Bias Meter

[benoit]

These look pretty cool, though price is prohibitive for the casual builder. Anybody use 'em?

http://www.compu-bias.com/order.shtml

I guess its just laziness since you can check that stuff with a good multimeter but probably a time saver if you build a lot?

[Richie]

Allyn has one, and he really likes it. It makes it easy to check alot of things at once.. something you can't do with one meter.. May not be for a one time builder, but for the road, or someone that builds a few amps, or to have to use on your own amp,it would be well worth it. People spend as much on some pedals. I believe these are from Mission amps, and Bruce Collins .. He builds some top notch stuff.

[Johnhenry]

If your into DIY here's what i use, I built this several year's ago, I also built a 9 pin for 6BQ5's, work's good even in a head where with that style of bais meter you'll have to pull the Chassis to test the bais.
Johnhenry
[IMG:800:600]http://i199.photobucket.com/albums/aa33 ... asprob.gif[/img]

[benoit]

So this DIY thing gives you just voltage? Could you throw a similar readout (but for current) in series or would you have to modify things significantly?

[UR12]

Basically all of the probes are using milli volt meters to read the voltage drop across a 1 ohm resistor. Since the current is the voltage divided by the resistance and the resistance is one, if your reading 75 mv on the meter that also corresponds to 75 ma of current so you are actually reading current.

[benoit]

Doh.. could you make the resistor switchable in and out and then add some bias test points like the ones on a Komet so that you could hook your multimeter up and get voltage too? Or is it simpler just to do the math?

[Johnhenry]

The meter i used on this read's in Millivolt's, if you look at the schematic you can see the 1 ohm resistor's on the cathode's of each tube, these do the math for you, when the meter is reading 75 millivolt's, this equal's 75 milliamp's.
if you wanted to skip the meter and use your own DVM you coud just put a couple of test socket's for your DVM to plug into and omit the built in meter.
if you build this, you need to know that which kind of resistor you may choose will work fine, C/C, C/F, M/f or ceramic, just make sure they are under or at 5% resistor's and they read exactly 1 ohm, i would reccomend a fire proof type resistor I take my DVM into part's store's and read them as I buy them, do this and you'll get a precise reading from all four tube's, you can build this for 1, 2, 4, 6, or as many power tube's as needed.
I also added 2 more test piont's on my build so i could read plate and grid voltage.
hope this help's you !
Johnhenry

[David Root]

Ted Weber's Bias-Rite does the same thing but displays plate voltage and cathode current mA to decimal 1 at the flick of a switch, on up to four tubes at once, and it's cheaper than compu-bias too. (Not as cheap as DIY tho'!). I've had one for a few years now, I use it for repair/adjustment as you don't have to drag the chassis out of a combo. Most heads you need to pull the chassis though.

[PRR]

It is "just wiring" to build an adapter to read voltage and (with a resistor) current.

The Compu-Bias is much more, because it tells Plate dissipation WATTs directly. While there are a couple ways to do an approximate V*I computation, this one probably has a little CPU inside. (Ah, it does.... "Re-Programmable".)

Further, you can connect it to the speaker output and measure "Output Power". They fully disclose the flaws in its power measurement: it measures from peak to peak and does not indicate distortion. You can't compare its numbers to published specs for the amp; not measured the same. However it is still valid for bench work. I used a similar technique on a Traynor which had not been touched in 25 years. Traynor claimed 24 Watts, I knew that in "clean" terms it was a 18W-20W amp, but my P-P meter implied 9W. Wobbling the tubes gave readings from 2W to 13W: dirty sockets at least. Clean sockets and new tubes (I figured 25 years was time enough) got me to 21W rms implied from the P-P reading, and I knew that was as good as ever and ready to go back in service.

I have a couple thousand bucks of test equipment and a slide-rule. I "can" do all these readings without no silly handheld CPU. But if I repaired more than a couple tube amps a decade, I'd probably "need" the Compu-Bias. It takes me days to get all my test gear set up, the amp opened, cathode resistors added.... with the CB I could get a Healthy/Sick reading in a few minutes and set any bias trim in a minute more.

[David Root]

Well, if it does all that I would agree it's worth the extra money. These gizmos sure save a lot of grunt work in checking out an amp.

FWIW I put one ohm cathode resistors and dual bias adjustment pots with meter tip jacks in all fixed bias amps I build now. I'm not sure I should be putting them on the chassis rear apron where less-than-knowledgeable types can mess with them, but I do put locking pots in.

I just finished working on an old ('83) M-B Son of Boogie, the 60W 2x6L6GC version, and it is fixed-fixed bias. 507V plate, 60mA cathode draw at idle. That's running pretty close to the bone, I guess M-B liked to run ALL the tubes hard, not just the preamps. At least they are "Made in USA", look like GE or Sylvania maybe. Good candidate for variable fixed bias I guess.

[Allynmey]

I have a Compu-Bias and it rocks! You can see two tubes on the screen at the same time and it displays Plate Voltage, Cathode Current, and watt dissipation. Very useful. You can see the relationship between current and voltage as you adjust in realtime. I wouldn't buy anything from BBQBoy.

The best part is you don't have to check plate voltage to range your bias only to have the plate voltage change as you increase/decrease current!

Allyn

[benoit]

BBQBoy?

[Allynmey]

BBQBoy=Weber!

[benoit]

Ahh.. not very popular around here, that Weber. Why is everyone so down on their stuff (or maybe its the company more than the products)?

[David Root]

I don't know either. I spoke with him a few days ago to correct a mis-shipped choke and he was very helpful.