Designing a PCB for a tube amp

[vk6zgo]

Some people swear by DC voltage applied to filaments, specifically on high gain stages, to minimize hum.

It was never necessary in the old days, but has become a "sacred cow" amongst those who are "semi-educated" in tube technology.

Possibly this is because of the poor quality of most tubes available today, some of which are tarted up "pulls" from operating equipment, which may well have been removed from service due to heater-cathode leakage.

I've never been a fan of PCBs used with tubes, as the heaters produce quite a lot of heat, sometimes leading to damage to tracks & board delamination.
With the old phenolic boards, "sometimes" could be replaced by "inevitably"!

Ps: Oops! Didn't realise this was a zombie thread!

[schmitt trigger]

In the good old days, there was a trim pot inside the amplifier, which was called AC balance or something like that.

It was a low valued trimpot, whose cursor was grounded, and the two endpoints were connected to the 6.3 volt heater supply.
One would adjust the trimpot for lowest hum. It usually was only applied to the high gain phono preamp stages.

I have a H H Scott amplifier with that adjustment.

[TimFox]

Besides the tube sockets, another potential heat problem with vacuum-tube PCBs are the power resistors, which may run hot.  Heathkit used to tell the assembler to cut a small piece from the cardboard box (maybe 5 or 6 mm thick) and use it as a temporary spacer between the power resistor and the board while soldering, and then remove the spacer so that the body of the resistor did not heat the board material directly.

[dazz]

Thanks for the great ideas and suggestions everyone.
In this design, I have the filaments referenced to ground on one end of the 12.6VAC, so that it can be rectified to some 16VDC which is what I use for the grid bias. I guess I could have used the plate or screen supply for the fixed bias to have the possibility of adding that trimpot in the filaments supply.

We've already built this amp, only that it was the original version with the tubes in the same side as the components, and there was no hum according to my friend, the guy who put it together and tested it.



Also, I have the largest power resistor mounted vertically. I think I'll do the same thing for the other power resistors, I mean, why not. 

[dazz]

Back again with a question about the power transformer needed for the EL84 push-pull amp.
Going by the simulation, I estimated that I needed 220VAC and 200mA in the secondary for some headroom. Does that seem about right? I'm having trouble finding a Hammond tranny with those specs here: https://www.tube-town.net/ttstore/components-10662/transformer/power-transformer/audio/

There's this Hammond 369HX with a 150mA 225V winding. Would that provide a full 300V for my anodes at máximum gain & volume?
Thanks