Elevated DC filament Supplies

[megawhat]

Hello,

I'm working on a new build and I have a choice to make... sort of.
I'm using one of the more simple DC heater circuits for V1 & V2 for a high gain amp. A bridge rectifier with some 4700uf caps and a faux centre tap on the DC side.

My AC heaters will be warming the power tubes and two preamp tubes. I'll be using a cathode follower in this part of the circuit so I thought I'd use DC elevation on the AC heaters anyway.

Reading Merlins PSU book, I could tie the Faux Centre Tap of the DC heater circuit to the reference voltage for the AC heater elevation.

(both AC heater FCT and DC heater FCT tied to the same reference voltage).

I can't really think of an advantage to doing this since the AC heated valves and DC heated valves will be totally separated anyway.

Would there be a reason NOT to reference the DC circuit to a DC voltage? I'm just working on my layout.

[markr14850]

You must be sure to *not* directly reference both the AC and DC portions of the circuit to ground. If you do, you'll instantly smoke a rectifier.

Elevating AC while "center tapping" the DC will break this rule.

If you really want elevated DC, you can do one of the following:
A) Elevate the AC section. Remove the "center tap" from the DC section, and tie the negative leg of the DC to ground via a large (6800uf or greater) cap.
B) Get rid of the center tap and connect the elevated reference to the DC as it's ground reference.
C) Build a totally separate DC supply, just for your 9 pin tubes, and elevate it.

--Mark

[megawhat]

My post might not have been clear. its two different 6.3V winds on the PT.

one is higher power and im using it for four power tubes and two 12AX7's. There is easily enough current to run KT88's and one of the triode stages included in this is a cathode follower.

The other 6.3V tap has less current available, but enough to run another two 12AX7's.

The the 'AC heaters' and 'DC heaters' are discrete, they are not chained together with AC heaters fed before the Bridge Rectifier.

If they were all on the same 6.3V wind, I could put two 100R resistors on the output of the bridge rectifier from the + and - and tie them to a reference and elevate the whole circuit. Merlin suggests using a 'humdinger' pot in his book.

given that the circuits are discrete, I don't see a need to elevate the DC Heaters really, but it would be really easy to do on my layout thus far. However with DC, is there really any pro's or con's to doing it? Should I just whack two resistors to ground?

[megawhat]

I attached a sketch on what I mean.
I'm sure there is no real advantage to doing it like that right? But is it a bad idea?

[martin manning]

The main reason for elevating the AC heaters is to reduce the cathode follower's filament-cathode voltage right? I'd just do that and run the DC heaters to a virtual centertap on the DC side to ground. They're isolated circuits though, so I don't see any problem either way. You drawing shows both the AC and DC heaters on a single winding, so I'm going by your description of two separate 6.3VAC windings.

[megawhat]

sorry! you're right. they are two different windings, that is a mistake how i have drawn it.

The advantage to doing it that way for me personally is I can get the board layout to work a bit easier. Otherwise I'd do it exactly the way you suggest, which was my original plan. I just wanted to check I wasn't missing something out of consideration. cheers.