So I let the magic smoke out of my hum balance pot...

[guiltless]

So I'm working on a Bassman 135 that I've torn apart and put back together a little different. One thing that I kept was the hum balance pot since the power transformer doesn't have a center tap for the heaters.

So today I was turning it to try to see what the difference was from one max to the other. When I got all the way one direction it started arcing. I immediately shut everything down. I don't think I did any damage to the tubes, but I'm pretty sure I fried the hum balance pot. After disconnecting the leads I don't get any resistance from one leg to ground, and I get really weird resistance from the other leg to ground. As in that one max starts at 400 ohms then goes down, then goes back up two 200 ohms at the other max. My guess the carbon Avenue now has a bunch of extra carbon on it.

I'll post a picture of the schematic at the bottom of this post. I took photos of the amp before I took it apart, and I definitely noticed a lack of two resistors that appear to be after the hum balance pot. From what I can tell, it is a pair of 4.7 ohm resistors. My guess is that they are there to keep the pot from arcing when it is turned from one extreme to the other.

Any thoughts?

http://i180.photobucket.com/albums/x96/oc2tennessee/cde8e530.jpg

[Hwoltage]

How is the pilot light? Pull it out and see if it popped.

[guiltless]

Pilot lite works, amp still works. There is now effectively no CT for the heaters now. I may have lucked out and be out only a 100ohm pot.

[Hwoltage]

There's not really much else that can happen (that I know of), assuming your grounding of one side of the heater tap is what happened. If you're getting power through the 6.3v winding, then it's probably ok. I would replace all the resistors in the circuit however.... :)

Check your tubes carefully because it's possible you might also have been arcing between the heater and cathode in one of the tubes.

[guiltless]

I am definitely replacing the pot. Am I reading that part of the schematic correctly? Are those 2 4.7ohm resistors off the one leg of the pot?

[Hwoltage]

Yup those are 4.7 ohm.

[trobbins]

If you can, then check if any part of the heater or bulb circuitry is not being grounded somewhere. Otherwise the only likely reason for magic smoke is a poor valve.

[donnyjaguar]

I wondered first whether it was a cathode to filament short myself. Otherwise this shouldn't have happened. In my experience power tubes are more likely to short through but there's no reason why a small-signal tube can't suffer from this.

[guiltless]

Interesting, I thought if the Pot was bad, or in this case whoever was in the amp before me didn't put the 2 4.7ohm resistors back, going max to one direction would essentially create a dead short of the heater voltage to ground.

[guiltless]

Another question, since I have the pot at the end of the heater chain, how would I wire those 2 4.7ohm resistors. I really don't want to run heater wires all the way back to the lamp.

[mr coffee]

it's an "upgrade" to the artificial center tap that the silver faces had..
http://ampedia.redbeartrading.com/data/schem_layout/twin_reverb_aa270_schem.gif
the two 100 ohm resistors to ground on the pilot light

[guiltless]

I went ahead and replaced the pot. All I could find was a 2w at the local MarVac. And the only 4.7 ohm resistors I could find were 2w as well.

In the mean time, I wired it up but could not really find a good reason for the way the resistors were wired. Heck, the schematic even says that they weren't always used. So, I wired them in series between V1 and the pot to make sure there is at least some resistance in the event that it gets turned all the way to one direction.

It works great. Near dead center is where the hum disappears. I'm happy.

[Hwoltage]

2 watts are better than 1. :D