Fender 30 fro 1980 channel switching 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 just very slight 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

 

[slaweks222]

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).
- 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

 

[joekool]

Greetings ,
Your pop problem is related to the t ( the ECG 40938 ) transistors shifting impedance and DC voltage changes.
will cause the dc pop problem to occur . the fix in this case is to use Vactrol devices or to build and install a mute circuit which is only active when you engage the foot switch . many tube amps use this mute circuit such as fenders EVH brand and Mesa Boogies and Peavy products just as some examples of brands that do this . hope this is of some help to you .
Regards Joe

 

[musipol]

maybe try a 1meg or greater Res between 1000pf cap input and gnd. That would be on the side of that cap that goes to C and B optos.

 

[idylldon]

Go here for a solution that worked brilliantly:

https://groupdiy.com/threads/fender-30-channel-switching-pop.60572/