pcb amp...

[chief mushroom cloud]

I really need some advice from the experts (someone who has done this). If I did a pcb tube amp preamp...how wide should the signal traces be, and what thickness copper should I ask for? Should I use a ground plane or dedicated traces? Also, I need engineering explanation of why specific trace widths and copper thickness. My math is good...throw it at me.
TIA guys

[jelle]

Equivalent of 22awg solid core wire worked for me, you do the math. And thanks again for the books!

[JazzGuitarGimp]

Chief,

I haven't tried it yet, but it's on my list if things to do. I like the idea of ground plane on the side of the board farthest from the chassis, with signal traces on the side closest to the chassis. That way the signal traces have shielding on both sides. I think I would still keep each ground node (ie, the negative connection of each filter cap in the PS string) as a separate ground plane, so that they can all be grounded at one point on the chassis (or two points, if that is your preference). So, each ground plane is situated beneath only it's associated components, with perhaps 25- to 50-mils of space between planes. Also, my thinking is that all the preamp tubes should use standard chassis-mount sockets, with an oblong hole in the board to allow access to the nuts that secure the sockets to the chassis, as well as the pin connection terminals. Then use short wires to wire the sockets to pads on the board - grid resistors could be flown between a pad on the board, and the socket terminal. These are just some things I've ruminated on in the past. I'm not sure trace impedance (ie, width and thickness) is going to be critical. I like Jelle's idea of sizing them to the same cross-sectional area of a 22-awg solid copper wire. I imagine 1oz copper is more than adequate.

If you jump in with both feet, please let us know how it turns out.

Cheers,
Lou

[TUBEDUDE]

One ounce copper is mighty thin. The equivalent trace width would be quite wide. The thicker the trace, the more thermal abuse it will endure, as you drill or swap out components repairing or modifying the circuitry. I'd go as thick as possible, although I haven't seen more than 2 oz offered by the companies I've seen. Chassis mounting of sockets is just common sense for a durable design. But then again I'm based against PCB amps anyway.

[chief mushroom cloud]

2 Oz trace > 22awg wire is .194in (194mils)
but according to IPC-2152, a 194mil trace can conduct 2.5A w/o breaking a sweat. Using stripline calc, this amounts to ~33ohms impedance (FAAARRR lower than necessary for tube circuit).
Has anyone here done a blind A/B listening test (apple to apples...exact same circuit and components, pcb vs ptp)?
I've been approached to do this for an amp co, and they want metrics.

[chief mushroom cloud]

Equivalent of 22awg solid core wire worked for me, you do the math. And thanks again for the books!

no prob Jelle....

[JazzGuitarGimp]

I'd go as thick as possible, although I haven't seen more than 2 oz offered by the companies I've seen.

I'm pretty sure most board houses will plate up to 3oz. But since the current levels in tube circuits are so low, I wouldn't go beyond 1oz. A 25-mil-wide, 1oz trace will get you 3-Amp capacity with a 10º rise in trace temperature, which is way more ampacity than needed.

I like the promise of repeatable, predicable "lead dress" afforded by a pcb, though it may take a handful of layout tweaks to get the circuit to behave the way you want it to. I would suggest laying in copper pour on the signal side to shield grid circuit traces from other traces / pads that lie next to the protected (grid circuit) traces.

[jelle]

Tracy, if something catastrophic happens in the amp, would you like the board, or some resistor to be toast?

I know what I prefer.

Jelle

[TUBEDUDE]

The current capacity of a thin trace may be ample, but again, that is not the only quality to consider.

[chief mushroom cloud]

Tracy, if something catastrophic happens in the amp, would you like the board, or some resistor to be toast?

I know what I prefer.

Jelle

Yes Jelle...for my own builds, I do ptp...but this is info I need to explain to a potential contract job offer....

[chief mushroom cloud]

The current capacity of a thin trace may be ample, but again, that is not the only quality to consider.

Go ooon? Do tell......