Elevated heater voltage on cathode follower

[deci belle]

I read in Merlin's preamp book about elevating the heater voltage in a cathode follower (mine's a 5751 in a 5f6 circuit) to avoid straining the heater-cathode insulation.

I raised the grid current by reducing the cathode load R from 100k to 56k and also installed a 47k R and diode in series between pin 7 (grid) and pin 8 (cathode) of that tube to protect against arcing.

Could I elevate the heater voltage by connecting pin 9 (heater center-tap) and pin 8 (cathode) of that tube's socket?

[soundguruman]

"installed a 47k R and diode in series between pin 7 (grid) and pin 8 (cathode)"
Why would you want to prevent arching? Is this 5751 arching???
"avoid straining the heater-cathode insulation."
I was not aware that was a problem...
No, connecting heater to cathode is not advisable, I don't think so.
Perhaps it will prevent arching but the audio path will be mussed up?
What are you connecting it to? Just curios why the concern w/ arching.

[Chuck H]

SGM is right. The only preamp tubes I know of that have trouble with cathode follower circuits (even the vintage circuits that exceed specs for the tubes used then) are the Russian made spiral filament tubes. Your 5751 is probably NOT a new Russian spiral filament. And so it should be fine.

Elevating the heater CT is usually done either with a voltage divider on the HV rail or at the cathode resistor of a cathode biased amp. Cathode biased EL84 amps have very little cathode voltage so in that case it;s best to make a voltage divider on the HV rail. The "dropper" should be bypassed with a large value capacitor so that any 60Hz can take the dirt nap.

[frus]

Could I elevate the heater voltage by connecting pin 9 (heater center-tap) and pin 8 (cathode) of that tube's socket?

and you don't do it like this either; pin 9 is NOT the heater center tap, unless you run the heaters in series, and off 12.6V supply
but agree with the others, no need. the cathode on CF on my 5F4 sits even higher (over 200V all the time), and I never had any trouble with any tube I tried there (JJ, chinese, Philips, Telefunken, RFT,...)

[Alex/Tubewonder]

Putting heaters on some 50-70VDC above ground is not absolutely necessary but it's advisable if you agree that Murphys Law applies. If you run the cathode at high DC potential close to max spec for that tube than it means the tube CAN exhibit excessive cathode-heater leakage. Murphys Law says it WILL eventually. I can confirm it is so.

[pdf64]

"installed a 47k R and diode in series between pin 7 (grid) and pin 8 (cathode)"
Why would you want to prevent arching?

I think Merlin's concern is that at switch on, before the tubes have warmed up, the grid of the cathode follower will be pulled up to ~400V above the cathode, which is probably way above the level that the grid to cathode spacing was intended to withstand.
Merlin's idea limits that voltage without impacting on signal handling once the tube has warmed up; it seems wholly beneficial, what's not to like?
Pete

[Alex/Tubewonder]

That is correct.

[soundguruman]

SGM is right. The only preamp tubes I know of that have trouble with cathode follower circuits (even the vintage circuits that exceed specs for the tubes used then) are the Russian made spiral filament tubes. Your 5751 is probably NOT a new Russian spiral filament. And so it should be fine.

Elevating the heater CT is usually done either with a voltage divider on the HV rail or at the cathode resistor of a cathode biased amp. Cathode biased EL84 amps have very little cathode voltage so in that case it;s best to make a voltage divider on the HV rail. The "dropper" should be bypassed with a large value capacitor so that any 60Hz can take the dirt nap.

Hey, don't agree with me. That breaks traditions.
I've come here for an argument. No you haven't.
You're not arguing with me, you're just disagreeing with me. No I'm not.

[Chuck H]

I always elevate the filament supply in newly built amps that have a cathode follower. It's just good practice and prevents failure if the user ever plugs a new and less sturdy tube into that socket.

Better?

[deci belle]

Ding! …and in the white trunks~ haha!!

I was just diddlin' with my amp while I read that book and trying stuff…

The "dropper" should be bypassed with a large value capacitor so that any 60Hz can take the dirt nap.

What is the "dropper", Chuck— the resistor coming off the HV rail?

[soundguruman]

Ding! …and in the white trunks~ haha!!

I was just diddlin' with my amp while I read that book and trying stuff…



What is the "dropper", Chuck— the resistor coming off the HV rail?

Screw it, I'm using DC heaters.

[deci belle]

Screw it, I'm using DC heaters.

Hey, I wanna learn something here. Do you mind?

[Chuck H]

DC heaters are all the rage now. It seems everyone but me is doing it!?! Once upon a time the regulation for DC heaters in tube amps was considered clumbsy and prone to heat failures. New products seem to make it much less cumbersome than it use to be. I just haven't caught up yet. There's a few threads here if you look for them.

[Alex/Tubewonder]

Regulated DC heaters AND elevated 70V above ground.

[Chuck H]

Boy know how to throw down!!!

Circuit?