Chinese octal bias probe opinion

Winny_Pooh

I saw this on ebay which has 4 sockets and shows the milliamp readout of each tube and is low priced. I'd like to check the bias of my Marshall, any possible downsides? 
@ICBM & @jpfamps ?

http://pages.ebay.com/link/?nav=item.view&id=281863726806&alt=web

martinw

  Don't buy one.

I did, and discovered that it only has 1 x 1 ohm resistor fitted, in the control unit. Consequently the valves not being measured are 'off', i.e. not conducting which obviously affects the readings. It also makes any other testing you might want to do with this device fitted impossible.

You would normally expect there to be a 1 ohm resistor in each valve socket, and the selector just chooses which one to read.

I emailed the supplier about this and they had no idea what I was on about. They are just a manufacturing house, and they had paid an external engineer to design it for them. I asked for a refund as the device didn't do what it should, and they offered me 50% refund if I would redesign and rework the unit for them, then send pictures and drawing of what I'd done. I declined!

ICBM

Also, if the 1-ohm resistor is in the control unit, that means that the valve current (rather than just the test voltage) is passing out of the amp via the leads. This is a good way to cause instability - which will at the very least further disturb the readings.

On the bright side, they saved the cost of three 1-ohm resistors...

Winny_Pooh

Thanks gents, that is truly nuts.