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I read in older posts that unmatched tubes give a sweeter tone. I made this experience a few years ago already. For example, in my 18 the tubes are about 8mA apart and give a really nice tone. My PT got no CT for the heating, so I made it by using two 100 ohm resistors. So here's my question: Would a linear pot with 250 ohms help to compensate the hum caused by dismatched tubes? And is there a upper limit in using unmatched tubes?

Thanks a lot
musicheals

Be careful about which noises you try to eliminate. Often the annoying noises on idle are just part or side effect of what makes the amp sound so good in the first place. Do you want that polite Toyota or the anxious Ferrari?
Heater hum can be avoided on wiring properly

With a pair of matched tubes there is indeed no hum. But with a unmatched pair it definitely sounds much better but with a noticeable hum. Not much, but a bit annoying. I'm not sure if it's worth to drill another hole in chassis....

Thanks

Try elevating the heaters first. Disconnect the two 100 ohm resistors from ground and connect them to a voltage divider from B+. Anything from about 50V to 70V is sufficient. If this doesn't help, it might be a bad tube. Some just have excessive H-K leakage.

Years ago, I cross-wired a DPDT switch and used it to change the phase of the heater wiring tube-by tube in a preamp circuit. I found that it was possible to achieve a degree of hum cancellation if the heaters were each phased a certain way. Those were octal tubes though. Nothing like that should ever be necessary with 12AX7s and other tubes designed for high gain service.

Jack

The tubes are ok. One measures 30mA and the other is 38mA.
But my question more was "could a humdinger pot eliminate the hum caused by different anode current?"
I tried heater elevation on EL84s cathode, but no change...

Unbalanced current at the anodes could theoretically allow power supply ripple to modulate the output transformer. If that's the source of the hum, I don't see how a hum pot would help. A hum pot only balances the EM field across each cathode.

Jack

Ok, thanks a lot Jack

musicheals wrote: ↑

Mon 05/24/21 11:09 am

The tubes are ok. One measures 30mA and the other is 38mA.
But my question more was "could a humdinger pot eliminate the hum caused by different anode current?"
I tried heater elevation on EL84s cathode, but no change...

The EL84s cathodes are only at around 12 to 14V, you should be using around 50V for elevation. I've found that works extremely well, even with non-matched tubes. Otherwise, you need to look into layout, grounding and lead dress to cure your hum.

FYI, as I never buy matched tubes, I sometimes just pick a random pair of tubes from my stash which give least hum. That gives some great tones

Thanks a lot ZP!
I will try a higher voltage for heater elevation.

musicheals