DIY bias probe

[gingataff]

After looking at the GT probe at Hotrox and the Weber and Eurotubes probes, I stumbled upon this DIY version.
http://music-electronics-forum.com/t4976/
What do you reckon? It looks straightforward enough.

[HTH AMPS]

if your amp has an external bias adjustment, then go ahead and buy/make a bias probe.  however, if you need to remove the amp chassis to set the bias internally, then I really don't see the point in a bias probe as you can measure the voltage straight from the pins on the valves bases.

[MrBump]

I've been having this debate with myself for ages.

I want to revalve my Orange, as it's only running with 2 EL34s at the moment - I don't want to pay an amy tech to do it because I'd like to play around with different valves in the comfort of my own home.

I think that the Weber probe or the CompuBias thing might be the way forward.  Only the price of a couple of trips to a decent amp tech.

Mark.

[Dmoney]

i have the weber probe.
its easy to use and yes, you do have to take the chassis out to make bias adjustments, and to even stick the valves in the top of the bias tester, but i feel safer using it that probing around inside the chassis

[gingataff]

if you need to remove the amp chassis to set the bias internally, then I really don't see the point in a bias probe as you can measure the voltage straight from the pins on the valves bases.

Fair point...

but i feel safer using it that probing around inside the chassis

+1

Besides which, don't you have to faff around putting a 1ohm resistor in place of the ground wire? I figure the probe would take care of that.

I'm planning a re-valve of a 20 year old Laney AOR 30 watt combo that's still got all the stock tubes in (I know it probably needs to be re-capped too but I'm not sure I have the right schematic)

[Keven]

Speaking on that whole bias thingmajig. my amp has bias test points and a bias trimpot, the user's manual mentions that i should bias it so the test points red 25mV DC.. and the service manual says to bias to 220mV DC. now i'm not used to seeing amp readings in mV, but if i recall correctly, isn't it a trick with a resistor in that allows you to measure voltage but in reality tells you the bias current? like 25mV would be 25mA ?

[jpfamps]

The conventional bias probe inserts a 1 ohm resistor between the cathode and ground. The voltage drop across this resistor is numerically equal to the current flowing through the valve (Ohm's law) and is the sum of the anode/plate current and screen grid current.

Screen grid current is much smaller than plate cuurent, so this is a good approximation of plate current. Furthermore any biasing "error" will be on the safe side.

It's actually quite easy to build a bias probe that can measure plate voltage and current without opening the chassis - we built our own to save money!- however, as you are inferring with the highest voltages in the amp then I would only recommend this if you are fairly confident you know what you are doing.

A bias probe that doesn't allow you to measure plate voltage means that you will either have to open the amp up to measure plate voltage, or you will have to make an educated guess (which most of the time will be OK).

[Frank]

Not surprised to see a post on that forum about someone zapping his hand with high voltage DC. Take note, anyone who's ever thought about having a poke around inside a valve amp. Or indeed, any kind of amp. Or anything electrical. Sooner or later everyone gets a teeny bit careless and gives themselves a nasty jolt.

Although I do know someone who tried to blutack the top back onto a preamp tube when the glass bottle broke. And he didn't even switch the amp off while he did it.

Yes, that's blutack. The stuff you use for putting posters up on your bedroom wall. On a red hot tube with about 200 Volts DC on the plates. I'm surprised he's still alive.