Metropoulos Forum

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

we need more readings. measure voltage and bias

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.

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

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?

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.

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?

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.

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

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