Is this motorboating? (sound sample)

[BDM]

I've posted a short MP3 of my failed Deluxe Reverb. Is this sound what is referred to as "motorboating"? The sound is here:

http://www.bmumford.com/tmp/motorboat.mp3

[Mars Amp Repair]

HMMM,
could be motorboating....needed to last a bit longer to really tell. Almost sounds more like tremolo with weird hi freq overtones.

Motorboating is usually caused by the latter filters wearing (drying) out in the power supply. They work to decouple the amp stages from each other preventing the low freq oscillation (motorboating). How old is the amp? glen

[BDM]

I wasn't sure it was safe to run it longer. The amp is a 1972 silver face Deluxe Reverb. It worked until it started making this sound. I've got caps on order. The plate voltage on the preamp is very high --- around 400 volts.

The voltage on the two bypass caps before the standby switch is also very high, at 480 volts. But the caps are only rated at 450 volts. I wish I had ordered 500 volt caps instead of 450!

How come it's so high?

[Mars Amp Repair]

450 B+

Well as I see from this forum,
the line voltages have increased as time has gone by...what used to be 110v 30yrs ago is not 120 or so. The amps were designed to work on 105vac -110vac or so, so now everything in the B+ of these non-regulated amps is higher.
You also could be reading the voltages unloaded. That will raise them up, too. Many of those old Fenders have 500VDc rated caps from the factory, so 450V would be low anyway. good luck, g

p.s. remember the one hand behind your back rule when working with those hi voltages!

I wasn't sure it was safe to run it longer. The amp is a 1972 silver face Deluxe Reverb. It worked until it started making this sound. I've got caps on order. The plate voltage on the preamp is very high --- around 400 volts.

The voltage on the two bypass caps before the standby switch is also very high, at 480 volts. But the caps are only rated at 450 volts. I wish I had ordered 500 volt caps instead of 450!

How come it's so high?

[R ski]

trouble shooting

I would start pulling the preamp tubes, wondering if the reverb tube

If the PI and power tubes remain in and the motor boating is gone......one of the stages befor.......must be the culprit

[BDM]

Hey! I pulled the third tube, a 12AT7, and the popping sound stopped. Both channels now amplify, but the vibrato doesn't work.And, though I can hear the reverb springs if I tap them, the guitar doesn't seem to go through them.

So what does this mean? The bypass capacitors must not be at fault because the amp works, and the 12AT7 that I pulled tested good. It seems odd, because according to the schematic the 12AT7 seems to be associated with the reverb, not the vibrato, which seems to be implemented by a 12AX7 (the fifth tube).

What do I look for that would cause popping sound in the tube circuit?

[stokes]

The 12AT7 is the reverb driver or input tube,that is why you hear the springs and not the guitar,the reverb out put tube is still there,so the sound of the spring gets out but the missing 12AT7 stops the guitar signal.That 400 volts on the preamp tube is bad,check the plate load resistor on that tube for starters,that 400v should be under 200v's,that aint caused by the slightly higher 120vac at the wall outlet.