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Yes, the chassis is ground, but the bias supply is providing a negative voltage to the grids of the power tubes to keep them biased. Since all the voltages involved along the bias rail are lower than ground (negative), you must keep the positive end of the caps on the high side (ground) and the negative end of the caps on the bias rail.

The - designation on the cap doesn't automatically mean that end always goes to ground. It just means that end must be at a lower voltage potential than the end marked +. We're so accustomed to working on circuits where all the voltages are above ground, we assume the - end of the cap is always towards ground.

I changed that cap around but now the voltage is even higher it pins my analog meter on the 10volt scale and is still positive, maybe I damaged the pot?

If you have a digital meter, it should show a negative voltage at pretty much any point along the bias supply circuit. Clip the black lead to the chassis.

If you are using an analog meter, you will need to clip the red lead to chassis. Probe the bias circuit with the black lead. You should see voltages from about 75 volts at the diode, to 67 volts at one lug of the bias pot, to 42 volts at the other lug.

Maybe the schematic helps?

I got my DVM out and it is a negitive voltage of 70 volts at the #5 pins on the tube sockets.

the voltage at the pot is 74 on power side and 47 on the ground side 70 on the center with it turned full counterclockwize no tubes ,do I plug the tubes in to get the voltage at the bias spot on the preamp socket?

FATMAN wrote:the voltage at the pot is 74 on power side and 47 on the ground side 70 on the center with it turned full counterclockwize no tubes ,do I plug the tubes in to get the voltage at the bias spot on the preamp socket?

Sounds perfect. Yes, you'll need the power tubes in before you will get a millivolt reading at the bias test pin on the preamp socket.

Did you check the 10 ohm resistors? Are they still reasonably close to 10 ohms?

Yes I checked the 10 ohms they still seem fine, should I replace them anyway? I have some extra's should I try it again with the tubes? do you think I damaged the tubes?

If they're 10 ohms, then they're 10 ohms. I wouldn't worry about them.

I'm sure the tube's plates and cathodes are fine. It's not good to run them at 200 watts (400 mA * 500 volts) but a half minute probably didn't kill them. I'd be more concerned about the screens, and what might have happened to them.

It might be good to check the output transformer to make sure you didn't burn the primary winding. Just measure the resistance with the ohmmeter from each lead (blue, green, green/white, and blue/white) to the center tap (the red one which goes to the SDS board). Keep the amp powered off and unplugged while you do this. Compare the readings you get to your other output transformer. The blue and blue/white leads should measure a couple hundred ohms to the center tap. The green and green/white should be a little less than half of the blue and blue/white. It is normal for Dynaco output transformers to have asymmetrical DC resistances - the blue will not be the same as the blue/white, nor will the green be the same as the green/white.

In all likelihood, if the 10 ohm resistor didn't fry then everything else is probably OK too.

OK i will try it again, hope for the best. Thanks for all your help much appreciated.

SWEET SUCCESS!!! It biased out nice this time and no damage due to the one reversed cap. I have it burning in now on a cheap speaker,it sounds good so far but i will know better after hooking it to the horns. No hum or noise. Iwill measure the wave form after I borrow a scope and signal generator. Thanks for all your help, I don't think it would have been this easy with out your help, Best Regards Andrew

Yay! Good job. I love happy endings.

I will make the second one tomorrow and give them a good crank on the horns then try the triode mode to see if it is still enough power. All the best, Andrew

TY just woundering about testing it I want to put it on a scope and signal generator and I need to know what the output voltage should be using a 8 ohm resistor on the output, and what the imput voltage should be to reach maximum output. What should the frequency response be?
thanks Andrew

If you feed it a sine wave, a 60 watt amp should make about 31 volts zero-to-peak. (31 volts / sqrt(2))^2 / 8 ohms = 60.1 watts. Make sure the load resistor is beefy. It's gonna get hot.

I'm not sure on the exact input voltage required to drive the amp to clip. It's probably somewhere around a volt, give or take. I'd look at Shannon's assembly manual for his ST70 board. The Poseidon is really half of his ST70 board (or is his ST70 board two Poseidons?). There is a comprehensive set of graphs and test data at the back of the manual.

Edit... has anyone noticed the Test & Measurement data looks kinda funny? For example, on page 12 the first line shows 20 Hz, 17.31 volts = 37.45 watts @ 8 ohms. By my math, that should be 18.73 watts instead. Am I doing my power calculations wrong, or am I reading the chart data wrong?

Well, I got the second one together it took about 8 hrs start to finish It went with out a hitch,biased and balanced up quickly. I hooked them up to the horns and that is the sound I was hoping for. I think I will leave them in the UL mode for now I love the power they are very solid sounding they sure wokeup the horns. I will burn them in and then test them. so is there a kit for the 120 watt...