6G2 amp - volume and tone play up on one input only - diyAudio
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Old 16th October 2012, 04:11 PM   #1
diyAudio Member
Join Date: Oct 2012
Default 6G2 amp - volume and tone play up on one input only

Hi there - i'm a newbie here so please be nice if I tread on any toes And please take a deep breath - I have a fair bit to say, so go get some popcorn as I search for a troubleshoot

I've just built my second amp - based around a 6G2 brownface Fender Princeton. I've basically used up a few spare parts I have laying around, and largely based around a modified tweed Princeton chassis.

Click the image to open in full size.

As the chassis only has room for two controls, i've opted for a volume and tone and removed the tremolo circuit completely.

I've made a few adaptions (to suit my 240V requirements, etc), but here's the schematic i've followed for the most:

Click the image to open in full size.

The amp is seemingly working properly on one input, but I have a couple of issues to sort out before I can call this amp 'done'.

Firstly, everything works fine and sounds great on my 'outside' instrument input with the 1 meg resistor. But when I plug into the 2nd input and dime either the volume or tone, I get a very unnatural note decay - almost like a noise gate trying to kicking in. It seems to happen quite high on the pots - say from 80% - 100%. I'm completely lost as to why.

Problem #2 - this amp falls silent with my current pedal setup. With the initial startup of my 6G2, I eventually kicked on my delay and compressor pedal, and the amp seemingly worked fine (although there was quite a large AC pedal power supply hum occurring). But with the second and subsequent power ups of the amp, i'm getting nothing. The pedals work (and guitar signal passes) in 'bypass' mode, but engage them and all I get is a big fat silence.

I have two point of concern in the amp, but i'm not sure they're causing my problems. Firstly, i've not grounded the empty triode in the 12AX7 (created by the deletion of the trem circuit). Should I need to? And secondly, I need convincing the diode in the bias circuit is facing the right way - i'm not sure if the Fender schematic is wrong or the Wikipedia source i'm using to try and orient it. Either way - what would be the impact of a bias diode with the wrong polarity? I wouldn't think the amp would work at all.

Click the image to open in full size.

I'd dearly love to put an end to these quirks, so if you have any suggestions for a fix i'd love to hear them.

Thanks for your time.
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Old 17th October 2012, 03:38 AM   #2
diyAudio Member
Join Date: Oct 2012
Here's a video I made to try and better illustrate my operating issues.

6G2 build... a few issues to resolve - YouTube

Another thing that comes to mind is maybe I have a faulty grid resistor to the crackly input - can can see no other reason why one input is fine but the other isn't. Can anyone tell me what the symptoms are of a suspect grid resistor?
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