!

[alexradium]

about the reply,it makes sense because from a technician point of view the correct way to get rid of parasitic oscillations should be using a scope and get the exact point where it occurs first...

BUT

being this amp as many others a classic circuit with just 3 stages of amplification,the field is restricted usually to the second or third.
My approach would be to add a small ceramic cap to a plate resistor and see what happens,it will shave off some high frequencies,maybe just enough for the gain setting you are using.
the other solution is using grid stopper resistors,but it seems to me they already are in circuit.
Best of all,if you can trace just this channel with the optional boost circuit i can see what is the best way to solve the problem,can you do it?

Alessandro

[demonufo]

Also I can't make sense of this reply

Basically, by the sound of it, he's saying it isn't a fault, it is a characteristic of the amp and you should learn to live with it.
Sounds like a cop-out to me. Or a lack of knowledge on how to make his own circuit work properly.

Without seeing what's in the amp it's pretty hard to tell you how to go about fixing it though.

[Yngve]

[alexradium]

Sure go ahead,especially the gain channel to the phase inverter and the pot arrangement

[Yngve]

[stef]

Shielding inputs and moving all input resistors on V1 tube usually works best - look at metroamp 2203 layouts (wiki) and suhr SL68 photos

[demonufo]

I concur. Mounting input grid resistors directly on the sockets of the first stages and using RG174U should yield big improvements. That amp is certainly a major candidate for parasitic oscillation by the looks of it.

[julkke]

Interesting how a professional builder says that oscillation is normal and at the same time doesn't use shielded wire...

[Yngve]

[alexradium]

I concur. Mounting input grid resistors directly on the sockets of the first stages and using RG174U should yield big improvements. That amp is certainly a major candidate for parasitic oscillation by the looks of it.

if you look to the sockets 1 and 3 you can spot 68k resistors soldered directly on the grid pins.
the white cable you see is usually a shielded teflon coaxial type,Brunetti uses it from the 90's.

[alexradium]

Any proper directions etc exact value caps and resistors so l can do this myself. Please give me direct instructions

in order to provide any info about possible mods it is mandatory to have a schematic circuit with actual values,i mean the path from input to V3 through to the end of the tonestack/master volume gain channel.
even if you provided more photos its impossible to see the underside board traces and my crystal ball is out of order...

[demonufo]

I concur. Mounting input grid resistors directly on the sockets of the first stages and using RG174U should yield big improvements. That amp is certainly a major candidate for parasitic oscillation by the looks of it.

if you look to the sockets 1 and 3 you can spot 68k resistors soldered directly on the grid pins.
the white cable you see is usually a shielded teflon coaxial type,Brunetti uses it from the 90's.

Funny, because I can see a grid resistor over by the input jack, with NO resistor on the grid of V1. And that cable definitely doesn't look like shielded cable to me.

[malig]

Hi can you post a closer photo of the board, because I can't see the branding on the capacitors ?
I want to know what capacitors are inside !

By the way, I can see a lots of things that makes it squeel :
- Microphonic V1 tube (try change it)
- To much noise at the first gain stage (carbon comp and hi gain is not a very good idea)
- No grid resistors in v1a input (you should add a 33k just after the input jack, as close as possible to the tube socket)

Thanks and good luck

[tonehunter101]

I'm bumping this old thread as I'm interested to know if Yngve got the amp sorted and what he thinks of the Brunetti Mercury in general. I tried to send him a PM but he doesn't have an email listed so I am hoping he'll chime in if he sees this.

Brunetti amps are super rare here, but I am interested in the Mercury.