Quick question about EZ81 voltages

[aaron]

Hi guys,

I got a mid 60's radio, which I would like to convert to a guitar amp, working with what's there.......

So what I've done is basically gutted it, since none of the tubes, except for an EL84 output tube really match up for a guitar amp.

Starting at the transformer, I've got a center tapped 300V-300V, 6.3V transfomer.

These measurements were made at the rectifier socket, with the neg lead of the DMM connected to the chassis. The rectifier tube/socket is isolated from the rest of the amp except for the PT.

The problem is that I'm only getting 130V at the output pin of the EZ81.

So, do I have a bad EZ81? Am I not measuring voltage correctly?

(I'll make the decision about buying a new rectifier tube vs. putting in 4 diodes later.)


Thanks!

[Chuck H]

It may be the tube. It could also be something else like a wiring fault or even just a dirty socket. We'd need a schematic to determine what all else it could be. Your power supply may be offering only half wave rectification right now. Meaning that only half of your rectifier is operating and passing current. The good news is that you're measuring 300-0-300, so that should mean the transformer (a more expensive part) is likely ok.

There are other concerns with old radios as well. Some types of capacitors deteriorate with age whether they are used or not. Particularly those in the power supply. The more information you can give, the better. What is the tube line up. Is there a brand/model you can see?

[aaron]

It may be the tube. It could also be something else like a wiring fault or even just a dirty socket. We'd need a schematic to determine what all else it could be. Your power supply may be offering only half wave rectification right now. Meaning that only half of your rectifier is operating and passing current. The good news is that you're measuring 300-0-300, so that should mean the transformer (a more expensive part) is likely ok.

There are other concerns with old radios as well. Some types of capacitors deteriorate with age whether they are used or not. Particularly those in the power supply. The more information you can give, the better. What is the tube line up. Is there a brand/model you can see?

OK, I'll start with cleaning the tube socket..

re: the rest of the radio, I've totally disconnected everything beyond the EZ81 tube, so it's not hooked up to anything that might affect the voltage test.

Do you know what the voltage drop for a EZ81 tube given my transformer readings should be?

thanks

[Chuck H]

EZ81's can be somewhat variable but you can probably expect 20V to 25V voltage drop from the rectifier. That would be from tube rectification vs diode rectification. There will be a further voltage drop with the amps operating current. We call this the "loaded voltage". This drop is more nebulous because you need to know the PT DCR and a good estimate for how much current your finished circuit will draw through that resistance. But if you have those numbers you can multiply your current by the resistance (learn Ohm's law) and that would be the approximate voltage drop to expect.

[Jazz P Bass]

"The problem is that I'm only getting 130V at the output pin of the EZ81.'

It seems to me that 1/2 of the rectifier is not working.
Replace the tube.

[aaron]

EZ81's can be somewhat variable but you can probably expect 20V to 25V voltage drop from the rectifier. That would be from tube rectification vs diode rectification. There will be a further voltage drop with the amps operating current. We call this the "loaded voltage". This drop is more nebulous because you need to know the PT DCR and a good estimate for how much current your finished circuit will draw through that resistance. But if you have those numbers you can multiply your current by the resistance (learn Ohm's law) and that would be the approximate voltage drop to expect.

thanks!


I tried cleaning the tube sockets, and still only got 130VAC coming out of the EZ81, so I guess it's a bad tube.........