Amp Crackle: Testing Resistors

[nihoneiga1960]

I've got an old Teisco tube amp that I used at gig for longer and harder than I normally play and when I got it back home I noticed it had developed a static-like crackle that comes and goes even when nothing is plugged in. I tested all the tubes without finding an bad tube. So I moved on to checking the resistors that bridge around the socket.

What I found were a couple of resistors that don't immediate go up to their rated value, like a 500K resistor that starts at around 250K and slower climbs up to 500k if I leave the DMM alligator clips attached, finally reaching value after about 1 minute. What I'd like to know is: is this resistor bad and should it be replaced? I found one other resister in the circuit (which is just point to point) which was rated at 250K but did the same thing, slowly creeped up to 250k after about a minute.

Your wisdom is appreciated.

[R.G.]

Resistor values measured in-circuit are generally wrong in the low direction because of the other parts connected in parallel in the circuit. the creeping up you mention is your ohmmeter charging up caps around the resistors.

[JoeM]

You might try cleaning tube pins and sockets with some Caig Gold. I've fixed some minor sizzling type sounds using this. At least eliminate the easy stuff before doing the resistors. (Also a common noise source, especially older CCs)

[nihoneiga1960]

I sprayed with with some point cleaner and it seems to have quieted down for the most part. Also, it seems to act worse with different cables but the cables soldering seems sound. Wonder what it is?

[Resonator Guy]

I have a couple of Teisco made Beltones and they have very weak tube sockets that need regular re-tensioning to avoid a similar problem to yours. Check to see that all the tubes are firmly in their sockets. If not, the sockets will need to be re-tensioned. Also check that the wire tube retainers are properly shaped to help hold the tubes securely. I bet this will fix your problem.

[nihoneiga1960]

Cool, yeah the sockets definately are a bit loose, how does one go about retensioning the socket? The wire retainers are good, I replaced them with springs so that side of things are snug.

[Resonator Guy]

I use a dental pick, but you can use an open safety pin or some such. Unplug the amp and drain the filter caps. Take out a tube and check inside the pin holes of the socket. There are tiny clamping devices around the inside of the holes that hold the pin. Gently pry them together a little to hold the pin more securely. You have to do this with each pin hole for each tube, all the while being careful not to break anything as these Japanese parts are none too robust. After doing this for all the tubes, another cleaning will probably be in order and you should be good to go. The looseness can cause arcing which can sound as you described and it leaves a burnt deposit on the pins and in the pin holes.