The Steel Guitar Forum

...no, it's not my breakfast cereal, it's my amp.

After fixing my amp (I hope) after my last unfortunate episode (see the recent thread "What the **** did I do to my amp?") I now have noticed this "crackling" noise on channel one only of my silver face Twin Reverb. It sounds like I'm frying bacon in there, or like a really dirty pot might sound if you rotated it back and forth.

It is barely noticeable when the amp is first turned on and gets louder as the amp warms up. Since I never really use channel one this problem may have developed before the shorting incident and I just never noticed it.

I have cleaned the inputs and tube socket on channel one and replaced preamp tubes to no avail.

Any ideas or suggestions?

------------------

It sounds to me like a broken wire, resistor or PCB trace, particularly if it can be generated by banging on the amp.

If banging on it doesn't affect it, notably if, as you say, the pops get worse as it warms up, I would suspect a failing capacitor or transformer winding.

A frying sound that is NOT caused by a loose connection or tubes,is generally thermal noise generated in an overheated resistor. The resistor would have changed value to cause this. At any rate,it sounds like a job for a technician unless,as Dave pointed out,you can change it by hitting the amp.

Could that have been caused by soldering? (Might narrow down the possible failed component.) I try to use heat sinks on all components when soldering, but sometimes there just isn't any room to get even a small one attached.

What wattage soldering iron is best for working on these amps?

What components are most susceptible to heat damage from a soldering iron?

BTW, soldering anything to the chassis is a pain without a really honking soldering iron...

------------------

Definitely could be a cold solder joint if you have been in there lately. Maybe go back and check all your work if you have.

Any size iron can do damage if left in place long enough. A 35 watt iron with a very small tip is about the minimum for most electronics. I wired audio gear occupationally for some years and prefer a 50 watt iron with a small tip for wiring and a large one for chassis and speaker wire connections. A 50 watt iron may be a bit hot for most folks, though.

You can solder with any wattage iron and control the temperature at the point you are soldering by the amount of time you hold it against the metal. Hold it there long enough for the solder to flow and get shiny. I use a temperature controlled iron but they are not needed unless you keep them on for long periods of time. A 30watt iron is a good choice for soldering component leads.
It is not likely that you damaged anything by soldering.
A technician should know how to troubleshoot the amp. You must first localize the noise to the stage where it originates. One way is to start at the first stage and short the control grids to ground(using a cap is a safe way...about .04 to .1 mfd will work). when you short a grid to ground and the noise disappears, the noise came from the preceeding stage or stages. You then need to check the offending stage for faults.
If the problem is in the power supply the noise can be found throughout the amp. A scope is useful here to check the DC supply waveform for noise.

Dave, looks like we were typing at the same time, AGAIN.. Double you pleasure....haha

John: We no longer "hit" amplifiers as it sounds too crude. The politically correct term now is Percussive Adjustment.

I had an invisibly cracked resistor in my 65 bassman. The sound exactly simulated a blown speaker. I located it by looking at the chassis in very low light (dangerous - not recommended) and I could see the arcing at the crack. New resistor = fixed.
You might take a dry insulating chopstick and poke around the resistors carefully. You should be able to affect the sound by poking the offending part.

The single most cause of that is the 100K plate load resistors on the preamp tubes. Pull tube #2 (preamp tube for two gain stages of channel #2). Turn the amp on. If the noise goes away the tube or one or both 100K plate load resistors could be at fault. Now swap that tube with tube #1 (preamp tube for two gain stages if channel #1). If the noise now appears on channel #1 you have a bad tube.

If the noise still occurs after removing tube #2 it could be a plate load resistor at tube #4 (one stage is reverb tank recovery, the other is the wet/dry mix gain stage for channel two). Sub tube #1 for tube #4 and see if the noise goes away, if so replace tube #4.

If tube substitution fails to rememdy the problem, then you need to test the plate load resistors. Applying a ground to the plates of the tube (one at a time) will cut off the tube and stop conduction through the 100K plate load resistor. That will find it quickly. This is a technique borrowed from Gerald Weber, I can't take credit for that one!

Jim Bob... good point. Thanks for the reminder. Dave was the one who brought that up. When I was in electronics school, the instructor said "when all else fails, clear off the workbench and slide the thing across the bench and into the wall". He called that "the wall test".

quote...."When I was in electronics school, the instructor said "when all else fails, clear off the workbench and slide the thing across the bench and into the wall". He called that "the wall test"...unquote!

Way back when I had the same problems described above with my Fender Bandmaster, I found that lifting the amp about 12 inches off the floor and then dropping it would usually temporarily solve the problem. The problem was that it never really solved the problem until I started playing through solid state amplifiers!

www.genejones.com

What Ken said. I always just change these in old amps with new precision resistors.

Also you might try unscrewing the circuit board cards from the chassis. Lift the top section up just enough to spray a good grade of tuner cleaner the entire length of the card. Wash em down good and let dry. I have solved weired noises like that before.

The fact that you say the noise gets worse as the amp heats up points to the 100K resistors.

on non point to point wired systems, ones with IC's, and other solid state equipment, the Percussive Adjustment or dropping the unit a few inches is also referred to as "re-seating the chips"

Gene, sounds like we went the same route. When I used a tube amp, six nights a week, the first thing I did every night was pull each tube out, push it back in and wiggle it.
That amp was a Fender which had the tubes hanging down under the chassis. The heat from the tubes caused the sockets to oxidize. Mark, I suppose if you dropped the amp from a high enough place, instead of "seating the chips", you could "chip the seats". hahaha

Gee, I haven't worked on tubes in 40 years. I saw a unit where I knew the tube was bad, but it tested good. Finally it occurred to me that the tube was vertical in the tester, but horizontal in the unit. So I turned the tester on its side and sure enough, it tested bad! That was in the military, so we had a bit of freedom: anytime we weren't sure, we could replace this or that assembly. We called that "shotgunning": just blow something away and hope you get rid of the problem.

Not knowing anything about the circuit, I would guess a tube has gone bad. Thump it with a pencil eraser and see if it stops the noise. It's also good to switch tubes and see if the noise changes channels. It's very doubtful that a resistor would cause such a problem, although I have seen resistors fail in some amazing ways. A capacitor? Maybe the filter in the power supply.

From John's post:

<SMALL>One way is to start at the first stage and short the control grids to ground(using a cap is a safe way...about .04 to .1 mfd will work). when you short a grid to ground and the noise disappears, the noise came from the preceeding stage or stages.</SMALL>

From Ken's post:

<SMALL>If tube substitution fails to rememdy the problem, then you need to test the plate load resistors. Applying a ground to the plates of the tube (one at a time) will cut off the tube and stop conduction through the 100K plate load resistor.</SMALL>

Just want to make sure you are each talking about different things here since one mentions "grids" and the other "plates".

Can I make two "jumpers", one with a capacitor attached and one with just a wire, and do these tests by clipping to the appropriate preamp socket pin and ground?

Will any mylar film or polypropylene film type capacitor work?

------------------

I use a jumper to chassis and ground teh plate, not hte grid! That cuts off the tube conduction and loads the 100K from B+ to ground.

Sounds good.

I'm having trouble finding 1Watt 100K carbon composite resistors. Should I use carbon film, or something else?

I don't think they make composite resistors any more. Carbon film is ok.

Thanks.

You can get carbon comp resistors from several places on the web but a source may not have all the values you are looking for, and none of the ones I found have the 1watt 100K value...

Metal film is better. I've heard thay you can use an AM radio to find amp noise, although I can't say that it's worked for me. Your're supposed to tune to an empty station and wave the antenna over the suspect component, and pick up the noise on the radio. Anyone do this? (Please, no one die trying!)