12AX7 filament/cathode hum weirdness in some high gain circuits.

[Mars Amp Repair]

So, everyone has encountered 12AX7 tubes that produce a hum in some high gain circuits. So, has anyone experienced that the spiral filament tubes correct this issue? I kinda forgot about the spiral filament types. I'm going to order a few to have around, tho I usually either swap the V1 tube with another one to remedy it or just find one that doesn't have the issue.
Glen

[g1]

I would say it's circuit dependent and pretty much trial and error. I have had hum issues that were caused by spiral filaments, and going to a regular filament type cured it.

[The Dude]

So, everyone has encountered 12AX7 tubes that produce a hum in some high gain circuits. So, has anyone experienced that the spiral filament tubes correct this issue? I kinda forgot about the spiral filament types. I'm going to order a few to have around, tho I usually either swap the V1 tube with another one to remedy it or just find one that doesn't have the issue.
Glen

I've experienced that myself, as you say, especially in high gain circuits.

[Chuck H]

There is NO integrity in newly manufactured tubes. Anyone saying the "spiral filament" is an effort to reduce noise is only playing on the idea that it DID or it COULD. That absolutely doesn't mean that it DOES!!! Trying the spiral filament tubes for myself I've noted that they are generally good sounding, but not at all balanced. So hum is still an issue and not being mitigated by the filament design at all. They also tend toward some microphony. Not as much as the worst, but enough that you're not likely to find a "spiral filament" tube that will work for V1 in a high gain preamp.

I hate to be "THAT" guy, but... The only tubes I've found to work consistently in high gain preamp applications for V1 in amps are NOS/UOS (test good). Nearly ALL modern preamp tubes I encounter just aren't up to the task of being V1 in a high gain amp. It's troubling and my own stash of good, non microphonic tubes is dwindling and I have never found a good new production option.

[soundmasterg]

There is also another thing to consider. Probably when the NOS tubes were being made new, the spiral filaments were likely a benefit, but with the production of tubes nowadays, who knows if a 'spiral filament' is really in the tube, and if the manufacturer really did it correctly or not.

I'm with Chuck on the V1 NOS recommendation if you have them. They WILL work better and last longer. That said, they are a finite resource and will eventually all be gone. I used to go to a local surplus place R5D3 weekly for a decade and would pick up used OS tubes for $3 - $4 each. I have a mutual conductance B&K tube tester and I would bring that and test tubes and grab the good ones. Found a bunch of Telefunkens which I usually use in V1 as they last longer than any other 12AX7 type and are really balanced sounding. Also found some 50's and 60's Tung-Sols, Tungsram, Raytheon, RCA, GE, Sylvania. Haven't been disappointed with any of those yet. Unfortunately the former owner of R5D3 passed about 10 years ago and it was closed and there aren't any other surplus places around anymore.

Greg

[Mick Bailey]

I've had some frustrating amps where I've auditioned maybe 20 or more tubes to find one that didn't hum. Long plate, short plate, spiral, NOS, recent production. Sometimes NOS would be the answer, or suprisingly sometimes short plate Chinese tubes which I used to buy in bulk as factory test then grade them myself. Disappointingly, some of the NOS tubes had much higher hum in high gain amps than new stuff, the worst of all being JAN Phillips tubes.

[stokes]

There is NO integrity in newly manufactured tubes. Anyone saying the "spiral filament" is an effort to reduce noise is only playing on the idea that it DID or it COULD. That absolutely doesn't mean that it DOES!!! Trying the spiral filament tubes for myself I've noted that they are generally good sounding, but not at all balanced. So hum is still an issue and not being mitigated by the filament design at all. They also tend toward some microphony. Not as much as the worst, but enough that you're not likely to find a "spiral filament" tube that will work for V1 in a high gain preamp.

I hate to be "THAT" guy, but... The only tubes I've found to work consistently in high gain preamp applications for V1 in amps are NOS/UOS (test good). Nearly ALL modern preamp tubes I encounter just aren't up to the task of being V1 in a high gain amp. It's troubling and my own stash of good, non microphonic tubes is dwindling and I have never found a good new production option.

I used to do a lot of work on Mesa hi gain amps and like Chuck says the only tubes that are consistent in these hi gain circuits are old stock tubes. But even a lot of these are noisy in hi gain apps. Tubes that are dead quiet in Fender circuits are unusable in hi gain circuits.

[Chuck H]

The GE 12ax7 (EDIT: short, gray plates and halo getter) is one of the more affordable old stock options and are usually well behaved and quiet for V1. Though they do seem a little lower in gain to me than most others. How a tube sounds in a circuit is up for some debate. Some here say tubes don't sound different because they all perform well enough in the audio range that it's just a matter of gain. Not sure about this from own experience. I do think this is mostly true but I also think it's possible that the distortion characteristics or operational responses to shifting voltages may change the way a tube sounds and feels because I really have noticed differences here with different tubes beyond gain. JM2C on that. That said... While well behaved, the old GE tubes sound a little bland to me. But I've resorted to using them several times as an only option.

[Chuck H]

OOooh... I actually DO remember a new production tube that has been good for V1 more often than not. The Sovtek 12ax7WA. Also a very short plate tube, interestingly. Also seems a little vanilla in dynamics and tone. But typically less microphonic than others.

EDIT: This was maybe a decade ago that I cottoned on to this tube. So I can't speak for current production.

[tedmich]

I've had some frustrating amps where I've auditioned maybe 20 or more tubes to find one that didn't hum. Long plate, short plate, spiral, NOS, recent production. Sometimes NOS would be the answer, or suprisingly sometimes short plate Chinese tubes which I used to buy in bulk as factory test then grade them myself. Disappointingly, some of the NOS tubes had much higher hum in high gain amps than new stuff, the worst of all being JAN Phillips tubes.

No offense Mick, but is your lead dress, shielding and ground scheme done well?
Who made this amp?

[Chuck H]

FWIW I'll second Mick's post. In that I had much better luck with Shuguang 12ax7's for a long time than I did with other eastern Europe/western Asia tubes. I remember a time when I wasn't happy about the Chinese Shuguang preamp tubes being the most ubiquitous and rebrand product. Now I miss them terribly.

[Mick Bailey]

No offense Mick, but is your lead dress, shielding and ground scheme done well?
Who made this amp?

These were Hughes & Kettner - I think the Triamp heads. Pretty complicated PCB amps with grounding schemes established by the factory. A local dealer took them on, so there were quite a few H&K amps that came my way. After the first one it became clear that fitting tubes that were not in current production and easily available at moderate cost wasn't going to be a good idea.

[Chuck H]

Ok, not to spin the thread too far off the original topic (so I'll write something about the original topic in a sec), but...

I've been out of the game for years. So I just looked up what's going on with Shuguang and I see they started production again last year. There was some speculation that new Shuguang tubes might just be rebranded PS Vane tubes but that doesn't seem to be the case. Current reviews look pretty good and it seems Dave Friedman likes them so that's a pretty strong endorsement. From what I'm reading the new 12ax7's don't sound quite as big and bright as the old square getter tubes from the 90's but still very good. Not sure when I'll get to trying them but it's good to have options.

Ok. Time to get somewhat back to the original topic (sort of).
This isn't about hum but it still applies to unwanted noise relative to the cathode. In some newer preamp tubes, especially those from eastern Europe and western Asia I've noticed a lot of what I think is just emission noise. A sort of hashy sounding white noise but at a lower frequency. Not sure what causes this. Low quality or purity cathode metals or coatings maybe? It's loud enough on some tubes to be very audible in the V1 socket for high gain preamp designs. This is especially so because most of these designs use a partial cathode bybass on the input triode. In my own R&D I've found that fully bypassing the input triode cathode circuit virtually eliminates this noise. So that's just what I do now. Obviously you don't want to swap out the .68uf bypass cap on a plexi Marshall for a 22uf or 100uf because that's not THE sound for these amps. But when designing from scratch there are ways to work around not being able to use partial bypass for voicing.

My own humble experience and I hope this can help.

[Chuck H]

Also, the 12AD7 tube (old stock only) is a ruggedized, low noise 12ax7. Sylvania datasheet actually says "no microphonics". They go for about the same or a little less than old stock 12ax7 tubes. They draw a little more heater current for some reason. I don't think it would be an issue. I've never tried one but it sure sounds like an option for V1 sockets.

[tedmich]

Some Hughes & Kettner amps were (are) notorious for having a dumb ass HV CCF lighting system that can inject a horrid BUZZ into their preamp.

I'd lose the entirety of this circuit (lamps and HV components) before trying to silence this thing.

Then try to find the magical 12AX7?