best way to diagnose motorboating?

[Icetech]

Hi all... i was working today on my dumble inspired amp trying to track down a squeel, which ended up being the ground on the reverb transformer, weird eh?

Anyway.. got that sorted, turned up the amp and soon as i hit a chord or anything and i get the PUTT PUTT PUTT PUTT PUTT and both power tubes (el34) i can see a flash in each of them that goes in the same time with the putts.. i tried changing the leads from the pi to the power tubes around, they just made the amp scream soon as i started it:(

I know this is pretty general without a layout or voltages (company showed up.. hes waiting while i write this)

Could crap power tubes cause this? or is there a general diag for motorboating? thanks guys...

[Blindog]

This is from the Tube Amplifier Debugging Page: http://www.geofex.com/ampdbug/ampdebug.htm

"In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)"

Hope you get it worked out.

Mark

[Icetech]

yeah, its basically a new amp, built it awhile back.. i moved it back to its normal 4x12 with the airbrake and the problem went away... will figure it out if something smokes:)

[strato17]

I have an early 90's AC30 TBX that seems to have this same issue, but only after I play the amp for a while. I never knew there was a a term for it. It really does sound like an idling motorboat. I suppose this is the same problem that you are having?

Is there any harm done to the amp when this happens? Any negative effect on tone, besides the annoying noise?

[Alexo]

Motorboating can be caused by having resistors of too low a value in the B+ supply. I.e. if you have a 1K5 between two preamp stages' filter caps, you may get motorboating that could go away if that R was increased to 10K.

It can also be caused by a power supply that is not decoupled enough (too many stages being fed by the same PSU node).

HTH.

[Luthierwnc]

For motorboating I'd check the B+ supply. There was an excellent comment above me that you need to have enough resistance in each decoupling stage. Temporarily you can either hang another cap in parallel or increase the series resistance in the string.

One caveat, motorboating happens too fast to see in a tube as a rule. If the usual techniques don't help, I'd also look for a wrong connection in a gain stage or going into the inverter. That can also produce a popping but much slower. A quick check for that is adjusting a control will reduce or intensify it. Food for thought,

Cheers, Skip

[Icetech]

thanks guys, i will have to look into that.. as my B+ setup is rigged at best:) You can see it plain as day in both power tubes when it happens... will figure it out eventually:)

[Luthierwnc]

If you can see it, do they flash at the same time or alternate?

[Icetech]

same time, a nice little blue flash in the middle of the tubes...