Only JJ 12AX7 buzzing in Cathode follower position, Why?

[Gaz]

I also posted this over at the Amp Garage, but wanted to ask the brilliant minds over here as well:

I noticed an annoying little buzz coming at the CF position of a 5F6A. The buzz would completely go away when the CF's driving stage was grounded.

The amp would still buzz with no guitar and the gain controls turned all the way off. The amp has a "humdinger" (or "hum balance") pot across the heaters, and I noticed that I could null out the buzz completely! But...

For any given setting on the gain controls, the humdinger had to have a different setting. WTF?!

Now, good thing for me, I had another amp that was almost the same without the buzz, and the only significant thing I could find between the two was the tubes - one had all JJ 12AX7s, and other had a Tung-Sol reissue, Mullard re-issue, and NOS RCA black plate.

Sure enough, I switched the tubes and buzz went completely away. And sure enough, the buzz came back in the other amp! After some more tube swapping I narrowed it down to just the CF position. Every single JJ would buzz, while any other tube would be quiet as a mouse.

Any ideas?

[Merlinb]

Probably heater-cathode leakage; since the CF has a very high voltage between heater and cathode it tends to be most susceptible to this. Some valves can withstand it better than others, so it becomes a case of hand selecting. Looks like your JJ is the lucky one!

Even if you elevate the heaters, the peak heater-cathode voltage during signal conditions can still be pretty high, and could conceivably erode the h-k hum performance over time. Nothing lasts forever.

[soundguruman]

modify to DC heater

[überfuzz]

modify to DC heater

Or lay off the JJ 12AX7 tubes.

[kg]

i use dc heaters. the issue isn't hum injection directly, it's, as merlin said, the HK insulation breakdown.

the only way to solve it by using "better" tubes or by decreasing the HK voltage.

[clyde1]

Try Sino 12AX7's.

[soundguruman]

You may buy 6 Sinos at a time, so you can pick just the right one. The JJ has a spiral filament which seems better for a plate follower.
But it's still a good tube...like the workhorse standby.

[Gregg]

Or lay off the JJ 12AX7 tubes.

You should do it also with Tungsol, Sovtek LPS, EH, Mullard (new production) and others as they tend to die when used as CF:

http://music-electronics-forum.com/t9514/

Try Sino 12AX7's.

As strange as it may sound chinese tubes can withstand the higher h/k voltages when used as CF.

Elevating the heaters usually cures the problem.

[Bruce / Mission Amps]

I can remember when just about every other cheap Russian SOVTEK 12AX7A would do this and eventually the cathode follower circuit, (cathode to filament voltage issue in Marshall amps) would blow the flippin' tube up.

[Gregg]

Maybe that's why they started elevating the heaters in their amps.

[loudthud]

Refering to the RCA tube manual, the 12AT7 is rated H-K +/- 90V peak. The 12AZ7A is almost identical except the H-K rating is +/- 200V peak. The difference is the heater for the 12AT7 draws 300mA at 6.3V and the 12AZ7A draws 450mA. Apparently the increased H-K rating is at the expense of lower heater efficiency.

[Gaz]

For some reason I wasn't getting notifications for this thread... Sorry! A little update: I settled on a non-spiral filament Chinese tube, and it's as quiet as a mouse. As I said, other spiral filament tubes were quiet too, just not the many JJs I auditioned. I failed to mention before, but the wiper of the humdinger is elevated to a well-filter DC voltage of 70vdc.