12AX7 HUM

[big_teee]

Occasionally I come across a 12AX7 tube that Hums in my amp.
I just bought a $16.95 new issue Mullard 12AX7 and it hums in V1.
I had some 3 year old Tung-Sols, and they are very quiet.
What causes the Hum on certain tubes, and can't that be found when the seller tests the tubes?
Terry

[MWJB]

It can be found by the seller if they check for it, in high gain amps & reverb channel (V" in BF/SF Fenders springs to mind)amps. Sometimes sellers advertise "7025/low noise" variants of 12AX7, sometimes these are just a low gain tube like the Sovtek 12AX7WA or sometimes a regular gain 12AX7 that has been screened for noise/microphony. Try the noisy 12AX7 in a less noise critical position in the amp like a phase inverter, or a tremolo tube and buy another screened tube for V1.

[Chuck H]

It's been my experience that when the same 12ax7 has very dissimilar gain or bias on each triode there can be an increase in hum. Like the cathode follower tube in Bassman/Marshall type amps or the cold clipping stage in high gainers. The solution the high gainers use is to employ DC preamp filaments or sometimes a tricky sort of false lead off one side of the filament wind used to tune out the hum by manipulating where it's radiant field is in the amp for more effective cancellation in twisted pair circuits. There was the "hum balance" control used on SF Fenders too. Limited, but an improvement over nothing at all. Then there's the DC biased twisted pair. Can be very effective if the DC elevation is high enough to render the imbalance insignificant. Usually 30-40 volts does it. My solution has been to select tubes that are imbalanced in the right way that the hum balances in these troubled positions. I can usually find one that's pretty quiet for any troubled socket.

FWIW I have noticed a huge increase in new tubes being prone to hum. I think it's sloppy design. On he up side I also think that overall preamp tubes are sounding better.

[big_teee]

It's been my experience that when the same 12ax7 has very dissimilar gain or bias on each triode there can be an increase in hum. Like the cathode follower tube in Bassman/Marshall type amps or the cold clipping stage in high gainers. The solution the high gainers use is to employ DC preamp filaments or sometimes a tricky sort of false lead off one side of the filament wind used to tune out the hum by manipulating where it's radiant field is in the amp for more effective cancellation in twisted pair circuits. There was the "hum balance" control used on SF Fenders too. Limited, but an improvement over nothing at all. Then there's the DC biased twisted pair. Can be very effective if the DC elevation is high enough to render the imbalance insignificant. Usually 30-40 volts does it. My solution has been to select tubes that are imbalanced in the right way that the hum balances in these troubled positions. I can usually find one that's pretty quiet for any troubled socket.

FWIW I have noticed a huge increase in new tubes being prone to hum. I think it's sloppy design. On he up side I also think that overall preamp tubes are sounding better.

Your correct on the type of amp.
It's my new 2204 on High input, which uses 4 gain stages I believe.
I have had a tough time sorting out the hum, but some tubes are better than others.
Right now I have the Filament center tap taped up and connected 100-200 ohms to ground to each side of the 3.1Volts.
The 200ohms seemed the quietest.
Re did the grounds and that helped.
I think the DC would be the best bet, but don't know how to accomplish that with what's in the chassis!
I've re-routed and checked everything.
It's coming from V1.
You can ground the sig input to V2 and all is quiet.
Terry

[Gtr_tech]

A master vol (jcm800 type) 220* has 3 gain stages in the front end.

[big_teee]

A master vol (jcm800 type) 220* has 3 gain stages in the front end.

I believe you, but can you explain!
I thought on high it uses both V1A, V1B, and both V2 Stages.
Where am I going wrong?
Terry

[Gtr_tech]

When you plug into the "high" input, all 3 stages are used. With the "low" input, the first stage is skipped and signal goes straight to the second stage. Thats why it sounds muffled and crappy plugging into the low in.

[Bruce / Mission Amps]

I like the reissue Mullard and Tung-Sol 12AX7s but I also have had what I think is an abnormally large number of them being hummy... I don't know why.
They still sound pretty good but the hum us objectionable to some people... until they play on stage or with other musicians...

[Chuck H]

Yes, we (guitar players) get a little fussy when we're practicing at home and are allowed to focus on the constant hummmmmmm... Probably not as big a deal as we make it out to be. Still, there are a lot of reasons amps hum. If the tubes are hummy I can get over it a little better, but... I built a new amp, my standard model, for someone and as soon as I flipped the standby switch "HUMMMMMM". I spent about an hour going over my circuit and grounds. I've built six of these so the fact that it was built, grounded and layed out just like the others and working fine had me confused. Finally the light went on. I tried a different V1 and 'ta da' no more hum. It was a brand new Sovtek right out of the box. I've unboxed several others since with similar issues as well as a few other brands. I use to get all my preamp tubes via salvage at a local electronics junk yard. As in 'used tests good' for about a buck each. Then I would test them in an actual amp for microphonics and tone. Even these "junk" tubes never had the hum ratio per capita that current new tubes do. It's sad IMHO.

[soundguruman]

Higher gain tube more hum, higher gain amp more hum,
But all you had to do was make the heater supply for the first two tubes into DC. Now the hum is gone and the gain is on?

You can learn a lot from those Hi Fi Guys. Ceramic disk capacitors can also induce hum into the audio path, from the AC field around the heaters.

[Koreth]

You can learn a lot from those Hi Fi Guys. Ceramic disk capacitors can also induce hum into the audio path, from the AC field around the heaters.

I don't follow this. I understand ceramic disk capacitors can introduce noise into the audio path through being microphonic, but how does the AC field around the heaters play into that? Microphones sense the vibrations of a sound and turn them into electrical signal. So if the caps were vibrating, I could see them being a source of hum. But an AC field is not a mechanical vibration. If the hum were entering into the signal path via some kind of capacitive coupling from the heaters to the ceramic caps, I might buy that, but then wouldn't *any* capacitor be susceptible to such, not just ceramics?

[soundguruman]

OK you asked for it...
Ceramic disk caps induct 60 cycle fields, and inject that 60 cycle right into the audio path. That's why you should replace them, particularly, if they are used in the audio path.

These are most noticeable in the application of the "bright" cap used across the volume control, or on the "bright" switch. Also the hi pass filter, used for the treble control. You will also spot these in other places, within the circuit. The closer the cap is located to the input stage, (or IN the input stage) the more 60 cycle will be introduced into the audio path.
The 60 cycle surrounds the heater wires, where these are used for the preamp, is inviting 60 cycle to be amplified.
That's one big reason we use DC heaters, especially for the first two stages of the preamp. The 60 cycle does not have the opportunity to enter the highest gain stages of the preamp, and therefore, is not amplified.

You are puzzled as to why a capacitor can INDUCT? I thought capacitance was the opposite of inductance.
They do, in fact modern capacitors are deliberately designed to be "non-inductive." The capacitor acts as an inductor at frequencies outside of it's intended operating range. It's just like an antenna. The antenna picks up 60 cycle, from the heater wires mainly, or the power transformer. There is no shielding to prevent it on a ceramic disk.
Your guitar pickups, pick up 60 cycle, from electrical equipment, and that 60 cycle is amplified also along with the music. Same exact problem.

Trivia: In silver face and black-face fender amps, you will always notice a low, 60 cycle hum, especially when the amp is cranked up.
Guess where at least half of the hum be coming from? If you do not know by now, start at the top and read again.

Notice: In an amplifier the parts talk to each other. They do not care where the wires go. Try for a moment to realize there are many other signal paths besides the ones connected by wires.

Any time you put two wires side by side, you have made a capacitor. Now the AC traveling through those wires is mixed together, it does not care if the AC is 60 cycle, music, anything, it still mixes together. Not the current, but the voltage, yes. Now that the 60 cycle has entered the audio path, it is amplified, you hear hum.

There is AC voltage on the chassis of your amp. It is not a lot, but it's there. Now you are laying a wire on the chassis. That tiny little AC voltage is inducted into the wire, and amplified.

that is why, if we want low noise, we need to start with non inductive capacitors and non inductive resistors. Then the parts talk to each other less.

[big_teee]

OK you asked for it...
Ceramic disk caps induct 60 cycle fields, and inject that 60 cycle right into the audio path. That's why you should replace them, particularly, if they are used in the audio path.

These are most noticeable in the application of the "bright" cap used across the volume control, or on the "bright" switch. Also the hi pass filter, used for the treble control. You will also spot these in other places, within the circuit. The closer the cap is located to the input stage, (or IN the input stage) the more 60 cycle will be introduced into the audio path.
The 60 cycle surrounds the heater wires, where these are used for the preamp, is inviting 60 cycle to be amplified.
That's one big reason we use DC heaters, especially for the first two stages of the preamp. The 60 cycle does not have the opportunity to enter the highest gain stages of the preamp, and therefore, is not amplified.

You are puzzled as to why a capacitor can INDUCT? I thought capacitance was the opposite of inductance.
They do, in fact modern capacitors are deliberately designed to be "non-inductive." The capacitor acts as an inductor at frequencies outside of it's intended operating range. It's just like an antenna. The antenna picks up 60 cycle, from the heater wires mainly, or the power transformer. There is no shielding to prevent it on a ceramic disk.
Your guitar pickups, pick up 60 cycle, from electrical equipment, and that 60 cycle is amplified also along with the music. Same exact problem.

Trivia: In silver face and black-face fender amps, you will always notice a low, 60 cycle hum, especially when the amp is cranked up.
Guess where at least half of the hum be coming from? If you do not know by now, start at the top and read again.

Notice: In an amplifier the parts talk to each other. They do not care where the wires go. Try for a moment to realize there are many other signal paths besides the ones connected by wires.

Any time you put two wires side by side, you have made a capacitor. Now the AC traveling through those wires is mixed together, it does not care if the AC is 60 cycle, music, anything, it still mixes together. Not the current, but the voltage, yes. Now that the 60 cycle has entered the audio path, it is amplified, you hear hum.

There is AC voltage on the chassis of your amp. It is not a lot, but it's there. Now you are laying a wire on the chassis. That tiny little AC voltage is inducted into the wire, and amplified.

that is why, if we want low noise, we need to start with non inductive capacitors and non inductive resistors. Then the parts talk to each other less.

I don't follow any of that Logic Either!
Chuck, Bruce, JazzP Help!
B_T

[Jazz P Bass]

The subject of hum, as related to the heater & cathode is covered very well in this piece.
There is talk of "why" the 7025 was what it was.
Selected.
I do not know if the current tube manufacturers have the "tact" that was developed at the height of tube production.
What with "grading" output tubes for this or that quality (when actually the tubes should be pitched)
And GT with all of there 12AX7 xxx tubes.
Maybe they are "selecting" them.
The full booklet is here.
http://music-electronics-forum.com/t26087/

[km6xz]

Have you measured this effect? I find it hard to believe that considering differential mode active stages have so many "difference" noise sources that such minuscule contribution by ceramic caps several diameters away from the twisted lead heater wires. The only effective difference field is the imbalance of current between the two twisted wires. That is why they are twisted, to cancel such field induction. There are too many real difference mode sources of higher amplitude in the circuit to be that concerned with tiny common mode imbalances.
There are other reasons that can be measured as to why ceramic caps are not advised for audio signal paths such as their piezoelectric properties producing varying instantaneous values of capacitive reactance. The non-linearity that results increases inter-modulation in complex signals. The same piezo property of ceramic caps results in micro-phonics.
I would be interested in seeing and studies or measurements that was the basis for the conclusion that ceramics are any more susceptible to fields than say wires or other passive components.

By any criteria, differential signal paths in such high gain circuits as guitar amps, which usually strive for very high input impedance, it is a miracle that amps are so quiet. Only a few builders have tried to do the logical thing of creating an all common mode path, balanced line, pickups, H-attenuation and gain stages but it never caught on despite all its advantages.