few questions on elevated heaters

[12AX7]

I am thinking of doing this from the merlin site... http://www.valvewizard.co.uk/heater9.jpg
(the upper one, i have a CT)

It's basically a 2204 type build buy 6V6 and 18 watt transformers. Typical marshall JCM series fixed bias SS rectified, 3 gain stages then cathode follower. Is there anything i should know before trying this? Just wanna be safe. I was thinking of taking B+ from the PI node (around 300v i think) and a 220k/33k to 56k for the voltage divider and a 100uf cap and tweak the lower resitor till i get around 40v.  Am i missing anything ?

Also, is there a way to do this for V1 only? thanks.

[sluckey]

Sounds good to me.

Also, is there a way to do this for V1 only?

Sure, but why??? You'll need a separate 6.3VAC filament source. The only tube that might benefit from this is the cathode follower that drives the TS.

[12AX7]

Had a question in this post but i'm overwriting it after finding out for myself by doing the mod. Works fantastic. I used a 27k on the bottom to get just over 30v and it's dead quiet. I use tung sol reissues in V1 and i have 5 of them. But they hum like mad and this removed that 100% so i now have 5 good TS's instead of one barely usable one and 4 that are hideously noisy. Thanks for the help slucky !

[trobbins]

Are you saying you tube-rolled five tubes in to V1, and they all exhibited a noticible hum on the output?  And then when you elevated all heaters in the amp, and tube-rolled V1 with those same valves, that there was no noticeable hum?

[12AX7]

5 tung sol reissues, yes. No other brands/models hum at all, just the tung sols. And yes, the tung sols are now as quiet as any other tube.                         

[trobbins]

Hmmmm, that's interesting - they must do something consistently different than vintage manufacturers did with their heater elements.

[12AX7]

It's a well documented issue with the TS reissues. It's only a problem tho with cascaded stages. You can use them in a low gain preamp or PI etc and they're fine But in V1 of a high gain amp they hum horribly. I didn't know if elevating the heaters would work but happily it did, and extremely well. I can't find another current production tube that feels and sounds as good to me in this amp at least as the TS reissue,  so i was on a mission to make them usable. Thankfully it was a successful one.

[Joel]

I've added heater elevation to all my amps.  It's especially useful in my JTM45 clone.  The Cathode Follower had a very high heater to cathode voltage which was causing intermittent noise.  Elevating 60V cured that. 

[12AX7]

Another question....now that i have elevated the heaters by 30v is it ok to now run spiral filament tubes like the tung sol 12ax7 in the cathode follower position?