rocket cathode follower tube

[JoshBernstein]

Ive read that the cathode follower position tends to blow tubes, and i have found this to be true. Ive popped and electroharmonix 12ax7 and a groove tubes 12ax7. Both of those popped in about 2 days. My greatest success was with a ruby 12ax7, which lasted a couple of weeks until i dropped it on the floor and it shattered. I have an nos jan sylvania 12au7, which sounds pretty good in that position, and im wondering if its alright to leave it there, or if there is danger of it popping or frying another part. Thanks
-josh

[JoshBernstein]

Also, how about an amperex 12ax7? I really like both of these tubes in that position, but i want them to hold up.

[Littlewyan]

I believe its only new valves which really suffer in the Cathode Follower position.

How old was the Electro Harmonix? As the Electro Harmonix is basically the same valve as the RI Tung Sol, Mullard and the Sovtek 12AX7LPS I think. I know for definite the Tung Sol was bad as a Cathode Follower but I heard they resolved this issue. Been too scared to try one in V2 in my JTM50 though!

The only current production valves that I know work as a Cathode Follower are any of the JJ 12AX7s and any of the Chinese 12AX7s.

NOS valves should be fine.

[Colossal]

The thing with cathode follower tubes failing is when the heater-to-cathode voltage is exceeded in operation. NOS tubes had a published value of 200V. Current production tubes are about 100V. Vintage amps still operate cathode followers at cathode voltages that greatly exceed that 100V limit. I elevate heaters to 50-80VDC to create an offset that keeps the follower operating within limits. The amp runs very quietly and I have not had a follower fail.

[FUCHSAUDIO]

It's new RUSSIAN tubes....they don't meet spec. Chinese or NOS or JJ are fine.

[JoshBernstein]

Alright, sounds good. Will stick with jj/chinese for the cathode follower. I appreciate the help!

[10thTx]

I've never heard of 12A_7 cathode followers being a problem with failure. I've never had a cathode follower fail in any amp I've had.

Fender __________ Reverb amps have in excess of 400+ volts on the plates of the reverb driver. And 300+ on the plates of the vibrato.

http://el34world.com/charts/Schematics/ ... ematic.pdf

http://el34world.com/charts/Schematics/ ... _AA165.pdf

Twin Reverb has in excess of 200+ volts on the plates of the LTPI 12AT7

http://el34world.com/charts/Schematics/ ... ematic.pdf

D'Lators have in excess of 200+ volts on plates of 12AX7's

https://tubeamparchive.com/download/file.php?id=27463

I've run in excess of 200+ volts on a cathode follower on several amps I've built for yrs including the Texas Raptor (6V6 Plexi) and the D'Mars ODs and never had an issue.

With respect, 10thtx

[martin manning]

It's the Vh-k that is a problem. The Fender and Marshall amps with high plate voltages and large Rk on the CF can have ~200V on the cathode. Some tubes are spec'd at ~100V. This is a well-known failure point.

[FUCHSAUDIO]

10thTX I think you may have just been lucky. Honestly, in production, mods, and repairs, the Sovtek 12AX7's simply don't hold up in CF or PI locations with higher heater/cathode differential. I think the RCA manual says 200 or 300 volts is ok, but a PI is 80 and a CF can be up to that, and they die....seriously, it's a pandemic, you just missed it

[10thTx]

10thTX I think you may have just been lucky. it's a pandemic, you just missed it

OK, fair enough .......... I'll have to figure out what I want to do in light of that. Maybe switch to JJ's. Maybe lower the voltage?

I've also used mosfets as a CF with fairly high voltages and haven't had problems with those either?

With respect, 10thtx

[FUCHSAUDIO]

Mosfets are cool. As long as you aren't counting on them for distortion, they generally are quite transparent and decent sounding.

I'd try another tube before diddling voltages. What's the point of replicating a circuit and then changing it to work around something this silly ?

[martin manning]

I've also used mosfets as a CF with fairly high voltages and haven't had problems with those either?

There is no heater, so no problem.

[strelok]

+1 to elevated heaters. Its very easy to do. 2 resistors and an electrolytic cap. Even if you don't have a CF it helps cut back on hum induced by the heater wiring. That being said I have a 100 watt metro that I built before I knew about this technique and its run fine for years without it with JJ's and NOS Sylvania's. I still plan on adding at some point though.

[Mark]

Try the diode and the 10K resistor. I haven't had an issue yet.

http://www.valvewizard.co.uk/dccf.html