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View Full Version : Bias'ing your own amps! (valve)

28th October 2003, 18:49
For a bit of fun, thought Id investigate the costs involved in getting the kit Id need to bias my own valve power amp.

From the proffessionals Ive spoken to you need an oscilloscope, frequency generator and a multimeter.

Well, the multimeter Ive got.... thought Id price the rest, so gave RS components a call...

Oscilloscope... from £400 upwards!!! (most around £1k!!)
Frequency generator from £1000 upwards !!!!

think I might be paying out £35 once every 18 months to a workshop instead, what do you think?? If I bought the kit it should pay for itself in, oh....60 years!!

Unless Ive missed something?

28th October 2003, 19:10

I an no expert, but if all you want to do is bias the valves after replacing old ones, I dont see why you should need a scope or a frequency generator. A multimeter is all you should need IMO. I saw one from RS fror about £35 which should do the job nicely.

I got some instructions from audio research on how to re-valve my amp and it basically involves putting the new valves in, turning the amp on and allowing it to stabilise for a while, then check a couple of voltages across the driver valves to make sure they are within tolerance (replacing driver valves until a match is obtained if they are not).

I then need to measure the power valve bias voltage for each channel on test points in the amp and adjust the value to 65 mV using a couple of small trimpots. And thats it!

I cant imagine your amp would be drastically different to mine.

Heres a link to someone who did his VT100 amp (more complex than mine to do as it has 4 power valves per channel)


Cheers, Robbo

28th October 2003, 22:07

Remind me what amp you have? Oh yes the JH50! Didn't I get hold of a manual with a schematic and an explaination of how to bias the tubes inside :confused:

As Neil say there is no need to get the pro stuff to do this. The best way is to follow the instructions or get somebody else to do it.

28th October 2003, 22:27
scope around £100 for a simple lcd unit from maplins,i bought one a few years ago and hardly use my proper scope now.
signal genarator,make it your next project :D http://www.maplin.co.uk/products/module.asp?CartID=031028212148932&moduleno=2399&Products=1
But you dont need either to bias valves,a decent high impedance voltmeter will do

29th October 2003, 08:38
Originally posted by LiloLee

Remind me what amp you have? Oh yes the JH50! Didn't I get hold of a manual with a schematic and an explaination of how to bias the tubes inside :confused:

As Neil say there is no need to get the pro stuff to do this. The best way is to follow the instructions or get somebody else to do it.

Yup! thats right (thanks for that btw)
The Lectron manual tells you how to adjust the bias, but reccomends the use of a signal frequency generator and a scope so that it can be set for minimum distortion.

Good to hear they can be had for as little as £100 - guess I'd need to learn how to use it first!! (the hard bit)

Thanks all


10th November 2003, 11:00
free app for frequency generating


Lt Cdr Data
13th November 2003, 20:41
yes all you need is a volt or multimeter. Make sure you know how to do this first.

You don't need to faff around with signal generators for distortion,

its frowned on, 70% power dissipation is a very common amount. See below

2 kinds of power valve bias..

1/grid or fixed bias american brute force technique

2/ cathode bias ince polite british method

I have the feeling you have grid bias, which is actually variable, tho' its also confusingly called fixed, cause you wouldn't need to bias if it wasn't grid bias!

all you do is measure the voltage across the resistor at the cathode of the power valve.

Find this by looking at the valve socket, and checking with a bottom diagram for your valve on the web.

Usually the resistor is about 10 ohms for fixed bias, and it will be a big high watt type.

Then calculate like this....voltage= whatever you find it to be.

current thro valve= volts you measure/ the 10 ohm resistor.

Find the maximum anode dissipation for your valve. el34=26 watts, 6550=35 I think, look on the web.

Measure volts at the anode, you have the current, so anode volts multiplied by current must equal less than 100% of the power or the valve will go kaput.

Typically adjust the pot for your grid biased amp so it gives a value 50-70% anode dissipation for your valve. vary it and see which sounds best, you can go to 95% if you wish, but valve life will be less.

great care, as there could be 600 volts at the anode, have your multimeter on dc volts . Use rubber gloves and don't touch!!

lets say its a 6550 which the yanks use a lot, as its big sounding and they like big

say the anode diss. is 35 watts, anode volts is say 500, so current is from


35=500x current,, thus current= 35/500= 70 milliamps..this is the maximum, so bias to say 70%= 0.7x 70 mamps
=50 milliamps.

If your cathode resistor is 10 ohms, you tweak the pot until the volts is..
v=current x resistance.
v=50 mamps x 10R
v=half a volt measured across the resistor.

if your cathode res is a different value, you will need a different voltage. don't go adjusting for .5 volt, it all depends on the values in the circuit.

That is how to do it without any circuit info. anything from 50-90% power dissipation is what the bias is for.
aren't valves lovely?

If you can find the value of the cathode resistor, and the anode volts, I can tell you what your measured voltage should be...simple as that. Alt, check on the web for this amp, or phone the maker for info.