Cathode follower/DC filaments - issues?

[EFK]

Thinking of some mods to the heater line of a JTM45. I'm curious as to whether or not there may be any issues with the cathode follower in V2 if I were to (1) elevate the filaments on @ 30-40V dc or (2) use a bridge rectifier and cap and just convert the the heaters over to 6.2/6.3 v dc. Anyone have any experience with this situation? I swear I read somewhere of issues w. the CF stage but can't find it now. Thanks!

[Gregg]

May this is the thread you had in mind.

http://music-electronics-forum.com/t9514/

There's no problem running elevated DC heaters - I'm doing it all the time. Depending on CF configuration you may need to elevate heaters more than 30-40V.

[FUCHSAUDIO]

The RCA tube manual says 200 V h/c differential. It's not a circuit issue, the EH tubes cannot handle the specified heater/cathode differential. I use Ruby in the driver and CF locations and do not elevate my heaters, and I do not have a problem with failures any longer. BTW: I use the tung-sol and EH 12AX7's in the front ends of my amp successfully. It's loop, tone stack driving cf's and drivers where you have issues turning up.

[EFK]

Thanks guys - I read up on it a bit. I've got some chinese tubes and I've got some Ruby's coming. I've got loads of the new Tungsols as I think they're great tubes but they seem to be hit or miss for a CF position. Might elevate the heaters or might not - seems like it may help, although if there are no hum issues then...

[Gregg]

The RCA tube manual says 200 V h/c differential. It's not a circuit issue, the EH tubes cannot handle the specified heater/cathode differential. I use Ruby in the driver and CF locations and do not elevate my heaters, and I do not have a problem with failures any longer. BTW: I use the tung-sol and EH 12AX7's in the front ends of my amp successfully. It's loop, tone stack driving cf's and drivers where you have issues turning up

It's both circuit and tube issue if your anode voltage is 350V and more as in most high gain amps where you're either very close to those 200Volts or exceeding them.
The average musician doesn't know about H/K differential and CF that's why I think it's a good idea to make the amp foolproof in case you're into experimenting with different tubes' tone.

[FUCHSAUDIO]

Gregg: Not to argue the point, but the anode voltage isn't the concern here. It's heater/cathode differential.

In a typical preamp, the difference is 1 to 3 volts. In a Bassman/Marshall tone stack cathode follower is might be 175 to 200 v over ground and filaments. In a typical driver circuit, it's likely 80 or less. I've seen quite a few failures in my FX loop and drivers using EH's and none using Ruby tubes.

Sure, lifting filaments a little to lower noise is cool, raising filaments up to 100 or more volts is just silly, as you are trying to correct a problem that using a tube that's up to spec would prevent. Correct me if I'm missing something.

[Gregg]

Gregg: Not to argue the point, but the anode voltage isn't the concern here. It's heater/cathode differential.

As I mentioned in some circuits this differential can be exceeded. If you take a typical Mesa DR CF for example (220k plate, 1k8/1uF cathode) with 350V anode voltage the differential is already ~180Volts. If you have the voltages they have or use different value resistors you get more than 200Volts between cathode and heaters.
I've used EH a lot and I mean hundreds and they only fail in CF position. Some times they will last a month sometimes they fail instantly. Some batches from 2007 and 2008 are OK bust most of them are not.
I have some Ruby tubes that are chinese and don't fail as CF. Mesa also uses the same chinese tubes I guess so they are OK for CF.
Also note that not only EH fail as CF.

[Steve Conner]

Yes, the point is that the classic tube amp circuits violate the Vhk spec of the original 12AX7/ECC83. There might not be 200V DC between heater and cathode, but the DC+signal peaks surely exceed it.

What you're doing now is saying that the original datasheet should be torn up and "Must survive in a Marshall CF" is the new de facto spec, and that strikes me as a little unfair to the tube manufacturer, because these failing tubes would probably meet the original 12AX7 spec fine.

Either way the most sensible thing to do, it seems to me, is to elevate the heaters to half of the datasheet Vhk rating. (Unless the preamp tubes then start making popcorn noises...)

And in the long term, maybe EH/Sovtek should introduce a new part number for tubes with super heater insulation guaranteed to withstand the overvoltage: 12CF7 or whatever.