Fender Champ Low Volume

[acorkos]

Mid 70s Silverface Fender Champ...new power transformer, new cap can, new cathode bypass caps. Resistors all within spec...voltages all present and correct. Low volume....at 10 it's as loud as another blackface champ on 4...through the same speaker. Lacking in high end also. I subbed in a new output transformer, no change. New tubes make no difference either. I retouched the solder on the pots, tube sockets and circuit board. Chassis grounds are solid, pots and jacks are tight. I'm stumped...please direct me to a solution. Thanks for your help!

[Helmholtz]

Start with posting the schematic.

Always a good idea to check circuit voltages.

[acorkos]

Voltages all present and correct

[Helmholtz]

Voltages all present and correct

Does that include screen voltage and all 3 cathode voltages?

Do you have a scope?

[acorkos]

Does that include screen voltage and all 3 cathode voltages?

Do you have a scope?

Yes and yes

[Helmholtz]

Ok then feed a 100mVrms/400Hz signal to the input and scope the plate (pin 1) of the input triode using AC coupling.

[The Dude]

Agree with Helmholtz- trace signal. I'll add: What replacement power transformer is installed and what exactly is your B+ voltage? Many of the Champs were running much higher plate voltage than what the schematic you posted says. Higher B+ would give you more output. If you still have access to both amps, I'd compare plate voltages from one amp to the other. Also, are both volume pots the same taper and/or have either been replaced? Check that the amp gives you expected power output regardless of volume knob position.

[Helmholtz]

Voltages all present and correct

The schematic you posted is for a blackface Champ. SF Champs typically have different (higher) voltages.

Was your amp modified to blackface specs?

What is the actual screen voltage measured at 6V6 pin 4?

Do controls work as they should?

[acorkos]

6v6 pin 4 370V
Pin 3 360V
B+ 374V
When I scope V1 pin 1, the input signal is increased 40x

When I scope the output at full tilt, I measure .78 watts

[acorkos]

I measure about 25mV on the .02uF cap at the 6V6 grid...is that significant?

[Helmholtz]

6v6 pin 4 370V
Pin 3 360V
B+ 374V
When I scope V1 pin 1, the input signal is increased 40x

Fine so far.

Now dime all controls and scope the 6V6 grid. If the signal is distorted lower the volume until you see a perfect sine.
Then scope the output (connected to 8R dummy load).

[Helmholtz]

I measure about 25mV on the .02uF cap at the 6V6 grid...is that significant?

No.

[acorkos]

Fine so far.

Now dime all controls and scope the 6V6 grid. If the signal is distorted lower the volume until you see a perfect sine.
Then scope the output (connected to 8R dummy load).

output scoped .5 watts w/4 ohm dummy load

[Helmholtz]

output scoped .5 watts w/4 ohm dummy load

Oh, I thought the amp had an 8 Ohm output.

Anyway, how does the grid signal look and measure at max clean output?

[acorkos]

Oh, I thought the amp had an 8 Ohm output.

Anyway, how does the grid signal look and measure at max clean output?

The sine wave at the grid is twice the height as that at the output