bias probes educate me please!!

bandmaster188

have been thinking about getting a bias probe so i can play with my amps a bit more at home.

what are peoples thoughts on these and any recommendations. i can use a multi meter and have a very basic understanding of amp stuff.

i saw this http://www.eurotubes.com/eurotubes-Bias-Probe-Pro-One-current-watts-voltage.htm but is it a bit o.t.t. for my needs?

cheers peeps!

ToneControl

because the valve has a 1 ohm resistor across 2 pins, by the miracle of maths/physics, you can measure the mV across the probe points, and from that you can set the bias to the right amount quite easily, I made dymo tape target values for my ones

bandmaster188

What kind of bias probe have you got tonecontrol. Just seen this on eBay. http://www.ebay.co.uk/itm/OCTAL-VALVE-BIAS-PROBE-6L6-6V6-EL34-5881-BRAND-NEW-/121050534586?pt=UK_Sound_Vision_Other&hash=item1c2f2c92ba

Clarky

bias probes educate me please!!

I'm sure I heard some totty say that in a skin flick once...

Handsome_Chris

Clarky said:

bias probes educate me please!!

I'm sure I heard some totty say that in a skin flick once...

http://i.telegraph.co.uk/multimedia/archive/01498/ET-The-Extra-Terre_1498982c.jpg

ToneControl

ah, I meant probe sockets permanently wired onto the amp chassis, across a 1 ohm resistor, fitted by a proper amp tech. The I just used my DVM

ICBM

Any bias probe that uses a 1-ohm resistor is as good as any other really. Just avoid any where you read *current* directly and not *voltage* - these don't have the resistor, and can be dangerous - some early ones were like that, I'm not sure any are still made, but it's possible.

You need a decent cheap-ish multimeter anyway, so you don't need a probe that has a built-in meter. You can get a good multimeter for guitar-amp purposes for well under £50, but it is worth spending a bit more than a real cheapo - you want detachable test leads and a 600V DC range at minimum, and preferably a tougher case the most of the 'pocket' ones have. (You *will* drop it!)

Maplin actually do some reasonable ones in their "Precision Gold" range - they're currently ( ) doing a special offer on the PG10B for only £20, which looks like a good one. http://www.maplin.co.uk/p/precision-gold-pg10b-digital-multimeter-gw18u

Van_Hayden

That looks quite good, as you need to know the anode voltage as well as the cathode voltage to bias an amp. And you can do it safely, there's one company selling a plate voltage tool, if you forget to plug in your meter you've got full HT on a pair of banana plugs on your bench.....

57Deluxe

Has anyone used just their bog standard Multimeter to do or check a BIAS? I got mine from Lidl ages ago... Branded Hi Lux

Van_Hayden

That'd do fine, if your happy to unsolder a few components or fit some 1 ohm resistors so you can measure current!

equalsql

That would be fine as you would only be measuring millivolts.

But don't go poking inside a valve amp unless you really know what you are doing as there's some mighty high voltages in there.. and if you do.. keep your left hand in your back-pocket (seriously).

jpfamps

There are plenty of companies offering inexpensive probes that allow measurement of cathode current using a current sense resistor.

eg:

https://www.tube-town.net/ttstore/product_info.php/info/p3132_Bias-Probe-Octal--assembled.html

There are some people who claim that this method is inaccurate as it measures the sum of anode and screen grid current; however the error is small and more importantly it slightly overestimates anode current so is on the safe side of error.

Ideally you need to know the value of HT in your amp. This is dangerous to measure, however for most amps this is easy to find out from the schematic, so you can use a ball park figure

It's perfectly possible to make a bias probe that measures the cathode current and HT without exposing hazardous voltage; indeed I've built one. This would not be a project for the inexperienced builder though.

ICBM

You can check the bias using just a multimeter, by measuring the primary resistance of the output transformer with the amp off, and then the voltage drop across it when on - which is in fact the most accurate of all methods, but is dangerous, because although the two points are only a volt or two apart, they're both at 400+V above ground. DO NOT do this unless you really know what you're doing.

Also do not EVER use the 'transformer shunt' method, which is similar - it's not only even more dangerous, it's inherently inaccurate and the error will always be in the unsafe direction, unlike the measuring the cathode current where the inherent error is in the safe direction.

The cathode current method is by far the safest, especially if done with an external bias probe, and the error doesn't matter for any practical purpose.

bandmaster188

toooooo much information ~X(

i have a 600v digi multi meter.

are these probes the same thing, one being alot cheaper than the other

https://www.tube-town.net/ttstore/product_info.php/info/p3132_Bias-Probe-Octal--assembled.html

http://www.ebay.co.uk/itm/OCTAL-VALVE-BIAS-PROBE-6L6-6V6-EL34-5881-BRAND-NEW-/121050534586?pt=UK_Sound_Vision_Other&hash=item1c2f2c92ba.

i'll deal with the technical info once i've bought the probe!

cheers for the input folks!

 

ICBM

They look effectively the same to me, except that the second one includes a lot of marketing BS about silver solder and the like, which probably explains the higher price. It's not necessary.

jpfamps

bandmaster188 said:

toooooo much information ~X(

i have a 600v digi multi meter.

are these probes the same thing, one being alot cheaper than the other

https://www.tube-town.net/ttstore/product_info.php/info/p3132_Bias-Probe-Octal--assembled.html

http://www.ebay.co.uk/itm/OCTAL-VALVE-BIAS-PROBE-6L6-6V6-EL34-5881-BRAND-NEW-/121050534586?pt=UK_Sound_Vision_Other&hash=item1c2f2c92ba.

i'll deal with the technical info once i've bought the probe!

cheers for the input folks!

 

The second one has a CE mark too!

bandmaster188

Cheers guys, think I'll buy the cheap German one cause its cheap! Might be back for more help soon if I haven't electrocuted myself first.

EdGrip

I bought one from a Hong Kong-based dude on eBay - it's got one output for measuring the mV with the high-precision resistor, and another that reads out the plate voltage divided by ten. Haven't had a chance to test it yet, and the seller is currently not selling any.

Budwiser

Here's a little maths to work out bias voltage on the marshall dsl 2000 ..50 watt ..

Ht is 460 vdc cathode resistor is 1 ohm on pin 8..marshall say 45mv per tube..way hot and it's near max wattage of the tube so I bias them at 38mv that works out about 17.48w per tube.. so 460 times by 0,038mv =17.48w or near to 17.5 watt.

Budwiser

If need just check pin 8 on fender amps this will feed back to ground point so here's were you add a 1 ohm 2w resistor it can be left in and the amp will be ok.. never mind forking out for over priced on line bias probes...

bandmaster188

i've just paid out for an over priced biasing tool! but hey i'm i bit new to this poking around inside my amp stuff and the bias tool is pretty much idiot proof which is right up my street.  Until i get to learn a bit more about the inner workings of my amps, this was christmas cash well spent!

cheers for the input though