12AX7EH CF Failure

[Daver]

CF=cathode follower.

[Steve A.]

The Sovtek 12AX7LPS is known to fail in the CF position, so it doesn't surprise me that the EH 12AX7 does it too. I'd use different tubes in that spot myself, especially after 4 of them fail for you. Save the EH for different locations in the amp and they'll work fine.

Greg

Ah, c'mon- let's sacrifice 4 more tubes to the Vacuum Gods!

Thanks for the heads up on this! Perhaps we can start a database here on problems and quirks of specific tubes...

Steve Ahola

[GainFreak]

I asked two European online shops about this issue. One of them confirmed about 12AX7EH adding to the list Tung-Sol, Sovtek LPS and LP. The other wasn't aware of the problem. Then I asked a major distributor and they also confirmed the 12AX7EH issue adding to the list any other spiral filament type tubes like 12AX7LPS, JJ ECC83s, Tung-Sol 12AX7, and Mullard 12AX7.

[myfoot]

This was posted over to the Metroamp forum. Sounds like it works fine anywhere but the cf stage

http://forum.metroamp.com/viewtopic.php?f=13&t=20150

[GCA]

I asked two European online shops about this issue. One of them confirmed about 12AX7EH adding to the list Tung-Sol, Sovtek LPS and LP. The other wasn't aware of the problem. Then I asked a major distributor and they also confirmed the 12AX7EH issue adding to the list any other spiral filament type tubes like 12AX7LPS, JJ ECC83s, Tung-Sol 12AX7, and Mullard 12AX7.

I'm not seeing that same problem with the JJs. The 12AX7EH, though... I'm wondering if there's an issue with that particular heater design.. poor insulation (material, thickness, etc). The one EH I have doesn't glow like the rest of similar construction, leading me to believe I may have received a slightly defective tube. Not all CF designs elevate the heater, so I'm wondering if the "less than happy heater" that I'm seeing is related to the CF death of those tubes due to insulation failure. Morgan Jones mentioned this on page 266 of Valve Amplifiers (3rd edition):

"Increasing voltage across the heater/cathode insulation increases leakage currents. Although Vh-k(max.) is specified on data sheets as being anywhere from 90V to 150V (except in some ruggedized and 'P' series TV valves), this is a very 'soft' limit, since it is usually given at an arbitrary leakage current, nevertheless, a suffieciently high voltage will [his emphasis, not mine] punch through the insulation to rupture the heater. Heater failure due to heather/cathode insulation breakdown is uncommon, but is most likely in cathode followers with high signal voltages or output stages with distributed loads (such as the McIntosh design)."

"High signal voltages" sounds a lot like guitar / bass amps to me, where gain is god!

Is speculation, no more, and I can't speak for the rest of those in the list. Perhaps raising the heater to the cathode level in CF might save a few tubes? Worth a shot, but probably not.

It's my understanding that the EH is a Tung-Sol reissue. Again, could be wrong.

[GainFreak]

The one EH I have doesn't glow like the rest of similar construction, leading me to believe I may have received a slightly defective tube.

This is normal for them. I've used 12AX7EHs alot - you can barely see their glow.

Perhaps raising the heater to the cathode level in CF might save a few tubes?

I've tried it and it works /heaters elevated at 80-85V/ but I don't use more than 360V on CF now /220k plate resistor/ because with higher anode voltage and/or lower value resistor you'll exceed the heater to cathode voltage again. The other option is to elevate only the heater of that triode section in question as much as necessary because often voltages on cathode are well above 200V. This makes me think about some big amp companies using mainly chinese tubes which don't fail even at higher voltages.

[GCA]

[quote=GainFreak;110284]

I've tried it and it works /heaters elevated at 80-85V/ but I don't use more than 360V on CF now /220k plate resistor/ because with higher anode voltage and/or lower value resistor you'll exceed the heater to cathode voltage again. The other option is to elevate only the heater of that triode section in question as much as necessary because often voltages on cathode are well above 200V. This makes me think about some big amp companies using mainly chinese tubes which don't fail even at higher voltages.

Mhm. I'm mainly using a 320 B+. Thinking of giving a voltage divider off of that a shot instead of trying to elevate the whole heater supply for now since I have plenty of mA to spare in the pre. See if the EH lives thru it. If they live with the elevation then I suppose we can assume it's weak/thin insulation on the filament and be on our way.

You know, if that works nicely I could probably just simplify the supply on my pre to do that for all the heaters... then just pop a relay like a turkey timer when they're hot and the resistance climbs Save a regulator in the process.

Oh, well... maybe in a few days.

[GainFreak]

See if the EH lives thru it.

After elevating the heaters all my EHs are still OK more than a year now.

[GCA]

After elevating the heaters all my EHs are still OK more than a year now.

Nice. I think we can wrap up the mystery, then... smaller than normal Vh-k. I wonder what the real rating should be?

[novosibir]

This was posted over to the Metroamp forum. Sounds like it works fine anywhere but the cf stage

MetroAmp.com Forum • View topic - Tung Sol Reissue 12AX7 not for CF stage !!!

Yep, this was me, who started this topic there, because I was so pissed about failing 3 tubes one after the other

But I've tried meanwhile Tung Sol reissues from 2009 production again in V2 of Marshall amps, after the vendor told me, that they've changed and upgraded the insulation issue of their preamp tubes - and... no more failure yet

Yes, I like this tube very much soundwise and it's IMO the best alternative, if you don't go NOS. I've put this 2009 production tube in 6 or 7 Marshalls (w/o elevated heaters), amps of people, who're playing 30-50 gigs a year - and no claim until now! Hopefully no claim will be coming in future

Larry

[GCA]

Nice. This didn't go out as a email notice to those with dealer accounts... they should be at least emailing about changes to their product lines. They don't, so I wonder how your vendor found out they changed insulation?

Other than that, yay!!

--GC

[novosibir]

... so I wonder how your vendor found out they changed insulation?

I'll ask him tomorrow, today it's already too late here in Germany, business time is over.

He's one of the biggest tube sellers here in Europe

BTB Elektronik-Vertriebs GmbH - Preislisten

and a few times every year he's in Russia, Saint Petersburg & Saratov. So I guess, that they've told it to him there in the factory?

Larry

[GCA]

That would definitely explain it!