Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Grantorino on January 05, 2026, 08:29:17 am

Title: Fender Blues Deville - Low Volume on Both Channels
Post by: Grantorino on January 05, 2026, 08:29:17 am

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.

Title: Re: Fender Blues Deville - Low Volume on Both Channels
Post by: SEL49 on January 05, 2026, 08:41:20 am

Quote from: Grantorino on January 05, 2026, 08:29:17 am

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

That's normal for that circuit.

Title: Re: Fender Blues Deville - Low Volume on Both Channels
Post by: BrianS on January 05, 2026, 11:48:25 am

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

Title: Re: Fender Blues Deville - Low Volume on Both Channels
Post by: Grantorino on January 05, 2026, 12:40:12 pm

Thanks Brian.

Quote from: BrianS on January 05, 2026, 11:48:25 am

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.

Title: Re: Fender Blues Deville - Low Volume on Both Channels
Post by: stratomaster on January 05, 2026, 01:23:30 pm

Quote from: Grantorino on January 05, 2026, 12:40:12 pm

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.

Title: Re: Fender Blues Deville - Low Volume on Both Channels
Post by: Grantorino on January 06, 2026, 05:22:15 am

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.