Bias supply Problem, NOT TYPICAL

[isaac]

Hey surprise, I'm 15 BUT DON'T WORRY ABOUT SAFETY AND KNOWLEDGE, I GOT THAT, I've been doing this for several years now... It's probably the age, but I Won't be asking about attenuators... Besides, as my dad has a lot of nice stuff, in there somewhere he has a really nice attenuator called the "Alex Attenuator." The greatest one there is, look it up. Hey you are from Buenos Aires, I know a guy who lives there. ;-) I built this amp before and had 550VDC average on the plates of my 34's, and calculated it out to 75W almost... Can't imagine peak voltage there. Wait minute, stupid me, peak voltage is double the average...

[frus]

yes, if you've got square AC voltage in your area

[isaac]

Ok thanks.

[isaac]

Hey guys got it all good. Amp works great except for a preamp problem I think I addressed, except couldn't test as I ran out of time... Anyway, thanks. It has 340VDC on the plates of the el34's with a two diode rectifier and grounded center tap, and then a typical bias supply. I calculated it out to 55W at max output. ;-) Thanks. Will try to get a schematic soon.

[isaac]

Here this should work...

Yup looks good that's what I have. At the OT/EL34 plates, I have 340VDC. That wasn't marked there.
Thanks,
Isaac

[loudthud]

I'll bet that 47K gets hot. What wattage did you use?

[isaac]

The 47k on the bias? Its a 5W I believe, although, I felt it after it was running for a little while, (CAREFULLY) and it didn't get hot. The 25kb is the same kind of pot you'd use for your presence control or whatever, and that one doesn't get hot anymore either. So I thought that was pretty cool. I have had myriad problems with resistors in power supplies in that amp blowing, I have NO IDEA how, but at one point in time, I had three 9W resistors in parallel equaling 300ohms and 27W in there right after the main rectifier and they smoked so quickly. I guess since that was before the output transformer HT feed, that is sensible, but the amp at that time was only making a total of 29Watts or so and it was drawing WAY over the 27W through those resistors, so I don't know, but that's gone and behind me, not a problem anymore, so I'm REALLY happy. I may even try to get a transformer with a slightly higher voltage than 290 so I can get a little more wattage out of the 34's. Like 460VDC max voltage on the plates of the 34's is about where I would like it, but It sounds really nice as it is now, just its really not half as loud as I thought it would be though. Could just be me... My dad made the same analysis, though. Right now the amp is 340VDC on the plates, biased 49mA each tube. I got that calculation from WeberVST. Still would like more voltage on plates, but a 55W amp is pretty good. ;-) Will probably also, soon install a PPIMV so I can have gainy tones at low volumes though. ;-)

[nickb]

Bias supply Problem, NOT TYPICAL - power resistors


I know you're over the hump with the resistors, but I think you will find this interesting. The important points are (1) that a resistor dissipating power will get very very hot, even when it's dissipating nowhere near it's full rated load (see heat rise chart, too hot to touch at only 10% of rated power) and (2) that you might not be able to operate it at the full power if the surrounding air gets hot - see derating curve.

Happy to hear it all worked out.

[isaac]

Yea that is cool.

[g1]

As has been mentioned a few times above, with 400VDC on a pair of EL34's, you will not make 50W cleanly. You have 340VDC on the plates, so you should get even less, but you say you are getting 55Watts. Is that heavily distorted? How are you measuring the power output?

[isaac]

Well, thats what I though, But I have 340VDC, and I put that into webers calculator, and it said to do 51mA per tube, so I did that, and the amp isn't "super" loud but tis pretty good, and it gets pretty mean with humbuckers, but I put it with a scope signal generator and dummy load, and I did voltage squared times impedance. so I got 21.07V across the dummy leads, and I calculated it out... 21.07 x 21.07 = 443.9449, 443.9449/8=55.493W I have been told that my math is weird before, but what do you guys think? As Also stated before, I would like to get a transformer with HV of 345-0-345 so I might get about 400-450VDC So I can achieve better output, and such. I also need to change my preamp HT dropping resistor, its too high, but otherwise, thats what I got.

[J M Fahey]

but I put it with a scope signal generator and dummy load, and I did voltage squared times impedance. so I got 21.07V across the dummy leads, and I calculated it out... 21.07 x 21.07 = 443.9449, 443.9449/8=55.493W

Were you looking at the waveform in the scope screen?
Was it a perfect sinewave or some other shape? (A picture would be excellent )
Did you use a resistive load or a speaker?

[nickb]

21.07 x 21.07 = 443.9449, 443.9449/8=55.493W

The real answer is half of that (for a sinewave). You have calculated the peak power.

[isaac]

Dont have a pic of the wave. It was just a badly clipped version of what was coming out of the signal generator. It was a resistive load, so I guess its not technically ...times impedance, but the load is 8 ohms, so...

[JoeM]

... 21.07 x 21.07 = 443.9449, 443.9449/8=55.493W I have been told that my math is weird before, but what do you guys think? ....

How did you measure 21.07 volts? If that's peak than RMS would be 14.9v, or about 28 watts. I would think this is more in the ballpark given your B+ of 340V.