Blackface Bandmaster; power sags abrubtly at times

[Modorange]

I got this Bandmaster and was trying to get it working well enough for my son to use it. The tubes in it were apparently original RCA, power and preamp tubes. Unfortunately when we hooked it to the speaker cab it sounded fine at first but then started moaning after he played a cord.. Barely moving the amp caused more noise. I tapped on the power tubes and one of them created more howling and noise. In fact this one tue was so microphonic that I could just speak into it and there would be sound coming out the speakers as a result.

So we put some EH power tubes in it. The howling went away. Another problem was when he played a power chord with pick attack, or plucked strings with the pick, some static distortion or popping was heard. The newer tubes alleviated the problem for the most part, but another problem was observed:

When volume is up, and he plays a power chord, volume dips to around half the gain before.. Then after he's playing for a few more seconds, it jumps back up abruptly. Any ideas about what could be the problem? It is an old amp and possibly needs caps and/or could have some dirty contacts in the tube sockets although I blew them out with contact cleaner and put the tubes all back in place. Another remote possibility is the preamp tubes are not in the correct order.. I just thought I'd post this on this forum first, before taking it to the local tech. Thanks for any knowledgeable input up front.

BTW The speakers are replacement speakers that I wired into the 212 cab, 4 ohms.

[Jazz P Bass]

Blackface Bandmaster

Does it matter which channel that you plug into.
Normal or Vibrato?
I have attached a schematic for tube placement.
I don't know if this is your exact model electrically, but the tube layout should be the same.

[Modorange]

That is the model. We're going back over to the studio to try out both channels,, hopefully later in the day.. It was primarily on the vibrato channel, although I haven't got a footswitch yet so I jumped the switch jack and taped the ground end of the little wire to the chassis. Overall an incredible-sounding amp. I'll check tube placement--I have a printed schematic..

[Jazz P Bass]

Normal Channel

Can you try it on the Normal channel.
If it does not do it on the Normal channel, that would narrow it down somewhat.

[Modorange]

I've tried both channels and the problem is not more pronounced on either channel. Also if you tap on the amp a popping sound occurs. The amp is plenty loud but every now and then a pop happens and then it gets slightly quieter; then a few moments later, another static "pop," and then it gets louder again. Otherwise the sound of the amp is great.

BTW I powered the amp up and left it on with volume up around 6-7 and turned the guitar down, and there is no noise at all.

[Jazz P Bass]

Easter egging

To try & tell if it is in the preamp:Non techy style.
With the amp on, lightly tap each tube with a pencil.
This may tell you immediately which tube is not happy.
If not, start Easter Egging.
Take out the Vibrato 12AX7. The 3rd one in towards the power tubes.
Plug into Normal.
Remove the fourth tube. 12AT7 phase inverter.
Put the Vibrato 12AX7 in there (I know it is the wrong tube)
Plug into Normal.
Problem should be gone.
If so, the 12AT7 is bad.
If not, time to get techy.

[ThermionicScott]

I wonder if there's some ultrasonic oscillation going on, triggered by strong signals. Does this Bandmaster have grid-stoppers on the power tubes?

- Scott

[Modorange]

To try & tell if it is in the preamp:Non techy style.
With the amp on, lightly tap each tube with a pencil.
This may tell you immediately which tube is not happy.
If not, start Easter Egging.
Take out the Vibrato 12AX7. The 3rd one in towards the power tubes.
Plug into Normal.
Remove the fourth tube. 12AT7 phase inverter.
Put the Vibrato 12AX7 in there (I know it is the wrong tube)
Plug into Normal.
Problem should be gone.
If so, the 12AT7 is bad.
If not, time to get techy.

I'll try this.. I'll get back w/in a day. Grid stoppers? Good question. First I've heard the term used. If they had them what would they look like? An extra resistor or something?

[Bruce / Mission Amps]

Just some more thoughts:
Check the grounding bare wire(s) from the eyelet board to the brass plate under the pots.
In older amps like this, sometimes the solder joint get oxidized and the wire is loose where you can actually pull the wire right out of the solder joint with a good tug... or even broken off.
If it does this with both channels I'd suspect the phase inverter ground at the NFB 820 ohm and 47-100 ohm resistor (which should be grounded through one of those bare wires).
Another thought is the 1w 470 ohm screen resistors on the power tube sockets... again bad solder joint on the socket lugs or even an overheated screen resistor going open with heat.

[Modorange]

Thanks.. I'll check these things.. They could definitely be a factor, since a similar popping sound happens when the amp is tapped, (although I didn't check to see if volume loss happens at the same time). Overall I feel pretty lucky that there's not more wrong going on. The sound of the amp is otherwise fantastic.

[Modorange]

OK...First off Thanks to everyone who responded. (Sheepish grin) The problem, I believe was due to operator error

I didn't mention a key element of the amp that I had done when trying it out. I took a small wire and stuck it in the vibrato pedal RCA jack, and then taped the other end of it to the chassis. This was to activate the vibrato and play it "on" or to turn the intensity up or down and then play it, just for testing it.

A lightbulb went off in my head(!) and I disconnected the wire. Problem solved. I will get either an rca plug and jump it or a proper footswitch and test it later, but I think the vibration of the amp playing and or when I tapped on the top of it, the wire would lose connection and this caused the vibrato circuit to come in and out of the signal path, thus causing the sudden lowering of the gain with the popping sound.

I had my son play his guitar into the amp last night without the wire and it sounded loud and proud. Still may need to check the sound of it with the vibrato footswitch connected, and there may indeed be a loose solder joint or two in there.