If you haven't heard of the AX84 project, it's a collaborative guitar amplifier blueprint that anyone can build.

So, I'm trying to extend the design to use a solid state ZVS power supply (This module: https://smile.amazon.com/gp/product/B010UGJF1I). Everything is going swimmingly, except I realized I'm going to vastly exceed the cathode-to-heater voltage limit of a 12au7 with my cathode follower.

For the heaters, I was going to use this module: https://smile.amazon.com/gp/product/B00C4QVTNU . I just checked both modules, and they both tie their output ground to their input ground.

I'm currently stumped and a bit discouraged. How can I regulate 6.3v, but have it be referenced against the b+ voltage of the tube, instead of referenced to ground?

Certainly this is done "in the real world." There are isolated solid state DC supplies that can sit on top of power lines and what not.

Also, please be kind. This is my first amp build. I'm not an electrical engineer, but I did take a physics class in college. I'm here because I'm trying to learn and fill in holes in my knowledge.

Thank you for any help!


Block Diagram of my amp: Block Diagram of my amp


simulate this circuit – Schematic created using CircuitLab

PSU block diagram for OP to edit.

Actual Schematic: Actual Schematic

  • 1
    \$\begingroup\$ Use an isolated DC-DC converter instead. \$\endgroup\$
    – user16324
    Aug 5, 2016 at 19:24
  • 1
    \$\begingroup\$ @JonathanS.Fisher: I think you haven't got an answer yet because nobody can understand your requirements. Your, um, childish schematic doesn't help either - the wires are unlabeled and it's not clear what the problem is. I've added a schematic for you to edit. I don't know what the 24 V is for and can't remember how to hook up the grids. Play with that and clarify your question. There are a few old-timers here who are familiar with valves / tubes. \$\endgroup\$
    – Transistor
    Aug 6, 2016 at 10:23
  • 2
    \$\begingroup\$ Let's be adults and avoid name calling [childish] and maybe be a bit more objective. I do appreciate your input, but as we learned in first grade, "if you don't have anything nice to say, don't say anything." \$\endgroup\$ Aug 6, 2016 at 15:57
  • 1
    \$\begingroup\$ @JonathanS.Fisher most well know tube guitar amps connect the heater voltage to ground (or 0V). See the Vox AC30. \$\endgroup\$
    – Steve G
    Aug 6, 2016 at 16:54
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    \$\begingroup\$ This isn't an answer and perhaps a bit late, but if you want proper valve advice, diyaudio.com is a good site. They have people there who understand valve amps pretty well including heater elevation and can read colour coded schematics. They're not as bitchy either. \$\endgroup\$
    – Paul Uszak
    Mar 15, 2017 at 22:03

2 Answers 2


In your question, you are attempting to connect the "heater" to the B+ tube power supply.

The B+ in your system is essentially connected to the Anode (Plate) of the tube (P, pin 5 in symbol below).

enter image description here

The heater in a vacuum tube is more closely associated with the Cathode ( K in the tube symbol above). The first vacuum tubes used a directly heated cathode before filaments (heaters) were created.
Filament heaters INDIRECTLY heat the cathode (electrically isolated).

In most common vacuum tube circuits, the cathode is more closely related to ground (common of power supply), much as the emitter of a NPN transistor in a common emitter configuration.

Vacuum tube data sheets can show "Maximum Ratings" such as (12AU6) :

enter image description here

Notice that there is a maximum 180 voltage rating (12AU6) for "cathode to heater". I did not find a maximum in the specifications for the tubes you have selected. But beware anyway.

I have not seen any circuits where filaments (heaters) have been referenced to the B+ power supply.

For further information on audio and filaments (heaters), Here is a good page to start with .

EDIT : ECC81 is same as 12AT7 : Maximum 90 volts Heater to Cathode.

  • \$\begingroup\$ Bottles! :-) (Long time no see.) \$\endgroup\$
    – Russell McMahon
    Aug 26, 2016 at 13:16

"How can I regulate 6.3v, but have it be referenced against the b+ voltage of the tube, instead of referenced to ground?"

You can't with non-isolated power-supplies as shown in your CircuitLab block schematic. You need an isolating power supply so that you can connect the + or - terminal to b+ as required. Even then you need to be sure that the isolation is rated at > 500 V DC and probably 1000 V DC to be safe from breakdown between the secondary and mains or secondary and earth.

enter image description here

Figure 1. The O'Doom amplifier schematic shows valve heaters are ground referenced.

I always understood that heater filaments are referenced to HT negative. This is confirmed in the schematic. Why do you want to connect them to the high-tension +?

  • \$\begingroup\$ Thank you, any idea where I can read up on such a thing, or how they work in theory? \$\endgroup\$ Aug 6, 2016 at 16:09
  • \$\begingroup\$ See the update. Isolating power supplies, whether they use regular mains transformers or high-frequency switched mode power supplies use isolated coils to provide the electrical isolation between the primary and secondary sides. \$\endgroup\$
    – Transistor
    Aug 6, 2016 at 16:13
  • \$\begingroup\$ Thanks. I think what I'm going to do instead is replace the cathode follower with a MOSFET and dodge the problem completely, provided I can calculate the cathode voltage in the output tube correctl.y \$\endgroup\$ Aug 8, 2016 at 17:56
  • \$\begingroup\$ Why are you ignoring several comments questioning referencing of the heaters to HT+? You haven't explained why you are doing this. \$\endgroup\$
    – Transistor
    Aug 8, 2016 at 18:03
  • \$\begingroup\$ See that's where I'm confused :/ The way my modules work right now, the 24v+, 6.3v+, and 376v+ all share the same ground. I must be missing something obvious, because it would seem they indeed are in fact already referenced to HT negative. \$\endgroup\$ Aug 8, 2016 at 19:20

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