Bandmaster ab763. Thump or "waa" sound when standby is disengaged.

[Jimmybjj]

...on for a brief moment. I am relatively experienced so i don't know if this is normal. I got new power tubes and never rebiased the amp. I finally got around to doing this and found it was at around 7ma i bumped it up to 32ma @ 380v. Before i rebiased the amp there was no (or at least now enough that i remember) and now there is a noticeable thump, and a very slight hum (i don't know if this was there to begin with). The amp plays fine, its just the thump that bothers me. Is this normal? Thanks for any help.

[Chuck H]

Most BF Fenders pop or thump when the standby switch is flipped. The switches are under rated, but seem to last just fine, thumping away for many happy years.

But...

I do find it strange that a new set of tubes only drew 7mA of current without adjustment. Unless the bias was already off with the old set of tubes. What method are you using to measure bias?

The hum isn't normal. The amp should be relatively quiet unless something is wrong IME.

Chuck

[MWJB]

"I finally got around to doing this and found it was at around 7ma i bumped it up to 32ma @ 380v." This is most worrying, 380v is suspiciously low. Are you sure you don't have 480?

[GregS]

Is this normal? Thanks for any help

I recently restored one. It also had the thump you describe when flipping the standby switch so I would say it is normal. The only noise type sound after that was a slight hiss from the carbon comp resistors - no hum.

If I remember correctly the voltage on the power tube plates was around 458V so that 380V your getting sounds like a problem.

Greg

[Jimmybjj]

Most BF Fenders pop or thump when the standby switch is flipped. The switches are under rated, but seem to last just fine, thumping away for many happy years.

But...

I do find it strange that a new set of tubes only drew 7mA of current without adjustment. Unless the bias was already off with the old set of tubes. What method are you using to measure bias?

The hum isn't normal. The amp should be relatively quiet unless something is wrong IME.

Chuck

I don't know if it makes a difference but when i got the amp it had 6l6wgc rather than 6l6gc's. I am not very savvy when it comes to electronics, although i am making a great effort to educate myself. I am measuring with a bias probe into my meter from eurotubes.com.

"I finally got around to doing this and found it was at around 7ma i bumped it up to 32ma @ 380v." This is most worrying, 380v is suspiciously low. Are you sure you don't have 480?

I am sure. The highest it got was 405.

I recently restored one. It also had the thump you describe when flipping the standby switch so I would say it is normal. The only noise type sound after that was a slight hiss from the carbon comp resistors - no hum.

If I remember correctly the voltage on the power tube plates was around 458V so that 380V your getting sounds like a problem.

Greg

It is definitely a hum and not a hiss. Is there somethings that you guys recommend checking out or trouble shooting for? Again i am not very savvy when it come to electronics but i am making a great effort to learn and I am VERY cautious in everything I attempt. I have put together a "color by numbers" type of champ amp kit.
One other thing I just noticed, and don't know if it is normal. In a darker room when I flip the standby switch the tubes will flash blue/white for a brief moment.

[Jimmybjj]

i don't know if this makes a difference, but its something that i just noticed. the resistor across pins 4 and 6 of the power tubes looks new so i checked the schematic and it says the resistor is 470ohm but the resistors on the tube sockets are 1k does that make any difference?

[GregS]

In a darker room when I flip the standby switch the tubes will flash blue/white for a brief moment.

I don't think that's an issue. That's probably just the result of the voltage suddenly appearing on the plate and tube starting to conduct current.

the resistor across pins 4 and 6 of the power tubes looks new so i checked the schematic and it says the resistor is 470ohm but the resistors on the tube sockets are 1k does that make any difference?

That's the screen resistor. The higher value may lower the voltage on the screen slightly but it does not explain why the voltage on the plate is so low. Perhaps the power transformer is either bad or not the original?

Greg

[Jimmybjj]

Please correct me if i am wrong or misunderstanding as i can't even be considered a newbie yet. The schematic for the amp says the voltage coming off of pin 3 of the power tubes is 440v with a +/- 20%. So does that mean i can be as low as 352v and still be consider normal? Thanks for any help.

http://www.ampwares.com/schematics/bandmaster_ab763.pdf

[52 Bill]

Yes, based on the wording. But the reality is that when that was drawn up, voltmeters were far less accurate than they are today. Generally, I've never seen one off by that much, and they usually are on the higher side.

[Jimmybjj]

Yes, based on the wording. But the reality is that when that was drawn up, voltmeters were far less accurate than they are today. Generally, I've never seen one off by that much, and they usually are on the higher side.

That makes sense. So i think ill take a bunch of measurements and start a new tread to see if anyone can help.

I just remembered that the previous owner install a oversized "allen" output transformer, would that cause the lower voltages?

[Enzo]

And the filter caps? Same old 40 year old caps it was born with? Weak filter caps could explain the low voltage AND the hum.

Look under the cover of the cap vault. Any of those caps leaking out the vent hole on the end, or are any of those vents bulging out?

You measure about 380v B+. I(s that what you get on the standby switch terminals? Nothing special about them, I just find them easier to get at. Flip your meter to AC volts and measure B+ again. You are now measuring ripple. How much is there?

[Jimmybjj]

And the filter caps? Same old 40 year old caps it was born with? Weak filter caps could explain the low voltage AND the hum.

Look under the cover of the cap vault. Any of those caps leaking out the vent hole on the end, or are any of those vents bulging out?

You measure about 380v B+. I(s that what you get on the standby switch terminals? Nothing special about them, I just find them easier to get at. Flip your meter to AC volts and measure B+ again. You are now measuring ripple. How much is there?

The filter caps seem to be the only 5 capacitors that have been replaced 2x 100uf 350v and 3x 22uf 500v. The resistors connected to these are original. B+ is the same measure at the standby switch, although today everything is measuring slightly lower at 370v. Measuring AC ripple as you suggested gets me 69v ac. Is that bad? I've included a pic of the filter caps. i can include any other pics that anyone suggests as well. thanks for your help.

[Bruce / Mission Amps]

The filter caps seem to be the only 5 capacitors that have been replaced 2x 100uf 350v and 3x 22uf 500v. The resistors connected to these are original. B+ is the same measure at the standby switch, although today everything is measuring slightly lower at 370v. Measuring AC ripple as you suggested gets me 69v ac. Is that bad? I've included a pic of the filter caps. i can include any other pics that anyone suggests as well. thanks for your help.

Looks like one of the two filter caps on the left is in backwards... better fix that asap and replace them both as they hate being hooked up backwards.
It's the right one of the two far left ones.... unless I'm sleeping!

[Jimmybjj]

Looks like one of the two filter caps on the left is in backwards... better fix that asap and replace them both as they hate being hooked up backwards.
It's the right one of the two far left ones.... unless I'm sleeping!

Bruce, i think you are right. Damn thats a keen eye you have. Both of the positive ends of the capacitors are connected together. I took some pictures the second capacitors end is lifted. The first pic the lifted cap is the middle cap, the second pic its the bottom pic.

Now should I replace all of the caps or just the two 100uf's? Could this have damaged anything else?

I am very upset about this, I just paid a local tech to go over my amp because it was new to me, and he suggested that I replace the filter caps. I agreed and he installed them, like the picture depicts.

[Bruce / Mission Amps]

Bruce, i think you are right. Damn thats a keen eye you have. Both of the positive ends of the capacitors are connected together. I took some pictures the second capacitors end is lifted. The first pic the lifted cap is the middle cap, the second pic its the bottom pic.

Now should I replace all of the caps or just the two 100uf's? Could this have damaged anything else?

I am very upset about this, I just paid a local tech to go over my amp because it was new to me, and he suggested that I replace the filter caps. I agreed and he installed them, like the picture depicts.

Well, all techs make mistakes here and there so don't wig out on the guy.
It's hard to say if the backwards one is completely wrecked but, I think it will be stressed enough that I wouldn't trust it at all.
I'm not really sure about replacing both though. Maybe not.
The two capacitors form a capacitive reactive voltage divider at half the total capacitance (those swamping resistors across each one help with the division and swamp out any odd reactances either one of the capacitors might have so the voltage is divided better).
I'd call the guy and let him have an opportunity to buy/give you one new 100uf-350v cap and maybe do the reinstallation yourself.
That backwards cap could be the reason you are getting weird voltages.
The other ones are correct...