50 watt build...low voltage V1 V2 V3

[kevrockitt]

Hey guys...50 watt build. Low voltage readings, about 50%, across all three pre tubes.
Fired it up and played for just a few moments and power tubes got really hot. Power tube readings are
good at all pins. Any ideas?
Thanks!
Kevrockitt

[Roe]

we need more readings. measure voltage and bias

[kevrockitt]

Just went to check readings again. They seem to be creeping up slowly with the amp powered up.
I am using a bunch of NOS caps. Any thoughts?
Bias is at 38. And power tube readings are right on target.

[kevrockitt]

Power tubes are still raging hot. Going to let it cool off for an hour or so and then recheck all readings.

[kevrockitt]

Ok...after a good cool down here are the readings.
V1 pin 1= 127 V2 pin1 = 139 V3 pin1= 190
pin2= pin2= pin2=
pin3= .77 pin3= 1 pin3=35
pin4=3 pin4=3 pin4=3
pin5=3 pin5=3 pin5=3
pin6=163 pin6= 145 pin6=185
pin7= pin7= pin7=
pin8=1.6 pin8=145 pin8=35
pin=3 pin9=3 pin9=3

Dc voltage all around is good. All pins on power tubes read right on.
Bias is weird...V4 pin 8 is way higher than V5 pin 8. Almost 50% higer and climbs the more you leave power to the amp.
V5 bias reading stays constant.
Tubes are new and matched. Bad valve maybe?

[toner]

Preamp plates are a little low. For V2 pin 6 I assume you meant 245, not 145. Also looks like you skipped V2 pin 7, which should be the same as pin 1 (139V).

What are your power tube plate voltages (pin 3) and bias? Is this a Metro kit or at least the same layout? Did you use 10k and 8k2 for the first voltage droppers (right next to NFB resistor)?

Regarding the bias, make sure pin 5 of both power tubes have the same negative voltage. Try swapping V4 & V5 around to see if the same tube has higher bias current or if there's something up with the V4 socket or 1 ohm bias resistor. The climbing V4 current could be pulling your preamp voltages down.

[kevrockitt]

Toner...I did mean 245 not 145 for V2 pin6. And yes V2 pin 7 is the same as V2 pin1.

It is a Metro type layout. I did use an 10k and an 8k2 as droppers.

The bias is at 38. Both V4 and V5 pin 5's read -34.

I did switch the power tubes around and the problem followed the tube.
Should I assume just a bad tube?

[toner]

Does the bias reading on the hot tube ever stop climbing when sitting idle? I've never seen one do that. It sounds like a bad tube.

What brand tube is it? Just curious. I've read about recent JJ EL34's failing.

[kevrockitt]

Interseting that you say that. It is a JJ. E34L. Slightly hotter than an EL34. Been using them alot and never had a bad one.
First time for everything though. I will change it out and see what happens. I will let ya know. It is weird though...it rose like a thermometer slow and steady.

Thanks again for all the help and input.
Kevrockitt

[kevrockitt]

OK..here is the update. Changed the tube and all appears to be well. I will play it for a few more hours and monitor the settings as I go. Again thanks for all the input.

Everyones help and ideas are very much appreciated!
Kevrockitt