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Old 22nd March 2009, 07:17 PM   #1
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Default 6AU6A - Micropowered tube amp

Hi everybody,

I have a few Westinghouse 6AU6A tubes sitting around and was looking at building a low powered tube amp. I plan on connecting it in triode mode. I know there is probably better devices to do this with, but because I have these, I wanted to try to build something useable out of them.

Right now I am looking at the output stage only. I intend to use this as a guitar amp and want it micropowered so it doesn't get very loud when I crank it up. I also want to use only 1 tube for the output.

The datasheet on this tube says to use 250V for the plate voltage and 330 ohms for the cathode resistor. The plate resistance is ~7.5k ohms it says, so I would use a transformer with a 7.5k ohm primary when it has an 8 ohm load on the secondary.

I have found some information in some books on building Class A transformer coupled stages. Using this to crunch some information I have found that the power output would be ~0.25 watts if I move Ec1 between -1 to-7.

What I am asking is, is there a better bias point for this tube? The datasheet says the maximum plate voltage in triode mode is 275 volts. The numbers I have crunched show that it will peak at 300 volts if I pull the grid to cathode voltage down to -7.

Thanks.

P.S. - Datasheet
http://www.mif.pg.gda.pl/homepages/f...93/6/6AU6A.pdf
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Old 23rd March 2009, 12:04 AM   #2
ilimzn is offline ilimzn  Croatia
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Default Re: 6AU6A - Micropowered tube amp

Quote:
Originally posted by davidallancole
The plate resistance is ~7.5k ohms it says, so I would use a transformer with a 7.5k ohm primary when it has an 8 ohm load on the secondary.
Because a triode is not a real voltage source with an internal resistance equal to the plate resistance the usual rule of best power transfer when load equals source impedance will not apply. Usually the transformer primary impedance will be about 3x the plate resistance at your chosen operating point. Of course, you may woork out any desired load line using the trioded curves in the datasheet...
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Old 23rd March 2009, 12:18 AM   #3
kuroguy is offline kuroguy  United States
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This thread from two weeks ago might be of interest
http://www.diyaudio.com/forums/showt...46#post1761946
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Old 23rd March 2009, 01:29 AM   #4
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Thanks for the responses.

I read that thread the other day. I was kind of wondering if a 10k load would work better. I guess I will have to try and see.
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