CHoosing a Bias probe ?

[hebaton]

I am in the habbit of changing amps often. They come and go as I try different things. I like to get new tubes on them usually and so need to bias them properly.
Not every amp is built equally and some times, getting to the contatcs to make the measurement is a pain.
Enter the idea of Bias probes. I am looking at those among others :

https://www.amazon.com/Nobsound-Vacuum- ... r=8-2&th=1

THereis an option to get Plate current or Cathode current. So, what to get and why ?
If I use the Robrobinette Calculator How does this work ?

[Stevem]

Save yourself money and a headache, whatever number tube you need new get it in Groovetubes brand and in a middle of the road # 5 out of 10 rating.

This is what Fender and many other manufacturers do so they can just pop in the output tubes and box the amp up.

[Stephen1966]

I am in the habbit of changing amps often. They come and go as I try different things. I like to get new tubes on them usually and so need to bias them properly.
Not every amp is built equally and some times, getting to the contatcs to make the measurement is a pain.
Enter the idea of Bias probes. I am looking at those among others :

https://www.amazon.com/Nobsound-Vacuum- ... r=8-2&th=1

THereis an option to get Plate current or Cathode current. So, what to get and why ?
If I use the Robrobinette Calculator How does this work ?

I believe RobRobinette explains the difference between cathode or plate current biasing. Check it out and come back if you still have any questions.

[hebaton]

Hey thanks, very helpful !

[dorrisant]

All of the parameters you need can be accessed by pulling the chassis out and proving the voltages and resistances needed for calculations.

That said, if you are not willing to go that far then you have to live with limited results.

[Stephen1966]

All of the parameters you need can be accessed by pulling the chassis out and proving the voltages and resistances needed for calculations.

That said, if you are not willing to go that far then you have to live with limited results.

Pulling a chassis out is five minutes of a job - nothing! Whether you decide to include the screen current in the cathode biasing or go straight to source with the more dangerous plate current biasing is a matter of taste. No more. The difference is rarely heard nor matters. @Hebaton, please explain what you find helpful. Explaining it will help you to understand it better, I find.

PS. Never used a bias probe - multimeter and a bit of maths works just fine.

[hebaton]

THis is not going well. I don'T know why anyone got the idea id didn't want to go trough the paces.
IT is just that changing power tubes is something I do ofeten and most amps do not have a 1ohm resistor acroos pins 1-8 aO I measure voltage drop and tranny resistance etc.
I am looking for a quicker ay to do this and Bias probes seem to be a good answer. It is just that I'm not sure wich version to get and exactly wat they measure and how to use that info !

Sorry if this is to trivial for professionnals. I got very good and veru helpfull support here many times and I am very grateful for it. I guess some are growing tired of a neofite trying to find his way around and asking basic questions some find anoying...
I am not a tech by any means, I'm just an old amateur guitar player hopin to get the most out of the few playing years I may still have in me. And, I do admit I'm not willing to learn a new trade at my age. But having a tech go trough my stuff s often is something I cannot afford, not to mention I have to drive 1 hour to the nearest guy.

Please accept my apology

[sluckey]

Read this... https://el34world.com/charts/BiasChecker3.htm

Then either build two of Hoffman's probes or buy two of the Amazon cathode probes. I would just buy the Amazon probes.

[hebaton]

Why do they have two versions. One for plate current, the other for Cathode curent ?
Maybe I am dumb, must be age !

[martin manning]

Generally people are targeting plate dissipation, so measuring plate current and plate voltage is the direct route. If cathode current is measured, screen current has to be estimated and factored into the target current to get accurate plate dissipation. Screen current varies, and it might be 5% of plate current for beam tubes (6L6, 6V6, and KTXX), or >10% for pentodes (6K6, EL34, and EL84). I always do my own math (it's trivial), and pretty much everyone of grade school age or higher has a scientific calculator in their pocket these days. Note the bias calculator you linked uses a fixed 5.5%, so it will be in error for pentodes.

I like the idea of measuring the plate current directly, but the banana plugs on the item you're looking at are not shrouded, and you will have plate voltage on them. Measuring the current at the cathode is safer, and you're less likely to send the amp into oscillation. That means you have to factor the screen current into your target, and you still have to measure plate voltage somehow to get dissipation. That might require removing the chassis from its cabinet. Speaking of that, sneaking this probe in while the chassis is still in its cab might be difficult, and might lead to burned fingers.

I think I would get the plate current type, and replace the plugs with shrouded ones. That assumes your DMM will accept them. If it doesn't, get a new one. Stacking type plugs would let you connect another DMM and get plate voltage and plate current simultaneously.

[hebaton]

I do have the chassis out and I have measured plate voltage, Center tap voltage and OT resostance...
But My multimeter is not precise enough. I don't get decimals o any of those readings so calculating Voltage drop and then calculating VD/R is not workable because my results are not precise.

[Stephen1966]

I do have the chassis out and I have measured plate voltage, Center tap voltage and OT resostance...
But My multimeter is not precise enough. I don't get decimals o any of those readings so calculating Voltage drop and then calculating VD/R is not workable because my results are not precise.

Talk us through what you are doing. The results you are getting don't sound normal. But providing the readings you are getting and where you are taking them from will help us understand better. Auto-ranging multimeters sometimes display readings you expect in one scale, in another scale. A missing decimal in the hundreds of volts is not going to rock the boat.

[lonote]

I built my own nearly the same as you linked, as I had some parts on-hand, but the Amazon units seem worth getting for the time saved & materials involved.

Those adapters & some cheap multimeters would work fine. Be aware that the meter has to stay connected to the adapter for the tube to conduct, so you need a dedicated meter for each adapter (if you want to do both tubes at once).

I usually do one at a time anyway; one meter in series with tube cathode, by way of the adapter & one meter to measure the plate voltage.

[Phil_S]

If you don't have a holy grail amp and are willing to drill a couple of small holes in the chassis, you can install a couple of tip jacks. Assuming this is a 2-tube push pull arrangement -- do the same for each power tube. Break the existing cathode ground. Wire a precision 1Ω resistor between the cathode and its ground. Install a jumper between the tip jack and the cathode. Put the hot lead from your meter in the tip jack and find a ground anywhere on the chassis. Now you know the voltage drop across the resistor. From this, you can determine bias. If you are unsure of whether you get good resolution with a 1Ω, use a 10Ω and just adjust your bias calculation by a factor of 10. With 2 meters, you can check both at the same time, but it's very quick to move the meter lead from one tube to the other.