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Tubes for 5E3?
This is my first post, so bear with me... I recently built my first tube amp, a 5E3 kit from Marsh. I only have basic electronics knowledge, but years of soldering experience. The build went off without a hitch, and the amp sounds wonderful. My question concerns tube choice. The first few days the amp seemed to get quite hot. It came with a Sovtek 5y3 and JJ 6v6's. The JJ's weren't glowing or anything, just putting off a lot of heat. Some web surfing lead me to a NOS JAN 5y3, which cooled things down significantly. Some more surfing gave me the method of calculating power dissapation for each of the power tubes. I tried all combinations(I also have some TungSol reissues and some '70's Westinghouse NOS 6V6's), and here's what I came up with: Sovtek/JJs - 17.13watts per tube; Sovtek/West.- 15.28watts; JAN/JJs - 13.89watts; JAN/West.- 12.74watts; JAN/Tung.- 13.57watts. My calculation was: Power for both tubes=Ic x Vp, where Ic= cathode to ground voltage / cathode resistor value, and Vp= Plate to ground voltage minus cathode to ground voltage. Then I did some more reading and found that my wattage figures may be a little high because I didn't take the grid into account. I've settled on using the NOS rectifier, but I need to know if I can use the TungSol's and the old Westinghouse tubes without burning them up prematurely(I'm not worried about the JJ's). Incidently, the cathode resistor is the 270 ohm that came with the kit and I don't really want to change it unless it's really necessary. I appreciate your input!
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You shouldn't have any problems with the NOS 5Y3. I'd allow the best part of a watt for grid current.
So long as your tubes are <40mA each I wouldn't worry.
Cathode biased amps often generate more heat than equivalent fioxed bias amps (when biased properly). -
MWJB - Thanks for the quick response! Both the TunSols and the NOS Westinghouse are just under 40ma(cathode voltage of 21 and 20 respectively), so I should be good to go.
I bought the Westinghouse 6V6's back in the early 80's for a DR that I ended up selling. I was pleasantly surprised to find them in my workshop. I'm not sure who actually made them. They have "Made in England" silkscreened on them and are mostly black glass. They sound nice at moderate volume but I haven't really cranked them yet, being concerned for their well being. Guess I'll be cranking them today to see how they sound.
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I finally broke down and bought a bias probe and found that the idle current was still a little higher than I wanted - hovering between 40 and 41 mA for 3 of the 4 sets of tubes I have. So I tried Bruce C.'s suggestion of placing a 5watt 12 volt zener between the PT center tap and ground(cathode to ground) and it worked perfectly. I'm now running at about 38 mA per tube which is right where I was hoping to be. I'm not entirely sure why this works, but then I still have a lot to learn. Maybe someone could explain it to me sometime.
Chris - I keep forgetting to measure the +B. I'm sure I did a few weeks ago but I neglected to write it down.
Thanks guys!
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