Blues Junior Project Mastervol Problem

[FredH]

Hi,
I proudly finished my first amp-project, the Hoffman Blues Junior and I think nearly everything is fine except two "problems":

1. The Amp isn't quiet, even when the masterpot is set to zero. If I play with the guitar with master on zero, I clearly hear the guitarsound (not loud but good to hear with nearly no bass). If the gainpoti is set to zero, the amp is quiet.

2. There is a little 50Hz-Hum (Germany) from the speaker, even when the Amp is only in standby-mode (where only the kathodes get their 6.3V).

Thank you for any help in advance,

Manfred

[shooter]

1.  what type MV, post PI, pre PI?  If it's the same as Doug's schematic, Pre MV, try disconnecting the NFB at the speaker and see if that does anything

2. Start with the 1st tube, typically assigned V1, pull then check, still hummin? leave out, pull V2, walk it to the area

[sluckey]

Use a gator clip test lead to ground the junction of R20 and C11. Does this totally kill the guitar sound?

[92Volts]

Only treble bleeding through sounds like a capacitive effect-- wires before and after the master volume are too close to each other.

Otherwise, small amounts of sound could be a bad MV pot, or resistance between the pot and circuit grounds. Maybe a bad solder joint, or relying on ground through the chassis instead of a wire.

It might be even worse if the pot is grounded together with "noisy" items (like tube cathodes) and that entire section of the circuit is then poorly grounded.

50Hz hum even in standby is from the OPT picking up magnetic fields from the PT. Rotating one or the other sometimes helps. Moving them further apart usually helps, but isn't possible in all chassis/layouts.

[FredH]

Use a gator clip test lead to ground the junction of R20 and C11. Does this totally kill the guitar sound?

Yes, I just tested it. This totaly kills the sound.

Manfred

[sluckey]

The problem is likely a defective MV pot or a poor ground connection to the MV pot. Try grounding the pot ground lug with the gator clip test lead. If this totally kills the guitar sound when the MV is turned to zero then you have a poor ground connection to the pot. Otherwise, try a new pot.