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RE: A few quick(ie) questions.....

b) Does one need to use a coupling cap to keep DC off the transformers if using the same Specos from the SEX amp?

You could in theory replace the 4K plate load resistor with the Speco if you want. Use the 8K tap (brown = "0.625 watt"). But it will put the full plate voltage (36 volts) on the headphone wires, since you must use the primary 10W tap as an autoformer. And it rasies the plate load - see below.

d) Would series resistors have to be used on the headphone jack if the output impedance ended up being lower than 250ohms?

If you are using 8k:500, that's effectively 4K:250, which matches the tube's desired 4K load. The tube's 4K plate resistance combined with the 4K plate resistor makes 2K, so the output impedance on the 500 ohm tap (which is now the 2540 ohm tap) is half the load, or 125 ohms. Close enough to 120 ohms!

You might have noticed the tube now sees 2K, which is on the low side. Obviously not a problem, since that has worked pretty well. Fortunately, triodes are pretty tolerant of load mismatches! You could replace the 4K resistor with the old SEX amp plate choke; that raises the output impedance to 250 ohms but increases the power and loads the tube better.

e) any ideas on what size Hammond box would cover the entire battery arrangement?

PB's suggestion of putting the batteries below the chassis is OK - that's how I did the prototype. Just don't glue the chassis to the base! Personally, I like the batteries on top, I think it looks cool, but gustibus and disputantum after all...


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  • RE: A few quick(ie) questions..... - Paul Joppa 22:53:28 09/13/09 (0)

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