Best practices for heater traces on a PCB

[thorpej]

I'm working on a line stage pre-amp that I plan to (mostly) build on a PCB. I plan to put two tube sockets on the PCB -- a noval for a 12AX7, half of which will be used for a tone control amplifier for each channel, and an octal for the output stage (a 6AS7 / 6080), again, half of which will be used for each channel.

The 6AS7 draws 2.5A of heater current, and a the 12AX7 draws 0.3A of course. I plan to have a switch on the board that can be set to configure the heaters for either a 9A base (12AX7) or a 9AJ base (Soviet 6N2P). I have thought about using DC heaters, mostly out of concern for having that high-current AC present on the PCB. But there are trade-offs in doing so, and so I'm willing to consider AC heaters if there are solid guidelines I should follow when routing those traces on the board (obviously I can't twist those leads ).

I am planning to have a ground plane on each side of the PCB used entirely for shielding purposes (i.e. brought out to a shield-GND pad that will connect to PSU ground / safety-ground). I'm using a toroidal transformer, so I'm not expecting stray EM emission from the transformer to be a significant source of noise.

If anyone has any advice in this area, I am all ears and ready to absorb.

Thx.

 

[Brice]

I run them close to each other, as short as possible, away from everything else and no components crossing above them.
And set the input at midpoint so the least current is flowing around.

So far I had no issues with heater supply in my PCB design, and I did not have a ground plane either.

 

[Lavane]

I run them close to each other, as short as possible, away from everything else and no components crossing above them.
And set the input at midpoint so the least current is flowing around.

So far I had no issues with heater supply in my PCB design, and I did not have a ground plane either.

I've built two amps using PCB's. One does use regulated DC heaters. The second uses AC and has a ground plane. Both are dead quiet. I'm working on a third that uses AC, but just has soldering eyes on the heater pads. I had to run twisted pairs from socket to socket. We'll see how that works out. I guess it might be easier to move the twisted pairs around if I encounter hum. I've seen a few PCB's with twisted pairs etched into the board too.

 

[thorpej]

Ok, for those that have done it, how do you handle the "hop" over the socket for the pin 9 connection on a 12AX7? For the 6AS7, it's easy to run the AC heater traces pairs parallel and away from everything else, but for the 12AX7, the other heater connection is on the opposite side of the socket. I suppose those traces aren't carrying nearly as much current (only 300mA), so small traces that can fit between the pads should be OK.

 

[Brice]

That's how I did it. I navigated on the outside. Follow the H trace.
And a board can feed the other just with a jumper. No twisted wires.
I hope it helps.

 

[Lavane]

The one I'm working on now shows two 6.3v windings used. One for the output tubes and another for the 6sl7 drivers. I guess that's how whoever designed the board dealt with that problem. But I'm running heaters for all six tubes from a single filament transformer. The winding on the power transformer I'm using can't produce enough current for all six tubes. I ran the heater wires to the first output tube at one end of the board, then from the last tube out past the end of the board and around to the 6sl7's.

 

[thorpej]

That's how I did it. I navigated on the outside.

Thanks, this helps me visualize what I need to do.

 

[thorpej]

So, just to close the loop, I've re-evaluated putting the tube sockets directly on the board... not for electrical layout reasons, but for physical layout / build reasons. Specifically, I'm having a difficult time finding octal and noval PCB-mount sockets that are the same height. I want both to be flush with the top of the chassis, so I'm reworking my design to simply use chassis-mounted sockets, building those bits of the circuit locally on a combination of tag strips / additional smaller PCBs (for example, I already have PCBs for a 2-channel constant-current load that I'm going to use with the 6AS7).

 

[gadget73]

The one dinosaur I own with lots of tube heaters on a PCB did it by not doing it. The heater string is done with twisted pair wiring that goes from socket to socket. This is 1970s tech when multi-layer boards were rarely done.

 

[Lavane]

The one dinosaur I own with lots of tube heaters on a PCB did it by not doing it. The heater string is done with twisted pair wiring that goes from socket to socket. This is 1970s tech when multi-layer boards were rarely done.

That is how the board I'm working with now is made. Eyelets connected to the solder eyes for the sockets. And just run twisted pairs.

 

[Lavane]

So, just to close the loop, I've re-evaluated putting the tube sockets directly on the board... not for electrical layout reasons, but for physical layout / build reasons. Specifically, I'm having a difficult time finding octal and noval PCB-mount sockets that are the same height. I want both to be flush with the top of the chassis, so I'm reworking my design to simply use chassis-mounted sockets, building those bits of the circuit locally on a combination of tag strips / additional smaller PCBs (for example, I already have PCBs for a 2-channel constant-current load that I'm going to use with the 6AS7).

Mixing octals and novals on a board isn't ideal. The Tubelabs board I used for my 300B had one octal, two 4 pin and two noval sockets. I had to use a pair of socket savers because the novals sat so low in the chassis. I've seen people use small diameter brass tubing soldered to the sockets to extend them, but it never looked very sturdy to me.

 

[gadget73]

I wouldn't use pin extenders as a noval spacer. Either use standard chassis mount sockets with standoffs between the ears and the PCB, or use a PCB type socket with a plastic or ceramic spacer under the center of the socket with a screw passed through the middle of it.

 

[Lavane]

I wouldn't use pin extenders as a noval spacer. Either use standard chassis mount sockets with standoffs between the ears and the PCB, or use a PCB type socket with a plastic or ceramic spacer under the center of the socket with a screw passed through the middle of it.

The board I used socket savers on had holes punched in the center of where the sockets mount on the board. So I was able to put screws through the socket saver and bolt them to the board. What do you use to extend the pins on the socket using a spacer under the socket? That does sound better than those up to 1 inch "legs" being added to the sockets I've seen.

 

[gadget73]

you can use the pin extenders to make the electrical connection, I just wouldn't want to rely on them for the mechanical attachment.