Bias probes or Weber bias rite for biasing tube amps?

[jalexquijano]

I have seen many of these bias instruments and / or tools to bias your tube amp:

https://www.tedweber.com/br-4

Not to bash this company, but i bought this same tool 3 Years ago and while biasing the second fender tube amp 2 of those terminals broke down. Isnt it better to buy just a single high quality probe and connect it to a digital multimeter?

Bias Test Equipment

Any opinions???

[bob p]

I've never bought any of those bias probe gadgets. I prefer to wire a current sensing resistor in the cathode circuit so that these devices aren't necessary. But I did create my own bias probe to use with amps that don't have current sensors.

If any of those commercial units broke in short order, I'm sure that the vendor would take care of you. I used to deal with Weber when Ted was still alive and he was a stand-up guy. I think his family still owns the business, so I'd imagine the support would be just as good today.

If it were me, and the leads became disconnected, I'd just fix them myself, and be cautious about tugging on them in the future.

[soundmasterg]

With the Weber Bias Rite, a lot of people were using them to bias the amp, and then they would leave them plugged in while they played, often at high volume and distortion levels. The Bias Rite was not intended to be used while playing the amp, so of course many failed after time. Weber is very good with warranties and what not on their products. You should contact them about the failure and see if they might be willing to do something. In any case, if you only use the Bias Rite meter to bias the amp, then disconnect it before you play the amp, then the Bias Rite is very reliable. That said, it is easy to add a resistor in the cathode and some tip pin jacks to be able to measure the bias, though I can understand not wanting to cut holes in vintage Fender amps.

Greg

[nevetslab]

I haven't yet adopted the method of measuring the plate current (total) by sensing the current thru the O/T Primary plate lead wires. I know you can do that with just a DMM, once you've measured the DCR accurately on both lead wires to the plates. I have a nice Tek DC Current probe system for that sort of thing, and at times, that's more convenient than using the Compu-Bias Probes that I have. I bought one of their meters, and two sets of probes....one set for 9-pin tubes, the other set for Octal tubes. After I had one of the octal probe bodies fail, I ordered another pair, and still managed to repair the one probe.

The down side with Compu-Bias probes is you can't operate them above 600VDC. Internally, they have a scaling network to drop the plate voltage by a factor of 1000. So, to deal with higher voltages that don't have current resistors in the circuit, I built an adapter module to plug into my HP 3467A 4-Ch DMM, so I could measure with the Octal Probes and the HP scanning multimeter. Still, the DC Voltage rating on the plate-scaling resistors within the probe aren't rated for real high voltages (I'd have to look at the Dale spec sheet for the rating).

You also can't run the amp under full power with the probes in. There's 10 ohm 1/2W metal Film resistors in the current path, so you could do damage to that resistor.

And, I've sometimes had a tube pull high current upon powering up (coming out of S/B), and shut it down immediately so I don't damage the instrument. I had the meter fail on me once from that problem

But, all in all, I use the Compu-Bias System as my daily dirt measurement system on amps lacking either plate or cathode resistors in the path, and it gives you Plate Voltage, Plate current and plate dissipation per two tubes on the instrument. Very handy.

.

[bob p]

That said, it is easy to add a resistor in the cathode and some tip pin jacks to be able to measure the bias, though I can understand not wanting to cut holes in vintage Fender amps.

I'm sure you know this, but the OP may or may not: You can install the current sense resistors without cutting holes in a vintage amp.

If someone is working on an amp that they own, I would recommend installing current sense resistors in the cathodes (measuring plate + screen current) or on the anodes. Voltage ratings on the resistors should not be a problem because the actual voltage drop across the low-Z sensing resistor is small. What you do have to worry about though, is exposing the person servicing the amp to B+. For safety reasons most people prefer to put the sense resistors in the cathode to ground path where voltages are low. In my Audio Research HiFi amps the sense resistors are in the anode load position, where a high degree of vigilance is required in taking measurements.

I prefer to drill the small holes required for the tip pin jacks. If you're concerned about devaluing a high value vintage amp then you're just going to have to bite the bullet and take of the back panel (tweed) or pull the chassis (BF/SF) and use micro-clip leads to take measurements.

If you don't want to cut holes in a vintage Fender amp for tip pin jacks, it's easy enough to just use micro-clip leads like these. You can get them from Pomona with a BNC fitting or banana plugs at the instrumentation end.

[TimmyP1955]

We've used the BiasRite for a number of years. I've broken a few guide pins, and super-glued them back on. I've had tubes short and take out the cathode resistor a few times - certainly not a fault of the unit. Weber has repaired them at a reasonable charge. I have no complaints with the units. I love having four heads, as it saves time, and it makes it easy to put pairs or quads together from pulls or NOS tubes.

[stokes]

I've been using the Weber Bias Rite for too many years to remember,never had any problems with it.Using a cathode resistor is usually not an option since I'm not working on my own amps and most clients dont want components added to the amp fearing it will change something.The Bias rite is just so easy.