ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Motorboating Princeton Reverb

10/15/2004 9:46 PM
Tom	Motorboating Princeton Reverb A friend brought me his Princeton Reverb to look at, with the complaint that it was motorboating. Since it had the original can multi filter cap from '71, I replaced it, expecting this to be the culprit. Unfortunately, the put-put remains, though it seems intermittent. Playing through the amp right away has staved off the noise, and it has at least once stopped cold after 15 or 20 minutes of the amp being on. The noise does occur consistently if the amp is turned on and left to sit; it kicks in after about a minute or two of warmup, starting high and cruising down to a steady rate within 10-15 seconds. The volume and tone controls have no impact on it; the reverb does it's thing to it, and the tremolo does, too. Pulling the 1st tube (first two preamp stages) prevents it (and anything else) from happening. Pulling the 2nd tube (rev. driver) doesn't stop it, and pulling the 3rd tube (rev. recovery and third preamp stage) silences it as well. So, it's originating in the preamp, but after the tone and volume controls. The B+ taps for three tubes are all off the same (last) filter cap, which seemed like an oscillation waiting to happen, but obviously this isn't a big deal in other PRs, and the caps are now new. Any ideas where else to look for the problem? Besides the filters and fresh tubes, the amp is stock. Thanks, Tom

10/15/2004 9:46 PM

Tom

Motorboating Princeton Reverb

A friend brought me his Princeton Reverb to look at, with the complaint that it was motorboating. Since it had the original can multi filter cap from '71, I replaced it, expecting this to be the culprit.

Unfortunately, the put-put remains, though it seems intermittent. Playing through the amp right away has staved off the noise, and it has at least once stopped cold after 15 or 20 minutes of the amp being on. The noise does occur consistently if the amp is turned on and left to sit; it kicks in after about a minute or two of warmup, starting high and cruising down to a steady rate within 10-15 seconds. The volume and tone controls have no impact on it; the reverb does it's thing to it, and the tremolo does, too. Pulling the 1st tube (first two preamp stages) prevents it (and anything else) from happening. Pulling the 2nd tube (rev. driver) doesn't stop it, and pulling the 3rd tube (rev. recovery and third preamp stage) silences it as well.

So, it's originating in the preamp, but after the tone and volume controls. The B+ taps for three tubes are all off the same (last) filter cap, which seemed like an oscillation waiting to happen, but obviously this isn't a big deal in other PRs, and the caps are now new. Any ideas where else to look for the problem? Besides the filters and fresh tubes, the amp is stock.

Thanks,

Tom

Replies:

Lee Try new preamp tubes.... -- 10/15/2004 10:49 PM