Can a normal run of the mill booster pedal damage a tube amp?

[liddokun]

Hey all,

Just a question that I coudln't find an answer to by searching. I built two SHO boosters and were trying them out. The SHO boost has its output hooked up to output jacks in parallel, so I'm running one output to a blues junior, one output to a 59 bassman reissue. 

My question is, is there any way in which a booster of this design (simple mosfet clean boost) will be able to damage an amp? What happened was, with the Bassman initially connected, we heard a strange oscilation sort of like whining/screeching noise, going up and down in pitch. We thought it was weird, so we bypassed the pedal, and found that it was still there, only fainter. So we switched to the second SHO that I built, and the oscilation disappeared, but after playing with the pedal for a bit, we hit the stomp switch and there was a pop and the Bassman just cut out. This worried me, so I tried running the guitar straight into the Bassman, sure enough, the Bassman was now putting out very low output volume. The pedals I built both produced switch popping that didn't go away with a 1.5M pulldodwn at the input or power filtering, so I just accepted it, therefore I can't remember if the pop we heard before the Bassman cut out was louder than usual.

I'm trying to rule out the pedal as the perpetrator.  Is there any possible way that the pedal could have done this to the amp? Even if I didn't wire the pedal properly? I built both pedals stock, with the addition of a 47uF cap on the power input for filtering, and a 1.5M pulldown on inputs.

[kurtlives]

Boosts don't damage amps.

Id check your pedals switch as well as the jacks.

Almost sounds like a parasitic oscillation in the amp.

[liddokun]

Check what exactly, Chris?

What should I check on the switches and jacks? DC? In the day and age where I was studying engineering and stuff, I'm sure I could figure this out myself. But my head is so over saturated with nursing and health care procedures now!

[petemoore]

  Only if you want to, and can stand terrible tones of speaker saying OUCH !
  Or...the amp needs it's weak spot identified.

[liddokun]

Well, like I said, I'm trying to rule out the pedal as the problem, which is why I'm asking this question in the first place. I'm almost 100% sure that it's not possible for the pedal to have done any kind of damage to the amp, however, I'm always learning so I wanted to be sure by asking here.

I did call two of the city's resident amp repair shops, and they both told me there's probably a problem in the Bassman ready to happen, and hitting the front end hard with the SHO just pushed it over the edge.

[kurtlives]

I'd check the switches and jack to make sure they are working properly. Check continuity, intermittent failure, etc.

Here is a long shot...
The Bassman uses 1/4W plate resistors, does yours?

The big input signal caused a huge swing across the plate resistor, too much power being dissipated. The plate resistor smoked up and was gone.

Take a look inside, see whats what.


Which amp places did you talk to btw?

[liddokun]

Switches and jack work fine on both pedals I built. We ended up running the pedal through another set of amps, and we kicked the pedal, shook it, wiggled jacks on the pedal, and abused the hell outta the switch by stomping on it repeatedly. No problems at all.

So I guess it's just a case of Fender underspeccing the plate resistor? I would open it up, but it was in a music store, so I'm not sure I'm allowed. Might though. Should be an easy fix if it's just the plate resistor, guess I should toss in at least w 1/2 watt this time.

For the record Chris, I dropped a line to Capsule and Superfuzz. Though the Superfuzz boys seemed more confident in answering my questions, haha. Go figure.

[Ronsonic]

Do check if the pedal is putting DC on the output.

An oscillating amp will melt down, but I doubt anything above audible going into the input will get far enough down the signal path to do real harm. Never tested that theory either....

[liddokun]

Well, is it possible for a pedal being fed only DC voltage to output AC voltage? I wouldn't think so. Was using a boss 9v adaptor.

[kurtlives]

Well, is it possible for a pedal being fed only DC voltage to output AC voltage? I wouldn't think so. Was using a boss 9v adaptor.

Ron was talking about the signal output. No the incoming DC voltage to power the pedal.

He is thinking you might have a faulty coupling cap. Measure the output side of your coupling caps and check for DC voltage.


Who the hell are Capsule and Superfuzz?

[liddokun]

I've got nothing coming out of the outputs in terms of DC.

Uh, Capsule Music and Superfuzz Audio, who both do a lot of amp repairs. Although I know Capsule Music do more guitar repairs and service.

Superfuzz was started by the guys who used to work at Songbird Music.

[Ronsonic]

Well, is it possible for a pedal being fed only DC voltage to output AC voltage? I wouldn't think so. Was using a boss 9v adaptor.

Okay, gotta spell it out a little. There were two things I was concerned about as the only ways I could imaging a pedal damaging an amp in a way that you wouldn't hear coming.

First, that there was a wiring fault or a bad capacitor allowing positive DC to get on the output. This would bias the first preamp tube into conduction causing freakout and failure.

The other is that the pedal is, for whatever reason, oscillating, that is turning itself into a signal generator. This can happen at ultrasonic frequencies. You won't hear it, but you'll hear, see, measure and smell the amp trying to amplify them. Guitar amps do a poor job handling radio frequencies. I have not experienced that particular fault, not from a pedal. I have seen amps go into RF oscillations from other causes and it isn't pretty. You would need a scope to know if this is happening. I do not know if your amp would be susceptible to damage from a pedal putting out HF.

I would just repair of the amp based on what you see on the bench rather than possible causes related to the pedal. THEN after seeing what the problem is, determine whether it is related to the pedal.

[liddokun]

Would a freakout in the preamp cause the heaters in one of the preamp tubes to go?
Also, there isn't any DC on the outputs of the pedals...not anything I can measure with my DMM anways.

Is the self oscillation in the pedal possible with the simple design of the SHO?

[Ronsonic]

Would a freakout in the preamp cause the heaters in one of the preamp tubes to go?
Also, there isn't any DC on the outputs of the pedals...not anything I can measure with my DMM anways.

Not in any way I can imagine.

Is the self oscillation in the pedal possible with the simple design of the SHO?

I don't know the SHO well, but it doesn't take much to oscillate, a bit of positive feedback and away you go.

This does sound more like coincidence than causation. Have you tried the pedal with anything else?

[liddokun]

Tried it with a blues junior and some cheap solid states as well as a vox ac4 and a vox ad15vt hybrid, no problems at all. clicked it in and out continuously to try and see if the switching was a problem.

Starting to seem like coincidence to me as well. I should hope it is.