Fender 75

[bsco]

God afternoon. Have this fender 75 which i just cleaned up...Somebody had 12AX7A's in place of the 12AT7A's that were supposed to be there. Noticed that when I pull the mid pot out the volume increases a fair bit and the sound seems overly bass heavy...Also once the pot is pulled out, there seems to be no adjustment on the mid range control. Push the switch back in and the sound returns to normal and the pot then controls the mid range freq. Looking at the schematic, this switch basically just shorts out a .0047 uf cap and a 100K ohm resistor... and everything checks out fine.....Is this normal for this amp as I have not had any experience with this particular model of Fender amp......One side of this pot is connected to the center lug and looks like it was original...The pot is definately original.....
Cheers,
Bernie

[g1]

Sounds fairly normal. When the switch is closed, the mid pot is grounded like a normal Fender circuit. When you engage the "mid boost", the switch opens and puts a 100K (paralleled by .0047uf) resistor between the mid pot and ground.
fender_75.pdf
Likely all tone pots will seem to have less effect when the mid boost is pulled. In a regular BF or SF fender tone circuit you get a similar situation if the connection is missing/broken between the mid pot and ground, the tone pots will seem ineffective.
In this case, the mid's ground is lifted deliberately, the 100K/.0047 is supposed to give mid boost.

[bsco]

Ok. Thanks g-gone...then this amp is ready to go...
Cheers,
Bernie

[pdf64]

Yes, it does get very 'woofy'; good for Santana type thick lead tones.
A lower value than 100k can help to get a better balance, eg 33k; maybe suggest it as an option.
What was the the original fault?
Pete

[bsco]

Thanks pdf64.....actually the owner wanted it cleaned up and checked out...I found two wrong tubes in it..Somebody had put 12X7A's in place of the 12AT7's so I changed it back....outside of that, just a good clean-up.....but I noticed that when the mid control was pulled out, it was extremely bass heavy...and I was not sure if this was normal or not for this amp...but I guess it is...I will experiment with a couple of different value resistors in this circuit once I get the ok from the owner....they might not want it changed but i will ask them...maybe if the resistor value was cut in half???? I'll have to experiment....I have to say, I have worked in electronics all my life but since I started doing music equipment I have learned so much in such a short time...I want to thank everybody for all their help and input...
Cheers,
Bernie

[pdf64]

The front panel switches (line, standby, hi/lo) can be a weak point on these.
Also check the power tube dissipation, as the bias voltage (as stock) can be low for modern tubes. It's not trivial to increase it much though, as it's also supplying the switching LEDs and so the dropping resistors to these will dissipate more if the voltage to them increases.
The electrolytics in Fenders of this era (plain aluminum Mallory) seem to be very long lived and so if they check out and look ok, I'm happy to leave them.
Pete