Randall RG50TC problem

[jrem]

I have this amp opened up on my bench, tubes 1 & 2 don't light up. There is voltage between pins 9 and 4&5 on both tubes (heater filaments), but something is dragging down the supply when the tubes are plugged in. A snippet of the schematic showing the circuit is at http://www.jrem.com/rg50tcsnip.jpg

I'm scratching my head. I tried fresh tubes, the caps aren't shorted, and the rectifier is working. I'm thinking of putting a load in place of the filament and measuring the current drop across it. Any suggestions?

Thanks . . .

[Jazz P Bass]

DC Heaters

Can you measure the DC voltage across C70 & C72? 300uf/ 10V capacitors.
This is a floating bridge. Don't measure to a ground reference.
Remove the tubes when you test this.

[jrem]

Can you measure the DC voltage across C70 & C72? 300uf/ 10V capacitors.
This is a floating bridge. Don't measure to a ground reference.
Remove the tubes when you test this.

yeah, will do tomorrow and will post the results. You thinking a cap is bad?

[Jazz P Bass]

DC Heater Voltage

Actually what I would be looking for is 6Vdc.
If you do not have it, the bridge rectifier is probably at fault.
Or the caps, or the tubes, or....

[jrem]

ok, I've got 7.07vdc with a 10k load, 7.85 with no load, and 70mv with a tube in the socket, no filament glow.

[Jazz P Bass]

DC heater voltage

Sounds like a bad tube.
Something is collapsing the 7 volts.

[jrem]

cha, got that. tried different tubes, good ones, brand new ones, nada. I think it's the rectifier.

[Enzo]

Your meter reads voltage with no tube because there is nothing to drag it down, but you have an open circuit somewhere. If you just grap the tip of your meter probe with your fingers, you will measure some voltage. You need the load the tube represents.

Put the tube back in the socket, and trace the 6v. With the tube in there, your meter won;t be fooled.

Now is there AC across the AC legs of the bridge? if it ain;t getting there, it can;t rectify it.
Is there DC across the DC legs of the bridge? If the bridge is open, there won;t be.
Is there DC on the filter caps, C70,72?

Measure voltage from the ends of the filter caps to the heater pins on the tubes. They SHOULD measure zero volts - a wire drops no voltage. If you get voltage between the cap and the tube pin, then there must be an open between those points.