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Millett Mosfet-MAX Power Supply & hum problems

post #1 of 11
Thread Starter 

Hi everyone,

My Mosfet-MAX has been acting up again recently. I have a quiet, but audible hum in the background, and for some reason I can't adjust my V+ with the trimmer - this used to work a while ago. When I gently press the tubes into the sockets while the amp is on, the hum is sometimes reduced a bit but it comes back as soon as I take my hands off.

 

Also, with some of my tubes, touching the tubes (I have 2 sets, 3 of those 4 tubes seem to do this to a certain degree) or even just giving the amp chassis a little nudge causes a strange microphonic effect; the touch is clearly audible on the affected tube's channel. I assume those tubes are faulty and should be replaced? Some also take much longer to start glowing with full brightness...

 

Any help would be much appreciated. If you need more information, I'm happy to provide it.

post #2 of 11
Quote:
Originally Posted by hitman47 View Post

Hi everyone,

My Mosfet-MAX has been acting up again recently. I have a quiet, but audible hum in the background, and for some reason I can't adjust my V+ with the trimmer - this used to work a while ago. When I gently press the tubes into the sockets while the amp is on, the hum is sometimes reduced a bit but it comes back as soon as I take my hands off.

 

It sounds like the regulator has blown.  There is no hum or ripple present if the power supply is operating correctly.  The fact that the trimmer seems to make no difference in the voltage supply sort of confirms this suggestion.  What voltage are you reading on V+?

 

As for the tubes humming, but then not when you press them down into the sockets, it could be one of the following:

1. Dirty tube pins,

2. Faulty tubes, or

3. Faulty sockets.

I think this problem is just tangential.  You may indeed have less-than-clean/optimum tube pin contact, but that's not going to cause the hum all by itself.  As I said, if you can't adjust V+ with the trimmer, that could mean a bad regulator (assuming the trimmer is OK).  If so, the amp will be getting un-regulated DC, which can be quite noisy and full of hum and ripple. 

 

Also, with some of my tubes, touching the tubes (I have 2 sets, 3 of those 4 tubes seem to do this to a certain degree) or even just giving the amp chassis a little nudge causes a strange microphonic effect; the touch is clearly audible on the affected tube's channel. I assume those tubes are faulty and should be replaced? Some also take much longer to start glowing with full brightness...

 

It depends on whether you can live with the noise when you "nudge" the amp.  Microphonic tubes are somewhat common.  Unfortunately, microphonics can't be tested with a tube tester - they have to be put into a device that produces analog sound (an amplifier).  It can mean the tube is worthless or it may sound pretty good as long as you don't nudge it.  It's up to you whether you think the microphonics are bad enough that they must be replaced.  If they are Beezar tubes and you purchased them from me, of course - I'll replace them.

 

As for varying brightness among tubes or some starting to glow quicker than others, it means nothing.

 

Any help would be much appreciated. If you need more information, I'm happy to provide it.

post #3 of 11
Thread Starter 

Thank you so much for the quick response. I'm getting around 27.5 volts, which, if I remember correctly, is what I got when I accidentally drove the LM317 out of regulation once, so it's very plausible that the regulator is indeed busted. I hope I can get a replacement locally, shipping that kind of stuff to Switzerland is always expensive and our Farnell chapter charges 15 bucks for shipping too.

 

Also, I did get the tubes from you, but I think I can live with the microphonics since sound quality isn't really affected as the amp is on my desk.

 

Thanks again!
 

post #4 of 11
Quote:
Originally Posted by hitman47 View Post

Thank you so much for the quick response. I'm getting around 27.5 volts, which, if I remember correctly, is what I got when I accidentally drove the LM317 out of regulation once, so it's very plausible that the regulator is indeed busted. I hope I can get a replacement locally, shipping that kind of stuff to Switzerland is always expensive and our Farnell chapter charges 15 bucks for shipping too.

 

Also, I did get the tubes from you, but I think I can live with the microphonics since sound quality isn't really affected as the amp is on my desk.

 

Thanks again!
 

Send me a PM with your name and address and I'll be glad to send you an LM317.  smily_headphones1.gif

post #5 of 11

Whilst Tom obviously offers amazing service, if you're looking for something a little faster then ebay doesn't seem too bad (as long as you're not against ebay, I know some people just wont use it). I would imagine that post for germany comes quickly to switzerland. A quick browse of ebay showed http://www.ebay.ch/itm/10-LM317-T-Spannungsregler-TO220-LM-317-T-Voltage-Regulators-TO-220-853410-/150876307454?pt=Bauteile&hash=item2320edbffe#ht_500wt_824 for example, yeah you'd have to buy 10 but i imagine that it's relatively fast and painless. I also had a quick look at conrad, but they're crazy expensive for small orders.

 

I'm fairly sure I don't have one but I'll have a rummage through my parts boxes tomorrow, since I live in france it could be quick and easy. (I know for certain that I have a long ton of 7805s, but they wouldn't help much as you'd need to change the resistor bias.

 

Ah well, hope you get it sorted out with a minimum of fuss

Cheers 

post #6 of 11
Thread Starter 

Thanks for the tip Goobley, I'd completely forgotten about ebay even though I use it quite often. I've even found a vendor selling 317s from Switzerland, so I'll just get some from that guy. 5 bucks for 4 pieces shipped is a pretty nice price. Edit: crap, just noticed that offer wasn't a buy now deal. I'll just get them from Germany.

 

Still, thanks for the generous offer, Tom! You never cease to amaze with your amazing service both on Beezar and here, I don't think many of my DIY projects would be working right now without your help.


Edited by hitman47 - 11/1/12 at 10:07am
post #7 of 11
Quote:
Originally Posted by hitman47 View Post

Thanks for the tip Goobley, I'd completely forgotten about ebay even though I use it quite often. I've even found a vendor selling 317s from Switzerland, so I'll just get some from that guy. 5 bucks for 4 pieces shipped is a pretty nice price. Edit: crap, just noticed that offer wasn't a buy now deal. I'll just get them from Germany.

 

Still, thanks for the generous offer, Tom! You never cease to amaze with your amazing service both on Beezar and here, I don't think many of my DIY projects would be working right now without your help.

Thank you so much for the kind comment!

 

I was offering to send you one for free, though.  I only have maybe 100 of them of them at the moment, so I could surely spare one.  Let me know if you change your mind. wink.gif

 

One thing to think about is that way back when, we had at one time suggested the use of the LM338.  It's more or less the same thing as an LM317, but is capable up to 5A.  In the early days, there were instances of the MOSFET-MAX (and AMB's M3) drawing too much current for the LM317.  I'm not sure I've heard of this issue lately, though.  You should have plenty if it's a "legitimate" LM317T, which is good to 1.5A.  I think there are some LM317's mfd by others (STM has one) that may only be rated 1A or less.  That may be a little too close and could blow it under the right conditions at turn-on, etc. (maybe).  Again, the "legitimate" National Semi-TI version has built-in protective circuitry, but it may not handle transients at near rated power, if they are large enough

post #8 of 11
Thread Starter 
Quote:
Originally Posted by tomb View Post

Thank you so much for the kind comment!

 

I was offering to send you one for free, though.  I only have maybe 100 of them of them at the moment, so I could surely spare one.  Let me know if you change your mind. wink.gif

 

One thing to think about is that way back when, we had at one time suggested the use of the LM338.  It's more or less the same thing as an LM317, but is capable up to 5A.  In the early days, there were instances of the MOSFET-MAX (and AMB's M3) drawing too much current for the LM317.  I'm not sure I've heard of this issue lately, though.  You should have plenty if it's a "legitimate" LM317T, which is good to 1.5A.  I think there are some LM317's mfd by others (STM has one) that may only be rated 1A or less.  That may be a little too close and could blow it under the right conditions at turn-on, etc. (maybe).  Again, the "legitimate" National Semi-TI version has built-in protective circuitry, but it may not handle transients at near rated power, if they are large enough


Thanks again, but I've already ordered a bunch of 317Ts from Germany now, it'll be faster that way and they only cost like $10 shipped for 10 pieces anyway.

post #9 of 11

Glad you've got them ordered and hope you get it sorted without any major issues smily_headphones1.gif

post #10 of 11
Thread Starter 

Well, I replaced the LM317 and all is well again. And while replacing it I noticed that I hadn't even soldered the old one in properly, somehow I had failed to notice that its pins were not in the holes while assembling it. I guess it made contact for a while so I didn't notice and at some point it must have slipped off or something, either way I mounted the new one to the lower hole so that wouldn't happen again and now everything works perfectly. Thanks for the support!


Edited by hitman47 - 11/9/12 at 11:44am
post #11 of 11
Quote:
Originally Posted by hitman47 View Post

Well, I replaced the LM317 and all is well again. And while replacing it I noticed that I hadn't even soldered the old one in properly, somehow I had failed to notice that its pins were not in the holes while assembling it. I guess it made contact for a while so I didn't notice and at some point it must have slipped off or something, either way I mounted the new one to the lower hole so that wouldn't happen again and now everything works perfectly. Thanks for the support!

Glad to hear it!smily_headphones1.gif

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