Filament balance pot

[lowell]

I installed a hum balance pot to the filaments in this 100w SF Twin Reverb. It fries the pot immediately... I've tried 2 pots now and both fried. This is a 100ohm 5w linear pot. I thought this was enought rating... is it not? My math if pot is in the center is 6.3v/50ohm=.126amps. .123a*6.3v=.8watts...?? I'm missing something.

[jrfrond]

Did you disconnect the original heater center-tap?

[Enzo]

What John said.

YOur math is fine, but what about when you turn the pot over towards one end? If the CT is grounded, there would be 3VAC to ground to either winding wire. Let's say the pot was turned over to one end with maybe 3 ohms left before the end. That means there would be 3v across that last 3 ohms. 1 Amp would flow through that tiny part of the pot. That is three watts trying to dissipate from that tiny part of a pot that is rated to dissipate 5 watts over its entire length. SInce 3 ohms is about 1/33 of 100 ohms, that would be like the whole pot trying to dissipate 100 watts.

Of course if there is no CT, then never mind.

[mac1amps]

The burning of a filament line "hum" pot. (OR the (2) 100 Ohm resistors, OR the heater center-tap) is [virtually] always the sign of a heater/cathode short in one of your power tubes.

[jrfrond]

As a precautionary measure, I've always used 5W wirewound controls for hum-balance controls. The extra couple of bucks investment is worth the piece of mind, just in case a tube develops a heater short.

[lowell]

Hey guys,
There is currently no CT in the amp... not sure if someone removed it or if it was never there... I see no sign of one having been there. I am using a 5W pot and clearly it is not enough (per Enzo's math - makes perfect sense). I tried putting 2 100ohm resistors in there and it made NO difference at all. I will try other power tubes and see if that makes a difference.

[lowell]

Ok I pulled power tubes and there is NO resistance to ground from heater windings. I then also pulled preamp tubes and the same result. What do we all make of this? The amp works... could the amp work w/ this scenario?

[Steve Conner]

I've seen transformers where the heater winding center tap shared a lug with something else that was meant to be grounded too, like the B+ winding centre tap. If that was the case, it'd need surgery to the transformer to disconnect it.

When you say "NO" resistance, do you mean zero resistance, or infinite resistance? If your transformer is like what I described above, then I'd expect zero resistance.

Does the hum balance pot/ resistors still burn with all the tubes pulled out?

Enzo's math was meant to show an abnormal situation, not an example of how to rate the hum balance pot. If there is already a center tap, installing a hum balance pot is wrong. It won't do anything except catch fire when you turn it all the way to one end. You need to disconnect the center tap, and then the hum balance pot will work, and only needs to be a watt or two. 5 watt pots are more expensive, and as Enzo proved mathematically, a short will burn out any wattage of pot if you turn it all the way in the wrong direction, which you will when you start tinkering with it because the heater-cathode short made it stop working.

[lowell]

Not sure if the pot would burn w/ tubes out... didn't try that.

Ok Steve thanks... yes I think this heater winding has a CT now that you say that... sorry Enzo and folks, the orange wire that is grounded on same lug as the yellow/red CT from B+ secondary... is that the CT for the heaters?

I just tried disconnecting the orange CT... if that's it, just to see. I then wired up a 5watt 100ohm pot and it fried again. It also did not seem to affect the hum at all for the little time before it fried.

So all in all you guys're saying that any wattage pot will burn out when turned all the way in one direction? Or is this a theoretical point you're making? Finally in your experience do you find it best to just keep the CT rather than modify to a balance pot?

[Enzo]

No no, any pot will burn out IF IF IF you also have a grounded CT AND you turn the pot anywhere but center. WIth NO center tap = and remember they can be hidden - then the pots work fine, and many amps had them.

[booj]

Leave the pot out. Measure from one side of the filament supply to ground. It it's 1/2 of your filament supply, then you have a center tapped supply already. You don't need the hum pot.

[jrfrond]

If the amp is operating correctly and the pot is linear, the pot wiper should never stray too far from the middle of it's travel.

It sounds like you have a CT to me, in which case, you probably DON'T need the hum-balance control.

[Bruce / Mission Amps]

Sounds like the pot is wired wrong.

None of this makes any sense to me.... especially if the tubes are out.
All SF twins I can see have a grounded center tap so you shouldn't use a pot that can ground one side out.
If you have a two wire 6.3v filament winding (no center tap, so NO reference to ground anywhere), then you should be able to use a 1 watt 100 ohm pot with one of it's outside lugs wired to one side of the filament winding respectively. That would be 100 ohms right across the filament supply.
If wired correctly a 5 watt 100ohm pot can not possibly burn out.... because the resistance of the heaters is MUCH lower then the 100 ohm pot so nearly all the current from the winding will be used up in the heaters no the pot.
However, I didn't read anywhere how the pot is wired and I suspect this is the issue.
You have 100 ohm across the two 6.3v leads.... max current across that would be 6.3v/100 ohms = 63ma, or, .063a^2 X 100ohms= <.4watts. 1/2 watt pot will be at it's limit.
But that configuration does nothing for hum balance, it just uses up a tiny but if filament supply current.
A filament hum balance pot, used in that configuration, should have the wiper grounded through at least a 47-100 ohm 1/2-1 watt resistor. (I'd use 1 watt)
Then you would turn the pot to where ever you want and it will balance or upset the 6.3vac voltage a little and afford some hum reduction/balance and still not draw enough current to do any damage to the pot or the resistor.

[Shea]

All SF twins I can see have a grounded center tap

All the SF Twin Reverbs I've ever looked inside (plus a Quad Reverb) have used a pair of 100-ohm resistors to ground as a virtual center tap. They're always mounted on the pilot light holder. Like in this layout diagram:

http://www.kbapps.com/audio/schemati...a270layout.gif


This is kind of interesting: http://www.schematicheaven.com/fende..._100_schem.pdf

That shows two revisions of the 100 watt / master volume schematic. The earlier one uses the two 100-ohm resistors to ground (like I've seen), and the later one uses one 100-ohm hum balance pot instead of those resistors.

So, my hunch is that Lowell's Twin has those two 100-ohm resistors mounted on the pilot light. He'd have to take them out if he wants to use a hum balance pot, but, IME, it's not really worth it. The resistors work well enough.

Shea

[Rob Mercure]

Also, the power transformer heater center tap should be green with a yellow tracer - occasionally brown/yellow. In anycase it should be the same color as the heater leads with, usually, yellow tracer. Orange is probably an internal shield on the PT to reduce hum although I've seem orange bias supply leads before - what color is your bias supply lead?

Rob