few questions on layout

[arjepsen]

Hey.
Just wanted to hear you guys' opinion on a few layout points:
On a typical gainstage-cathode follower using both sides of one 12ax7, there will be HT on one of the plate pins. Is there any ill effect (noise, oscillation,etc.) from having the regular 100k plate resistor straight on the socket? Like say from pin 6 to pin 1 (HT on pin 6)
And then a grid stopper (maybe 10k) from pin 1 to pin 7?
I wonder if it's too close to heaters which I have running straight over the sockets in a straight line.

Also, I'm fiddling with vvr for this amp, and since it scales the whole amp, I have a .1µF after the normal input grid stopper (33k) to the socket. But I also read people advising to keep that first grid stopper as close to the socket as possible. Am I right in assuming that there's no difference in whether I put the .1 cap before or after that 33k?

[TUBEDUDE]

The grid stopper has the priority on tube pin. I'm not clear on the cap install. If it shunts the input, fine, just attach to the pin also. If in series, before the resistor would be proper. What is the cap function here?

Also, i have mounted cathode follower components just as you have suggested, with no issues. Any heater hum should only be a problem with high gain builds, where DC heaters might be required. Otherwise, i've found wire routing, and sometimes heater elevation will make for a quiet amp.

[sluckey]

On a typical gainstage-cathode follower using both sides of one 12ax7, there will be HT on one of the plate pins. Is there any ill effect (noise, oscillation,etc.) from having the regular 100k plate resistor straight on the socket? Like say from pin 6 to pin 1 (HT on pin 6)

Fender did that on the 5F6A Bassman. Marshall did that on a lot of the old amps. Most of the clones I've seen do that. I do it. I think it's superior to running extra wires between the board and socket. I've never seen a grid stopper at that point but I don't see any reason not to do that if you want a grid stopper at that point.

[arjepsen]

@tubedude
The .1 is related to the vvr HT scaling. It prevents DC on the guitar pot.
In Dana Halls documentation he shows it located after the input grid stopper.
I'm just wondering if it would be better to put it before the grid stopper.

@ Sluckey
I hadn't seen a grid stopper there before either. It's something I found in Merlins books.

[Malcolm Irving]

If you have to run a hum-sensitive component or connection over a heater wire, try to cross over at a right angle. This minimises hum pick-up.

[TUBEDUDE]

@tubedude
The .1 is related to the vvr HT scaling. It prevents DC on the guitar pot.
In Dana Halls documentation he shows it located after the input grid stopper.
I'm just wondering if it would be better to put it before the grid stopper.

Can you post a schematic, i haven't used a VVR and need to see it to comment further.

[sluckey]

The VVR circuit does not directly put dc volts on the preamp grid. But when you use the VVR to drastically lower the B+, this low plate voltage condition on the preamp tube will cause dc voltage to appear on the grid, and this dc will usually make the guitar volume pot sound scratchy. The same will happen in other situations that apply a low voltage to the preamp plate.

[arjepsen]

schem

[TUBEDUDE]

It looks like there will be no grid stopper with that setup. With the grid stopper at the pin, the cap will still stop D.C. from getting to the guitar and the grid stopper will function. Grid stopper from tube pin to grid resistor and cap. Grid resistor to ground, and cap to input jack/resistor. You can check with Dana to see if he has found an operational issue that requires the circuit to be as drawn, or maybe it was just drawn following the way Fender typically drew their inputs schematically.

What is the advantage of whole amp starving, as opposed to just the P.I./ P.A. scaling?