PCB recommendation for Valves

I am making a stereo Mullard 3-3 on a PCB, are there any real precautions I should take for heat conduction down from the EL84 output valves to the PCB? Are things like big thick tracks from the anode and screen grid pins needed?

The PCB is FR4 glassfibre double sided, the top will only be a ground. The transformers also go through the PCB (drop through style) the whole thing being the top 'plate' of the amp, the sides being bits of wood strip with slots to hold the PCB.

[Herald1360]

If the top side is ground plane, maybe fit a normal chassis mount ceramic valve base to the PCB then use short BTC links to pads on the track side?

I already have 5 rather nice flush mount (you can't see the pins) PCB sockets, I quite like the 'no screws' look.

[jjl]

Wide traces for the hotter electrodes won't do any harm, but most importantly, the heater connections should use the widest tracks you can accommodate and should be routed as far away as possible from the g1 traces.

You probably already know this, but, g1 traces should be routed as far away as possible from other signal carrying tracesl, surrounding g1 traces with grounded areas on either side might be a good idea. Other than that, ensure that there is adequate spacing between traces with a large potential difference between them.

A PCB layout engineer once told me "Remember, we have to pay to remove the copper".

John

[kalee20]

I'd go for 3/32" or even 1/8" thick FR4 material. This'll also have the benefit of reducing capacitance from the underside, signal tracks, to the top-side ground.

As the top side is ground, it'll tend to spread the heat away from the valveholders, so don't worry about the thickness of tracks for thermal reasons (other than making them reasonably thick so as to be robust).

I'd also suggest, don't track the heater wiring, but use twisted pairs soldered to pads beside each valveholder (yes I know, makes a nonsense of having a PCB, but then you can't include such hum-cancelling techniques in your tracking).

jjl

ensure that there is adequate spacing between traces with a large potential difference between them.

Yes, going to do that and give a squirt of anti corona laquer.

kalee20

don't track the heater wiring, but use twisted pairs

Doing that as well, others will also be hard wired, like the grounds (to a star point) transformer conexions etc.. The PCB will end up more of a sculptured tag strip than a monolithic wiring thing. I did look into the capacitance issue and found that for a 2" x 0.1" track on 1.6mm FR4 is the order of pFs, not worried.

Picture of it so far below, all components (bar valve sockets) will be 'surface mount' soldered on the underside, the ground plane top is only a 'pretty'.

[Craig Sawyers]

I use these for octal http://www.diyhifisupply.com/node/285 , shipping from the US to UK isn't too daft. Needs largish holes in the circuit board to take the pins.

EL84's need B9A, http://www.diyhifisupply.com/node/284 for "premium" at $19 a pop, or moulded teflon a whole lot cheaper http://www.diyhifisupply.com/node/289 for $4

These take a whole lot of heat, being teflon, and the premium ones have serious turned sockets which conduct heat really well down into the board.

But as others say - make sure the heater tracks are chunky, and run them close and parallel to each other to prevent hum induction elsewhere.

Craig

[Alistair D]

The PCB on a valve amp will flex due to the heat generated. Keep the component holes to the smallest diameter acceptable to reduce the chance of joints cracking as the board bends.

Al