Bias supply Problem, NOT TYPICAL

[Dave H]

How are you measuring that 21.07V? From the power you have calculated I’d guess it’s the peak voltage on the scope? It should be the RMS voltage. If you’ve used the peak voltage divide the calculated power by two so it’s 55.5/2 = 27.5W which looks right to me. My EL34 amp has about 340V on the plates (when I subtract the 20V cathode voltage) and it measures about 28W.

A scope won’t measure RMS volts directly but you can measure it accurately enough on the AC volts range of a digital multimeter if the frequency is not too high. I use 400Hz. Turn the volume up until the scope shows a little flat on the peaks of the sine wave then measure the RMS voltage across the dummy load with a DMM and use that value in the V^2/R calculation.

(Joe and nick posted as I was typing.)

[isaac]

Yea I suppose, its probably about that, which Means I need a new power transformer... or maybe a way to double my voltage. Anyone have any ways to do that? Its a 290-0-290 tranny, I have two in4007's, will more diodes help? I can't use a bridge rectifier either so that's out of the question.

[JoeM]

It's not loud enough?

[J M Fahey]

It was just a badly clipped version of what was coming out of the signal generator.

It was practically a squarewave.
Clean RMS is somewhat above half, so 30/32W. (I'm considering supply sag here).

Dear Isaac: nect time you can repeat the experiment, the correct way is:
1) set all controls to "flat" or at last all to "5" which is somewhat close, *except* presence which should be on "0" .
2) if you have a master volume, set it to 10 .
3) inject an around 100mV RMS sinewave, classic frequencies are 1KHz or 400/440Hz , take your pick .
If no generator available, download an MP3 test tone sinewave (google it) and play it in any MP3 player, which put out 100/200mV at the headphone output.
4) first check that at low volumes your sinewave is still clean.
Then start raising volume until the waveform on the screen starts getting flattened top and bottom peaks.
Then slowly lower the volume until the tops are *barely* flattening.
Now you measure voltage and calculate.

FWIW don't worry, **many** well respected and famous "100W" amps actually have 80/85W under the very strict conditions I set above, yet they perform flawlessly, many "25W" amps with 2 x EL84 give around 15 and so on .
Won't give brands, but many "bench" Techs who post here will have met this many times.

[isaac]

It's not 50W loud and that's what I'm shooting for. Besides it needs to have a good clean tone, and it doesnt really have that with humbuckers, although my tele is pretty good with it. Believe it or not, I got two EL84's to do almost 27 watts, just two. It was subjective and crazy but I did it once, so I think two el34's doing only 27W is not good, IMO two el34's better do at least 40W, so 27W is kinda sucky... I'll get it there. But it did roar like the craziest wonderful Marshall you've ever heard... ;-)

[J M Fahey]

It's not 50W loud and that's what I'm shooting for. Besides it needs to have a good clean tone, and it doesnt really have that with humbuckers, although my tele is pretty good with it. Believe it or not, I got two EL84's to do almost 27 watts, just two.

Well, those EL84 weren't 27 W either

It was subjective and crazy but I did it once, so I think two el34's doing only 27W is not good, IMO two el34's better do at least 40W, so 27W is kinda sucky... I'll get it there.

If measuring hurts, don't do it

But it did roar like the craziest wonderful Marshall you've ever heard... ;-)

That's the point
Don't obsess with numbers .

As in "I have no clean sound with humbuckers" what did you build a Marshall clone for?
To play Country?

By the way, what speakers are you using?

I bet your head into a 4x12" loaded with Vintage 30 or similar will be *unbearably* loud.
No kidding.

[isaac]

It's not really a Marshall clone anymore, its got elements of a 18W marshall power supply, and thats about it... I have messed with all the components and preamp and stuff, now its just a 50W custom amp, but when cranked will put out "that Marshall tone." With the old Power transformer, which was a horrible tranny BTW, it had about 550VDC on the plates of the 34's and that was the volume I wanted, and It didnt get dirty until it was pretty darn loud, and with my calculations... it said almost 75W but it wasnt that much, I think it was actually around 50 or so. but that was great, except that PT was just a horrible PT... IT made so much noise you wouldn't believe it, and the voltages were too high for anything but the power tubes. and those were marginally high. Anyway, Yea, Will try to get higher voltages, for more power, since that's what life is about right MORE POWER!!!! Just kidding. ;-) Yea I'll see about the PT. I'll get back to you guys.

[Enzo]

Do you want more power or do you want louder? Those are not the same thing. 100w is only 3db louder than 50 watts.

[isaac]

Well I guess louder is more like it. Yea I heard of the 100W vs 50W 3db difference. But really I want it to be more power too. So 50W and little louder, because in this case they seem to be pretty well related, Because I had really loud but the tranny was a stupid tranny, so I had an insane amount of what sounded like 60cycle hum but really really loud, and not able to be gotten rid of. Not sure what caused that, but it was horrible, now this transformer is really nice, except low voltages. I want them to be about 450VDC, and I have 340. With a bridge rectifier, I had 497 VDC which is fine here, but I don't have a bias winding and with a bridge rectifier, you have to have a separate bias winding, otherwise you can't achieve a bias supply. So I now have 340VDC and a good working bias. So 50W and Louder would be great. I'm guessing a new power transformer would get me there. One that is wound probably about 345-0-345 or similar, I have 290-0-290. It's kinda weird, This is a power transformer for a 36W 1974X Marshall, and i has 290-0-290, and I have 340VDC, I have a power transformer exactly like it except for the regular 18W and that one with the same power supply, will do 412VDC. SO I'm slightly confused, except I think this one with the larger tubes, must have a larger load, so that is why. But I know It can handle the load, it doesn't heat up or anything, and its only running two el34's and two 12ax7's so its fine, maybe another 12ax7 but otherwise its fine.