Another BeePre 2 with Hum

[ScottAstroNut]

My next question, then, is how to go about diagnosing that without blowing another resistor once I get a replacement and have repaired the circuit?

As I mentioned, at one time I did electrical engineering, but it was all digital logic systems. I did my circuit design on breadboards or perfboards with lots of wire wrap. If I mis-wired something, usually it would not go up in smoke! I did learn about regulated power supplies, but only to the extent that I could build one that would provide the 6 to 12 VDC I needed for my digital IC's. You analog (and especially you audio tube-analog!) guys work in an entirely different world. One that I am very much enjoying getting to know a bit better by building your kits. I have a lot of respect for the expertise and experience you need to do your kind of job.

Having circuits that work only upside down or right side up is amusing to me! I love EE war stories!

 

[Doc B]

I think you have to look at this from two different perspectives. Yes, you can analyze the circuit and peruse the schematic to see if you can imagine a scenario where something that isn't exactly like the circuit drawing is causing the issue. But from a practical standpoint it's a lot easier to say "crap, something shorted" and look at the amp for signs. There are various ways this can happen. Gravity - when you flip the amp over something moves and shorts out. Gravity, part 2 - when you flip the amp over an offcut piece of wire that was hiding somewhere falls down and shorts something. Time - there was a miswire from the beginning that finally made the resistor give out from being overloaded.

If I was stuck I would measure across each filter cap and look for a low resistance reading to ground. If I found one I would start to investigate the components nearby. When a big resistor smokes there is a short somewhere. Miswire, leads touching things they shouldn't, caps/diodes in backwards - the list of possibilities is not too long.

 

[ScottAstroNut]

Hi, Doc! Thanks for the helpful insights.

I did check the caps in the HV supply, and did find that the 220 microfarad cap closest to the transformer has a very low resistance reading. So I guess I am going to need a new capacitor, then. I don't immediately see anything amiss in the vicinity of the cap, however. The diodes look to be OK. None appear to be oriented the wrong way. The other three 270 ohm  resistors don't appear damaged when measured. The other two large caps seem fine.

 

[Paul Birkeland]

I did check the caps in the HV supply, and did find that the 220 microfarad cap closest to the transformer has a very low resistance reading. So I guess I am going to need a new capacitor, then.

No, lift one leg out of the circuit then check again.  If the cap is still soldered into the amp, you can't say much about its condition.  The caps we provide are of exceptional quality, so it would be surprising to randomly get a dead one. 

 

[Deke609]

That seems like a really weird place for a resistor to fail: the last resistor in the return (-ve) rail of the B+ supply. Granted, I don't know much about this stuff, but I can't think of a way that could happen that doesn't involve an obvious serious wiring error - and I don't see one in your photo.

So a shot in the dark: how much physical tension do you have on those white cement capacitors? I can imagine that the internal connection between a lead and the internal wire coil (or whatever resistive structure is inside) might crack if you soldered one lead and then pulled the other really tightly before soldering it -- or installed the resistor suspended by the leads and then tried to push the resistor towards the chassis.  A cracked/flaky connection inside the resistor might easily overheat with even normal current running through it and then pop/smoke when it inevitably fails. I can also imagine it being noisy when it was still capable of passing current.  Just a thought. 

cheers, Derek

 

[ScottAstroNut]

Oh, right about removing the cap from the circuit to test it. Given the lateness of the hour here and the fact that I am tired, I think I will leave that for tomorrow.

The caps are of exceptional quality, and they worked just fine before, so they weren't dead on arrival.

 

[Paul Birkeland]

When you get the replacement resistor, can you remove the 22 ohm resistor with the parts installed on it and try putting it between 33L and 35L?

-PB

 

[ScottAstroNut]

Sure. No problem.

 

[ScottAstroNut]

I think I found it! The reason for the blown resistor, that is. Upon careful inspection of the HV supply, I found a solder run that looks like it shorts one of the 270 ohm resistors to ground. It looks to me like the excess solder barely touches the chassis, so that if the chassis were placed right side up, gravity might very well pull the assembly down enough that the solder run doesn't short. Upon inverting the chassis plate, gravity pulls the solder run to the chassis and a short occurs. This must have occurred when I re-flowed all the solder joints in the high voltage supply, and simply didn't catch this bad joint. Dolt!

I have also made the change Paul suggested of moving the 22 ohm resistor to terminals 33 and 35. We shall see what happens once I get a replacement 270 ohm resistor.

I haven't received a reply for a parts request I sent to replacementparts@bottlehead.com, so have gone ahead and ordered parts from Mouser.

Scott

 

[Deke609]

... so have gone ahead and ordered parts from Mouser.

Scott - Mouser is way behind on orders b/c of the Texas storm. I have an order from Feb 12 that still hasn't shipped, and two more in queue that haven't even been processed yet. If you want something fairly quickly, I suggest a local electronics hobbyist store. Or Digi-Key.  And then cancel the Mouser order.

And congrats on potentially having found the cause of the problem.

cheers, Derek

 

[Paul Birkeland]

In your photo I see UF4007 leads that look like they will short out the two power transformer terminals.  I would provide some separation for those while you're waiting.

 

[ScottAstroNut]

I think that the diode leads looking to be too close together may be a perspective thing with the photo. Here's a photo from the other side showing that the leads are not near each other and are constrained well enough, I think, that there is no chance of them touching. Please let me know if this looks OK. If not, then I will definitely make adjustments. Thanks!

 

[ScottAstroNut]

Derek, thanks for the heads-up on Mouser. I've gone ahead and placed an order with Digi-Key for the parts.

 

[ScottAstroNut]

I have a question on how to proceed once I get the replacement 270 ohm wire wound resistor...

Once I have the part in hand, do you recommend that I simply insert the new resistor and "give it a go?" Or, do you think a more deliberate approach would be better? For example, should I disconnect the UF4007 diodes from the transformer, then test the transformer output to make sure it is OK. Then, reattach the diodes, but disconnect the HV leads to the rest of the circuit and test the output of the HV supply before attaching the rest of the circuit? This latter approach seems safer to me, but more work. I love soldering parts. I am not so fond of de-soldering!

 

[Paul Birkeland]

That's some good photo work, those look great.

You will need to put the 270 ohm resistor back in place and fire the BeePre back up. 

 

[ScottAstroNut]

Well, the BeePre 2 is back up and running. I replaced the 270 ohm resistor in the HV power supply after fixing the other minor issues and sweet music is playing again. Unfortunately,  the hum/buzz issue is still present. I think it is slightly lessened by having moved the 22 ohm wire wound resistor to its new location as requested, but it is still there. Slightly, but still audible at normal listening levels through headphones using my S.E.X. 3.0 as the headphone amp. I wouldn't be too concerned about this issue except that my Moreplay, when used in the same location, is extremely quiet. I am pretty sure at this point that the BeePre 2 is correctly constructed. Also, it doesn't seem to matter which headphones I am using, from 32 ohm Grado PS 500e's to 600 ohm Beyerdynamic DT880's.

Thoughts? Suggestions? Again, this is a non-issue when listening with speakers. Thanks.

 

[ScottAstroNut]

On a positive note, I do have to say that the overall sound quality of the BeePre 2 is outstanding.  Very sweet and musical while also possessing amazing clarity and resolution. Makes me wonder what improvements I might hear if using a Mainline instead of the S.E.X.?

 

[Alby0521]

I did the changes suggested by Paul and I found myself in your same situation. Hum still there but to a slightly less extent.
I shipped the beepre2 to Paul yesterday. Hopefully he can find the problem.

Best

AB

 

[Paul Birkeland]

It's somewhat possible that the BeePre just isn't suitable to sit in front of a headphone amp.  It would not surprise me at all seeing that you have Grado headphones in your lineup which are incredibly sensitive. 

Yes, a Moreplay will have a lower noise floor since the 6V6 is an indirectly heated tube. 

If you use the BeePre as a preamp in front of a power amp driving speakers, how well does that work?

The upcoming upgrades will drop the noise floor a bit more as well. 

 

[ScottAstroNut]

Paul, the BeePre 2 sits in front of my Kaiju just fine. No hum at normal listening volumes. Sounds great.

Too bad to hear that the BeePre won't work with a headphone amp. I listen to headphones much more often than I listen to speakers. Guess I'll keep using my Moreplay.

Thanks for all your help.