Getting Feedback

[Geo]

I am getting nasty feedback when I try to put the volume up with this amp.

I was thinking of removing the master volume.

Any suggestions would be appreciated.

 

[psychodave]

I'm not sure I'm liking the tubes right next to the controls????

 

[paulyc]

I don’t see shielded cable on the input Jack wiring (grounded at only one end of course)

 

[fusedbrain]

You might have the polarity of the output section reversed, and you are getting positive feedback instead of negative feedback.
Looking at the pic, the easiest way to check this would be to reverse the 2 wires between the master vol and the power tube grids.

If you ever remove the master and the problem returns, then you will have to reverse the polarity elsewhere in the power section.

Also, agree on the odd layout. Not crazy about the filter cap mounting either, and where is the cathode resistor for the normal channel?

 

[7704A]

Not crazy about the filter cap mounting either

You mean their flying trapeze antics?

 

[Geo]

Thanks for all the info peeps. This not my build, but cool little amp none the less. Nice and neat soldering.

I have never seen anyone mount tube near the controls and have only seen filter caps like this in hi fi stuff.

It would be cool to figure out who did it.

 

[Geo]

 

[harddriver]

The LAR/MAR PPIMV should be using shield cable. Also if the amp is not running DC heaters running the buss wire straight through the lugs could be a source for AC heater noise since this amp does not have twisted wires that cancel the AC noise.

As others mentioned the input jacks standard unshielded wire running right over the top of the signal carrying lugs of V1 preamp tubes is asking for noise trouble.

Since the layout of the amp can't be changed a good start would be to install shielded wire on the input jacks and the PPIMV and see if the nasty feedback goes away. Since the Presence pot is on the back I would also route the green wire away from the output jacks not riding on top of them.

With all those flying leads everywhere they can act like Antenna's for noise as well.

 

[JackBootedThug]

I don’t know shit about shit but I would start with tubes. Every time I have had squealing issues or feedback it’s always been tube related.

 

[harddriver]

I looked at the pics again and the builder ran the 68K input resistors direct from the input jack lug to the preamp amp tube lug so you won't be running shielded wire there. Even if you don't run shielded wire for the PPIMV you can twist the wires to cancel as much noise as possible.

It look like a decent build definitely not a Billy Blades level build.......

 

[Geo]

I looked at the pics again and the builder ran the 68K input resistors direct from the input jack lug to the preamp amp tube lug so you won't be running shielded wire there. Even if you don't run shielded wire for the PPIMV you can twist the wires to cancel as much noise as possible.

It look like a decent build definitely not a Billy Blades level build.......

Thanks for the input, yeah definitely not a BB build.