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dscottguitars 14th August 2011 11:59 PM

power supply voltage
Hello all,

I built a bassman clone, aa864-phase inverter and power stage only- and a custom preamp. I have much higher voltages than I expected, with the transformer voltage: 350-700-350. I have another one with the same resistor setup and the voltages on this one are about 70v higher to the preamp and 40v higher to the phase inverter. The 6l6 plates are 425v, which is great. I have more than the 330v rating on the 12AX7's and I know that is too much.

Is this a current issue? I am assuming the higher voltage is a higher current rated transformer, but I'm not sure.

Should I just increase the first resistor to drop the entire rail?

Any advice would help, thanks...


razorrick1293 15th August 2011 12:26 AM

you mean 350-0-350? 400v on a 12AX7 is fine, from experience they start dying at around 450v. current is a big thing in HT voltage. as long as it sounds fine and components can cope id say you wouldnt need to do alot unless youd really get a big difference in tone.

only time ive been worried about HT too high is when my 0-350v transformer was giving me a HT of 500v with an idle current draw of 80mA... at an idle of a few mA voltages hit about 540v. that blew a few caps...

kevinkr 15th August 2011 01:08 AM

:cop: I'm going to move this to instruments and amplifiers forum where it should get more input.

In terms of adjusting the voltages I probably would reduce the voltage as it probably is affecting the tone you are trying to achieve.

Can you post a schematic of your power supply and amp (hand drawn or out of copyright please) but you should probably adjust the resistor in the supply that feeds the phase inverter rather than the last one in the chain.

jjman 15th August 2011 01:53 AM

Increase the resistor feeding the PI node for the voltage on the PI and the preamps to drop. This assumes a typical series-type dropping configuration. In a AA864 Bassman this would be the 1k after the choke. To keep the PI close to where it is, while reducing the preamp voltages, increase the 4.7k that feeds the preamp instead. Each "dropper" drops voltage on all the nodes it feeds as well as "down" the rail. It's a balance act. You can use a PS designer (like Duncan's) but real world trial and error may be easier since you've already built it.

I would try a 3k in place of the 1k as a starting point crap shoot. If the PI is then good but the preamp is still a little high, proceed to the 4.7k and maybe try a 10k. Assuming that's where you are starting from........

dscottguitars 15th August 2011 04:26 PM

5 Attachment(s)
Thanks all..

Kev: I didn't realize there was a better forum spot, thanks.

Here is a schematic of the amp and one of another that works perfectly. With the "same size transformers" the voltages are quite different. I guess I'll need a larger first resistor. I used a 400 ohm instead of a choke.

Is there a way I can test the transformer for how much current it has? I'd like to see how much difference there is between the two, but my knowledge and experience are limited to what I've read...

Thanks again...I love this forum!

Correction: The PI is a 12AX7, not a 12AT7.

I plugged in a Presonus tube preamp into the power amp and it sounds fantastic. (I'm still working on the preamp switching and stuff. Here is a picture of that...

indianajo 15th August 2011 06:15 PM

Cool, a tube PA amp. I've used a ST70 for PA (keyboard in churches) with better reliability than my diy-repaired ST120 (no schematic) at the time. Same volume or more than transistors at bigger numbers.
You can test your transformer by loading down the winding with successively smaller high wattage resistors. You measure the voltage across the resistor. I=V/R Power=V*I. When the power starts dropping pretty fast you have reached some sort of limit. Then you hook up the resistor that you think is okay and run it for a while. If the transformer gets hot in an hour you guessed wrong.

Triodethom 15th August 2011 06:24 PM

On a bigger chassis I would use a choke as well as film caps from some one like Angela instruments asc brand it does change the sound of the amp however . The color of the chassis is cool what is that robin's egg blue ? If you like now stay with this setup and play with the first resistor.

dscottguitars 15th August 2011 06:38 PM


Indi: I'm curious why you said PA amp? I kick myself for not thinking about your test, I know that already. Duh. Thanks for reminding me.

Trio: What caps are you referring to? And what does a choke do different? The color was an accident. I got the blue/teal color but couldn't keep it from crinkling so I sprayed dark red and black on paper and used a tiny brush to shake the spots all over it.

BTW: this is really quite too.

dscottguitars 16th August 2011 08:48 PM

To all: I found a problem that brought the voltages down. The cathodes on two preamp tubes were not grounded. When I did this, the voltages came down to values I'm used to seeing. I have sound through the amp, but it's very low. I think it may have to do with the foot switch, but I'll figure that out, hopefully.

Thanks for everybody's input.

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