B+ Plummets When Power Tubes Warm Up

[The New Steve H]

I got my new amp all rigged up and ran the smoke test. It doesn't explode, but the B+ goes from the more-or-less-correct 271 V down to ~170 when the power tubes heat up, and there is a ton of hum and not much amplification.

What is the most likely problem? This thing has parallel OTs and 4 output tubes, and it's based on the Little Wing, which has 2 output tubes and one OT. I just duplicated the output section and ran everything off the coupling caps.

The hum was expected, although I did not expect it to be the main sound the amp made. I left a lot of long wires in there so I could get the amp running and then trim everything later. Because I was afraid I would have to make changes, I didn't want to take all the slack up yet.

[diagrammatiks]

why do you have the output transformers paralleled?

[The New Steve H]

Well, I have four output tubes instead of two, and I didn't want to run them off one OT. I was advised that there would be an impedance issue.

[The New Steve H]

I assume current is squirting out someplace, but I haven't found it.

[diagrammatiks]

who made your power transformer? did you check your bias one the tubes?

you used the exact same specs as the power transformer for the little wing?

did you use a 5y3?

[The New Steve H]

Heh heh. Open solder joints on the power caps. I found the answer in an old post here. I read it, and I thought, "I know I didn't do anything that stupid." Then I went and looked.

I tack things down while I'm building an amp, just to hold things in place, but I try not to do more soldering than I have to because things may come up later, and I don't want to spend all day sucking out solder. I guess I missed these joints when I was finishing up.

Now the amp runs, and the hum is gone, but the volume is pretty low. Back to work. And I still have to adjust various voltages.

[The New Steve H]

The PT is not the same. It's a bigger one.

http://www.classictone.net/40-18085.pdf

[The New Steve H]

It seems like the output section is not doing its thing. There's plenty of distortion on tap, so I think the preamp is working, even though the voltages haven't been adjusted upward yet. The voltages on the output end are where they should be, so I can't figure out why the output is low.

I switched the wires around on the OTs to make sure I didn't have them reversed, but it made no difference.

On the up side, the amp is now extremely quiet, even with fluorescent lights above it and all sorts of wires hanging out.

[diagrammatiks]

what are you using for a rectifier?

the 5y3 can only supply 140ma max.

[The New Steve H]

I'm using a 5Y3, but I have switched a 5AR4 in, and it doesn't improve things.

I think the output voltages are off. I'm not too sure what the relationships should be.

Here's what I got, with the schematic values in parentheses.

B+: 292 (275)
Plates: 293 (275)
Screens: 272 (249)
Cathodes: 28.5 (23.5)

I notice the difference between the plates and screens is a little lower than expected (21 v. 26 volts). I fiddled with the cathode resistor, and it actually seems to sound better with the voltage a little higher than 28.5, so I don't think dropping the cathode voltage will help.

I know it can make noise, because I got a loud squawk while I was playing with a screen resistor. I thought maybe that meant I should increase the values of those resistors to lower the screen voltage. I raised them from 4.7K to 10K, which lowered the screen voltage to around 260. Not a big improvement in sound.

I don't think the power supply is worth worrying about, since I got that loud sound out of the amp.

The preamp voltages are low, but I don't think that's causing the output problem.

Preamp voltages:

V1 pin 1: 133 (150)
V1 pin 6: 136 (150)
Positive supply: 277 (325)

V2A cathode: 1 (1.5)
V2A pin 6: 161 (180)
v2B cathode: 162 (180)
V2B plate: 276 (325)


V3 plate pin 6: 178 (230)
V3 cathodes 30 (54)
V3 plate pin 1: 194 (235)

The higher than expected B+ is in line with my area's 125V AC, so I think the PT works fine.
V3 grid pin 2: 18 (22-23)
V3 grid pin 7: 20 (22-23)

[Structo]

What do you have for grid stoppers on the power tubes?

If you are too low in value you can get blocking distortion that will cut out the signal.

I'm not sure what power amp you are running in this thing but try a 5.1K on each grid in series with each grid, near the socket.

1/2 watt is fine.

[passfan]

Transformers have a polarity. You could be bucking it even though you swapped some leads. I've seen some dual OT designs on Schematic Heaven in the bargain bin amp section. Might be worth a little detective work to see if that's it.

[passfan]

Here is how Gretsch did the Super Bass.

http://www.webphix.com/schematic%20heav ... ch6157.pdf

[The New Steve H]

I have read that the heaters on 6BM8s have to be biased relative to the cathodes in certain circumstances, but I am having trouble finding information. I don't know if it's necessary when the triodes aren't being used.

Last night I found that the cathode voltage was not steady. Pin 2 went from 23 to over 200 volts, shooting lots of current through the cathode resistor. It's as though the screen voltage is being transferred to the cathode.

If you look at the output on the original Little Wing, you'll see all the resistor values I'm using. Up to the coupling caps, it's a Bassman, but to the right of the caps, I went with the Little Wing values.

[The New Steve H]

I don't understand the heater bias thing. The heaters have to be within 100 volts of the cathode. If the heater has a center tap to ground, then the voltages will always be less than 4 volts from zero. The cathode is between 20 and 30. I don't see how this can be a problem.