Hi, I'm a long-time lurker, first-time poster.
I have a Presto 1D wound for 500 ohms, and reading this manual plus slowly teaching myself electrical theory has helped answer quite a few questions I've had, especially regarding the large bump in response at 4.2 kHz I could never get rid of. It looks like the capacitor and resistor will help tame it, but I'd like to do a sanity check: they mean 4 MICROfarads, not 4 millifarads, don't they? As near as I can tell, a 4 mF film capacitor would be larger than a gallon jug of milk, and would cost upwards of $1500. Somehow that seems a bit off.
I have a few more questions:
In another thread, a poster talks about unwinding another 500 ohm 1D to find it contained 120 feet of AWG #37 wire. And other people are mentioning putting fuses between their amps and heads that seem to be rated roughly double the maximum current for the estimated corresponding wire gauge as per this reference - http://amasci.com/tesla/wire1.html
. This would mean if my 500 ohm head has the same #37 wire, I should be looking for a 250 volt, 125 mA fast-blow fuse. Does this sound about right?
Also, the DC resistance of my head is 92 ohms. I understand with inductance the reactance changes with frequency, but I'm still worried that through the impedence-matching transformer and lacking the recommended capacitor/resistor circuit, the setup I thought was presenting a 4 ohm load to the amplifier could instead be as low as 0.6 ohms.
That would explain the problems I've been having with clipping. I'll try using a different tap on the transformer and put a variable resistor in the circuit as an experiment, and see where that gets me. I wonder how many watts any permanent resistor ought to be rated for?
Finally, does anyone have the original factory specifications, or their own measured specs, for a 500 ohm 1D head, in terms of nominal DC resistance, capacitance and inductance? Once I can get my hands on an LCR meter, I can measure mine and plan how best to flatten the response curve, but in the meantime some ballpark figures would be helpful, plus they'd show how far mine may have drifted over the decades.