Bias current probe help :)

Fortheloveofguitar

Please can someone have a look at this and please let me know if it’s the plate current or cathode current version I need for a fixed bias amp?

Many thanks 

https://www.ebay.co.uk/itm/8Pin-Valve-Tube-Socket-Cathode-amp-Plate-Bias-Current-Probe-Tester-for-6L6-EL34-KT88-/263943680334?varId=563424162361&txnId=2327077640016

ecc83

I hope TC that red wire does NOT connect to the anode!  I would also like to point out that AFAIK all modern digital meters have safety 4mm sockets and those bare ass'ed jobs won't work.

Dave.

ICBM

It's also saying you need the meter set to the mA range, which means it's feeding the whole current through the meter - do NOT use this type, they're inaccurate and potentially dangerous.

You need one with a 1-ohm resistor built into the valve base, and the meter set to milliVOLTS - which gives the same reading, but the difference is that the potentially dangerous and amp-instability-causing connection is kept within the valve base and not run through the meter.

Fortheloveofguitar

Wow thanks guys that sounds bad

can you recommend one that’s safe as I would rather use this method? 

ecc83

Oh! Lazy moi did not read that IC. Yes, to be avoided.  Cheap enough though and if you can solder, easily modded.

Dave.

jpfamps

Wow that looks a hazard!

Copper for excellent conductivity!

Fortheloveofguitar

ecc83 said:

Oh! Lazy moi did not read that IC. Yes, to be avoided.  Cheap enough though and if you can solder, easily modded.

Dave.

I’m good with a soldering iron if I know how to mod it 

ICBM

They're actually quite hard to find, at least without buying a full system that costs over £100...

This one - perhaps surprisingly - does look OK though:

https://www.ebay.co.uk/itm/8-Pin-Tube-bias-current-Probe-Socket-for-Testing-6L6-6V6-EL34-KT88-5881/282934199413?hash=item41e0314475:g:YmYAAOSw-0xYfSSm:rk:30:pf:0

It's from Bulgaria, but looks OK.

BEWARE - the same seller makes other versions which meter plate (anode) current, or have a switch to select one or the other. This is EXTREMELY DANGEROUS. DO NOT under any circumstances use this method! It means that the leads and meter are live at 450V or more relative to earth. Measure cathode current ONLY - there is no need to worry about the difference caused by including screen current, as the error is small and in the safe direction.

NB - yes, I and many other techs use the plate current method by metering across the OT primary, but doing that assumes you know what you're doing and are happy working inside a live amp chassis. Feeding that sort of voltage out using a 'probe' style arrangement which gives a sense of protection is a very bad idea indeed, especially as the accepted way of doing that is with the cathode current, which is safe - it would be very easy to assume you were safe, when you aren't. The switched versions are even worse since you could flip from one to the other without thinking about it.

jacksparrow

Buy a Weber Bias Rite and be done, had mine for years now, and it has easily paid for it's self.

Fortheloveofguitar

The Weber bias rite looks great as it has a switch to read the plate current too. However I would get completely shafted on import duty and handling charges etc so would end up very expensive 

The TAD bias master looks the same but doesn’t read plate current. That’s £112 for 2 probe or £132 for 4 probe. 

Thinking I’m not going to get anything accurate without knowing all 7 of my amps plate readings 

Collings

ICBM said:

It's also saying you need the meter set to the mA range, which means it's feeding the whole current through the meter - do NOT use this type, they're inaccurate and potentially dangerous.

You need one with a 1-ohm resistor built into the valve base, and the meter set to milliVOLTS - which gives the same reading, but the difference is that the potentially dangerous and amp-instability-causing connection is kept within the valve base and not run through the meter.

There is an option to select if you want a version to measure anode current or cathode current (not via a one ohm resitor using mV) Is the cathode current version also not recommended?

ICBM

Collings said:

There is an option to select if you want a version to measure anode current or cathode current (not via a one ohm resitor using mV) Is the cathode current version also not recommended?

The cathode current ONLY version is the only safe one. It also must use a 1-ohm resistor or it could make the amp unstable, or - in the case of a valve short as well - allow high voltage outside the amp.

Metering plate current (ie at plate voltage) outside the amp is dangerous. It is possible that a high value series resistor has been used to prevent a significant current leak to the outside, but it doesn't say so and without either that or a look inside one I certainly wouldn't assume that.

The Weber one reads plate *voltage*, not current - again it needs a series resistor, but Ted Weber knows what he's doing.

This is all not quite as straightforward as it sounds - valves operate at potentially (no pun intended) lethal voltages.