Having a couple of Digital Multi Meters (DMM’s) is an easy way to set and monitor the bias voltage across a resistor. I have a couple of tube socket style bias adapters where you actually measure the current across the tube but this is somewhat cumbersome with a 2 or 4 tube amplifier. It’s a great way for cathode biased amps with no jacks to tap and monitor the bias, but I’m not a huge fan otherwise. With monitoring more then 1 tube at a time it’s easier to set the bias while watch both (or all 4) tubes.
The Bias Buddy came to thought much in the same way the EZLoad did. I have a couple of DMM’s but I was about to embark on building a Quad KT88 amplifier and figured either pick up 2 more DMM’s to move the total to 4 or make my own system for checking the bias on anything with 4 or less tubes. I didn’t like spending the extra money on the DMM’s and also though about the tangle of wires and such with all the equipment on the workbench. The idea was to simplify, while making something useful.
I had seen multi-color digital voltmeter modules on eBay and they seemed to have enough range and
resolution for the project. These were 5 Digit 0-33 Volts models. They also had 0-3 volt models that would have worked, but I figured if something went wrong having the 33 Volt capacity might save a meter. These meters were about 12 bucks each and looked like they would be perfect for the project. They included a nice panel mount bezel too!
The meters I used had the following specs –
- Dimensions:48mm x 29mm x 21mm
- Measure voltage: DC 0-30.000V
- Power Supply: DC 5-30V
- Measurement precision: ± (0.3 ‰ +2 digit)
- Display: five 0.36″ digital LED Tube
Parts used for the Bias Buddy Project –
4 – 5 Digit 0-33 Volt digital panel meters, Red, Green, Blue and Yellow
1 – Hammond diecast aluminum case
1 – 12Vdc wall wart
4 – Colored Tip Plugs
That’s about if for the parts except for some assorted colored wires.
I found a nice and small diecast box from A.E.S. and ordered a few different color tip jacks and plugs. They only issue was that they didn’t have a yellow tip jack so I had to use a white for yellow for the plug. Now that I had the size of the digital panel meters it was time to start cutting the diecast box. I used a simple CAD program to layout where the meters would fit, printed out a 1:1 template and began the marking so I could start cutting the holes. It was a bit more time consuming then a sheet metal case due to the necessity for rectangular openings. I drilled each of the 4 corners and then used a Dremel tool with a cut off blade to cut the lines. It was a bit noisy and messy.
This took a bit of time since I cut the holes small just to make sure I would have a nice tight fit, Then once that was complete I filed the rectangular holes clean and square to a tight fit so the meters would snap and and not move around. The bezels were nice as they covered some of the cutting and filing mess. And finally a hole for the power and test leads and was just about done. Wiring was very simple, and just follow your digital meters wiring for signal and power and it’s ready to test.
For testing I hooked it up to a bench power supply and check to see how it matched my Fluke 187 DMM. It was very close, and tracking to all 4 meters were better then expected, all were reading almost exactly the same with a single digit variance in one of the meters. Way better then I could ever expect for what I paid for the parts.
All in all a very good project. I’m all about building useful tools to help out with the larger projects. In actual use the Bias Buddy has been proven a very effective tool to help get the bias set on the Quad KT88 amplifier. It will work on any amp that has a bias tap with a 10 Ohm cathode resistor to read bias voltage directly. I guess it will also work with a 1 Ohm resistor just fine with some loss in accuracy, but 2 digits past the decimal is really good enough IMO.
I think the big win is the all tubes are concurrently monitored and in a small compact device that makes is simple to do the job of biasing you amplifier.