12AX7 H-K voltage

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Wayne
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Joined: Thu May 29, 2008 2:10 am

12AX7 H-K voltage

Post by Wayne »

Hi all. I know the maximum H-K voltage for a 12AX7 is given as 100V, but I routinely run into designs that exceed this. My YBA-1, for example, puts about 176 volts on the cathode of the CF tone stack driver with heaters referenced to ground. That's without even thinking of signal peaks!

I would imagine that among new production tubes some brands might take this abuse better than others, that is if any can take it at all. Is there a tube best-suited to this spot? Should I make circuit changes to cure this? I'd hate to fall in love with an amp (you all know this is possible :lol: ) and have it constantly eat tubes! I can either elevate the heater supply reference, or change the plate load of the previous stage to bring the voltage down. Either solution may cause problems elsewhere.

W
tubeswell
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Re: 12AX7 H-K voltage

Post by tubeswell »

Cathode insulation quality varies. I think on good old tubes, heater to cathode voltage could routinely go up to 140 or 180 no problem. Modern tubes seem to be using less expensive insulating material.

Having said that I build an amp earlier this year with a CF stage where the cathode voltage is 213V and the heater is only at 43, and there aren't any problems (yet), although I am expecting a shorter tube life there. I have heard there are issues with running modern (EH) tubes as CF stages.
FunkyE9th
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Re: 12AX7 H-K voltage

Post by FunkyE9th »

I looked at the spec of a 12AX7 and it says... "Heater Positive with respect to Cathode". How are you measuring the 176V? Cathode to Heater or Heater to Cathode? You probably should get approx -176V.

The cathode of the follower in a Bassman, per the schematic is 180V with respect to ground.

Edited to add:
It looks like the +100V max is under DC, no signal. And it can go +/-200V under signal conditions. So I guess if the signal swings more than 24V positive you'll exceed -200V...not much positive swing to work with.
Wayne
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Joined: Thu May 29, 2008 2:10 am

Re: 12AX7 H-K voltage

Post by Wayne »

OK. I guess -176 VDC describes my situation better - I just assumed everybody would know what I was talking about. Now - if I raise the heater supply to ~100 VDC, I'll have the necessary voltage swing in either direction, but all the other tubes will be operating at max. Come to think of it, there isn't even really a good way to keep both sections of the same tube happy.

The other option, lowering the plate voltage of the previous stage with a bigger plate resistor, changes the gain structure of the amp. Dunno if I'll like that or not, but may give it a try. Hey - if it worked for Jim Marshall...

Maybe I'll just leave it & see if anything "poofs", first.

W
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Structo
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Re: 12AX7 H-K voltage

Post by Structo »

I know the Marshall guys over at Metro were saying the new Tung Sol reissue didn't do too well in V2.
Tom

Don't let that smoke out!
Wayne
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Joined: Thu May 29, 2008 2:10 am

Re: 12AX7 H-K voltage

Post by Wayne »

So Tung-sols are out - that's good to know.

With quality and opinions constantly changing I never know which preamp tubes to buy, anyway. Sometimes I even take the crap shoot at the local music store.

"Tubes? Yeah, we got some. We got these big ones, here, and a whole bunch of these little ones too - whaddya need?" :roll:

W
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David Root
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Re: 12AX7 H-K voltage

Post by David Root »

I've run modern 12AX7s up to 200V in CF duty, but I could see maybe not all brands would hack that.

I think NOS (or UOS that measure good) are fine, and not just in 12AX7. I have an amp that uses a 6SL7GT in CF duty with 204V on the cathode and 6VDC on the heater, it is a NOS 6SL7GT Phillips ECG 1986 manufacture, which isn't exactly a "glory days" tube, and can run +70VAC signal too on the cathode.
morcey2
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Re: 12AX7 H-K voltage

Post by morcey2 »

Structo wrote:I know the Marshall guys over at Metro were saying the new Tung Sol reissue didn't do too well in V2.
The tung-sol reissues along with the Sovtek 12AX7-LPS need an elevated heater supply for use in CF stages, along with other configurations such as cascode, SRPP, and even LTPPI if the tail resistor is exceptionally large. I've only ever had one TS reissue fail and that was in an amp w/ a grounded heater. I elevated it about 80V and I haven't had any problem since then. (cathode sits at 150VDC).

If you don't want to mod an amp, then don't use them, but they're a really nice sounding tube. Brighter than the JJ's and their gain is on the higher end of the 12AX7 spec. And if you're using them in a normal common-cathode configuration, then you don't have anything to worry about.

Matt
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