Phil_S wrote:Really, though, the cathode method works just fine with the 1Ω sense resistor.
Here is a big bottle 5C1 with test jacks. The jacks are very useful for monitoring current directly. You want to use a good quality 1% 1Ω resistor.
Nice Trannies. Is this 6V6 tube?
Do you have the schematic for this fixed grid bias amp?
Use the above to get the max dissipation and reasonable current draw at about 330-370V but don't worry too much about a Champ. 35-50mA should be fine, you don't even need to calculate the bit of g2 current, if it's not red plating it's fine. Little difference btwn idle and full power in a SE amp, so just check it with the lights out at idle and cranked if you want. Really though keep the cathode resistor within reason (maybe 390-560R) if you have to use a large value to get proper current draw try going back to the lower voltage PT taps. Play it by ear, when it sounds good it is good.
chikov wrote:Do you have the schematic for this fixed grid bias amp?
The amp started as a stock Jones Octal One. The 5693 pentode is cathode biased, not grid biased. I found the stock arrangement to be too dark and so changed the values. The OT has three taps so you can rebias for most any power tube. The circuit is just 5693 -> 1MA Volume (bright cap) -> KT66. It is really a great sounding amp.
chikov wrote:Last time I checked the resistor was about 753 Ohms and the voltage drop was about 31V. Is it high?
31V/753R = 0.041A (41mA).
Plate current = tube current minus screen current. (Once you know the tube current, you can estimate plate current or screen current from the Ia:Ig2 ratio on a tube data sheet). Did you say this was a 6L6GC?, On the GE datasheet, the Ia:Ig2 ratio is about 21:1 http://frank.pocnet.net/sheets/093/6/6L6GC.pdf
In which case, Ia =~39mA.
389-31 = 358 plate-to-cathode voltage
358V x 0.039A = 14W (too low for a 6L6GC in Class A IMO)
