5E3 and 18W Lite Amp Hum with certain preamp tubes?

[rockybottom16]

I have a couple builds (one 5E3 type and one 18W Lite type) that have the grids tied together on V1. The cathodes are also tied on the 18watter, but not on the 5E3. Some new 12ax7's will hum like a bastard when you turn up the volume (nothing plugged in), and some are quiet as a mouse. On the 5E3, only the bright volume control makes it hum. On that amp, each half of V1 is biased separately, one with a .68uf bypass cap and the other with the standard 25uf cap. I've been scratching my head for a while on this. My best guess at this point is that, because the grids are tied together, the balance or imbalance of the two triodes is causing the hum. If this is the case, would it be better to use tubes with balanced or unbalanced triodes? Help!

[Jazz P Bass]

Don't know.
My guess is the way the heater is wound.

There are tubes marketed as 'balanced'.
I think they are labeled as such.
Sovtek is the 12AX7LPS.
Recommended (hmph) for LTP PI circuits.

[g1]

"S" for spiral? I seem to recall some spiral filament tubes that were supposed to have less hum in some circuits, but had problems in other circuits, it may have been that they couldn't handle the H-K voltage in cathode follower circuits?

Are the tubes that cause the hum/no hum any brands in particular or totally random?

[rockybottom16]

I have 3 brand new EH tubes that all hum. Another older EH and a new Tung Sol that don't hum. Incidently, the amps sound great otherwise with all the tubes. I'd like to get to the bottom of this so I don't have to buy a handful of preamp tubes just to find one I can use! Of course, there is the possibility that I just happened to get 3 bad tubes at once. I got them from "audiotubesdirect" on ebay, and I've had good luck with them in the past. Maybe I should stick with Tube Depot or Antique Electronics.

[Tom Phillips]

My comment is that the "balance" of the two triodes within one tube has nothing to do with the hum you are observing. The two grids being tied together should not be a factor either.

I think it is a problem with the tubes themselves given the evidence you have collected so far. One thing I suggest is that you pick the worst offending tube and try it in other amps. Perhaps a friend's amp. If the tube raises the hum level when used in the first stage preamp of another amp then that would pretty much show that the problem is the tube itself.

Tom

[Chuck H]

+1
I've made purchases of Rusky 12ax7's many times. Usually five or ten at a time. Sometime half of them will be unsuitable for audio applications because of excess noise, hum, microphony or all three! It's not unusual to get a couple of hummers in the bunch. Tubes that hum so bad as to be unusable. I once made a purchase that included four Sovtek 12ax7wa's. These are very low in microphonics and I needed ONE for a project. I figured that if I bought four I would get at least one good one. NOPE! All hummed like they didn't know the words. I've about given up on Rusky preamp tubes until I start reading reports that things have changed for the better. My last preamp tube purchase was Shuguang. Of ten there was one noisy tube that was unusable. The rest are in service. Two as the first stage in high gainers. That's WAAAAYY better than I've done with Russian tubes in ten years.

We use to have tube vendors that offered selection for microphonics and noise (and charged a premium of course). Looking around recently I don't see any of them doing it. There are a couple that offer their "select 12ax7" or whatever, but what If I don't like the Sovtek 12ax7wa (or whatever they sell)? Maybe I want them to select the best example of a 12ax7 of MY choice! Nope. Not anymore. I get so many bad tubes when I order Russian preamp tubes that I seriously doubt there is any testing whatever. Just a box swap and a service charge.

[rockybottom16]

Thanks for the input! It must be that I've just been lucky in the past. But with 3 new EH's and all of them hummers, I guess my luck has run out! Maybe I'll order a handful of the Shuguang and spin the wheel.

Chuck - Have you had much luck with JJ's?

[Chuck H]

I stopped buying JJ's about ten years ago when they developed a reputation for being noisy. MOST of them at that time were very noisy. Not just hissy from being higher in gain and not necessarily hummy. More buzzy and lower frequency hiss, crackle and popping. I've heard things have improved substantially since then but I haven't tried them recently, and I'll tell you why. What I've read is that the rusky tube brands all come from either of two factories and both have been responsible for high numbers of failed or problematic tubes. How the JJ's can come off the same presses and still grow positive reputation I'm not sure. I've had trouble with a couple of sets of JJ power tubes too. If I eat at a restaurant and get food poisoning from the meatloaf I don't go back hoping for better luck with the chicken. Maybe I'm missing out because I'm stodgy, but for now the Shuguangs and some select NOS are working out.

[g1]

How the JJ's can come off the same presses and still grow positive reputation I'm not sure.

JJ are from Slovakia, which is now part of "NOT Russia", no?
Otherwise, the only Russian factory still putting out guitar amp type tubes is Reflektor and all the brands now made in Russia come from there AFAIK ?

[Chuck H]

Oh. I thought only the old TESLA "equipment" was being used, but it looks like it's the old location is too. Shame they couldn't maintain the quality of the old TESLA product! I remember a lot of bad tubes coming from there after the original TESLA company got out of it. That would explain the early issues with noisy preamp tubes. Read above for my more recent experience with JJ power tubes