Power tranny output voltages.

[pinkphiloyd]

Admittedly, I'm a total novice when it comes to amp design theory. I've been reading a little off and on for several years. This is my current question. I've been comparing some schematics found easily on the web to some various layouts. I've been looking a lot at the 5F4, because it's easy for me to follow. So, looking at the schem, and the layouts offered by various kit builders, etc. So, I notice that with minimal, if any, changes in the circuit and/or layout, I see power tranny out put voltages that differ by, what would seem to me, to be a significant amount. I think I've seen anywhere from 330 to 360 volts. What gives? Are tube amps just not that picky? Is the voltage chosen arbitrary within a given range? I don't know, it just seems to me, in my extremely limited understanding, that that's a wide range to deal with. Could there be performance (or worse, safety) issues there?

[MWJB]

360-0-360VAC might give you up to 470-480vdc on the plates of the 6L6s with a 5U4G, depending on bias conditions, wall AC voltage and any drop accross the choke. 6L6s today have to contend with voltages over 500vdc in many amps, so you're perfectly safe. At the lower end you might be looking at <440vdc...without specific experience of the PT in question and all the tolerances that might affect the final voltage, it's pretty well impossible to tell you what you will get to the volt.

The higher voltage amp will make more power than the lower voltage amp, however the lower voltage amp might sound browner & warmer...if you're playing out, I'd go for the higher range.

Confusion may stem from the fact that some manufacturers take the voltages from schematics & others from real world amps that run on today's wall voltages which can be 10-15% higher than when these amps were designed. Additionally, personal taste plays a part & some builders may prefer a specific voltage. Typical Fender 2x6L6 PTs vary from around 345-0-345VAC to 380-0-380VAC (perhaps on the high side for a 5F4). Some of the Hammond Fender replacement PTs look to be on the low voltage side.

[pinkphiloyd]

There's no danger, then, to amp or life/property, in any of those voltages? Thanks, great reply.

[MWJB]

No danger...well, no more at the higher voltages than at the lower ones anyway ;-)

[Enzo]

360 compared to 330 may sound like a lot, but really, it is about a 10% difference. Like a 68 cent can of corn going up to 75 cents.

And no, tube circuits are not at all picky. The circuit that works at 450v will likely work just fine at 400v. And vice versa.

[pinkphiloyd]

So, I built this thing. 360 volts. I can get anywhere rom 368 to 426 on the plates. What about the actual bias? I've read so much stuff that I can't make sense of it. Between ground and pin 5, I can get anywhere from about -30 to -70ish volts. At first I thought that was where I wanted to set my bias, and I had it set at about -32 volts. But when I check mA between the output tranny center tap and pin 3 (pins 3 and 4), I get anywhere from -43.8 to -80.0. I'm so confused.

[MWJB]

You shouldn't get "-" anything when shunting the OT to determine bias, you should get positive dc mA, you want from 30-40mA per tube depending on B+, about 15-17W idle dissipation. You do not read from CT to pin 4, just to pin 3. You cannot set idle current by negative grid voltage alone. -32vdc is far too low, -50 to -60vdc is more typical, then fine tune by mA.

[jmaf]

You shouldn't get "-" anything when shunting the OT to determine bias, you should get positive dc mA, you want from 30-40mA per tube depending on B+, about 15-17W idle dissipation. You do not read from CT to pin 4, just to pin 3. You cannot set idle current by negative grid voltage alone. -32vdc is far too low, -50 to -60vdc is more typical, then fine tune by mA.

He may get a negative current reading if he placed the positive lead on the tube plate and the black lead on the +B when reading the shunted OT.

[pinkphiloyd]

But my OT center tap runs to pin 4. I clearly have no idea what I'm doing when it comes to bias. But it damn sure sounds good. NOt bad for a first attempt I'd say.

http://www.ceriatone.com/images/layo...4Ceriatone.jpg

[jmaf]

But my OT center tap runs to pin 4. I clearly have no idea what I'm doing when it comes to bias. But it damn sure sounds good. NOt bad for a first attempt I'd say.

http://www.ceriatone.com/images/layo...4Ceriatone.jpg

Oh, that explains it. It's just using your pin 4 as a bridge from +B to the OT CT. No screen resistors heh....unprotected tubes...that'd be a good modification, add 470 to 1k OHM 5W resistors to those pin 4's!

[MWJB]

Aha! You can't use the OT shunt method to determine bias on a 5E4A/5A/7/8A. Install 1ohm 2W cathode resistors, from pin 8 to ground & read cathode voltage there in mV...after installing the 470ohm screen grid resistors like Jmaf says.

[pinkphiloyd]

I just got back from the store buying screen resistors because I already planned on putting them in. Now you're telling me I have to go back! Shit. Ha ha, thanks for all the help.

[pinkphiloyd]

Just so I'm clear, I'm putting one 470ohm resistor between the B+ off the filter caps and pin 4 on that first 6l6, correct? The one I picked up is 2 watts.

[octal]

Nope, you want an individual 470 ohm resistor in series with the screen grid for each tube, so you'll need two.

Nathan

[jmaf]

Just so I'm clear, I'm putting one 470ohm resistor between the B+ off the filter caps and pin 4 on that first 6l6, correct? The one I picked up is 2 watts.

As octal said, 2 resistors, one for each grid, though an arrangement does exist for a single resistor two tubes, let's be conservative. 5 watts at least, 2 watts won't do. I wonder how the heck you found a parts store open on good friday....where I am it looks like Pripyat out on the streets.