300b faultfinding

[pre65]

Hi-i had a problem with my 300b amp when i noticed the stereo image was not central.

When i checked the fixed (adjustable) bias for the 300b one channel was less and the adjuster could not bring it up.

Checking voltages with the valves removed showed no variances between channels so i replaced the EH 300b with the original Chinese ones,reset the bias, and now all seems well.

So it looks like a dud EH valve !

One thing i had noticed was that the EH 300b valve (s) seemed very microphonic,is that usual ?

[Nick]

One thing i had noticed was that the EH 300b valve (s) seemed very microphonic,is that usual

Depends on the meaning of very, they all will be to some extent, but some more than others.

the 300b has a long fillament, so this will be prone to vibration.

[Greg]

My EH 300B Gold Grids can be a bit microphonic at first switch on but get much better as they heat up.

[Nick]

Yep, as they heat up the bits expand taking up the slack.

[Greg]

Hi Nick,

I'm very pleased you posted that. There was a similar discussion a while back on WD and some, Dr John in particular rubbished my corresponding view that you now offer. Therefore, I stand on this and say, don't consider the microphonics of any valve until it is well hot. The heating will expand the supporting mica plates which will cause them to physically interfere with the glass envelope and that expansion will make the overall structure more rigid, thus reducing microphony. On fire up, many tubes can sound microphornic, but give them a while and all will be tight and dull sounding to the 'finger flick' test.

Best wishes,

Greg

[Nick]

Hi Nick,

I'm very pleased you posted that. There was a similar discussion a while back on WD and some, Dr John in particular rubbished my corresponding view that you now offer. Therefore, I stand on this and say, don't consider the microphonics of any valve until it is well hot. The heating will expand the supporting mica plates which will cause them to physically interfere with the glass envelope and that expansion will make the overall structure more rigid, thus reducing microphony. On fire up, many tubes can sound microphornic, but give them a while and all will be tight and dull sounding to the 'finger flick' test.

Best wishes,

Greg

Seems to be common sense to me. After all we can hear them moving about as they warm up. Wouldn't do that if there wasn't differential expansion.