Bias Help for Rob Robinette JCM800 6V6

[rud85ny]

Building a Rob Robinette JCM800 6V6 amp and Im stumped on the Bias layout that's depicted on the site. https://robrobinette.com/RR2104_Master_Volume_Micro.htm

I have a 25k potentiometer available, but I notice there are 2 trims wired to what looks like on top or side by side to one another?
Rob has a Trim and Balance adjustment for the bias. Is this necessary? Can I get away with just installing a bias voltage trim pot of 25kOhms?

I would be more than grateful if someone can draw out (in layman's terms) a bias layout for this amp.

[sluckey]

I've attached a simple bias adjust diagram. But I urge you to study Rob's schematic because it allows you to adjust the bias level as well as balance the bias between the two output tubes. BTW, there is only one trimmer mounted on the big pot.

[rud85ny]

Thank you for the diagram @Sluckey! 
So the trimmer is just physically placed onto of the pot. Copy that sir, I will try my best to study the circuit, but will this work just fine if I have 2 NOS good tubes?

[sluckey]

The trimmer has 3 legs. One outer leg and the center leg (wiper) are soldered to the back of the balance pot. The other outer leg has the 7.5K resistor soldered to it. The bias circuit is a critical circuit so if you do a sloppy solder job your output tubes will likely burn up. Take your time and make a good solder connection to the back of the balance pot. Use enough heat to make a good connection but not so much that you destroy the trimmer.

Either of these two bias circuits will work fine with any tubes you have.

There are many styles of trimmers. Post a pic of your balance pot and your trimmer and I'll see if I can make a better drawing.

[sluckey]

Here's a pic showing a trimmer mounted on the bias balance pot in a Fender Bassman amp. I know it's a slightly different circuit but how you would solder the trimmer to the back of the pot is the same procedure. Of course, the style of your trimmer may require a slightly different mounting method.

[rud85ny]

Thanks again @sluckey. Quick question on the resistor values.
I noticed you have a 47k going to ground, is that particular to your circuit?
Ill be leaving my 220k resistors on the grid leaks.

Should I leave the 2x 10K resistors going from the POT to the DIOD and ground the POT as per the original diagram?
In other words, just leave it as is but omitting the 50k micro trimmer?

[rud85ny]

Should my total resistance going from the 220k Grid Leaks to ground be at approximately 75k?

[sluckey]

Thanks again @sluckey. Quick question on the resistor values.
I noticed you have a 47k going to ground, is that particular to your circuit?
Ill be leaving my 220k resistors on the grid leaks.

yes

Should I leave the 2x 10K resistors going from the POT to the DIOD and ground the POT as per the original diagram?
In other words, just leave it as is but omitting the 50k micro trimmer?

No! Wire the circuit as shown, either exactly as shown in my drawing or exactly as shown in Rob's drawing. Choose one circuit or the other. Do not deviate. The bias circuit is too critical to the life of your output tubes to monkey around.

Should my total resistance going from the 220k Grid Leaks to ground be at approximately 75k?

Which circuit? In my circuit the resistance can be as high as 72K or as low as 47K, depending on the setting of the single pot. In Rob's circuit the resistance can be as high as 82.5K or as low as 7.5K.

[rud85ny]

Copy that sir. Thanks a million!

[rud85ny]

Back to this same circuit. My power transformer does not have a 6.3V Center Tap (GN/YE) wire as described in Robs layout. Everything else matches.
The PT comes with standard 2 GN 6.3V Heater wires. Why do I need the GN/YE wire? I will be using the solid-state rectifier setup. can this be omitted?
should I tap in a 6.3V connection from the regular filament heater/ lamp circuit?

[sluckey]

My power transformer does not have a 6.3V Center Tap
The PT comes with standard 2 GN 6.3V Heater wires. Why do I need the GN/YE wire?

You need the center tap to reduce filament hum. So, make an artificial center tap using two 100Ω 1/2 watt resistors. Put the resistors on V5 socket. Solder one end of one resistor to pin 2. Solder one end of the other resistor to pin 7. Let the two resistors stand up and twist the two dangling ends together. Now solder a green/yellow wire to the two resistors. Secure this so the two resistors will not flop around and touch anything else. Now connect the other end of the green/yellow wire to the board as shown in Rob's drawing.

NOTE*** The wire does not have to be green/yellow.