How To Make a 6V6/6L6 Bias Test Probe From a Cheap Multimeter
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- Опубліковано 9 вер 2021
- In this video I go over how I made a pair of test probes for checking the bias of tube amps with octal output tubes in the 6V6/6L6 vacuum tube family (6L6GC, 6V6GT, 5881, 5871, 5992, etc.) To make your own you will need:
Cheap Multimeter
Broken 6v6 or 6l6/ New Octal Base
Amphenol Style Tube Socket
1" threaded PVC coupler
Super Glue
18 Gauge Wire
Heat Shrink
Solder
Tools:
Wire Strippers
Soldering Iron
Solder Pump
Drill
Small Clamps
Ball Peen Hammer or Similar (for breaking glass out of tube base)
This device measures a small amount of current at a high voltage so please use caution using these probes! Credit to Uncle Doug and Brad the Guitologist for giving me the inspiration to make the original bias probes i made several years ago. Thanks for Watching!
Great video! Been meaning to make two of these DIY probes so I can measure a Silver panel Bassman that my uncle gave me. Your video showed me how to solder the wires on those hollow tube bases and get rid of the excess solder on the pins. More confidently in making them now and parts are on the way as I'm typing this. Thank you so much!
Nice job!
There are a few clips out of order here, I’ll reupload when I get back in town, it’ll still be a little while
it would be best to measure the plate voltage with a more expensive multimeter instead of the cheap one? (for safety)
That’s what I do in fact. The test point is so you can measure plate voltage
I bought a Bias Probe device similar to this..it has a little switching box to select tube A,B,C or D….I only bought 2 probe cables,but was going to make 2 more…Mine has a 1ohm 2 watt resistor built into the cable between the socket and multimeter end…What is the purpose of the resistor,?….just wondering since yours doesn’t use a resistor,do I need it?
My way is different - even do not have to pull tubes from sockets. it requires knowledge like Ohm law and Kirchoff law. After I have access to pins of socket, knowing DC amplifier supply voltage I check voltage drop on transformer and have enough data to calculate from voltages the flowing current and fast compare of both PP tubes bias in few seconds. Only additional thing which is requires is to measure the resistances of output transformer windings which I did even without any disconnecting them. .
I know how to do that too. This can help check bias (or even set bias on some amps) without opening up/removing the chassis.
@@slothtronix3047 Thanks for comment. Tricky thing is that for normal transformers resistance of windings are by nature much different (about 25% ) for each tube so for the same plate currents the voltage drop is to be aligned to different values. .
@@Mikexception for sure although if you’re biasing based on plate dissipation during use (plate current*plate voltage), you can get very good operation of the tube regardless of the since these biggest reason tubes fail prematurely is their inability to dissipate enough heat! I don’t personally see the point of going through the more inconvenient procedure but your mileage may vary
@@slothtronix3047 I can't understand what you mean when you say plate dissipation. - in tubes not plate but cathode is responsible for dissipation Also cathode is wearing for that reason not whole tube so we need to care for heater power and that heating is independent from plate current and from resulting plate temperature which is unwelcomed side efect . Emmision from plate suppose to be restricted and in multigrid tubes it is prevented
I think my way is the simplest - once you calculated the proper volatge on transformer each time you make check you only measure that voltage and adjust to that individual required voltage for each tube - two values are the same for both channels with accuracy only how much idendical are resistances of windings in both channels. I observed that it is accurate about 1% so no problem . - easy even to remember .
@@Mikexception in a tube everything dissipates heat! The cathode is inside the plate and is also dissipating heat and has to be heated either directly or indirectly by the heater. My point is tubes break down much quicker when they are biased too hot because the plate cannot maintain a working temperature. The cathode wants to be hot and emit electrons, the plate wants to be as cool as possible and dissipate heat away from the tube through the envelope. The screens help limit secondary emissions which creates more heat. Cathode dissipation will be higher than plate dissipation as it includes screen dissipation as well as plate dissipation . Here’s the thing though-cathodes slowly wear under normal operation of the tube unless you are stripping the cathode via too high heater to cathode voltage or plate to cathode voltage differential which, as long as the circuit is well designed, happens mostly during the warmup of the tube from my understanding. Most bias in amplifiers varies with primary voltage in the PT so I don’t personally have a need to be as precise as you are recommending but need to ensure that we are maintaining the working conditions for the tube for a relatively long life. From there I like to bias by ear on the guitar amps I mostly work on. The goal for me is the sound and the limiting factors are the tube parameters.
Anyway that is my understanding of the matter, maybe you have more insight?
It seems to me your way is not the simplest for me since it requires opening the chassis. Otherwise it’s a great way to do it. These probes are for measuring plate voltage and plate current to make sure they fall in line with recommended operating conditions without opening the chassis. If I have an amp open already I will measure all the other parameters of the tube operation as well but I’ve used these to bias tubes for long life many a time
Where do you test the voltage at?
Where does the voltage test point connect? The little wire with the turret?
The plate of the tube-for measuring plate voltage
How about making one of these for EL 84 tubes. Much more challenging.
I did this on my channel......... making a 1-box solution for EL84 or 6l6 types.
Not really, you can buy novel tube base, that’s what I recommend