Weird EH12AY7 Problem

[Trout]

I use a pretty good number of 12AY7's in guitar amps, mostly Fender Stuff.

Just recently I have been getting an unusual hum issue with EH12AY7's

The hum increases with the volume control, and also if you put your hand anywhere near the glass envelope. The tube shield also increases the problem/noise.

It is almost like a ground is lifted, just not as loud.

If I replace the tube with other 12A_7's type tubes the problem stops.
So I guess it is safe to say they have a recent production issue?

I have 16 of these here out of the last 2 purchases. (10 one month, 10 more the next) 2 started out ok, but only for about 30 minutes.

Normally these are dead silent great sounding tubes which really had me thinking I was missing something.
They test fine in my testers, but clearly something is not right with the current batch from a Known Distributor.

Any thoughts?

[Miles Prower]

Looks like an RF oscillation. Grid stoppers?

[Trout]

Originally posted by Miles Prower
Looks like an RF oscillation. Grid stoppers?

This is in the V1 position of a tweed circuit, these have 68K resistors to grid.
Schematic

Thing is, any other brand tube, 12AX7's ect are not doing this. It is almost like they changed something or had a production mistake.

The ones I bought 6 months ago do not exhibit this problem at all.

[audiopro]

Sorry, I don't have a solution. But I ran into a similar problem with the 12AU7EH. In one particular line of stereo amp, in the phase splitter circuit, after about a month of use the amps started to hum. Not too loudly but audible. Replacing the tube, or even just swapping out the phase splitter tube for the first stage tube in the same channel would eliminate the hum. This problem still exists and is one reason I no longer use EH tubes. No other brand of tube does this.
Other than tube replacement, I have yet to find a solution.

[Trout]

Originally posted by audiopro
Sorry, I don't have a solution. But I ran into a similar problem with the 12AU7EH. In one particular line of stereo amp, in the phase splitter circuit, after about a month of use the amps started to hum. Not too loudly but audible. Replacing the tube, or even just swapping out the phase splitter tube for the first stage tube in the same channel would eliminate the hum. This problem still exists and is one reason I no longer use EH tubes. No other brand of tube does this.
Other than tube replacement, I have yet to find a solution.

Right now, EH is the only new production 12AY7 available.

Many of the Tweed era amps use these in the V1 position and other than hunting down older vintage tubes(not practical) there is really no solution I guess.

Using the 12AX7's in that position really limits the tonal quality.

Wonder what the heck happened, Those EH12AY7's were outstanding for seems like ages.

[audiopro]

They must be having some quality issues. I used all kinds of EH tubes for years. Now I can't get quiet 12AU7's, reliable EL84's or good 12AX7's from them. I've since switched to JJ for my purposes.
12AY7's are not that popular. I would imagine NOS versions would not be that expensive.

Have you tried a 12AT7 in that position?

[kevinkr]

Does sound like those 12AY7A are oscillating, checked the circuit with a scope to see? Some minor lead dress and plate/grid stoppers may help. Does sound like there may have been a recent change or it may just be their transconductance is just a bit higher than usual. (I have had NOS 12AY7 oscillate under some circumstances as well.)

[Trout]

Originally posted by kevinkr
Does sound like those 12AY7A are oscillating, checked the circuit with a scope to see? Some minor lead dress and plate/grid stoppers may help. Does sound like there may have been a recent change or it may just be their transconductance is just a bit higher than usual. (I have had NOS 12AY7 oscillate under some circumstances as well.)

I put a few 6021's, RCA 12AY7's, and GE 12AY7's in the amp last night, no hum.
12AX7's work perfectly also, but they have more hair than needed/wanted.

I guess it must be a batch issue, I contacted the distributor these came from, but no response yet.