bias testpoints, any schematics?

[zompzwin]

I'm planning to put bias testpoints a la Komet in my new build, but don't quite understand how they work. Any schematics or descriptions on the net?

[mhuss]

It's so simple, you don't really need a schematic.
1) connect the two cathodes to the red test points
2) connect 1 ohm 1% resistors between the red TPs and the central black TP
3) connect the black TP to ground (where the cathodes would have gone).

Now when you connect a voltmeter between a red and black TP the voltage in mV corresponds exactly to the mA flowing in the tube cathode. As a rough rule of thumb, 90% of the cathode current is plate current (the rest is screen current).

--mark

[dobbhill]

How about a photo:

https://tubeamparchive.com/files/kendri ... hassis.jpg

[mhuss]

By the way, I think the Komet used 'banana' jacks for test points. I prefer to use 'tip' jacks, because normal meter leads plug right in.

Edit: I just looked at the Concorde pictures on their website, and on close examination they look like tip jacks, so I stand corrected.

--mark

[David Root]

Me too. The first bias check & set-up layout I ever saw was on a '76 SVT I did some work on. All tip jacks and screwdriver adjust pots. That is the way to do it IMHO, and I can't see any reason to use anything else.