JCM800 Cathode Follower and Vhk ratings

[ECV02]

Upon glancing at the JCM800 DCCF it can be noted that the cathode voltage of the buffer stage is quite high, around 193v. This is well in excess of the Vhk(-) rating of a 12AX7.

The JCM800 did not use heater elevation here. Why is that? And, more importantly, could I get away without elevation as well? I am going to undertake a SMPS based build where heater elevation will be impossible. I was thinking JCM800 preamp into a single ended 6V6 or 6BM8 power amp, most likely the 6BM8, since I have loads of them.

I have read and heard that a JJECC83S can also handle 180v Vhk(-). I am starting to consider this a viable option.

Thanks,
E

[shooter]

Marshalls DCCF is a "standard" in my last few builds.  here's a good read on why bad design works well 

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

[Greg]

In your new single ended build, you are going to have lower B+ so instead of an Ebb of 350 volts in the preamp node, you are going to get an Ebb around 250 volts or a bit less and it's going to take care of that problem at the same time! Well, Marshall didn't care so much about EL34's screens, so why would they care about a 12AX7? Good luck

[ECV02]

In your new single ended build, you are going to have lower B+ so instead of an Ebb of 350 volts in the preamp node, you are going to get an Ebb around 250 volts or a bit less.

What makes you say this? I plan to use 300v B+. After filtering, I will still have a high enough plate voltage to send the cathode sky high. With total DC + peak AC, this could easily exceed 180v.

[Greg]

Ok, it was only a rough assumption having a smaller amp/power tube in mind. Of course, you're the boss on how high you want your preamp supply voltage. Around 220/250V on the first stages is what I had in mind. You will still get 120V-150V on the cathode but it will be under the max rating. In general I don't see a big problem if there isn't a condition where the tube gets the full «no-load» voltage at turn-on.

[d95err]

The JCM800 did not use heater elevation here. Why is that? And, more importantly, could I get away without elevation as well?

About 90% of all guitar tube amps produced the last 40 years or so has had this circuit (without heater elevation). If the lack of heater elevation was a significant issue, I think it would have shown up by now...

[pdf64]

I think that russian made 12AX7 having their h-k insulation damaged when used in higher voltage DCCF circuits is fairly well publicised issue.
Vintage, chinese and European made tubes seem to cope fine under the same conditions.

[PRR]

The typical 100V rating is statistical. If the factory did everything right, breakdown is over 1,000V. But it is a tricky process, and few tube circuits have high Vhk, so they cut themselves a lot of slack.

As pdf64 notes, some factories do better than others.

McIntosh ran a lot of 12AX7 with 225V swings on the cathodes.

Most good SMPSes can have their 12VDC biased-up 100V above their case.

[pdf64]

DCCF have a current surge at power up, apparantly russian made 12AX7 don't seem to cope with that either, see https://music-electronics-forum.com/showthread.php?t=46108

[shooter]

from Merlin;

The traditional arrangement suffers from the problem that on start up

he has "a useful" mod in his discussion on DCCF

[pdf64]

That grid arcover issue should apply to any 12AX7.