75 Deluxe Reverb

[Doc Stitches]

Just rewired chassis with pushback wire. Now I have a low hum when the variac hits 90 v. The hum fluctuates from quiet to louder.

All controls off. No guitar cord plugged in.

When I pull the rectifier tube, ....no hum.

Ideas ?

Thanks
Doc

[Tom Phillips]

Just rewired chassis with pushback wire. Now I have a low hum when the variac hits 90 v. The hum fluctuates from quiet to louder. All controls off. No guitar cord plugged in.

My first thoughts are power tube or biasing problems. Maybe one power tube going into runaway. Can you monitor the biasing of each power tube as you approach 90V? What happens if you warm it up with 120V line voltage then switch from standby to operate? Set the bias to max cold (most negative voltage) before doing this for the first time and check that the same negative voltage level is on each of the power tube grid pines (Pin #5). Also check for excessive ripple on the B+ and the bias supply.

When I pull the rectifier tube, ....no hum.

That's as expected since there will be no B+and all circuits except the heaters are off.

[Enzo]

Of course the hum stops when you pull the rectifier tube - the amp then has no power supply voltage.

Why are you trying to run the amp on 90 volts?

[Doc Stitches]

Progress with loud hum

Of course the hum stops when you pull the rectifier tube - the amp then has no power supply voltage.

Why are you trying to run the amp on 90 volts?

Just had some time to get back to this DR. and found and resolved 4 more issues

pin 6 not wired on tube 6 ....I had pushback wire right to the pin, BUT no solder !!!

Vibrato channel inputs not wired correctly

Removed .002K capacitor from reverb output

Replaced both 12AT7 tubes and got rid of another 25% of the hum

..........................this amp was a 75 Deluxe Reverb with 2 pushpull pots with a lot of mods...I am attempting to restore it back to AB763

Almost all hum is gone, but I think I can get it very quiet.............now if I can just get the treble and bass pots to work along with Trem and reverb, I'll happy

Thanks Tom and Enzo...I'll get to the voltage checks soon, ........................................ and Enzo................I was only going to 70 v with the variac thinking it was the best way to prevent possible damage, because at that voltage the

hum was soooooooooooooooooooo LOUD.

You guys are great
Doc