Testing rectifier tubes without a tube tester?

[1blueheron]

I picked up a large lot of tubes this past weekend.  Supposedly a guy got them from an old TV/radio repair shop that had closed.  I netted probably 30 or more 5U4 rectifiers in several shapes and brands along with about 500 other misc tubes.  Is there a good procedure to test these without endangering my PT if they are bad?   My thought is to check for heater/cathode shorts using a DMM and if its good there, pop it in and check voltages with all other tubes removed.  Is this OK?

[jjasilli]

Yes, but I would make sure to properly fuse the PT primary & secondary AC.  See:  http://www.valvewizard.co.uk/fuses.html

[sluckey]

My thought is to check for heater/cathode shorts using a DMM and if its good there, pop it in and check voltages with all other tubes removed.  Is this OK?

The heater ***IS*** the cathode for a 5Y3 or 5U4 and the cathode is connected internally to the heater in a 5AR4, so there is no reason to check for a heater/cathode short. There is no separate pin for cathode anyway.

I would highly recommend using a light bulb current limiter to test a lot of 5U4s. Just plug them into a known working amp and check for B+.

[jjasilli]

Agreed.  Though I would still use fuses.  However it occurs to me that circuit breakers may be a better choice than fuses for this application.

[1blueheron]

OK, sounds simple enough.  I wasn't thinking about the cathode being the heater. .

So I will get my current limiting lightbulb setup some fuses for the HT and test away.   

From what I understand,  all of the preamp tubes can be tested in a working amp without fear of damage correct?

[jjasilli]

Preamp tubs can short internally causing a short circuit. But the above suggestions will give protection. 

[1blueheron]

Preamp tubs can short internally causing a short circuit. But the above suggestions will give protection.

What fuse rating would be appropriate?

[jjasilli]

Look at the last 2 drawings on Merlin's site.  That will tell you whether you need one or two fuses off the AC secondary.  If your PT secondary has no CT, you can create one for the tube rectifier by completing a diode bridge behind it with SS diodes.


Each fuse must support the full current draw of the amp.  That's also true for the 2-fuse circuit.  The current is NOT 1/2 for ea HT leg.  It's full current on both legs.


Disregard preamp current draw.  It's not even a rounding error.


Verify your plate voltage.  Go to the tube charts for that tube type & plate voltage: add current draw for plate + screen under full signal conditions, combined for all power tubes.  Double that number = fuse value in mA or Amps.

[byoungblood]

Preamp tubs can short internally causing a short circuit. But the above suggestions will give protection. 

The plate load resistors and dropping resistors in the power supply will limit the current from a preamp tube that has lost its bias or shorted. So if you have 300v B+ across a 100k load resistor, you can't have more than 3mA current passing through the shorted tube. Not like a power tube whose load is a transformer with a low DC resistance.

[jjasilli]

Good point! But 300V X .003A = .9W.  This would blow a .5W plate resistor.  Maybe a good argument for 2W plate R's!  Still, to your point, it wouldn't blow any fuses, or dim the light bulb limiter.