Building a bias checker

[mr_sooty]

Looking into building one of these:
http://www.el34world.com/charts/biascurrent.htm

I'm getting a Fender 65 Deluxe Reverb Reissue in a week or two and I want to check the bias and if necessary adjust it for optimum performance. I don't really want to ship all the parts from the US. I can get the valve socket from JayCar, I can get banana plugs there too (although I can't find a dual one, but that won't matter, will just use two singles), I have a lead that will do the trick, but don't know where I can get the black tube base. Anybody know where I can get one of these locally? Or can I just rip one off and old valve? If so, anybody have a suitable old valve?

[robnobcorncob]

I used to have a schematic for a bias probe that had a resistor between one of the sets of pins (pin 1 I think?) I'll see if I can find it. (I haven't built one though, I'm confident with most things but when it comes to the finer details like biasing, I leave it to the pros.)

[Ears]

You might want to try this , bias them all up without having to change the probe.

[ant]

The problem here is you can get a socket easily, but you need the base part too, If you rip the base off a valve you may realise its quite fiddly,

What I would do is just permanently wire a 1ohm resistor between the cathode (pin eight) and ground, if the DRRI has chassis mounted sockets it will be a breeze but PCB - a nightmare.

[mr_sooty]

The problem here is you can get a socket easily, but you need the base part too, If you rip the base off a valve you may realise its quite fiddly,

What I would do is just permanently wire a 1ohm resistor between the cathode (pin eight) and ground, if the DRRI has chassis mounted sockets it will be a breeze but PCB - a nightmare.

Pretty sure the DRRI, while being PCB based, has chassis mounted tube sockets. Would this resistor have any effect on the sound? Would you only need to do this on one of the the sockets? Or both? I guess both if you want to check whether they're matched or not.

Any chance of getting a drawing or photo of where the resisitor goes? And how do you then measure the current?

Cheers. I've never measured bias before, but I've been reading up on it, and pretty keen to learn to do it myself rather than pay for it. The DRRI has a bias pot, and apparently they're biased pretty ncold from the factory. Pretty keen to warm it up just a fraction, from 16ma ish to 20ma ish.

[ant]

Lots of old valve stuff has exactly what you are after, an octal socket with a plug extension as they used them to hookup poweramp and preamp. I have chucked mine away though

Also you actually only need to make one bias meter because you can just swap it over and check the other tube, obviously easier though to have two. In the long run you may be better off with the bias probe cause you can swap it over to other amps etc, your call.

With the one ohm resistor method, unless you put tap points on as discussed below you will have to remove the chassis everytime to bias. But you could look at it as adding value to your amp if you decide to sell.

For the resistor method: Basically pin 8 is grounded normally, you make it so pin 8 now has 1 ohm to ground by breaking this connection and inserting the resistor in the path.

You measure with your voltmeter set to 200mV and 30mv = 30ma,

Alot of boutique amps now offer this feature, where you have probe points on the back of the amp, its just a wire running to a RCA socket from pin 8, with a 1ohm resistor in place as above.

Much much better than a crappy univalve where you are relegated to running tubes cold, as this method gives the user the option to adjust the bias to suit the tube

Hopefully not to technical but the measurement you are making at pin8 actually includes screen current - screens on 6v6 will draw a couple mA, so you subtract that from your measurement to get your actual current.

So I would bias a DRRI to 23ma-25ma, JJ's can go higher. Of course you can set it where you like the best tone and check your reading after, and make sure its legit .

[robnobcorncob]

16ma to 20ma is all relative to how much plate current is there in the first place. (Is it plate current? I might be wrong..) somebody chime in and correct me.

[the_haunted]

16ma to 20ma is all relative to how much plate current is there in the first place. (Is it plate current? I might be wrong..) somebody chime in and correct me.

Close, it's plate voltage. As a rule of thumb people use 70% of the max. for that tube.
So,
Bias target = 0.7 x (Max power dissipation / plate voltage)

For EL34 it's 20W Max power dissipation
So,
Assuming the plate voltage measured was 460v
0.7 x (20/460) = 0.030A = 30mA

[robnobcorncob]

That's it ! Thanks. How's the IIC+ clone coming along?

[ant]

El34 is 25w

[DC4]

Actually, bias adjustment is a bit more complicated than a simple current set, the optimum setting could be a few milliamps either way depending on the tubes and results wanted.
To do this:
Really needs a dummy speaker load, an oscilloscope and a sine-wive audiogenerator to examine the output at various levels, and the scope to see what is happening at clipping or cross-over, and a suitable meter to measure the output voltage to do a power out calculation and one to measure the current across the one ohm resistors you have inserted...
Then there is matching the tubes.
Also, tubes can be damaged if biased incorrectly, too much current and they'll run hot and be short-lived.
Not enough and you might not like the distortion.

Finally, the voltages around the output tube sockets can be lethal.
But hey, lots of fun, but be very careful out there.
I enjoy doing it, and my Vibrolux Reverb certainly improved once I set it up. I did need to add an adjustable negative bias circuit, but that's another story.
Oh yes, and the listening test of course! There is always that final tweak.

[ant]

You dont need a scope, you bias to a certain current limit, so tubes dont run hot thats the point. You dont bias to where the crossover notch disappears. generally the 70% rule eliminates crossover, (the point).

There is plenty of data around the net for what you should bias a certain amp too as well


http://www.aikenamps.com/Biasing.html

The only truly accurate method of consistently setting the bias is to measure the quiescent plate current and set it to a point within the acceptable range for the plate voltage the tube is operating at and the desired class of operation. This can be done in one of several ways.

[pedalboy]

Surplustronics have all the parts. I just bought the same thing recently.

[mr_sooty]

Surplustronics have all the parts. I just bought the same thing recently.

You mean you bought a valve base? I just checked their website and couldn't find one on there. Don't have one of those shops down here as far as I know.