Speaking of stompboxes, what can damage our amps?

[lopsided]

Hello,

I recently begun trying out high voltage tube projects (http://www.diystompboxes.com/smfforum/index.php?topic=94289.msg812943#msg812943).
It all worked fine on my test solid state combo, so I wanted to try it with a tube amp I play (old school Carlsbro CS 60, single channel, no master volume, all pretty simple). It sounded good for a minute, but then sound stopped even when bypassed. I panicked, but after unplugging the pedal and restarting the amp, all was okay again.

It made me ask myself, yes I want to experiment with pedals, but I definitely don't want to ruin the amp.
It's quite possible that something else went wrong, but if it was the pedal's fault what could it be?
I was thinking about:

1) DC on output. The pedal goes on about 150V but I have a 400V cap after the second gain stage, and did repeatedly measure 0 volts at the output. But let's assume it would leak somehow. As much as I know, amp do not have something like an input cap to protect them form DC, what woud happen with high DC on grid of the first preamp tube?

2) AC on output, "hot signal". I have measured about 10V AC with volume and gain maxed, strumming as hard as possible. Of course I was nowhere close to these setting. I remember reading in some Z-Vex's description of SHO (i think) that it could make too much signal and damage the amp. So, how much AC signal are amps designed for? Is there some limit between pushing the amp and breaking the amp?

3) High output impedance. My pedal has a pretty high series resistance at the output. O use a 100K volume pot (think it sounded a little better to me than 10K, but...) in series with 680K resistor to attenuate the signal. Now I know the problems of signal or treble loss caused by high output/low input impedance, but could it do any actual harm? I though about implementing a buffer, but it did not degenerate the sound quality on my testing amp, so I wanted to test it as it was at my real rig.

4) Something else I haven't thought of?

I would be really grateful for any comments, and think that this topic could be of interest to other builders too.

Thank you!

Jakub

[defaced]

1) I've never dealt with leaky coupling caps, but I'm pretty sure you'll hear a scratchy volume pot on the pedal which will be an indicator that there is an issue long before you'll get a real problem inside translating to the amp.  Being that this is a common failure in tube amps, and that is essentially what you're pedal is emulating, I don't think you'll have an issue.  If say the coupling cap completely failed and turned into a wire, the voltage would get knocked down by the voltage divider on the output.  Worst case, I think the input triode would conduct more and possibly over dissipate or the grid would exceed it's current rating (which is extremely small) resulting in failure, but at first glance I don't see how anything but the tube would be damaged.  You can add a DC blocking cap to the amp.  Some amps have them, some do not.  If it is significantly large, it won't make a difference in the tone of the amp.  I think my amp has a 220n up front.  I hear no difference with it in or out of the signal chain.  

2) Tube amps have massive signals inside them.  10v AC is nothing.  In a cascaded gain stage amp, say like a Mesa Recto or Soldano SLO, the signal can be upwards to 250v AC in the preamp.  Obviously not all of this signal is passed to the next stage, often half or more of it is thrown away via voltage dividers, but 10v AC is nothing.  

3) The input impedance of most amps is around 1M, so no, your output impedance concerns won't really do much more than possibly suffer from issues with cable capacitance.  If you want to add a buffer, go ahead, but I don't think you'll have an issue - experimentation will determine if you need one or not.  Further, if you add a buffer, high voltage MOSFETs will do the job nicely without the requirement of an extra hole in the chassis or heater wiring.  See R.G.'s GEOFX page titled "MOSFET Follies" for more info.  

4) Don't sit beer on top of your amp.  Warm beer is bad, and your amp prefers to stay dry 

[iccaros]

is it this amp http://www.chambonino.com/carlsbro/carlcs60tc.html

if so the first stage is biased hot.. While I was assume that 10volts (swing) would be a hot signal for an amp (2 -3 volts is what I am thinking) on the tube you could be going into cutoff and its quite, looking at the datasheet for a 12ax7 I would not expect that 10vac would hurt the tube, but that amp looks to be biased for a strait guitar, not for effects.

But I have a delay that does this, I thought the same thing, then I discovered a bad wire on the bypass switch.. which was causing it to cut out.. IT was weird that it only did it on one amp and not on the others, but that was the what happened by chance.

[Le québécois]

+1 on that issue. Mine was with my multifx DIYstompbox into a SGH Head. This was at our pratice local and it happen 4 times in the same gig. The amp just suddenly shut down but the power on, power off, trick make it back to life eachtime.  I tough it was because of a to hot signal coming from my pedals + onboard guitar preamp. Maybe it trigger a protection in the amp (if such thing exist!?).
Never solve this. We ask for another amp for that practice (they gave us a marshall don't no what head) and the problem disappear.

[petemoore]

  The simple answer is 'no' under 'normal' conditions.
  One guys 'normal' is another guys patch from an amp output to an amp input though, another might be a 220vac > 120vac adapter [they don't make 'em that I know of but we managed to get 220vac supply into our amps/mixing board at a gig once...the singer was mic'd through my Plexi that night...
   IOW you really gotta mess up or try to blow amps. Most of the amp-blows I've experienced involve an internal amplifier problem, external load that is too large, and usage where the amp is pushed past it's limit 'in private'...ie can't be heard struggling and sounding terrible.
   All amps do have their limits of course, they vary, but only so much AC or DC can be input without damage. These limits are generally well above what produces usable performances, but unusable performance does occur...on purpose or when the straining rig sounds are squelched by other loud noises.
  I've not read or heard about an amp that can't take pedal-abuse, generally speaking there's nothing a 9v pedal [in good working order] can do to a 'sturdy' amplifier. If there's a weak link such as an 'underrated' speaker, boosting can of course push more current or pressure to the coil or cone and cause damage. When this happened I realized I'd expected more than my 6'' featherweight cone/coil speaker was capable of producing and chalked one up on the 'speaker upgrade time' board.
   Most modern amps have protection of one or many kinds, they all have their limits of course, transistor has it's set of requirements and they're similar but different than tube amps. When an output transistor is overloaded generally something either prevents it, limits it...or...it'll bark your speaker to instant cone-blowout or bake it to death...I've had these events happen...tube amps are more forgiving about it, they only go as many watts as they're rated and no more. Transistor amps have headroom above where they clip and can produce huge spikes at the output when overloaded/clipped, these can easily heat up and burn a speaker coil or pop it right 'through' the cone during one wave cycle.

[lopsided]

Hi again.
First of all, thanks a lot for all your responses!

As I have said I am pretty positive I have no DC on output and I am not pushing the settings to extremes, so I probably wait after the recording session we have by the end of December and will try again, to see if it wasn't just some accident.

is it this amp http://www.chambonino.com/carlsbro/carlcs60tc.html

if so the first stage is biased hot.. While I was assume that 10volts (swing) would be a hot signal for an amp (2 -3 volts is what I am thinking) on the tube you could be going into cutoff and its quite, looking at the datasheet for a 12ax7 I would not expect that 10vac would hurt the tube, but that amp looks to be biased for a strait guitar, not for effects.

actually i believe it is this one http://www.chambonino.com/carlsbro/carlcs60pa.html
but given that it is more than 40 years old and I am at least the fourth user I can not be sure if something was not changed, to make it more a guitar amp . Reverb is disconnected for sure but that does not matter really.

Looking at the schematic:

grid resistors 3,3M

no cathode resistor

47pf cap on grid

But I for one do not understand the first stage at all. The fist triodes (first two inputs) plate seems to be connected only to B+ (through the 100k, but where does the signal go?) and the second triodes plate does not seem to be connected to B+ at all...
there goes my understanding of tube gain stages...
not sure if anyone can make a sense of this.
(btw the amp can be seen and heard here http://www.youtube.com/watch?v=3C0kdP84pZs&feature=related that's the guitarist my friend and drummer bought the amp from.)

  I've not read or heard about an amp that can't take pedal-abuse, generally speaking there's nothing a 9v pedal

when I have worked with my 9V builds I was not afraid of any damage, now with my first high voltage project I am trying to be as cautious as possible.  When I connected it for the first time to the solid state test amp, the transistors got overdriven very hard, and the combo played quiet loud even with volume on zero. So I have added the attentuating reesistor and volume pot at the end, and at least with the combo it plays with no problems.

4) Don't sit beer on top of your amp.  Warm beer is bad, and your amp prefers to stay dry 

Actually it is so cold in our current practice room (it is in an old rail way station originally from 1863, still in use...) so I am taking some hot tea to the rehearsals. So keeping it warm on the amp could be actually a good idea...

Thanks all you!

J.

[iccaros]

If that is the amp, its a PA head not a guitar head. So its bias for very small signals like a Microphone.  so 10 volts could way over run that amp.. @ zero bias you drive the grid positive very quickly


edit does it look like this?
http://www.chambonino.com/work/carlsbro/carl3.html

[lopsided]

If that is the amp, its a PA head not a guitar head. So its bias for very small signals like a Microphone.  so 10 volts could way over run that amp.. @ zero bias you drive the grid positive very quickly

edit does it look like this?
http://www.chambonino.com/work/carlsbro/carl3.html

so the problem is the no cathode resistor, am i right? this should keep the grid negative?

I knew it was original meant as a PA, but the last users I know used it for guitar with good results, so maybe we have finally found its limits - it does not like very hot signals (even though it sounds good when pushed a little)

The one on the photo is very similar, ecxept  instead the "echo" i got a reverb knob. I think mine could be Mk.I but who knows.