Help! Any ideas on my Fender Super Twin crackling?

Greg Milton · From: Benalla, Australia

Hello to all you tech-minded people out there in forum land!

My lovely Fender Super Twin has a crackling sound that starts about 20-30 mins after I start playing, and then seems to get steadily worse as the amp gets hotter.

Obviously there is some thermal component to it because it has to get hot for the fault to present.

I've had a tech go through it twice but without solving the problem - he replaced all the plate load resistors but it's still there. Previously he has replaced caps and tubes. He's now pretty much giving up, saying there's nothing more he can do.

I need this amp, and need to get it fixed! Any ideas?

Greg

Ken Fox · From: Nashville GA USA

There are also quite a few more resistors prior to the plate load resistors that are involved in the B+ voltages, have seen that as a noise issue before. If these have been getting hot it is usually a result of filter caps with a high ESR(equivalent series resistance). What that means is the filter caps act like a DC load to the power supply and cause those dropping resistors to pass more current than usual, thus damaging them.
Still there are other things that can cause the issues you are speaking of. Even a bad pre-amp tube, tube socket, filter choke, circuit board contamination and more.
It takes an experienced tube amp tech to chase down issues like this! Can drive a man to drink

Shocked

Dave Meis · From: Olympic Peninsula, Washington, USA

I've had pre amp tubes sound crackly, or like frying bacon..when it starts, go down the row of pre amp tubes with a pencil or chopstick, and tap each one.. you might find it that way. You could pull the PI tube (closest to the power tubes) and isolate the power section and see if it's on the power side.. lots of process of elimination available.. hope you find it soon! Smile

Bill L. Wilson · From: Oklahoma, USA

I've had brand new tubes be bad right out of the box. Blame it on the Russians.

William Kendrick · Posted 10 Mar 2017 3:58 pm

Listen to Ken Fox. This guy knows his electronics.
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Greg Milton · From: Benalla, Australia

Thanks everyone!

I'm well aware of Mr Fox and his amazing gifts! I just wish I wasn't on the other side of the world from him!

I'll mention this stuff to my tech and see if he can have another crack at finding the fault.

James Phillips · From: Michigan, USA

Make sure your tech checks the tube sockets,as well.
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Rich Upright · From: Florida, USA

Filter caps. They dry out after a while.
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Ken Fox · From: Nashville GA USA

Wiggle the [pre-amp tubes and see if you get a crackling noise. If so the tube pins are likely oxidized (as opposed to dirty sockets). Rarely have I seen the socket as an issue. Remove the tubes and wire brush the pins with a small brass wire auto parts cleaning brush. You would be amazed how well this works. Contact cleaner will not remove the oxidation.
Been there and done it, even in the last week on an old Ampeg amp!!

I have yet to see a filter cap cause a crackling noise. If they become open they will not filter the DC voltage and hum will occur. If they start to short (high ESR, equivalent series resistance) they will load down the power supply terribly and eventually the amp will blow the fuse. Some of the filter caps under the cap can are associated with the pre-amp circuits. These can cause a motor-boating noise in the amp when they go bad. rarely have I seen it, only on one amp over the years.

Things to check:

Turn down the volume controls and see if the noise goes away. If it does it is in the first two tubes (one is for the normal channel and the second tube is for the vibrato channel)

Do the tone controls affect the noise?

As stated above you can pull the inverter tube (small tube next to the first power tube. That shits off the pre-amp. If the noise goes away it is in the pre-amp stages. If the noise is still there in is in the power amp section.

Lots of possibilities! That's why most tube amp techs end up in little white jackets with really long sleeves!!! Off to the funny farm they go, ho, ho!!

Only worse thing to chase down for me is a similar noise in the older transistor amp power sections like Session 400 and even a Lab Series amp like I have on my bench right now for a friend. I would rather trouble shot a tube amp any day than deal with a noisy solid state amp. Pretty much Evans, Peavey, Webb, Lab Series and a host of others used that same classic power amp design. As Jim Evans once said to me it was a real balancing act to get them set up right!

Mark Fowler · From: Minnesota, USA

Bill Hatcher · From: Atlanta Ga. USA

found a few of these noises by taking a heat gun and directing the heat in very small areas. if the component is making the noise from heat build up it will start pretty fast when its directly heated up. a solder gun will work too, but the heat gun lets you not have to touch the components.

in the black face amps i just change the 100k resistors on the board. replace with precision resistors. always gets rid of noise.

Greg Milton · From: Benalla, Australia

Thanks for everyone's input. I've now got some ideas about how to proceed

Brad Higgins · From: New York, USA

I had the exact same problem with my Fender Vibrosonic, and it wound up being some of the solder joints in the power supply circuit, right above the 6L6s. The 6L6s produce so much heat with the inverted chassis that over years it literally destroys solder joints and components. After I finally found and fixed the problem, I put a small cooling in the cabinet, it made all the difference in the world, I could reach in and touch the tubes at the end of a long night and not burn my fingers. It makes tubes last longer too. That's one thing that I've never understood, they put cooling fans in solid state amps and there's no where near the heat buildup, but not in tube amps. Hope this helps!

Greg Milton · From: Benalla, Australia

Thanks Brad. That may be worth exploring.

One thing I have noticed is that the only knob to affect the crackling is the presence control.

It's still there when the presence is off, but only faintly, but when I turn the presence up to ten it gets very loud - does anyone know if this is indicative of something? Or does the presence just exacerbate any noise in the cabinet?

Ken Fox · From: Nashville GA USA

Pull V2 (12AU7) and see if the noise goes away, It should, as V2 is involved in the E.Q. controls for the amp and the presence is one of those controls. Try to change V2 to a new 12aU7. Check and change the plate load resistor for V2. The caps around the presence EQ control circuit could well be contributing to the noise.

Carl Mesrobian · From: Salem, Massachusetts, USA

As Ken mentioned about straight jackets, I keep several in my shop - some are even pre-CBS. Isolating it to a stage (preamp, PI, output, power supply) helps. The best order of testing, in my and most likely others experience, is

1. tubes
2. power supply
3. components (caps, resistors, etc)
4. input cables and devices
5. transformers, and speakers.

If you clean the tube sockets and pins, or better yet, make sure the tubes are good first, and you still have the noise, you've only used one straight jacket sleeve.

Then on to step 2, etc., etc.

Step 3 is fun Smile

I use an infrared heat sensor gun and aim it at stuff on the board with the circuit on. The "chopstick test" is handy, too, for checking physical things (bad solder joint, microphonic tubes).

Did your tech replace the 470 ohm screen grid resistors on the power tubes?

Also you can check the amp in a DEAD (totally discharged caps) state. Resistors go up in value as they get old and tired, and also if the temperature rises on them.

Hopefully you won't get to step 2,3,4 or 5. 3 is a trip in its own right.

There is a reason it happens - don't give up. BUT it could be anything - the thing you want to avoid is "shotgun" approach, like replacing all the tubes, coupling caps, filter caps, cathode bypass caps. And guess what? The crackling could still be there Wink

I had an old Ampeg b12 with reverb that had oscillation. I'd open it up check it, and it sounded fine. Close it up , and the oscillation came back! Turned out it was a signal cable shorting to ground when the amp was closed up. Anything..

Five straight jackets is plenty. - after 5 you're certified..
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"The better it gets, the fewer of us know it." Ray Brown

Greg Milton · From: Benalla, Australia

Thanks Ken and Carl, that is very helpful.

I've spoken about this to other Australian steel players - we feel very remote and isolated here, kind of like a pedal steel outstation! If anything goes wrong with our instruments or even our amps we're pretty much relying on you guys to help out because there's just no-one here to turn to, for the most part.

I really appreciate everyone's input, especially Ken's, because he is just an amazing source of knowledge.

I will keep you all posted.

Mark Fowler · From: Minnesota, USA

I don't know if this will help but I have seen many Australia and NZ amp techs on forums. http://www.ozvalveamps.org/valvetechs.htm

My friend lives in Wellington, NZ so that doesn't help you.

Mark

Greg Milton · From: Benalla, Australia

Well, two years (and several hundred dollars) later I finally fixed my Super Twin - it apparently was down to lifting a part of the circuit board up away from other circuits and, hey presto!, no more crackle.

The amp spent much of the last two years with the amp tech who tried everything under the sun to find the fault. Now I've finally got it back the crackle is gone but there is now some very loud hum! I couldn't bear to send it back to the tech again (I may not see it for another two years) so was wondering if anyone could help me trouble shoot it.

It only hums when a lead is in the input jack, whether a guitar is plugged into it or not. The volume knobs increase the hum as they are dialed up. I've tried different cords, guitars, power outlets, orientations, etc. and they have no effect.

Any ideas? Or do I need to bite the bullet and send it to someone again?

Carl Mesrobian · From: Salem, Massachusetts, USA

It hums only when the cable is plugged in? Try wiggling the plug that is in the amp jack and see if the hum goes away. It sounds like a worn out input jack.

Do both jacks act the same?

It could be that a solder joint is loose from the board being moved. Or something similar.. like a worn out jack or one that isn't grounding properly when plugged/unplugged in.
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--carl

"The better it gets, the fewer of us know it." Ray Brown

Steven Paris · From: Los Angeles

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Noise problems like that are easily found and solved with a cheap oscilloscope. You can often get working ones these days for the price of a stomp-box gizmo. Winking

Brian Hollands · From: Geneva, FL USA

Try tightening the jacks. They make ground through the panel and that joint can easily corrode. Just put a socket on the hex nut that secures the jack and snug it down good. If that helps but doesn't elimate the problem, you could take the jacks out and clean that area, or at least the front of the panel where the nut cinches down. May not be the problem but it's a cheap and easy thing to try
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Greg Milton · From: Benalla, Australia

Carl - it is both jacks

Thanks everyone - this gives me some ideas to go on with

Greg Milton · From: Benalla, Australia

Hi Donny,

I don't suppose you could talk us through how I could investigate this problem with an oscilloscope?

Greg

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