 100W hfe 10-35
KSC2752 NPN 400V 10W hfe 30-60 or 20-40
KSA1156 PNP 400V 10W hfe 30-60 or 40-80 or 60-120
ST93003 PNP 400V 40W hfe 16-100
BUL3P5 PNP 400V 60W hfe 18-40
MJE5731A PNP 375V 40W hfe 30-150
MJE5852 PNP 400V 80W hfe 15-150
Can also lower the Beta (hfe) by putting a resistor between the base and the emitter, or to the bottom of any emitter resistor)
Don
FJP5027 NPN 800V 50W hfe 15-30 or 10-20
FJP5321 NPN 500V 100W hfe 15-40
BUT11A NPN 450V 100W hfe 10-35
KSC2752 NPN 400V 10W hfe 30-60 or 20-40
KSA1156 PNP 400V 10W hfe 30-60 or 40-80 or 60-120
ST93003 PNP 400V 40W hfe 16-100
BUL3P5 PNP 400V 60W hfe 18-40
MJE5731A PNP 375V 40W hfe 30-150
MJE5852 PNP 400V 80W hfe 15-150
Can also lower the Beta (hfe) by putting a resistor between the base and the emitter, or to the bottom of any emitter resistor)
Don
Kuroguy, that's a cool little amp! 😎 I especially like the meters; I'm assuming they're VU indicators.
Could you post a schematic? I've got hundreds of 6AU6's, and I was thinking of selling most of them at a hamfest or something 'til this thread appeared...
Could you post a schematic? I've got hundreds of 6AU6's, and I was thinking of selling most of them at a hamfest or something 'til this thread appeared...
I don't really have a schematic. I salvaged most parts from a reel to reel. The schematic for the reel to reel is at http://www.tubesrit.com/audio/index.php?cat=5 . It is loosely based on that schematic.
Edit: Actually a cross between the r2r schematic and the radiotronics amp schematic also on that site.
Edit: Actually a cross between the r2r schematic and the radiotronics amp schematic also on that site.
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