Working on an old Ampeg SVT

[AmpegSVT]

I have replaced the volume pots in a '78 SVT (among other things) and stumbled upon a little issue. When the volume pot is turned about 95% CCW towards the the lowest volume setting a loud pop comes from the speaker. It also pops when leaving this 5% range. In looking at the schematic I notice that the volume wiper goes to ground when volume is at zero. Im thinking that the tubes are going into cutoff when the wiper gets around ground potential. The new pots have about 1 ohm between wiper and ground (full ccw), the old pots have about 45 ohms between wiper and ground (full ccw). Im getting 154v at the plate and 6v at the cathode of the 12DW7. The grid is tied directly to the wiper with no other components connected to the grid. Also the cathode jumps a half a volt up when this occurs.

[Zippy]

If you think that's an issue, insert a resistor between the pot and ground. I've done similar things to "idiot proof" some controls before when I found that ZERO was a bad place to be.

[AmpegSVT]

I might have to do that. When I place my voltmeter on the grid to ground the problem dissapears too. How would I know the tube has gone into cutoff? Wouldn't the cathode voltage go to zero?

[JazzGuitarGimp]

I'm wondering if perhaps the wiper blade is lifting off the conductive track at the point where it pops. Connecting the meter between grid and ground solves the problem and certainly, the meter's input impedance is much higher than the pot's resistance where the pop occurs. If you need to keep the new pot for some reason, you might try adding a resistor which is ten times the pot's value, between the pot's wiper and CCW terminal. I'd try a different pot, however. If the tube were going into cutoff, the cathode voltage would indeed be zero.

[Firestorm]

Make sure you haven't use "no-load pots." These cut the ground when turned down. Used in some guitars IIRC, but you never know.

It was supposed to say no-load I fixed it. The damn phone keeps changing it.

[jjman]

A bad connection from wiper to track would cause the tube to drift up in current, not cut off. You could verify it as an increasing cathode voltage. It's bad so no need to verify more than once. The problem is already verified by the meter preventing the pop from it providing an alternate path to ground for the grid, albeit distant.

[AmpegSVT]

After revisiting "Chapter1:Fundamentals of amplification" I see where my thinking was flawed. In order for that tube to go into cutoff the grid would have to be more negative than the catode is biased. My cathode is biased at about 6vdc, so i'd need too be a lot more negative than -6vdc at the grid to get into cutoff. So when the grid is at ground potential (0vdc), it cant possibly be in cutoff.

[AmpegSVT]

Turns out to be the pot. I dug up the old one, put it back in and it worked. A bit scratchy and hard to turn, but it works. The Alpha pot that didnt work jumped from 1 ohm fully ccw to 200 ohms with only a slight cw turn. That instant jump of 199 ohms was the culprit. I took the pot apart and you can see where the wiper goes past where the resistance material ends. Not sayin the alphas are junk, but they dont work in this application. Just bought some old CTS off ebay, and they even have the d-shaft cut the right way so the original knobs point the right way.

[Firestorm]

Yay! CTS pots you can mess with all sorts of ways to make them work. Alphas are harder.

[JazzGuitarGimp]

Turns out to be the pot. I dug up the old one, put it back in and it worked. A bit scratchy and hard to turn, but it works. The Alpha pot that didnt work jumped from 1 ohm fully ccw to 200 ohms with only a slight cw turn. That instant jump of 199 ohms was the culprit. I took the pot apart and you can see where the wiper goes past where the resistance material ends. Not sayin the alphas are junk, but they dont work in this application. Just bought some old CTS off ebay, and they even have the d-shaft cut the right way so the original knobs point the right way.

:thumbs_up: