'61 Ampeg Reverberocket oscillation

[hawkeyeinexile]

sounds like a 60Hz buzz. pulling the back panel & tapping all around w/chopsticks seems to point to the power supply area at the right side (looking from the rear). tubes (some more than once), caps & cap can have been replaced, everything else in that "power" section has been re-soldered nicely. thought it was fixed, but the buzz is back.

it has been exhaustively checked by our local amp tech, much replaced, cold solders found & corrected, etc. this buzz comes on after a few minutes of playing, esp. low E notes and down around there. at first intermittently, disappears after giving it a little slap on the top, then the noise comes back & stays on - is louder than the notes being played.

the effect can be reproduced by pulling the back panel & tapping around right side of the board (where the cap can, power supply stuff, etc. are). tube sockets appear to be fine.

amp worked fine for over a year of fairly hard playing (volume around 3-5 and with clean boost)

:confused:

[VacuumVoodoo]

If the amp is persueded to temporary good behaviour by a slap on the head it would indicate an intermittent open circuit either in the wiring or a component. All joints having been resoldered is not a guarantee that there's no accidental solder blob somewhere (no intention to discredit your amp tech), it happens.

On an amp that old I would double check all grounding and all components, voltage dropping resitors in power supply. Power resistors are usually wirewound and operate at elevated temperatures, an open circuit may not be directly apparent cold but makes itself known when resistor heats up - any crackling noises besides the hum ? Also the fact that the problem comes up on low notes seems to reinforce this suspicion.

If it's really only 60Hz hum it would point to grounding, perhaps intermittent open circuit in heater supply grounding/symmetrization.
120Hz hum points to post rectifier problem.

Which version of Reverberocket is it ?

That's all I can speculate about for now.

[hawkeyeinexile]

thanks, A.N.,

it's an early Reverberocket, 1961 looks like. this was reportedly the first production reverb combo (beat Fender by a few months). our tech went through everything, replaced all tubes, caps, and resistors where warranted, located & corrected a couple cold solders, tested w/scopes, his personal Strat (hey, nice Strat! ;)), ran it for a couple hours under a signal generator of some kind. thought it was ready to roll.

then the buzz started up again.

he's a good tech and a good guy, must've spent hours on this thing, but since the amp's still a little flakey, it's no charge to me. anyway, maybe it's time for an Ampeg specialist...

[BJF]

Hi Hawkeyeinexile,

Faults like this are the hardest to beat: works fine on testbench and then suddenly.

With the words of Kalle, grand old amptech: 'Come back when the fault is permanent'

Hm, going out on a limb here with some more meat on the bones provided by your tech:

Check poweramp for oscillations- this sometimes won't show up under dummyload condition, i.e. check with scope while buzzing is happening.

Know thy enimy ;)

I don't think you'd need an Ampeg expert to fix this, just patience and a bit of luck.
Ken Fischer of Trainwreck used to work for Ampeg just in case you'd feel you'd want to get hold of someone on the inside so to speak.

Just a few thougts
BJ

[hawkeyeinexile]

hey, thanks, Bjorn. a mutual friend of ours (who must have 5-6 of your pedals on his board ;)) is gonna call Ken soon and ask about the little Ampeg

:cool: