Fender Deluxe Reverb High Voltages

[everythingyousee]

I have what I believe to be an AB868 on my bench. Silverface with the push/pull volume boost on the Vib channel. 5U4 rectifier. All the small tubes are 12AX7, no AT's in there...

Amp sounds very muddy and distorted, especially in the lower frequencies.

It has had some work done before including most caps and some resistors.

I am consistently getting higher-than-expected voltages throughout the chassis. I have the amp powered up through my Variac at exactly 120v and here are a few voltages I've measured with the amp powered on:

-line voltage entering PT: 120vac
-primary wires to 5U4: 370vac vs 330 on schem
-B+ coming from standby switch, before reservoir and OT centertap: 468vdc vs 420 on schem
Bias pot connected to 6V6 output tubes: -60vdc vs -35 on schem
Plate voltage on output tubes: 450vdc
PIlot light: 4.2vac vs 6.3 on schem
Heater on V1: 1.3vac

Could this be a bad PT?

 

[corliss1]

The DC voltages don't concern me too much, but those AC numbers are very odd. What happens to those numbers when you pull the tubes? For true science, you could pull one tube at a time from the preamp and see if the AC numbers change, which could indicate a partially shorted tube or an issue with that particular socket/area.

 

[Old Gaucho]

The ammo box chassis stand is genius.

 

[AntonyB]

Remember that the schematic say all values in volts are 20%!
468 vs 420 is in range of 20%

 

[Ten Over]

I have what I believe to be an AB868 on my bench. Silverface with the push/pull volume boost on the Vib channel. 5U4 rectifier. All the small tubes are 12AX7, no AT's in there...

It's not an AB868 with the push/pull switch on it.

 

[Phrygian77]

Pull ALL of the tubes, take the pilot light out, and check the heater voltage again. Also, check the AC primary current with no tubes (no load). The easiest way to do this is with with one of those digital power monitors. There should be very very little current/power.

 

[schmee]

High voltages are very common. Every BF/SF Deluxe or DR I have had has been 450-465 volts on the power tube plates.
It needs a 12AT7 in the PI and it sounds like that one has an AX7. The AX7 makes it gainier.

The pilot light voltage is a concern. As is the bias voltage.
I wonder if the diode is bad on the little board?
I'm not aware of a pull boost on a 68, it's probably a '78.

 

[Dacious]

Needs ATs in V3 and PI. The mudiness and compression could be the reverb alone. I'd take V1 out and remeasure.

 

[Ten Over]

Now that I have looked at your picture, it seems you have version 2. It has a .01uF capacitor from the 10M roach resistor to ground.

 

[everythingyousee]

It's not an AB868 with the push/pull switch on it.

View attachment 1043755
View attachment 1043756

Thank you so much!!! This helps tremendously.

 

[Wally]

PIlot light: 4.2vac vs 6.3 on schem

This voltage is of concern, imho. Something is amiss….or is this a typing error?

 

[everythingyousee]

This voltage is of concern, imho. Something is amiss….or is this a typing error?

It is indeed 4.2vac at the pilot light and 1.3vac at V1

 

[Phrygian77]

It is indeed 4.2vac at the pilot light and 1.3vac at V1

Read my post. Pull the tubes, including the rectifier, and check the heater voltage again.

 

[Lowerleftcoast]

Welcome to TDPRI.

Just so we are on the same page, when measuring the heater VAC measure from pin to pin. V1 will measure from pins4and5 to pin9 for instance. The pilot lamp socket will measure from terminal to terminal.

If the tubes aren't getting enough heater voltage, they may not be conducting. If they are not conducting, the B+ current would not flow resulting in high B+ voltage.

 

[Wally]

If you get these odd heater filament readings, lift the filament winding from the pilot light and read the voltage on the winding itself. If you get a good voltage, then you have a problem in the filament supply circuit.

 

[D'tar]

Are you measuring across the heater or referencing to ground? What is resistance from heater circuit to ground. Should be about 50 ohms.

 

[peteb]

Bias pot connected to 6V6 output tubes: -60vdc vs -35 on schem

that is crazy cold. All DC voltages are going to be high.

 

[Wally]

We need to have feedback on the heater filament Voltage.

 

[D'tar]

that is crazy cold. All DC voltages are going to be high.

back to the comfort of having a complete voltage chart to answer questions that arise.... Is -60vdc "voltage available" at the bias board or is it actual bias voltage at the power tube control grid?

 

[everythingyousee]

Hello all, I recently disconnected the PT from the pilot light, diode, and removed all tubes. These are the voltages I came up with:


Variac @ 20vac = PT Primary @ 125vac
Variac @ 60vac = PT Primary @ 366vac
Variac @ 90vac = PT Primary @ 554vac
Variac @ 120vac = PT Primary @ OL on my Fluke 117 meter
Variac @ 100vac = PT Primary @ 615vac



Variac @ 20vac = PT Green Heaters @ 1.15vac
Variac @ 60vac = PT Green Heaters @ 3.39vac
Variac @ 90vac = PT Green Heaters @ 5.1vac
Variac @ 120vac = PT Green Heaters @ 6.7vac





Variac @ 20vac = PT Yellow Filaments @0.89vac
Variac @ 60vac = PT Yellow Filaments @2.75vac
Variac @ 90vac = PT Yellow Filaments @3.98vac
Variac @ 120vac = PT Yellow Filaments @5.32vac


Then I installed the 5U4 rectifier and got:

Variac @ 20vac = B+ From [email protected]
Variac @ 60vac = B+ From [email protected]
Variac @ 90vac = B+ From [email protected]
Variac @ 120vac = B+ From [email protected]


Variac @ 20vac = [email protected]
Variac @ 60vac = [email protected]
Variac @ 90vac = [email protected]44.7vdc
Variac @ 120vac = [email protected]