Lowered V1 12AX7 Heater Hum.

[HaroldBrooks]

I boosted the gain on my Gretsch 6156 amp a while back, and things worked out fine, but when I started rolling a few of my 12AX7 tubes into the first preamp position, I found one that sounded much better than the rest. It's an old Black long Plate RCA. The problem is it also hums quite a bit.

So just last night I had enough, and decided to have a look to see what I could do.

Gretsch 6156 Schematic :



This amp only uses the one half of the 12AX7 preamp tube, so on a hunch (the hum went to zero with the volume pot turned down) I just attached a 470uf 50V cap to cathode pin #8 (the unused side of the tube) and that cured it !

Boy was that easy, but I was surprised that even though that leg of the Preamp 12AX7 tube was not being used, it still injected hum into the circuit.

I realize I could have swapped tubes and gotten rid of the hum more directly, but believe me when I say this RCA is the best sounding one in this particular amp circuit (Bigger thicker sound but clear at the same time), and I got to use it !

I love this vintage amp even more now, and It's a joy to have it dimed, without much hum.

Anyone else come across this ? All and any comments are welcome !

[Leo_Gnardo]

Anyone else come across this ?

Wow, that's bizarre. And points to you for trying an unusual solution that worked so well, who would have thought of that? YOU!

[John G]

Hi,
Real interesting.
Just curious if you join pins 6,7,8 of the spare section (remove your cap of the cathode) do you get the same hum reduction ??
Thanks
John

[trobbins]

You could try to reduce the hum from that 12AX7 by dc elevating the heater supply. One simple way to try that is to move the heater CT going to ground, over to the output stage common cathode (as that is at some DCV above ground). The other typical method is to add a resistor divider off the lowest B+ voltage (eg. 22kohm and 100kohm, with a bypass capacitor across the 22kohm - that would give you about 1/6 of B+, so about +40V) and connect the heater CT to that divider.

Some valves have relatively low heater-cathode resistance, and that is often the reason why one valve will show some hum but others won't.

The disadvantage of adding more decoupling capacitance to the input stage cathode is that the tremelo level may be noticeably reduced.

[Helmholtz]

The disadvantage of adding more decoupling capacitance to the input stage cathode is that the tremelo level may be noticeably reduced.

Yes, but that's not what he did. He AC grounded the floating cathode of the unused triode system.

[trobbins]

Ta, I missed that.

Perhaps worth trying just a direct connection of un-used cathode pin 8 to pin 3 cathode resistor ground point (ie. the un-used cathode is floating, so dc coupling to ground should make no practical difference to ac coupling to ground).

The reasoning for the reduction is a little uncertain. If the hum was coupling from heater to grid, then the new connection could be acting as a better shield to reduce capacitance from heater to pin 2 grid - but that would indicate one tube had a different internal construction than the other 12AX7's tried.

If the hum was coupling through a degraded heater to cathode resistance interface in the operational triode, then shunting the other cathode to ground should have little effect as the heater supply has a CT connected to ground - which would indicate to me that there is hum voltage between the heater supply CT ground and the local triode ground. Interesting.

[HaroldBrooks]

Hi,
Real interesting.
Just curious if you join pins 6,7,8 of the spare section (remove your cap of the cathode) do you get the same hum reduction ??
Thanks
John

Will try this next time I have the back cover off. This amp (actually all of my amps...) have had the cover off far too many times, with me tinkering with the tone and I've now had to use tooth picks to fill in the screw holes a bit, and stripped the one screw that goes into the metal chassis. But I'm sure there will come a time when I dig in again, and I will try it just to see how it works. Thanks for the idea.

[HaroldBrooks]

You could try to reduce the hum from that 12AX7 by dc elevating the heater supply. One simple way to try that is to move the heater CT going to ground, over to the output stage common cathode (as that is at some DCV above ground). The other typical method is to add a resistor divider off the lowest B+ voltage (eg. 22kohm and 100kohm, with a bypass capacitor across the 22kohm - that would give you about 1/6 of B+, so about +40V) and connect the heater CT to that divider.

Some valves have relatively low heater-cathode resistance, and that is often the reason why one valve will show some hum but others won't.

The disadvantage of adding more decoupling capacitance to the input stage cathode is that the tremelo level may be noticeably reduced.

About a year ago I tried the heater ground elevation to a power tube cathode pin on another one of my Gretsch amps (6162) and it helped a little, but less than I would have thought.

That amp also had a deep hum that I resolved by simply connecting the bottom floor mounted chassis (the power section) to the upper pre-amp, tremolo, and reverb chassis. Valco split those two chassis in that series of 1960's twin amps. I had tinkered with that amp a raised the gain quite a bit, and I also raised the hum with the noise floor. Linking grounds from both of those amp chassis lowered the hum considerably, and again it was dumb luck, as I just had a hunch that the filament AC was creating the deep hum, and they have a cable that runs a long way from the power transformer at the bottom chassis, up to the preamp chassis. My hunch paid off.

I was just counseling someone a short while back that they should accept a little hum in any tube guitar amp, and now I'm the one with the obsession again !

LOL. Hey at 4:00am with nothing to do except think of the Coronavirus death and destruction, or fixing the hum in my amps, the amps won the battle !

I've had luck with using large cathode bypass caps on preamp tubes so far, and when I spotted that lonely #8 pin on my 6156 amp I figured, let's see what happens, the test was just an alligator clip away ! and it worked. There was also some hum on the other 12AX7 tube candidates and that took a nose dive as well, but this tube was the worst with hum, but now it's usable and sounds the best.

Guessing, and just a little bit of Luck. Responsible for half of my successes in Life !

[loudthud]

This amp only uses the one half of the 12AX7 preamp tube, so on a hunch (the hum went to zero with the volume pot turned down) I just attached a 470uf 50V cap to cathode pin #8 (the unused side of the tube) and that cured it !

I remember a version of the Blackface Bassman that only used half a 12AX7, they only connected the heater on the side that was being used. In your case that would be pin 5 not connected.

[Justin Thomas]

I'll have to look at mine to check that out. I never noticed.

For the record every 2x6L6 Bassman head all the way through to rhe SF ones & Bassman 70 us3d only one triode in an AX7, til it was retired in 80 or so.

Maybe that explains that deep hum in my Bassman 100. I added a tube so I could have an AB165 preamp... Hmmmmm...

Justin