Series 300 Simul-class switching?

[JOEY B.]

I replaced the half power switch on the MkIII today, and got a chance to trace down the wires to the half power switch. Both wires go through the filter cap board and then continue into the amp. The yellow wire goes to the preamp board area shown in the picture, right next to and connected to the 2 watt resistor. The orange wire is connected to the buss wire that runs to pin #4 of the 6L6 sockets, as the Class-A ones have a resistor connecting pins 3 and 4. When switched to "half power", the buss wire connected to pin 4 (6L6 sockets) is disconnected fron the preamp board. The voltage on this buss wire goes from 530 volts (full power) to 495 volts (half power). Pins 3 and 5 remain at the same voltages during the switching. Anyone know how this works? Better yet, how will this affect the tube life?

 

[AdmiralB]

I'm a little confused...on the Series 300s, the inner sockets are the EL34s. The outer and middle sockets are 6L6s.

You've described the inner sockets as being wired triode, which is correct for all but the last 'stripe'.

It sounds like the switch is lifting the screen supply from the 6L6 sockets, which is a (rather uncommon) method of taking the tubes out of operation.

 

[JOEY B.]

Original post edited for clarity. Oops The orange wire is connected to the buss wire that is soldered to the 470 ohm pin #4 resistors.

 

[Boogiebabies]

JB, shoot it like it's an 87 GN. Show us a little K guts in Hi-Def.

 

[AdmiralB]

The one I had wasn't a K-number, it had a regular five-digit serial.

 

[Boogiebabies]

The one I had wasn't a K-number, it had a regular five-digit serial.

Interesting. I personally, out of habit, call it the K because that is the series designation for these behemoths. Like in Joey B's circuit board shot where it is written in marker "KDRG". A Coliseum, Simul/Reverb and graphic EQ.

 

[JOEY B.]

Ask and ye shall receive. These shots are all pre-mod. The R119 resistor will be replaced next week with the proper 680 ohm 2 watt unit. The R117 slot will be filled by either a trim pot or a 75K resistor.




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