Tube Bias adaptor

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a customer who owns a few tube guitar amps bought himself a couple of "tube bias adaptors" complete with small DMMs.

The adaptor is hand made and consists of an octal plug and socket pair, with matching pins wired straight through EXCEPT for the two pin 8s which are brought out on leads terminated with banana plugs.
The idea is to monitor cathode current with the amp running at idle and the DMM set to the 200mA DC current range. Internal trims can then be adjusted to get the desired readings.  

My customer got some strange readings with a pair of new "Tung-Sol" EL34s so brought the Marshall amp and adaptors to me.  

The problem turned out the be with both DMMs which had dicky range switches,   the slightest movement altered the resistance of the range by a large factor. It should have been a steady 1 ohm.  

I opened each meter as gave the range switch a quick squirt of WD-40 and rotated it a few times resulting in a good fix - the resistance of the 200mA ranges showing a steady 1.1ohms on my Fluke.

Then I though about it, these adaptors are offered on Ebay complete with DMMs for US$20 each.

The DMMs are extremely cheap, selling for around US$3 each.  

But the PROBLEM is that the banana plugs are not shrouded and the user can receive a possibly lethal electric shock if the bare plugs are handled.  

It would be FAR better and safer if the adaptor itself contained a 1ohm resistor ( with diode protection ) so the external meter read the *voltage* appearing across it on its 200mV DC range.  

This way, the banana plugs are not live and tube current does not have to pass along the external leads. I suspect the makers have simply NOT thought through the design.  

Anyhow, be wary if you ever come across one and do warn any who owns is about the buy one of the hazards.  

.....   Phil  

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