Fender Blue Deluxe Problem

[GregS]

I have a 15 year old Fender Blues Deluxe that has started giving me trouble recently. The sound goes all static like and cuts out whenever you play a note or chord. It doesn't happen if you pick very softly but any medium to hard pick attack will cause it.

I measured the voltages at all the tube pins. Everything checked out normal except the difference between the grids and cathodes of the phase inverter tube. The cathodes are at 37V the grids at around 25V. It seems to me a 12V difference between the cathode and grid would mean that the bias is way off - unless this isn't so unusual for a phase inverter?

It is a long tail pair PI. The voltage on the other side of the 470 Ohm resistor from the cathode is 35.5V. From that point across the 1M grid leak to the grid the voltage drops to 25V. I also measured the resistance of these resistors with my DMM and they are still accurate to their rated values.

My question is is this 12V difference between the cathodes and grids of my PI normal and could it be the cause of my problem?

Thanks,

Greg

[Enzo]

The 12v difference is probably your meter. I bet you are measuring the grid to ground and the cathode to ground and subtracting. Try measuring the voltage directly between pins 2 and 3 or 7 and 8. See if it doesn;t fall into pplace at a volt or two.

We think of a meter as a uninvolved observer, but it is not. When connected to a circuit, it becomes part of the circuit. And the meter has resistance. That 1M resistor from the cathode and your meter form a voltage divider, which makes the voltage appear lower than it is. I'd be wagering that, anyways.


Look at the power tube sockets soldered to the circuit board. Look very close. Any tiny cracks in the solder around any pins? In fact I would resolder them anyway. All those latter day Fenders built like the Hot Rod DeVilles, I always resolder those sockets as a matter of course.

Try tapping on things. The vibration from the speaker is most likely what is triggering this, so send your own vibration through the amp. If it is not power tube socket solder, look for tube sockets not working well. Poke parts on the circuit board with a chopstick, and don;t rule out the speaker.

Plug your guitar into the FX return. Still cut out? And plug the guitar into the input, and fun a cord from the FX send to some other amp. Does that FX send cutout? Or does that continue while the speaker cuts out?

[GregS]

Thanks for the reply.

I bet you are measuring the grid to ground and the cathode to ground and subtracting.

That's exactly what I was doing.

That 1M resistor from the cathode and your meter form a voltage divider, which makes the voltage appear lower than it is.

Interesting. I never thought of that. I also looked at the voltage measurements I'd recorded from a working Fender Bandmaster and found a large difference in the cathode and grid measurements relative to ground as well. I was pretty confused!

The solder joints look ok but I'll re-solder them anyway as you suggest.

I'll try the other suggestions as well to isolate where the problem is occurring.

Thanks again,

Greg