(Fixed) Fender Blues Deville - Low Volume on Both Channels

[Grantorino]

I'd appreciate some assistance, as i'm feeling stuck on this one.

An original Fender Blues Deville is producing low volume on both channels.

Power tube pin 5 voltage is stable at -49V on new matched 6L6GCs, B+ healthy (~475V), heaters good (6.4-6.5V AC), OT resistance perfect (52/56.8/109Ω). Output previously tested to external speaker, volume was still low. Tried switching out pre-amp tubes, no difference.

Latest measurements (no signal, controls mid):

V3 pin 1: 266V DC
V3 pin 6: 268V DC
V3 pin 2: 23.8V DC
V3 pin 7: 24V DC
V3 pin 3/8: 36V DC
V2 pin 1: 254V DC
V1 pin 1: 248V DC
Power Tube pin 5: -49V DC both
Power Tube pin 3: 474V DC
Power pin Tube 4: 474V DC
Heaters: 6.4-6.5V AC

I had suspected leaking caps upstream of V3 and have been going through the schematic systematically testing and replacing drifted components. The amplifier was in a poor state when I got my hands on it and has since had all electrolytic capacitors replaced, wiring issues corrected, new dropper resistors, etc. I have reflowed solder joints across the board, including around the tube sockets, replaced ribbon cables with single wires.

Any ideas on common causes for positive DC on PI grids in original Blues Devilles? Tail resistor (R46 6.8kΩ) tests good, C52 (47pF) tested low but replaced.

Thanks for any thoughts – schematic attached if helpful.

[SEL49]

Any ideas on common causes for positive DC on PI grids in original Blues Devilles?

That's normal for that circuit.

[BrianS]

Check effects loop jacks.  Did you clean all the pots and jacks?

[Grantorino]

Thanks Brian.

Check effects loop jacks.  Did you clean all the pots and jacks?

Yes, all the pots and jacks were cleaned using Servisol Super 10. One of the broken input jacks was replaced. No FX loop on this amplifier. Power Amp In, Preamp Out, Foot Switch jacks all cleaned.

[stratomaster]

Power Amp In, Preamp Out...

This is the effects loop.

Try putting a short patch cable jumping these jacks.

Also the two plate resistors for the PI are under rated for voltage from the factory. I'd look at replacing them with appropriately rated ones.

[Grantorino]

Thanks for the input everyone. I tried jumping the fx loop, no change.

Simple issue, simple solution - the speakers were plugged into the external cab jack and I had neglected to check this. The amp came roaring back to life, no issues. Sounds killer.

Thanks again.

[tubeswell]

The low voltage +/- power supply for the channel relays/opamps etc is quite possibly the culprit- especially if someone already had to replace the 5W resistors previously. Some broken component powered from that supply could be causing the problem.


The issues with these amps are well documented on the inter webs.

[Grantorino]

Thanks for your reply tubeswell.

Thankfully this amp is sorted now.

There were many issues, including mis-wired CP leads from the power components (wired in the 240 V configuration, not 230 V), burnt out resistors, broken solder pads, failing caps, broken input jacks, mis-wired speakers and two dead speakers.

The issues seems to have arisen from work done on the amplifier, rather than the original design.

Two Jensen speakers have just been fitted - killer amp!