About the 6AU6s

Mosquito · 11th November 2009, 11:19 PM

Hi, everybody. I need some advice about the 6AU6's. I mean if some particular brand and/or submodel are preferable.
The intended circuit to build is a standard pentode voltage amplifier (after Rozemblit):

Geek · 12th November 2009, 01:56 AM

Hi,

IME, R1 and R2 are too low... try R1 ~ 220K and R2 ~ 470K-1.5Meg. C1 is way too high, it'll take forever for the voltage to come up and the tube to begin emission. Try 0.1uF there.

6AU6 sonic characteristics are as wide and varied as the brand out there. Some hum, some are microphonic, some are brilliant full mids, some are flat as a transistor. You'll just have to "roll 'em"

Cheers!

Bandersnatch · 12th November 2009, 10:30 AM

Quote:

Originally Posted by Geek

Hi,

IME, R1 and R2 are too low... try R1 ~ 220K and R2 ~ 470K-1.5Meg. C1 is way too high, it'll take forever for the voltage to come up and the tube to begin emission. Try 0.1uF there.

Cheers!

hey-Hey!!!,
6AU6 runs ~30% of the plate current to the g2. With all pentodes, the load line is critically dependant on the g2 voltage. fortunately there is lots of the required data on various g2 voltage( as in plate current curve sets ). Choose the dropping R to the g2 in order to deliver your desired operating condition.
cheers,
Douglas

Geek · 12th November 2009, 10:45 AM

Heya Douglas,

That's why I said "IME", because I took several hours with a few on the breadboard tweaking them for best sound

Quote:

Originally Posted by Bandersnatch

fortunately there is lots of the required data on various g2 voltage( as in plate current curve sets ). Choose the dropping R to the g2 in order to deliver your desired operating condition.

That reminds me, this tube is covered under "Resistance Coupled Amplifiers" in the back of any RCA tube manual

Cheers!

Original Burnedfingers · 12th November 2009, 11:15 AM

Gregg,

Just looked at a schematic that had a 6AU6 as the input tube and it shows the same values that you mentioned. Is there a possibility of fudging the C1
value? I don't have any .1 at this time without ordering some but do have some .22 handy.

The values I have on my schematic are C1 .2, R1 221K, R2 1.5M R3 470K

Geek · 12th November 2009, 11:22 AM

Hi 6BG6GA,

Oh, that cap isn't critical. I'd say between 0.047 and 0.47uF is good.

Cheers!

revintage · 11:30 AM

Hey Mosquito,
Do you want your 6AU6 configured for max gain or lowest distortion and what undistorded outswing do you need if we are talking about the latter alternative?

Also Zin of next stage must be taken in account when finding the workingpoint.

Many variables but they must be known before final design of your circuit.

The Ua/Ia curves will be of help:

http://www.mif.pg.gda.pl/homepages/f...93/6/6AU6A.pdf

Mosquito · 12th November 2009, 12:10 PM

Hi again. I much appreciate all your answers. I think the circuit (as seen) is optimized for minimun distortion. Gain is less important in my application. ( I guess is around 120/150, if i'm not wrong). In my intentions it is not for musical instrument, but for home fi.
Triodeel has some Telefunken not so cheap as for take a bunch to select...

Mosquito · 12th November 2009, 12:18 PM

To Revintage: I'm not sure if it is possible, but I'll like to try some 6Y6s that I have, in self-split output stage.
It looks that they need some 40Vrms at the grids.
The source will be a CDP and nothing else.

revintage · 12th November 2009, 02:38 PM

Hey Mosquito,
That could be something, the 6Y6 looks interesting! Have you seen Shoogs version with EL86? Can try to help you getting it together.

12th November 2009, 11:27 AM	#7
revintage diyAudio Member Join Date: May 2007 Location: Eskilstuna, Sweden	Hey Mosquito, Do you want your 6AU6 configured for max gain or lowest distortion and what undistorded outswing do you need if we are talking about the latter alternative? Also Zin of next stage must be taken in account when finding the workingpoint. Many variables but they must be known before final design of your circuit. The Ua/Ia curves will be of help: http://www.mif.pg.gda.pl/homepages/f...93/6/6AU6A.pdf __________________ Brgds Lars Last edited by revintage; 12th November 2009 at 11:30 AM.

12th November 2009, 02:38 PM	#10
revintage diyAudio Member Join Date: May 2007 Location: Eskilstuna, Sweden	Hey Mosquito, That could be something, the 6Y6 looks interesting! Have you seen Shoogs version with EL86? Can try to help you getting it together. __________________ Brgds Lars

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12th November 2009, 01:56 AM	#2
Geek diyAudio Member Join Date: Sep 2004	Hi, IME, R1 and R2 are too low... try R1 ~ 220K and R2 ~ 470K-1.5Meg. C1 is way too high, it'll take forever for the voltage to come up and the tube to begin emission. Try 0.1uF there. 6AU6 sonic characteristics are as wide and varied as the brand out there. Some hum, some are microphonic, some are brilliant full mids, some are flat as a transistor. You'll just have to "roll 'em" Cheers!

12th November 2009, 11:15 AM	#5
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Gregg, Just looked at a schematic that had a 6AU6 as the input tube and it shows the same values that you mentioned. Is there a possibility of fudging the C1 value? I don't have any .1 at this time without ordering some but do have some .22 handy. The values I have on my schematic are C1 .2, R1 221K, R2 1.5M R3 470K

12th November 2009, 11:22 AM	#6
Geek diyAudio Member Join Date: Sep 2004	Hi 6BG6GA, Oh, that cap isn't critical. I'd say between 0.047 and 0.47uF is good. Cheers!

12th November 2009, 12:10 PM	#8
Mosquito diyAudio Member Join Date: Sep 2009 Location: Santa Fe, Argentina	Hi again. I much appreciate all your answers. I think the circuit (as seen) is optimized for minimun distortion. Gain is less important in my application. ( I guess is around 120/150, if i'm not wrong). In my intentions it is not for musical instrument, but for home fi. Triodeel has some Telefunken not so cheap as for take a bunch to select...

12th November 2009, 12:18 PM	#9
Mosquito diyAudio Member Join Date: Sep 2009 Location: Santa Fe, Argentina	To Revintage: I'm not sure if it is possible, but I'll like to try some 6Y6s that I have, in self-split output stage. It looks that they need some 40Vrms at the grids. The source will be a CDP and nothing else.

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