Fender 30 from 1980 switching channel pop

[slaweks222]

Hello everyone,

I own rare Fender 30 amp fro 80's.
Schematic attached.
I spend lot of time recently building footswitch for that amp and small maintenace in overall.
I used kind suggestions from experienced users of this amp, and checked and replaced possibly leaking 0.047uF cap also 1000pF cap from switching circuit, and used 10meg resistor from junction of 0.047uF and 220k resistor to ground.
Attached drawing.
Now the channel switching pop via footswitch is lower, noticable but I can live with that , also independent from volume level on both channels, that is good news.
However in case when footswitch in unplugged there is huge pop during channel switching in both ways, increasing together with volume levels on both channels.
When volume is 0 on both channels, there is no pop.
Normally, with footswitch unplugged and channel switch in "off" position this amp offers sort of "blend" of both channels, both are active simultanously and you can shape sound with tone and volume on both.
In channel switch is in "on" position, only reverb channel is active.
Could you please share with me your opinions?, any technical advice how to get this pop sorted or just minimize?
How it looks/ looked on your own Fender 30 with footswitch unplugged when channel switching?
I'm very grateful or any reaction in advance.
Regards Slawomir

[loudthud]

I think you need another 10Meg on the 1000pF side of "B".

[slaweks222]

loudthud, thank you for you suggestion, I tried, no changes. Would be grateful for more tips. Will try to replace this DPDT channel switch, but I dont think that is the issue.
Regards Slawomir

[Helmholtz]

It is interesting that there's no popping with channel volumes at zero.
That might mean a bit of DCV at the input(s).
Does it pop with
- nothing connected to the inputs?
- either V1 or V2 removed?

[slaweks222]

Hello,

Helmholtz,thank you for response. Well, below some research I did this morning.
I focused on checking one more time if there is really no pop at all with both channel volumes at 0. In totally quiet room, with my ear close to speaker, I could actually hear some barely hearable pop during switching from OFF to ON position ( from both blended channels to one).
Sorry for wrong information.
- with nothing plugged in any of inputs, still pops.
Now, I started pulling amp preamp tubes, and amp still pops even with all V1-V5 preamp pulled out....
With only V6 , PI tube amp still pops.
After pulling V6, switching is quiet.
I replaced V6 12at7 with another one, still pops.
I double checked and reflow soldering point on eyelet board.
I checked soldering on V6 pins, looks rock solid.
As always, Im looking forward for some more tips.
Regards Slawomir

[loudthud]

Check the Plate Voltages on V6. That's pins 1 and 6. Compare those to Voltage on pins 3 and 8. If the Voltage difference between the two is too low, Grid current will result.

[slaweks222]

Thank you loudthud,

Results as follows:

Pin 1 to ground 245V ( schematic shows 228V)
Pin 6 to ground 224V ( schematic says 213V)
Pin 3 to ground 79V (schematic says 85V)
Pin 8 to ground 79V (schematic says 85V)

Regards Slawomir

[slaweks222]

Meanwhile I temporary replaced both of 0.1uF 630V caps from plates of V6 to pin 5 grid on both 6L6GC, ( just see that one of power tubes on schematic is called V6A instead of V7).
No changes