Thoughts on Tung Sol reissue 12AX7 and reliability?

[Steve Conner]

There is no negative side, it's just a single connection. If you go to scopeboy.com/amps then click on Ninja Corvette, then scroll down to the bottom of the page, there's a link to a PDF schematic of an amp I built using this trick. The PT had no centre tap for the heater winding, so I used the two 100 ohm hum balance resistors. Instead of connecting the midpoint of them to ground, I connected it to the cathode of the power tube. That's all there is to it.

I know the preamp part of the circuit looks crazy. I've since rebuilt it with smaller cathode resistors and a better tone stack driven by a MOSFET follower.

[Leo_Gnardo]

I suggest you try elevating the heater circuit with a positive DC voltage if you haven't already. It can really help.

In a cathode biased amp, you can just connect the heater winding CT (or the midpoint of your two 100 ohm resistors) to the power tube cathodes, assuming they're bypassed.

Steve's plan may cut filament-induced hum, also "floating" the filaments on negative V from the bias circuit of an amp that has this feature, or placing the midpoint of the "balancing" resistors on a capacitor (typically 0.1 uF 200V) as Ampeg did in the mid-60's. Also using a 100 to 500 ohm pot as a "hum balance" control. Sometimes I've found the sweet spot for hum reduction with fixed resistors to be not balanced but slightly off, say 82 and 120 ohm. Heck, you could build a DC supply to run that pre tube filament. BUT this all falls into "panel-beating" the amp in order to make faulty preamp tubes work. Sorta puts the cart before the horse don't it. Welllll, sometimes you have to "wag the dog" to get things to work to your advantage.

[Steve Conner]

No, negative voltage will only make the hum worse.

The argument is that the hum is caused by the heater emitting electrons because some cathode material has got onto it, or simply because it's so hot. As electrons are negatively charged, and opposite charges attract, a positive bias encourages them to stay stuck to the heater rather than emitting.

I found it useful in this guitar amp and also my hi-fi amp, using NOS 12AT7s in the first stage. I tested with a bias voltage from a bench power supply. No bias voltage, it hummed. Turn the bias on, the hum vanished. I got a SNR better than 90dB referred to full power, with AC heaters and no hum balance pot.

[daz]

There is no negative side, it's just a single connection. If you go to scopeboy.com/amps then click on Ninja Corvette, then scroll down to the bottom of the page, there's a link to a PDF schematic of an amp I built using this trick. The PT had no centre tap for the heater winding, so I used the two 100 ohm hum balance resistors. Instead of connecting the midpoint of them to ground, I connected it to the cathode of the power tube. That's all there is to it.

I know the preamp part of the circuit looks crazy. I've since rebuilt it with smaller cathode resistors and a better tone stack driven by a MOSFET follower.

Ok, i'm going to ask what may be a very stupid question. But i looked at some schematics for DC heaters and they use rectifiers. On your schematic i see nothing like that and the tubes appear to be wired normally with 100R resistors to simulate a center tap your PT doesn't have. How is that DC? I'm not trying to question your method, you know this stuff like the back of your hand and i know nothing. But i just see AC heaters as far as i can tell. What am i missing?

[Steve Conner]

This method is still AC heaters. You just superimpose a positive DC bias on them to discourage them from emitting electrons. No current is drawn from the DC bias source, the AC is still providing all the heating power. Read my last post for details.

Some people call it "elevating the heaters". As a bonus, it reduces the heater-to-cathode stress on any cathode followers.

[daz]

The PT had no centre tap for the heater winding, so I used the two 100 ohm hum balance resistors. Instead of connecting the midpoint of them to ground, I connected it to the cathode of the power tube. That's all there is to it.

But the power tune's cathodes are grounded, so are you saying to unground them and attach the heater's center tap wire to them ? If i did that how are the power tubes cathodes now grounded to the circuit? And i take it i can no longer use the 1 ohm resistor method to bias my amp? If the latter is correct, this isn't an option for me. I need that bias method and have dual bias and probe jacks on the amp. I'm not going to trash that if i have to in order to do this. I'm probably way off, but you know what you were getting yourself into trying to tell me how to do this ! You know i'm an electronic moron.

[Leo_Gnardo]

No, negative voltage will only make the hum worse.

The argument is that the hum is caused by the heater emitting electrons because some cathode material has got onto it, or simply because it's so hot. As electrons are negatively charged, a positive bias encourages them to stay stuck to the heater rather than emitting.

Not meaning to gainsay you Steve, because you are exceptionally technically adept and knowledgable, but through actual experience I have found that in SOME cases I get hum relief by referring the filament to a negative DC supply. And it may simply be that there's a filter cap there to shunt hum to ground, besides the voltage biasing effect. Amps that have a refractory (hard to fix) hum problem - I try one thing after another until I'm satisfied. I've been called an empiricist before - doesn't bother me.

Once our OP has found a way to cut the hum, maybe those TS's will sound splendid. Until then, TS's look like a nice piece of cake to me, covered in ants. Hmmm, maybe that would be tasty, without the ants.

[Steve Conner]

I qualified my explanation earlier by saying "in a cathode biased amp". With a fixed bias amp, you'll have to find some other source of positive voltage for heater elevation. Maybe 4 9v batteries in series would do for a test, so you can find out whether it's worth the bother.

[km6xz]

Leo, if a heater has a negative potential and it reduces hum there is something wrong elsewhere in the circuit. It could help if the "right" defect or leakage is present.

I have had it with 12AX7s made by New Sensor, my latest 12 has a terrible ratio of good to bad tubes. No problem with power tubes right now but I have to convince clients to accept my recommended and thoroughly tested Chinese 12AX7's which have been quite and reliable.

With such bad hum problems, it suggests a oxide coating on the filament is not insulating the cathode from the heater.
I plugged in a new EH 12AX7 last night into a Pignose G40v and the otherwise working amp blow the GFI breaker, twice. Putting in another brand 12AX7 worked fine. Gotta figure out how this modified amp is generating ground currents when a tube is plugged in. I hate modified amps, 90% of time they are less reliable than stock.

[tubeswell]

Steve, got any schematics to show how i could do that? You don''t use the circuit ground for the negative side do you?

Here ya go



Referencing the heaters (either by way of the heater winding CT or other ground reference mechanism like the 100R resistors etc) to the output tube's cathode voltage (in a cathode-biased output stage) should also do the trick.

[Steve Conner]

Stan, heater winding shorted to primary in the PT? :-O

[daz]

I have had it with 12AX7s made by New Sensor, my latest 12 has a terrible ratio of good to bad tubes. No problem with power tubes right now but I have to convince clients to accept my recommended and thoroughly tested Chinese 12AX7's which have been quite and reliable.

With such bad hum problems, it suggests a oxide coating on the filament is not insulating the cathode from the heater.
I plugged in a new EH 12AX7 last night into a Pignose G40v and the otherwise working amp blow the GFI breaker, twice. Putting in another brand 12AX7 worked fine. Gotta figure out how this modified amp is generating ground currents when a tube is plugged in. I hate modified amps, 90% of time they are less reliable than stock.

I think this is the problem here as i first said when i started this thread. Before i even bother with DC heaters or anything of the sort, i have to use logic to determine if thats even a possibility for a cure. But logic tells me that when every single TS i have use (about 7 now) every one has hummed where no other tube has at all, the amp is fine and the issue is the tubes. Doug told me he hasn't seen this problem, but maybe few people use them in V1 of a cascaded preamp style amp, and those who do are a small enough segment that they generally don't bother returning them. They DO stay relatively quiet is a PI spot or probably in a fender or other non hi gain distortion stage. All i know is this tube design is either defective or there is something unusual about it that only works right in some amps. We already know for a fact that they aren't usable in cathode follower spots due to thier inability to handle the cathode voltage So what does that tell you? Tells me the design is flawed but they weren't able to fix it without sacrificing the sound that is what is selling that tube. (they really do sound great)

[Steve Conner]

Sure. But the fixes we recommended will make your amp more tolerant of hummy tubes. Maybe tolerant enough that you could use some Tung-Sols from your collection that would have been wasted otherwise. Then again maybe they hum so bad that the fixes won't help. You'll have to try it and see for yourself.

[daz]

If i was still playing out i'd be desperite. But at home volumes i can tolerate some of them and even have one thats almost as quiet as a different tube till you turn it up. The one ting that really confuses me tho is the way they go from one level of hum to another very different one for no obvious reason. Like i said, i tried that one that hums louder than any other them to make sure it was the really bad one b4 i dent it back, and then it was fairly quiet. So i just don't know what to think. (and yes, i wiggled it while in the socket and no change)

[Pedro Vecino]

daz,

have you tried the TS in another amp(s)?

Some Russians 12AX7s are especially sensitive in some amps used as V1. For example, in the low sensitivity input of a Marshall 2203 many units of 12AX7WA, LPS, EH, Mesa Russian II, Tung Sol .. can generate abnormal hum. It also tends to occur in 1959 but in a lighter way.
Those same tubes ca