Power supply low voltage

[e178453]

I am troubleshooting a Crack amp built by a friend.  The first problem I found was a 6082 tube supplied with the kit rather than a 6080.  The 6082 requires a 26V filament supply.  Got the proper tube, the 6.3 filament voltage is fine, tubes light up.  Did the resistance check, all looks good.  Now I have a power supply problem.    Measured very low voltage, almost 0V, where I should be seeing 170V.  Disconnected the power supply from the rest of the circuit to check it.  I get only 30V.  Disconnected everything except the first filter cap and diodes, if I leave it powered up for 30 minutes or so, the voltage climbs to 180.  Disconnected everything from the transformer, I get 150VAC from one side, but only 127VAC from the other.  Diodes check fine in the circuit.  My friend did a great job soldering this together, so I don't want to start dismantling the entire power supply until I asked for some help here.  Just FYI, I worked in electronics for some time, have built a couple tube guitar amps and had good luck troubleshooting a few others, built numerous other projects, but this has me baffled, at least for the time being. Any thoughts you guys have would be greatly appreciated. 

[Paul Birkeland]

Can you post some photos?

The black/red twisted pair of wire between the two pairs of the high voltage transformer terminals needs to be wired in phase to make proper high voltage DC.  If they are wired backwards, this would give approximately your symptoms. 

If it takes ~30 minutes to reach proper voltage, I sure would suspect a bad solder joint somewhere in that diode bridge. 

-PB

[e178453]

Thanks, Paul.  Hope this photo attaches.  It is not very good, but shows the secondaries connected together.  A resistor and the ground to the filaments are disconnected to isolate the power supply back to the first filter cap.  Something else I neglected to mention:  When I check for resistance across the B+ secondaries, I get Megohms, really an open circuit.  The primary shows 12 ohms on my meter, the 6.3V winding shows .5 ohm, which sounds correct.  I would expect the 150VAC secondaries to show about 18ohms.  My friend's soldering looks good throughout the build.  I hesitate to blame the transformer yet, not quite ready to make that leap, but I am really puzzled.

[Paul Birkeland]

That resistance reading is very odd, especially considering there are two secondaries there, and you only need one for the transformer to function.

It's also very odd, because a reading in the megaohms would mean that you shouldn't get any AC voltage reading out of those windings. 

Can you check to be sure that the wires wrapped around the base of the transformer solder tabs are still connected and not broken? 

-PB

[e178453]

The HV secondary windings appear to be connected in parallel, which would yield 150VAC(?)  But I need 300VAC going to the rectifier circuit, correct?  I must not be understanding how the connected windings combine.
Thanks again for the help.
-Scott

[Paul Birkeland]

161V AC is the specified AC voltage that you should see, 150V is still plenty for the amp to operate. 

Paralleled windings give more current but the same voltage.

Windings in series give twice the voltage but only the current that a single winding could deliver (the PT-3 in the Crack must *never* have its windings wired in series).

-PB

[mcandmar]

(the PT-3 in the Crack must *never* have its windings wired in series).

Why is that?   I remember Doc mentioned something about that being the reason you never sold the transformer on its own.

[Paul Birkeland]

I'll let PJ chime in on that one, I don't quite remember the specifics.

-PB

[e178453]

Thanks, guys.  I was thinking 300VAC P-P would yield about 212VDC.  This must be 150VAC RMS?  The transformer on this build looks good from the outside, wires coming out of the windings are soldered to the tabs.  I still get an open circuit across both 150V windings no matter what combination I check.  Even measured the resistance on windings of the power transformer on another build I have to make sure I knew what they should look like.  Primary and 6.3V look good.  I sure don't understand how this can be, but there it is.  Will ask my friend who put it together to call or email Bottlehead and see what can be done.  Thanks again for the help, will post again on this puzzler if I find anything out.

[Paul Birkeland]

What we could do is to take the amp in for repair.  If the transformer is defective, it's a warranty item.  If it's not the transformer, we can fix the amp under our flat rate repair service and get it take care of for you.

-PB

[Paul Joppa]

The high voltage windings are bifilar - two wires wound together, to make two identical windings. The only insulation between them is the varnish on the wire itself, and since the wire is manipulated extensively in the winding process, I do not trust the insulation to be, or to remain, intact when there is a high voltage between the wires. Therefor I have tried to tightly control what designs are used with this transformer.

[mcandmar]

That is interesting, thanks Paul.   Was this done to minimize noise vs a single secondary winding of higher amperage?

[e178453]

I brought my friend's build back to him today and demonstrated to him with a multimeter the open 150V windings.  Had him measure the resistance on all the windings himself.  The ends of the windings can be seen soldered to the tabs.  This is simply a bad transformer.  If he contacts you, will you send him a replacement?

-Scott Harper

[Doc B.]

We can replace it. However it's not too often, if ever, that we have seen transformer with open windings from our winder. So there might be something else going on in the amp that stressed the secondaries, and it would be a drag if you went to the trouble to replace it and the next one was stressed to the point of blowing secondaries too. I'm trying to figure out if the 6082 could have caused this when it was installed, but I  haven't come up with a reason for that happening.

It might be best to send the trans back to us for postmortem just in case it was a winding problem, and we can send out another. But please double check that there is nothing else going on that might take out the next one.

The other option would be to send the whole amp to us to look at. If we can't find anything else wrong we will repair it for free. If it's a miswire or other assembly error we would fix it and only charge for the replacement transformer.

[Paul Joppa]

That is interesting, thanks Paul.   Was this done to minimize noise vs a single secondary winding of higher amperage?

The origin of this design is a convoluted technical story pertaining to a now obsolete product, and this is not a good place to discuss it. A long evening in a good pub with lots of good beer and many many napkins to draw on might be more appropriate ...  :^)