Coping (or not) with microphonics in express clone

[jurgen]

I've had about 12 diffent tubes in V1. All were reasonably well behaved in other amps; some new, some NOS etc. All are doing a good impersonation of a ribon mic in my homebrew express. I've replaced the socket, tried stranded wire from the volume pot to the grid. Damping the chassis helps a bit. So before I drop $200 on A Brimar C4004, anybody got any ideas?
I read that some tube radios had rubber isolated sockets. Anyone seen this stuff?

EDIT voltages all in the ballpark , all components measured in spec,
good continuity between any two ground nodes, layout and lead
dress are pretty much francesca


Thanks

[drhulsey]

...anybody got any ideas?

One amp builder, whose name escapes me, put rubber O rings around the bottle about 2/3 of the way up from the base. They had a green color and HAD to be heat tolerant. I don't know how valid his idea was, but he thought it helped cut down on microphonics

[gearhead]

When you say microphonic, what are the circumstances? Am sure you've already thought of these, but might be worth a second mention:

Is it microphonic when no instrument cable is plugged in?
If not, have you physically separated the cab and chassis?

[novosibir]

... but he thought it helped cut down on microphonics

And he probably though, that this is also a good idea, to sell HIS tubes to the folks, although it was only a regular chinese. The idea is it, what brings the money in - not the product

Long ago I've once buyed 'matched output tubes' from this company - and after I've put it on my tester I was asking myself, after which values they have 'matched' the tubes - maybe after lenght and diameter, but not after electronically parameters

Larry

[drz400]

I've had about 12 diffent tubes in V1. All were reasonably well behaved in other amps; some new, some NOS etc. All are doing a good impersonation of a ribon mic in my homebrew express. I've replaced the socket, tried stranded wire from the volume pot to the grid. Damping the chassis helps a bit. So before I drop $200 on A Brimar C4004, anybody got any ideas?
I read that some tube radios had rubber isolated sockets. Anyone seen this stuff?

EDIT voltages all in the ballpark , all components measured in spec,
good continuity between any two ground nodes, layout and lead
dress are pretty much francesca


Thanks

Sometimes the routing of the wires going to the power tubes and their proximity to the speaker jacks will make the amp overly sensitive to microphonics. try routing the wire to the the grids of the power tubes on the front side if you dont already

[drhulsey]

... to sell HIS tubes to the folks...

They were put on by an amp builder not a tube seller. I have no opinion on their effectiveness.
There's a discussion on 18watt.com about microphonic dampening:
http://www.18watt.com/modules.php?name= ... 811#175146
You be the judge

[jurgen]

Thanks for all your replies

I've pretty much chased this down to the second stage grid wire.
When pressed very firmly to the chassis, the problem is reduced by about 90 %.

[rfgordon]

jurgen,
Ah, then in that case you have the dreaded parasitic oscillation. If you can get the wire to stay put in a good spot, then by all means glue it there. You could also use shielded wire or do a wire-wrap shield.

[David Root]

+1 on shielded wire for that. RG174 coax is a good cheap option.

[pamaz67]

Viton o-rings ( the green ones) are an old trick from the hifi world to dampen mechanically the tubes. Sometime is better to use 2 or 3 of them.
In hifi somebody was making some tube dampeners ( do a search in google)
It works more or less.
The easy solution is to get siliconic tube ( that can withstand hi temperature) and cut some rings out of it to be plaed around the preamp tubes.
Ciao
Paolo

[jjman]

Perhaps a microphonic coupling cap?