Series heater wiring, series/parallel heater wiring

[diagrammatiks]

Hi,
So I've got no problem with the normal heater wiring in amps...
They are in parallel and I can visualize that.

I've read Merlin's book and website on series heaters but my lack of engineering knowledge is leading to some confusion...

How exactly would a string of heaters in series work?
What would the schematic look like if say...I wanted only 1 preamp tube to have it's own series branch off the main parallel heater supply?

Would I then be able to elevate just that heater run?

For example, when Merlin talks about the SRPP he says that the upper valve should have its heaters elevated by 30vdc.

In the section about phase-inverters it's suggested that the phase inverter heaters could sometimes benefit from being elevated by 30 or so vdc.

How would I go about implementing something like that without also elevating the rest of the heaters?

[octal]

Why would you not want to elevate the rest of the heaters? It's common practice and cuts hum. Attach your CT or virtual CT to +30V, done.

[diagrammatiks]

I just reread Merlin and I guess I misread it the first couple of times. Seems elevating the entire chain is totally cool too.

[Wilder Amplification]

What would the schematic look like if say...I wanted only 1 preamp tube to have it's own series branch off the main parallel heater supply?

It wouldn't. The only way you can run series heaters is if you have double the voltage available.

Of course the other option is to use a 12.6VAC center tapped power tranny for the heaters. This would give 6.3VAC - 0 - 6.3VAC (with "0" being the center tap reference), then you'd hook pin 4 to the "start winding", pin 5 to the "finish" winding, and finally hook pin 9 to the center tap. This would give superior hum cancelling I would think.

How would I go about implementing something like that without also elevating the rest of the heaters?

As everyone else has mentioned, just elevate them all. I run elevated heaters on every amp I build, although I use about a +55V reference myself. Not only does the phase inverter benefit from this, but DC coupled cathode followers also benefit (and even more so than the PI does) as elevating them reduces the high heater-cathode voltage differential that exists on the DC coupled CF circuit.

How I do it is with a voltage divider comprised of a 330K/2 watt and a 47K/2 watt connected to the screen node of the B+ rail. The 330K gets connected to the rail, the 47K goes to ground, and then they join together. I then use a 10uF 160V electrolytic across the 47K (very important!!! you will have LOTS of ripple hum without that cap!!!) and use the junction of the two resistors as the elevated reference.

[redelephant]

Interestingly elevating the heaters increases tube life, (reference: Getting the Most Out of Vacuum Tubes, by Tomer. Available from PMillet's wonderful site.)
It seems for optimum life 90V marks the spot, but 50-60V is recommended even tho it is not said why, my guess is b/c that's a safer voltage level and the difference in life is marginal between 50 and 90V..

[kg]

As everyone else has mentioned, just elevate them all. I run elevated heaters on every amp I build, although I use about a +55V reference myself. Not only does the phase inverter benefit from this, but DC coupled cathode followers also benefit (and even more so than the PI does) as elevating them reduces the high heater-cathode voltage differential that exists on the DC coupled CF circuit.

Interestingly elevating the heaters increases tube life,

i think these two things are saying the same thing, and i agree. in my experience i've broken down quite a few HK insulations by excessive CF Vk. tough to troubleshoot too since they break down intermittently.

[diagrammatiks]

yup sounds good to me.

I was just worried because some of the text made it sound like I should keep some at 0, elevate the pi to 30 and ac cf's to 60...was very confused.