Microphonic Valves, is there a cure?

[Toppsy]

The Mullard ECF80 valves on my KaT6550 power amp have become very microphonic. Is there anything that can be done with these treasured valves to cure this?

They are fitted with 2-damping rings per valve. Though this has a little effect the noice from the valves can still be heard through my speakers.

Am I doomed to having to buy new valves? If so what recommendations, please.


Colin

[steve s]

Originally Posted by Toppsy

The Mullard ECF80 valves on my KaT6550 power amp have become very microphonic. Is there anything that can be done with these treasured valves to cure this?

They are fitted with 2-damping rings per valve. Though this has a little effect the noice from the valves can still be heard through my speakers.

Am I doomed to having to buy new valves? If so what recommendations, please.


Colin

hi colin, don't think theres much you can do.. i have a few of those i can let you have cheap.. pm if you interested

cheers steve

[Toppsy]

Thanks Steve, you have PM

[Ianm2]

Just to dispel an illusion, the dampers are useless.

The mircophony is due to the strucure of the valve vibrating, when tapped or via vibrations, the plates, wires, nothing to do with the glass.

Isolate from the floor.

Some old ones seem worse in this respect.

No little expensive rings goning to make any difference to this I am afraid.

[Greg.]

Another sweeping statement Ian?
I don't agree with you. In many valves the internal structure is in direct contact with the glass envelope. Have you never noticed that some valves are microphonic when first switched on and cold, but lose that microphony as the valve warms, no doubt owing to the minute clearances between the internal structure (mica plates) and glass being taken up by heat expansion. Damping rings do have an effect on sound, but you need to have the sensitivity of hearing to recognise the difference.

Best wishes,

Greg

[John Caswell]

Hi all,
I am on Ian's side here, one only has to look at the internal stucture of a valve to see that it is bound to be prone to microphony and no the internal structure is not in direct contact with the glass Greg, the grids are ellipsoidal shaped and sitting effectively in free vacuum only attached at each end of the major axis. Tahe a valve apart to see what I mean! The ECF 80 along with other valves WAD used are primarily RF types and nothing has been done to them to reduce microphony whereas EF86,7571, 12AY7 etc care has been taken. As for the ECC88 and 6AU6 well nuff said.
That should put the cat amongst the pigeons

John aka Dr John

[Greg.]

Hi John,

I really respect your knowledge and would not generally want to argue with you, but I can't accept what you claim. Whilst I don't have the specific and comprehensive valve knowledge you have, my eyes do not deceive me. I'm now looking at a GE JAN - 6072A. All the internal construction is spaced by two top and bottom plates which are undoubtably designed to be in contact with the inside of the glass envelope to secure and centralise position of the metal parts. Cold, this interference will probably be very loose, but IMHO tightens up with heat and consequently reduces any microphony present at fire up. That's not just my theory.... that's the effect I've found from listening to and checking valve microphony when a system is first fired up and then later when well warm. I've found a microphonic valve when cold will often loose this effect when it's once warmed up, and that applies to both signal and output valves (EH300B goldgrid included).

My conclusion is that not all valves are microphonic, but some are and others can become so over time. The heating of the amp/valve can reduce or irradicate this feature unless it is the end of life for the valve. Going on from there, if the effects of heat expansion do reduce the microphony of the valve, the addition of damping rings makes total logical sense, and in my experience are a benefit worth doing.

I'm happy to discuss further because I feel very strongly on the point I'm making.

Best wishes,

Greg

[John Caswell]

Hi Greg et al,
Whilst your comments about the supports are quite correct, the wound grids that are attached to them are free to flap about and no amount of damping applied to the valve glass with fix this.
Even with frame grid valves where the grid is wound tightly formed and stretched between supports because of the closeness of the grids to anodes and cathodes (required for high gm) the slightest movement will tend to give you microphony, this is why although the ECC88 is a good audio valve microphonically it is ****, crikey I am beginning to sound like Paul B.
You can liken it to a diving board or bridge fixed at one or both ends. Those parts may be stationary but the bit in the middle will move under some influence. The heater also is not rigidly fixed inside the cathode but just placed there, this can and will add to the microphony. The ONLY way to stop everything from being microphonic is to encapsulate it rigidly (Transistor?)
If you have a look in the RCA valve data manuals it gives a sectional view of a valve and you will see exactly what I mean.

John

[NickG]

Yes, thats all very good, but for the grid to move, the energy needs to get to it from somewhere. The only place, is outside the valve, so whatever happens inside the valve, the conditions on its pins and envelope (only two bits that connect to the outside) MUST have an impact on its microphony. So by using damping to reduce the Q or the glass, must then alter the internal movement.

So "those expensive rings" have every chance of changing things.

What if the resonant frequency of the grid, was the same as the glass, must be worst case, just by changing the Fr of the glass, the effect would be to break the coupling between them.

IMHO.

[Shane]

Looking at this from the point of view of someone with no experience of valves but knowing a little about mechanics, I wonder how much of the sound getting into a valve is airborne through the glass envelope, and how much is structure-borne through the amp chassis, feet, shelf, floor etc. Are amps with microphonic valves more affected by positioning and things like cones or spikes? Is there a case for isolating valve bases in the same way that we do with arm boards? Could this also sometimes be a source of hum, if a mains transformer is vibrating the chassis of an amp at 50Hz? Extrapolating that, do output transformers ever vibrate at audio frequencies, and could that cause some sort of low-level feedback effect?

Returning to the effectiveness or otherwise of damping rings, would it not be possible to analyse the output of an amplifier with no input signal whilst it is standing in a powerful sound field? If there is a difference, it shouldn't be too hard to see or hear, surely?

Lots of questions, and I have no idea of the answers, but an interesting research project if anyone has the inclination and the wherewithall.