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master volume

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1/27/1998 7:09 AM

master volume

Can anyone help me with this?

I built a 50 watt amp with pretty much standard fender circuits and upon completion it didnt work.After testing I found that the pre-amp section was working fine and that the problem resided in the phase splitter.Measuring voltages on the 12ax7 tube of the splitter revealed that he input half of the tube had all the wrong voltages when compared to the other half of the tube.I rechecked the wiring and everything seems fine.I then bypassed the master volume which was installed between the output of the pre-amp and the input of the splitter by connecting the output of the pre-amp directly to the input of the splitter and the amp began to work.It seems that the pot gronds to pin #2 of the splitter and shuts down that half of the tube because when I unsolder the ground wire from the pot the amp works but the master volume does not.
Can anyone tell me how to correct this problem I sure would like an operational master volume.

1/27/1998 10:22 AM
GVB	It sounds to me, like a bad pot. Try checking the pot with an ohm-meter. GVB

1/27/1998 10:46 AM
Dave H	Try replacing the capacitor between the master vol wiper and the phase splitter grid (pin 2). It may be leaky. If the cap is OK the pot cannot upset the dc conditions of the phase splitter..

1/28/1998 12:45 AM

Gary , I think Dave has this one.
It sounds like the type of occurance when the grid on the first half of the phase inverter tube is connected to the output of the prior stage with no DC blocking cap at all.
You did not specify ....
I assume you "do" have a blocking cap from the wiper on the Master volume control to the #1 grid of driver tube?
If not. it will never work quite right and if it does make a sound, the amp sounds like it has a MAJOR case of Godzilla's version of an envelope follower.
Bruce

1/28/1998 7:15 AM
Gary	I,m a little embarresed to tell you guys this but you hit the nail on the head,I dont have a cap between the pot and the input of the splitter.My only excuse is that this is my first attempt at building an amp and I love it. Thanks for your help it's nice to know that there is a place to turn when you're stumped. Gary

1/28/1998 11:13 AM
Bruce	Never be emarassed to ask a technical question! It's the questions that never get asked that are embarssing. Besides, the ony reason I knew to ask about the cap is because I've made those same mistakes a few hundred times too! Bruce

1/28/1998 11:23 AM
Bruce	Now that's embarrassing too.... I'm already good looking so if I could type and spell, I'd be real dangerous. Here is the corrected version. sheesh!

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