Fender 75 wont go low?

[6v6heaven]

The high/low switch doesn't reduce the amp volume. Supposed to be 75 vs 15 watts. Plate voltages switch ~500v to ~250v and bias -56v to -24v. Took a look at the output on my scope with a 1kHz, 200mV input and the output sine wave doesn't change in amplitude between high and low setting. Apart from verifying that my ears are working anyone out there got a suggestion as to what is happening and how to get the 15 watt output? Thanks.

[olddawg]

Are you measuring the amplitude of the output at clipping? Or do you have the amp turned down and just expect a lower volume when you flip the switch?

[6v6heaven]

I've measured the peak to peak output, near but not clipped, and about half that, but I get the same peak to peak where-ever the high/low switch is. I had hoped for lower practice volume in low mode. Thanks

[g1]

You need to measure at onset of clipping, preferably across the load.
If your plate voltage is switching from 500 to 250 volts it is not possible to get the same peak to peak value at clipping.
Remember that a 15W amp cranked is still pretty loud.
The idea behind the switch is to be able to drive the power amp into clipping at a little less brutal volume.
To our ears the lower setting would still sound like more than half the loudness of the high setting. It takes 10 times the power to sound twice the loudness to our ears.

[Jazz P Bass]

Fender 75

A more realistic input voltage would be 100mv.
15 watt output you should see an 11Vac sine wave at the output.
75 watt would measure 24 Vac.

[dodona]

I owned a F75 in the early 80th and the high/low switch doesn't reduce the amp volume. It appears to be normal with this amp.

[pdf64]

Yes, the power switch shouldn't affect the volume unless the amp is clipping, the 15W setting is fine for a small gig.
I advise against switching from high to low while the amp is operational, put it into standby first. The B+ side of the switch might close while the bias side is switching over, and therefore open circuit. These switches aren't rated for high Vdc @ high Idc and are prone to failure.

[6v6heaven]

Thanks for all your responses.

Thanks to all for your responses. All good advice. I will try the 100mV and measure output today. I also think someone has rewired the speaker Jacks. They are different than the 135. It appears to be right with Green secondary doing the 8ohm honors and Green/yellow doing the 4ohm, but someone has strapped the white secondary tap to ground, so its not giving the full match Green/black for 8 ohm and Green-yellow/black for 4 ohm. Any comments? The switching jack also looks bent and looks as though it should function as a SPDT, but isn't because one pole is bent well away. Does anyone have a picture of how this should look? Are those switch jack sockets available anywhere? Any mods available on this amp? Thanks again this is really useful for me.
6v6heaven

[6v6heaven]

Any thoughts on that speaker wiring?

Any thoughts on that speaker wiring? Thanks.

[acorkos]

if it were mine and the switching jack was trashed, i would wire the main speaker jack for 8 ohm operation (green wire) and replace the ext. speaker jack with a standard speaker jack, connecting it like a typical blackface fender amp for parallel operation...leave the 4 ohm tap taped off and unused.

the white secondary looks like it's a 2-ohm tap and shouldn't be grounded. according to the schematic, the ext. speaker jack switches between the 4 ohm tap (gr/yel) when unplugged, and 2 ohm (white) when a plug is inserted.

but like i said, if the switching jack is bad, i would connect green to the tip of the main speaker jack (8 ohm), black to ground and its switch, and run a wire from the tip of the main jack to the tip of a new ext. speaker jack and call it done (tape off green/yel and white)

or wire the ext. speaker jack with the green/yellow and have that jack as a 4 ohm tap, with the main jack as an 8 ohm tap.