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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

The plate of the tube-for measuring plate voltage

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

That’s what I do in fact. The test point is so you can measure plate voltage

try a fender 5F2 with a el 84 tube

Thanks! The output transformer is linked below. Best single ended OT around! The model number is 40-18031 www.classictone.net/40-18031.html

Here’s the Hammond 1444-9552 chassis: www.mouser.com/ProductDetail/Hammond-Manufacturing/1444-9552?qs=XJfXErqHgA5ga42xYSZzCg%3D%3D

The model number for the Classitone Output Transformer is 40-18031 www.classictone.net/40-18031.html