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Discussion Starter #1
I've started on a JCM800 Micro build that uses a single 12AU7 power tube in push/pull configuration.

On the drawings, I'm a bit confused over the heater center tap of the power transformer being routed to ground thru a 10uF electrolytic capacitor in parallel with a 39K resistor. It's labeled "65 Volt heater Elevation". This is new to me, no idea what to make of it. To add to my confused state, the drawing shows a 180K resistor, connected to a main 360 volt power feed, also connected to this "Heater Elevation" junction and labeled "Bleeder/Divider".

I wanted to use a PT that doesn't have a heater center tap. That kind of eliminates this whole thing. Is that advisable? What the hell is it all about anyway?

JCM800 Micro


And on another drawing of the same amp, it shows the heater center tap connected to the cathodes of the power tube before going thru a 820 ohm resistor to ground. I don't get it.
 

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Discussion Starter #2 (Edited)
A quote from the notes above the schematic:

"The 6.3v heater center tap is elevated by connecting it to a Bleeder/Divider circuit at 65 volts. This heater elevation makes life much easier on the cathode follower and reduces heater hum (see layout below)."

Here's the schematic

RR2104_Master_Volume_Micro_Schematic.png
 

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The filament voltage in every amp must be referenced to some common point. Typically this is ground. It can be done with a center tap, or one side of the filament secondary or a few other ways.
In the case of this amp it is referenced to the HV supply via a resistor network.
The idea of elevating the filament is not new. You don't see it often in guitar amps though.
From the schematic I see the cathode follower before the tone controls is direct coupled to the previous stage. This will put significant voltage on the grid of the follower and thereby putting significant voltage on the cathode. All tubes have a spec - heater to cathode voltage. It's not a critical spec, but if exceeded can possibly shorten the life of some tubes. The idea then is to elevate the filament voltage by some amount so that the H-K is not exceeded.
The 180k and 39k form a voltage divider creating a point that is about 65V. The capacitor across the 39k is to shunt power supply noise to ground. It's a filter cap.
"The 6.3v heater center tap is elevated by connecting it to a Bleeder/Divider circuit at 65 volts. This heater elevation makes life much easier on the cathode follower and reduces heater hum (see layout below)."
The reference to that center tap is the two 100R resistors. That schematic is not showing a center tapped transformer

One other benefit to the raised heater is the possibility of better noise performance. Because all the noise from the the filament power supply is raised above ground it isn't circulating around in the chassis and ground circuits. There are no ground loops like this and if your ground is not perfect (none are) it can be quite a bit quieter.
 

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And on another drawing of the same amp, it shows the heater center tap connected to the cathodes of the power tube before going thru a 820 ohm resistor to ground. I don't get it.
The 820R is the cathode resistor for the power tube. Tying the heater CT there is just another type of elevation.
Like Dan said, the two 100R's create a 'virtual' center tap. It may be tied to ground, or some elevated point. The higher voltage elevation (off a voltage divider from the HV) is usually seen where there is a cathode follower circuit.
If you don't have an elevated heater winding when you have a cathode follower, some modern tube brands are more prone to failure than other brands in that cathode follower spot.
 

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Discussion Starter #5
I gained a lot of understanding out of all that. Thank you both. I've still got some reading to do on the subject, but I'm comfortable enough to proceed with the build now.

I made my own eyelet board. I still prefer them to turrets. Things were cruising along well until I hit the end of the board where the elevated filament stuff was.

JCM800M1.jpg
 
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