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Thread: Bias supply Problem, NOT TYPICAL

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    Bias supply Problem, NOT TYPICAL

    Hey, I'm building a 50W two el34's and I have Power transformer that doesn't have a bias winding, just a standard high voltage 290-0-290. So I used a standard kind of bias thing by putting a resistor off of the main HV winding right before the normal rectifier like-unto Almost ALL vintage examples, ;-) but its not working right for some reason. I drew up a schematic of what I have, and then I drew up a schematic of what I'm thinking of. I've got a bridge rectifier for the main HT throughout the amp, and since it doesn't have a grounded CT on the transformer, I measured the voltage on the CT (not connected to anything now) and it was about 129, so I am wondering if I can draw bias voltage off there. See attached schematic.

    Number 1 is what I have, and number 2 is what I'm thinking of. I wrote the voltages by the wires. I have no idea whether it would work or not, but I measured the voltage there and it looks like it might work. I just don't know if the way that these things work would allow for this. so #1 is what I have that won't work for some reason... and #2 is what I think might work but need help with.

    Click image for larger version. 

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    Also, that is a standard chassis mount pot for the bias, and it is overheating a little, what kind of pot, except a trim pot hopefully, should I use? I might just find the average amount of bias i need and put a fixed resistor if I have to.

    Thanks guys,
    Isaac

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    Supporting Member loudthud's Avatar
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    Neither 1 or 2 will work because the transformer winding never swings negative (more than 1 diode drop) to turn on the 1N5400 diode. FYI you should use a diode rated at 1000PIV. Look for a Marshall 9005 schematic. It uses a capacitor in series to get the bias.

    Link: http://www.el34world.com/charts/Sche...eramp_9005.pdf

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    Last edited by loudthud; 05-20-2013 at 09:46 PM.
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    Thanks. I'll try that. What exactly does the capacitor do, though? I wondered why #1 didn't work. I haven't had a problem in using the IN5400 vs IN4007, but I have more of the IN5400's so I try to use them more. But thanks. Will try that.

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    Dear Isaac, please do not design, but faithfully follow some established commercial circuit.

    The PSU you drew creates 820V DC.
    As soon as you turn it on, those poor capacitors will explode as grenades, not kidding.


    PS: as a side note, please do not post 6500x5000 pixel drawings.
    The Internet Browser will show it too small or unreadably large.
    For general use try to draw screen size or smaller.

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    It was doing 620VDC right on the positive lead of the bridge rectifier when I had the standby off. Once I turned it on, it went down to 497VDC, so I didn't worry. If I have to I can use a half-wave (or do I have my names mixed up... ?) two diodes on the windings and CT grounded. I had a transformer kinda like this that had, probably 325vac on the windings, and it did 560 with this same power supply design. and I lied, I just realized in both of those drawings I drew in a 32uf+32uf capcan, I really have 50+50.
    I'm not "really" designing here, as these things have been done many times, I just gotta find which ones have this and how they are done, but I had a similar design before, except on the older power transformer I had with the higher voltages, It also had a bias winding on the transformer, so since I replaced that one with this one, I don't have a bias winding anymore. And will keep drawing size in mind for the future. Sorry about that. Thanks.

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    (1) Your meter is misleading you. In DC mode it will indicate the average voltage. So with the standby open you are not seeing the peak value which Juan correctly points out is 820V.
    (2) Don't worry about it? Think about what happens if you have the standby switch on when you apply power. The heaters are cold and it will be some time before the voltage will fall from 820V. I for one would not want to be in the same room when that happens if your caps are not rated for 1000V.

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    My meter is definitely not broken, so Why would it display a wrong voltage. Its a very nice Fluke meter. So I should revert to this? Last time I tried this style power supply, I had about 325VAC on each side of the winding, but I only had 215VDC TOTAL at the diodes. Why would that be. I think it must have been that transformer, as it just always seemed to be problematic. Thanks for your help guys, as always it is very helpful and knowledgeable.
    Click image for larger version. 

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    Also, that bridge rectifier I have, is a bridge rectifier module, its not me putting together diodes and caps. its a little black box with four leads comin gout of it. probably about 3/4 inch square and 1/4 inch deep or so. With one corner lopped off for teh positive, although they are all marked, +, -, and two of these ~ (AC). I haven't had any problems with that before though.

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    OK last post for now, (three in a row, sheesh... ;-)) I just re-read what you said about average voltage, like on a wave, the peak is 820VDC, but average would be what the meter reads. That makes sense. Thanks.

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    Quote Originally Posted by isaac View Post
    OK last post for now, (three in a row, sheesh... ;-)) I just re-read what you said about average voltage, like on a wave, the peak is 820VDC, but average would be what the meter reads. That makes sense. Thanks.
    Exactly!

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    Ok thanks, I got the last diagram right, correct? That's what I should try? I will once I'm done working on a highly modded other single channel marshall 18w based amp. Which could be a while, so maybe I should sneak this in sooner or later. THanks guys.

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    Quote Originally Posted by isaac View Post
    Ok thanks, I got the last diagram right, correct? That's what I should try? I will once I'm done working on a highly modded other single channel marshall 18w based amp. Which could be a while, so maybe I should sneak this in sooner or later. THanks guys.
    That last version will give around -~55V for bias.

    But their are other issues. Under load you'll get 300V or so on the HT. You can't expect to get 50W out of a pair of EL34s with that low a voltage. You need nearer 400V. We don't know what the pre-amp consists of so the 100K resistor might might not be right - it's kinda high. Lastly we know nothing about the transformer - does it have sufficient power rating for the job?

    The best advice has already been given earlier - go with a known proven design. You'll be much happier with the end result.

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    OK yea, I was wondering about that. How can I get more voltage from it, so as to be able to do enough to do 50W? And just since I'm wondering, what happens when you lift the ground on a bridge rectifier? Would you then want to re-establish the Center tap to ground. Also, could I put a big resistor in series with the rectifier, after the bridge rectifier to lower the voltage, but still have enough for 50W. Because it needs to have at least 50W for me to be happy. ;-)

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    You have what you have, it will put out what it will, it's not positive thinking what will bend laws of Nature.

    290V AC will give you solid 410V DC, perfect to make a fine amplifier.

    No, it won't put out 50W RMS (almost no "50 W RMS" labelled does, by the way, with very few exceptions) but certainly >40W which is perfect by a drummer or onstage.

    Just build and enjoy

    EDIT: by the way, soon you'll post asking about attenuators because "it's too loud )
    Not kidding.

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    Quote Originally Posted by isaac View Post
    .. what happens when you lift the ground on a bridge rectifier? Would you then want to re-establish the Center tap to ground.
    I think you are asking what happens if you disconnect negative terminal of the bridge. Yes, you will need to connect the center tap to establish a return path for the current. OTOH, half of the bridge is not being used.

    Quote Originally Posted by isaac View Post
    Also, could I put a big resistor in series with the rectifier, after the bridge rectifier to lower the voltage, but still have enough for 50W. Because it needs to have at least 50W for me to be happy. ;-)
    No - that won't work as you need to have current flowing in the resistor to drop the volts and you will have zero with cold heaters so you'll still face the 820V cap breakdown problem.

    As JMF says, 290VAC will give 400V and you will get 50W on peaks - just not continuously.

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    Last edited by nickb; 05-22-2013 at 02:25 AM. Reason: typo
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    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...

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

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    Ok thanks.

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    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.

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    Here this should work...
    Click image for larger version. 

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

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

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    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. ;-)

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    Bias supply Problem, NOT TYPICAL - power resistors

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    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.

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    Yea that is cool.

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    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?

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    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.

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    Old Timer J M Fahey's Avatar
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    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?

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    Quote Originally Posted by isaac View Post
    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.

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    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...

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    Quote Originally Posted by isaac View Post
    ... 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.

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    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.)

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    Last edited by Dave H; 05-23-2013 at 06:04 PM. Reason: added comment

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    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.

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    It's not loud enough?

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    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.

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    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... ;-)

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    By mbailey in forum Theory & Design
    Replies: 5
    Last Post: 03-01-2009, 07:22 PM

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