67 Super Reverb

[J Luth]

New question time.

A friend just picked one up and brought it by, nice amp it’s been recapped and a few resistors changed. I found a few things more it needs but there are a couple of things that don’t make sense.

The rectifier tube has been removed and diodes added to the tube socket for rectification. OK I get that.

But on the power transformer the green/yellow ground wire has been lifted and taped off, the 5V filament wires for the rectifier tube from the power trans have been moved to two unused pins on the tube socket. The filament wires measure 20V when checked at the tube socket.

Shouldn’t the filament wires measure 5V not 20?
What winding of the power trans is the green/yellow ground wire for and should it be connected to ground?

The amp works well and the voltages appear to be OK. I haven’t checked them all yet.

[The Dude]

Green/yellow is usually center tap for the 6.3V filament winding. You can check it with your meter to the outside windings for verification.

Are you measuring across the 5V winding (wire to wire), or to ground or something else for checking the rectifier filament winding? Is the transformer original? Maybe someone swapped transfomers and the new one did not have a 5V winding? That would explain why they might nix the rectifier tube.

[Enzo]

I may be wrong, but here is the mental image I have from your report:

They pulled the rectifier tube and put diodes in its place. They moved the 5v wires so if someone stuck a rectifier tube inther it wouldn't do anything. Putting them on unused pins was just a way of securing them. Better than tape.

When you say 20v on filament wires, my head went blank. Do you mean you measured the 5v wires and they had 20vAC between them? Or did you measure 20vAC to ground from them? I hope you were not measuring them for DC.

However if you meant the 6v wires, that COULD be different. They removed the center tap for some reason. Did they replace it with a false center tap, the two 100 ohm resistors, somewhere else on the amp, such as the lamp socket? But what stuck in my mind was the possibility they cut the center tap then referenced the 6vAC line to a +20v somewher for hum abatement.

But I think you meant the 5v wires.

[J Luth]

Had a feeling it might be the center tap for the 6.3V. They have cut it off from it's ground point and added 2 100r resistors from the pilot light to ground.

On the 5V winding, I measured each one individually to ground not across the wires. The transformer is marked 606 718 Schumacher 18 week of 67 so I would say it's the original.

[J Luth]

"They pulled the rectifier tube and put diodes in its place. They moved the 5v wires so if someone stuck a rectifier tube inther it wouldn't do anything. Putting them on unused pins was just a way of securing them. Better than tape."

YES FIGURED THAT'S WHY THEY MOVED THEM

"When you say 20v on filament wires, my head went blank. Do you mean you measured the 5v wires and they had 20vAC between them? Or did you measure 20vAC to ground from them? I hope you were not measuring them for DC."

YES 5V WIRES EACH MEASURE 20VAC FROM THE WIRE TO GROUND NOT BETWEEN THEM. WELL ACTUALLY AFTER A RECHECK ONE MEASURES 19VAC AND THE OTHER MEASURES 16VAC TO GROUND.

"However if you meant the 6v wires, that COULD be different. They removed the center tap for some reason. Did they replace it with a false center tap, the two 100 ohm resistors, somewhere else on the amp, such as the lamp socket? But what stuck in my mind was the possibility they cut the center tap then referenced the 6vAC line to a +20v somewher for hum abatement."

NOT THE 6.3V, 5V FOR RECTIFIER. YES THEY ADDED 100R RESISTORS TO GROUND OFF THE PILOT LIGHT.

[Enzo]

OK, your 5v wires to ground measurement is essentially meaningless. The 5v winding is a continuous piece of wire wrapped around the iron core. Same as any other winding. So the voltage it makes is from one end of the wire to another. Measure from wire to wire, and I bet you get 5-6 volts unloaded. Closer to 5v when loaded with a recto tube. Any voltage to ground is just an induced voltage. Ever grab just the red probe tip on AC volts and see that your body picks up some voltages from the air? Same sort of deal. neither end of the 5v winding has a connection to ground, so ther will be no useful reading when measured to ground.

Think of a 9v battery sitting on your bench. Measure voltage from one terminal to the other, should get like 9vDC. Now measure from a nearby ground to one end or the other of the battery. Probably get zero on DC, and probably get some number of volts AC. But the battery is not connected to anything, so those numbers are not useful.

I have no idea why they replaced the center tap with a virtual center tap. Both work. In amaps with a humdinger, sometimes an off center setting works best, but with two 100 ohm resistors, that is like a centered hum pot, so...

Can I assume this is just an academic question? I mean the 5v is not in use, and doesn't seem to cause problems if the amp sounds OK. Or were you just concerned since the readings LOOKED funny?

[J Luth]

Thanks Enzo I get it now.

Or were you just concerned since the readings LOOKED funny?

Yes wasn't sure if I was measuring the voltage correctly. Across the wires it measures 5.4.

Not sure either why they clipped the ground wire.

He would like to put a GZ34 back in it. If he does I'll remove the diodes and put the wires back where they belong. Or move the wires and leave the diodes if that works.

Found a few other things such as the 3M RA speed pot on the trem was the 50K RA which belongs on the intensity and the intensity pot was a 100K linear.