These values should give a lot of flexibility in designing a cathode "resistor," and it seems like if you worked out the series/parallel math correctly, there's no reason you couldn't run 3 or 4 preamp tubes as RK. For your example, this would indicate a 168 ohm RK for your E元4's, which is pretty close to what you would choose with a traditional resistor on that application.
#Do you have to bias guitar amp pre amp tubes series
Using Ohm's law, it looks like each triode in a 12AX7 has a resistance of 42 ohms, (21 ohms parallel, 84 in series per tube). Thanks for posting this Andy, I've been considering doing this in a build where, by some warp in the space time continuum, heater noise is coming in through the grid of the pi. Now the draw backs are you have a longer warm up before using, but its a soft start for the power tubes,Īnd you can only design around pre amp tube types a. I've seen example with 2 6l6 and 4 6v6, and with the addition of a resistor 4 6l6. With some room to spare, and the cathode is rated at 150ma., all design center values. The 6CA7 have a 25w plate rating and 8w screen grid rating making the 29w figure possible The heaters in the pre are simplified to just a couple wires, no big resistor to mount in the chassis or It works and is gig ready, it even sounds good, with less parts to boot. The pre amp heaters glow and run DC with first stage heaters dressed to ground. give a static dissipation of around 29w for each tube. Ground side of the heater string is 157ma. to ground and 375v ref to the cathode.Ĭathode measured 26v and tot. Plate voltage for the pair of 6CA7 is 402v ref. The amp I have set up and used for data has a 022798 equivalent PT,Īnd a 018343 equivalent OPT and is set up with 6CA7 power tubes,īiased through the heaters of two 12ax7 dressed series to ground. Of the amp in the schematic were biased with heaters of the preamp tubes. I finally built a couple, being tired of fender, the power side You look at the bias, once you've got that figured,Īnd it don't look right. There's a couple ward's airline, danelectro, and silvertone schematics, I think, We seem to start with old fenders, if its notįender based, too many shy away from trying other notions of amp design. Pre internet, one of the few available sources for old schematics wasĪspens Pittmans tube amp book.
Every now and again you find something that's not veryĬommon in the technical resources you scrounge up while self educating. Tube electronics is an old technology, nearly all DIY'ers have to try to figure it Of a couple preamp tubes to obtain cathode bias for a tube amp, here goes. Bill Ellis sent me a PM asking to explain a little more about using the heaters
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