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April 24th, 2009, 02:16 PM   #40 (permalink)
<jbc>
Tele-Holic

Join Date: Dec 2007
Location: Switzerland
Posts: 823
Quote:
 Originally Posted by TelZilla It helps a ton. I'm learning a lot. Though the fog is clearing, I do have some other questions, so I'll bullet them out: I haven't had a chance to check the cathode voltage yet. I'll do that tonight. So the voltage at the first filter cap is the plate voltage? I sort of understand this, as pin 7 of the ELs is connected to the First filter cap, but I'm sort of confused by it too, because the connection is not direct, as it's going through the OT. What am I missing?
Check out Steve Bench's website. http://greygum.net/sbench/sbench/outstru.html

The section on output types re Parafeed explains how the OT in your amp is acting both as a choke and an impedance matching element.
Quote:
 Originally Posted by TelZilla Also, would you expect the Voltage at the Cathode resistor to change as the value of that Resistor is changed? I can't figure out if that resistor going from 120 to 200 should have any effect, or if maybe changing the screen resistors from 100 to 470 would affect it.
Most certainly, but the screen resistor relationship is much more complicated.

Quote:
 Originally Posted by TelZilla Another weird thing. In the process of researching this, I was looking for filter cap voltages for other 2 x EL84 amps. One thing I found was a layout of Hoffman's 18 Watt stout ( attached at the bottom). If I'm reading it right, he's got a Plate voltage of 350V, a cathode voltage of 11.3V, and a 130 ohm cathode resistor. I haven't done the math, but the values are similar enough to the ones you used that it seems like this "18 watt" is more like a "30 Watt". Am I understanding that right?
Consider if that amp were pure class B. Then each tube would have a 50% duty cycle.
Quote:
 Originally Posted by TelZilla I totally appreciate you answering all these questions. The TDPRI is such a great resource...
Yeah, theres lots to learn here. But its hard to stay satisfied what with all the temptation around the place.
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les cheaz