B+ Plummets When Power Tubes Warm Up

[Firestorm]

Last night I found that the cathode voltage was not steady. Pin 2 went from 23 to over 200 volts

Check the solder joints in the cathode circuit.

[The New Steve H]

Can't find any problems with the joints.

[The New Steve H]

I took out a faulty PPIMV pot and bypassed the PPIMV. I stuck a couple of 220K resistors between the coupling caps and ground, but there are only two resistors, and there are 4 tubes, so I will need to fix that. Right now it's only rigged up so it functions. I was in a hurry, so I didn't finish it up right.

The cathode no longer runs away, and the voltage is set at 24V, but the amp is not developing the full voltage any more. When I switch it on, the B+ shoots up where it should be, but then it drops quickly to something like 240. I think current is leaking out somewhere, but I can't find any shorts.

The preamp seems to work perfectly. On the oscilloscope, I get a nice 3V wavy line at the coupling caps. It responds to the volume controls.

I don't think the OTs are getting enough juice, and I don't think the PT or rectifier are the problem, since you can run a Bassman off a 5Y3.

I have gone through the power caps, and they look fine.

[diagrammatiks]

bassman's run 5u4s or gz34s.

you can do some simple calculations to see if the rectifier is going to put out enough current.

a single 6bm8 draws around 35-40ma on the pentode side.

4 would need 160ma idle current to develop full b+.

a 5y3 is maxed at 125ma.

Use that 5ar4 you have.

[Jana]

And, the PT is only rated at 100ma.

[The New Steve H]

It is true that the 5F6A Bassman does not normally use a 5Y3, but as I said, you can run one with a 5Y3. It may not be right, but the amp will function, and some people use a 5Y3 because they think it gives more sag. I once stuck a 5Y3 in my 5F6A and forgot it was in there.

Changing the new amp's rectifier (5AR4) made no difference. Neither did running a single OT with two tubes. I tried all that. That stuff is a dead end.

I really do not understand these tubes. I picked the PT based on the wattage; it never occurred to me to look at the current, and it apparently didn't occur to anyone I consulted when looking for a PT. How they can suck all that juice and only come up with 3.5 watts is beyond me. Lesson learned.

Anyway, I don't think the PT is the immediate problem. The amp should still work until cranked to the point where the current limitation kicks in, right? I believe I should be able to get this thing to run, even if I need a bigger PT to perfect it. I assume that's right, because it's capable of making loud noises as it is. It just doesn't do it when running normally.

The B+ used to be 292, I think. I am not looking at the figure at this moment. Anyway, it was right where it was supposed to be, and the amp was doing exactly what it does now. While "fixing" it, I have apparently done something to make the B+ fall. I think there must be an unwanted current-sucking connection in there somewhere. Either that, or when the B+ was high, the current draw was somehow lacking.

I guess I should get a new PT, since I'm going to need one even if the amp starts to work.

Thanks for the help.

[The New Steve H]

Oh, hey: what about replacing the choke with a resistor? The choke size is not specified on the schematic, so I picked what I thought was right.

[diagrammatiks]

I didn't catch that link to the PT when you posted it before...

looking at the little wing schematic...

puts out 7.5 watts with a 100ma pt.

this datasheet http://www.r-type.org/pdfs/ecl82.pdf

puts typical operating conditions as 31ma per tube plate and 5-7 screens.

so that's about 80ma + the triode sections leaving a bit of room for over drive current draw.

If you use 4 you'd need at least 160ma + current on top of that for the triode sections.

If you try to draw significantly more current from a transformer then what it is rated for then the voltages will drop.

[The New Steve H]

Here's something crazy. The 6BM8 puts out way less power when the plate voltage goes above 200.

I do not pretend to understand datasheets, but I have one that says that in class A operation, the maximum dissipation per pentode goes from 7.5 watts to 5. Surely that means the current also drops. But why would it say "class A"? I don't understand why this wouldn't apply to push-pull.

Better not to get into it, I guess.

It looks like finding an optimal PT for this thing is going to be a REAL pain. I can't find anything with 550V and over 140 mA.

The triode sections are not wired up. The preamp is all 12AX7 and 12AY7.

Anyway, I think these problems are secondary. If not, the amp would work with one OT yanked.

[diagrammatiks]

does it work when you yank 2 tubes?

[The New Steve H]

It has never worked with 2 tubes yanked.

Here's something weird, though. I decided to turn it on and put in one tube at a time. It works when then tube closest to the PT is inserted. It doesn't work turned all the way up, but the loudness and sound are okay until around half-volume. The bass, treble, mid, and presence controls work.

So it works with any other single tube installed, right? Wrong. If I insert any other tube, all by itself, I get squeal.

Now I'm trying to figure out what's different about this socket. As far as I know, all the sockets are wired up the same.

I'm hoping I will find something wrong with the other 3 sockets, and then I can put a different PT in, change some resistors, fix the PPIMV, and move on.

[martin manning]

Here's your PT:
Hammond 370HX
Secondary: 275-0-275 V, 200 mA
Filament winding 1 : 5 VCT, 3 A
Filament winding 2 : 6.3 VCT, 6 A
Mounting centers: 2.5" x 2.94" vertical mount

[Structo]

If the amp squeals, temporarily disconnect the negative feedback (presence).

You may have the phase backwards and it is positive feedback, thus the squealing.

[The New Steve H]

It only squeals on THREE TUBES. Changing the wires makes no difference.

Thanks for the PT recommendation.

[The New Steve H]

Anyone who wants to kick my ass, please make an appointment.

It turns out I had the OTs wired wrong.

You have to run each end of the phase inverter to the same end of both OTs, and of course, I had them crossed. In other words, I had two tubes hooked up to each coupling cap, but one tube on each cap went to one end of the OTs, and the other went to the other end. I turned that around, so it's like a Twin Reverb, and now the amp plays.

It sounds excellent, with virtually no hum, even though the amp's guts are all over the place. My only concern is that the volume is lower than I anticipated. You can crank it all the way without making yourself deaf, and there is quite a bit of distortion. Maybe the PT is still limiting it? I'm using a single 10" 8-ohm speaker.

I can't put the PPIMV back in until I get a pot that works. Stay away from the 250K pots Triode Electronics has on sale right now. The shafts are very long, and one of the three I bought crapped out.