Ampeg B-15 N - Weird noise

Hi there,

i've got myself an old Ampeg B15 N which had his original 110v power transformer switched to an Ampeg european 220v one.

It all works, i mean, i can plug a bass/guitar and ear it coming out of the speaker.
but i have a relatively quiet but unbearable oscillating noise at all volume. (The noise's volume changes with the amp volume.)
i've changed the pre tubes and the can cap, still the same problem.
Here's attached a sample of it recorded through my phone pointed at the speaker.

does anybody know this sound ? and of course, does anybody know from where it could come out ? what do i need to check ?

i'm not a high level electronic guy, and don't have a variac or an oscilloscope.
only a multimeter, an iron and some common sense.

thanks in advance,


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so i replaced the 5 capacitors (marked red below),
and there's a noticeable improvement, thanks a lot for this !
but still, when i increase the volume i still get the same, but quieter, noise.
anything i can do to further improve it ?

thanks again !


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turk 182

2012-10-26 3:03 pm
i would clean or replace the extension speaker jack (common fault due to corroded contacts).

the only other caps i would replace are the phase inverter output caps the .022's.

is the feedback resistor off tolerance?(10k from output transformer secondary) while your at that try replacing the cathode bypass cap in the phase inverter (25uf 25 v)
Last edited:
... a Few mounths later.

So, i've changed it all and nothing worked.
But here's an interesting evolution we discovered.
When i took and plugged it back home the amp was dead silent.
Back to the studio, oscillation's back.
We also found that the oscillation gets considerably quiter when we plug the amp in the 220 outlet through an extension cord (10m, around 32 feet).
And even more quieter when plugged into one particular outlet (we've got 6 of them in the same room, all on the very same electrical circuit).
i don't think anything's wrong in our electrical installation (while i can't be 100% sure), and none of our other tube amps show anything like that.

any idea ?
Then it´s 98% possible it´s coming through the mains cord.

I fear some terrible grounding mistake, including making the amp dangerous, was made when power tramsformer was replaced.

The original transformer was Two wire (hot and neutral) - UNGROUNDED - with a Death Cap rated for 120V mains, VERY DANGEROUS , specially with 240V European mains.

You need a THREE wire plug and power cord, connecting Live and Neutral as the original ones and the Ground wire, safely grounded to chassis, following accepted norms, to its own terminal.

Start with that and report results.

IF problem continues, we´ll troubleshoot the amplifier, making a few tests.

Random guessing won´t carry far.
Hey JMFahey,
thanks for your reply, and once again sorry for the delay ...

So, a 3 wire plug has been installed when the amp got switched from 110 to 220.
But the guy who replaced the transformer indeed left the death cap in place.
following your message i got rid of it yesterday.

now everything's how it should be at this point.
But the oscillation remains ...

let me know how to "troubleshoot the amplifier, making a few tests" and i'll do it.

Thanks again for your answers, i feel pretty frustrated with this one !

Hi picoDumbs,

I checked every pots, every grounds, resoldered when in doubt, secured every moving part and double checked with an ohmmeter.
still the same noise.

Also, two things that may be interesting to consider :
- when i ground pin1 of the first preamp tube (in betwin the two amp stages) the noise disappears. and such on both channel
- the amp is running pretty hot voltages compared to what the schematic shows.
for example Plates on V1 are supposed to run 165 and 175 volts but i read 195 and 200v on my amp.
V4 and V5 are supposed to run 430v, mines are 470v.
dunno if it's interesting or not.

any other ideas ?
Thanks again for diving in picoDumbs,
as i said in one of the firsts post of this thread, transformer is new, every filter caps have been changed, tubes are all new, included the rectifier tube.
As turk 182 suggested i also changed the phase inverter output caps.

Also in a previous post, the amp is dead silent when plugged at home, it's motorboating when plugged at my studio.

I'm pulling my hair off on this thing ...

turk 182

2012-10-26 3:03 pm
is it oscillating or rectifying?

if the sockets are dirty and there's leakage between pins it's possibly oscillating.

but you mentioned it was only doing it at the studio location, could there be enough RF from a switch mode supply or other source that is being received as rf in the high gain preamp stages? likely AM band...
"oscillating or rectifying ?"
i said it's oscillating because its a cyclic sound (sound clip at the beginning of the thread), i don't know what difference you're implying betwin rectifying and oscillating.

"could there be enough RF from a switch mode supply or other source that is being received as rf in the high gain preamp stages?"
no i don't think so, we've got multiple old things in the studio that are working just fine, at the beginning i thought the led lights we have there could be the cause but no.


Paid Member
2003-06-12 7:04 pm
Maine USA
...dead silent when plugged at home, it's motorboating when plugged at my studio....

So what is the difference?

Wall voltage?

Power smoothness? A couple of times I have heard-of or experienced fluttery power. Often a mis-adjusted distribution regulator undecided whether to use the "+3V" or "+6V" tap, so jumping 3-6-3-6-3-6 all day long. Oddly, if the taps are small enough, this is not obvious in incandescent lighting (I saw this in a inky-lit theater lighting lab) but the jumps may come through a lightly-filtered DC supply.