Great video Stu, and a beautiful amp too!

Really good video - thank you!!

Yes on most 'fixed bias' amplifiers the cathodes are tied to ground. Sometimes theres a 1 ohm resistor in series across which you can handily measure the bias as a voltage. E.g. 45mV = 45mA. Of course with cathode biased amplifers (usually the lower powered amps) the cathodes are not grounded but go via a resistor to ground.

Stuart, excellent video and explanation of the 1 ohm shunt resistor to measure the bias. I would like to email you to discuss the bias on the new Fender Pro Jr 1V. That is a different scheme of it. Thanks in advance for your reply. 😀

So in 100W amps, position 1 and position 4 control the other two as a pair?
I’m curious how to use my prob which only has two inputs like yours

Hi Pete You're half right and half wrong (but I'm willing to be corrected). I believe tubes 1&2 are a pair and tubes 3&4 are a pair.
So... if you remove the middle 2, you have removed ONE from each PAIR - which is correct. If you remove the outer two, again you have removed ONE from each PAIR - again correct. BTW Marshall caution against doing this and say it may damage the amp. Also, doing this will give you barely any noticeable volume drop, so it's not worth doing in my opinion. If you want a 50W head, I'd ... buy a 50W head!

Excellent tutorial... many thanks. Are the cathodes always tied to ground?

Does adding the 1 ohm resistors change the tone of the amp in anyway?

Hello Stuart ! We talked through a slight problem with my 1959 SL 100 shocking me. I was actually just adding a master volume kit from Mojotone and pulled the chassis out and put it on my chassis stand and noticed the two inboard tubes where the JJ logo was faded almost off the glass and the two outboard tubes still look like new. When biasing these two had about 2mA higher than the two outboard tubes or the two just in front of the power transformer. I biased them as I said in and earlier post about 40mA but the two outboard are about 38mA. Any ideas why this is happening as the amp has been doing great ?

I've seen tutorials before that used bias probes, but it still required a multi-meter to measure the plate voltage. I see that your probes can measure both plate voltage and plate current, which feels a lot safer to me. Where can I buy some of these?

Read; THE TRAINWRECK PAGES
By Ken Fischer. The safe operational range of EL34 is between 10-40mA per tube. You set it at 35mA... maybe this particular Marshall sounds better at say, 15mA ? I always have the customer play through the amp, using their speakers... I then adjust the bias within the safe operational range... until they find the "sweet spot" they like. You can then repeat this same bias adjustment every time you change the customers tubes, and save yourself time and money.

Regards 🎸

Potresti spiegarmi perché io ho costruito un bias probe doppio con bottone che si collega al tester....se uso probe quello singolo segna 36 m.a mentre con il probe doppio segna 18 m.a sia su una valvola che l altra . grazie

I have a 1964 AC30 that may need your help, would you be interested?

Thanks for this! very clear and easy to follow video. Gonna use this when i go take my amp back for service, I've been using a voltage converter to run my SL at 100v but with the bias that was set at 120v with the tech, I think he set it at 36ma, does that mean my amp has been running cold this whole time? It sounds good to my ears but then again I've only ever owned 1 100w super lead haha

What pins do the Bias probes attach to on the tube socket that allow you to measure the Plate voltage (Pin 3 I think) and the plate current simultaneously? Does the Bias probe calculate the bias current  via the I=V/R equation with internal resistance?

That bias adjuster, it’s only on the reissues correct? Because I don’t see it on my ‘72 Super Lead.

Hi Stuart, I tried to email you, but to no avail, I don't have the correct address.  However, have you tried to rebias the new Fender Pro Jr 4? It may make an interesting video. Thanks Stuart 😊

Hello Stuart, i just took your advise and added a 1ohm resistor in all four positions off pin 1 to 8 to ground and i got a good shock off the on off switch, when I took them back out the shock was gone. This does not sound right to me but it happened. This was on my 1959 super lead.kit by Mojotone. Which also works and sounds great.

Does it need to be a 1ohm resistor? Or can any value be used?

I have one of these with 6550 power tubes in it. I'm still figuring out where i need to be. 42ma - 50ma?
It has 150K resistors on the grid coupling cap section and a 47K bias trimmer pot resistor. 1ohm r on all caths.
The Party likes his 6550's. I say put EL34 tubes back in it as it would be more compressed and controllable.
6550 tubes to me are for a Bass amp or using in a mortar to bomb the other side!😂 A very well made amplifier.