Need Help

[chard]

Put my stereomour kit together. The resistance checks are good. The A side 2a3 Is drawing too much current. Pin 5 is reading 167vdc, pin 1 is reading 289vdc. The A side 2a3 cathode resistance reads 2.2k, when I divide the 167vdc cathode voltage drop by the 2.2k cathode resistance I get a cathode current for the A side 2a3 of 136ma, this is way more then the 50ma the tube is supposed to draw. The other tubes check out as expected. I switched 2a3 tubes to see if the problem followed the tube and it did not. I reflowed the solder joints around the cathode circuit for the A side 2a3, it did not help. Does anyone have any suggestions on what may be wrong?   

 

[chard]

I don't know why but the voltages are now reading right. The only thing I did was switch 2a3's to opposite sockets a couple of times. The plate voltage is a little high at 383vdc compared to the desired target of 350vdc. The tubes are also biased on the high side. A side 2a3 cathode voltage drop of 63vdc divided by the measured cathode resistance of 1235 ohms equals current flow of 51ma x (383vdc-63vdc) or 320vdc equals 16 watts. The b side 2a3 has less cathode resistance therefore it draws more current and and is biased even higher at almost 17 watts. I think a 2a3 is rated at 15 watts and 300vdc max.. Will running my tubes on the hot side be a problem? As the amp breaks in what are the chances of the plate voltage comming down?  If I have to I can change the 360 ohm and 1.2k ohm pairs on the dc power supply rail to 820ohm 10watt resisters this should get my plate voltage to about 350vdc and get my dc plate dispensation to about 14-15 watts. 

 

[chard]

Got my amp up and running it sounds very good, real smooth. Don't think I'll make any changes because if I try to lower the plate voltage to the power tubes this would also lower the plate voltage of the 12at7. The 12at7 has plate voltages around 190-195 and that's probably better then around 160. The power tubes seem to be find disipated a little high, no sign of red plating or anything. So everything is fine.

 

[Paul Joppa]

The high cathode voltage initially suggests that the grid was not grounded through the 249K grid resistor. Since swapping tubes fixed it, it may have been just dirty pins, cleaned by re-installing tubes a couple times. But do check the 220 ohm grid stopper at pin 3 (they are a little fragile) and the related solder joints, just to be sure.

The tube ratings are design nominal and have some allowance for individual variations - normally 10%. In your case, I expect your power line voltage is a bit above the nominal 120 volts. The supplied tubes are actually capable of more power, 22 or 23 watts being widely quoted.

If you have especially fragile or rare tubes and want to apply a lower stress to them, you can operate them as if they were 45s. At the other extreme, there are a few 300Bs with 2.5v filaments - the JJ 2A3-40 is popular - which can be operated very conservatively as 2A3s.

The driver is loaded with a constant-current source, so its plate voltage will not change with the power supply voltage. The plate voltage may drop during the first 100 hours of operation as the cathode forms more fully - that depends on the individual tube though.

 

[chard]

Thanks Paul for your invaluable insight. I will be looking closely at the grid resister and grid to ground connection. I won't worry about the higher then expected plate dispensation. 

 

[chard]

Looked around the grid circuit of the A side 2a3 and found that a 1 inch metal leed from one of the components hadn't been cut off after the component had been soldered in. It looked like it belonged there until I looked really close and found it had no purpose. It must have been touching somewhere it shouldn't have and then by pulling and reinserting the 2a3 it was disloged from its unintended contact point and the amp worked. I have cut the leed and amp has worked perfectly for about 3 hours. Cd's sound great and HD TV sounds like I'am there I can clearly hear everything thats going on, all the incidental sounds.  Thanks Paul for pointing me in the right direction.