New Crack Build Voltage Problem [resolved]

[jminassin]

Hi Folks,

I solved my resistance check problem, but now I have a voltage issue.  I’m following the directions on page 49 of the Crack manual.  I plugged the amp into my dim bulb tester and the dim bulb tester into my variac.  As I bring the voltage up above 35 volts the dim bulb tester starts to get brighter and brighter.  It appears I have a short somewhere.  I checked all the connections.  Nothing obvious is touching anything it shouldn’t.

Any thoughts?

John

 

[Thermioniclife]

I must admit that I have not followed this post closely but you stated that you passed the resistance test after re soldering.
If I may ask what wattage lamp are you using? I usually use a 200w lamp as I can't seem to find higher incandescent lamps anymore.

 

[jminassin]

Ahhh, good point!  I used a 60 watt bulb because the last thing I tested drew only 30 watts.  What is the Crack’s wattage at idle?  I need to size the wattage of the bulb in my tester appropriately.

I do appreciate your help,

J

 

[Deke609]

Well, you've now passed the 5 post mark, so you should be able to post pics.

Failing that, I'd carefully scrutinize the power supply, looking for miswires, or unclipped leads that may be shorting B+ to ground/chassis.  A power supply diode installed backwards would cause a high current short.

cheers, Derek

 

[Thermioniclife]

I would guess it's aprox 60 watts after warm up and higher at a cold start up. If your resistance tests are good I would not use a dim bulb and only use a variac and look and listen for trouble as you are probably fine. A 60w bulb will glow brightly at 500 ma and you will not get accurate voltage readings.

 

[Paul Birkeland]

Can you plug the amp into the wall and turn it on?

 

[jminassin]

Ok, thanks to all of you.  I will go through the circuit one more time and check the power supply diodes, unclipped leads etc.  I can plug it directly into the wall of course.  I’m just afraid of blowing something up.

John

 

[Paul Birkeland]

That's what the fuse is for. 

 

[jminassin]

Thank you Paul.  I’ll give it a once over look, plug it into the wall and hope for the best.  I’ll let you know.

John

 

[jminassin]

Hi Folks,

I had a transformer diode installed incorrectly (polarity).  I corrected it.  I put the tubes in place and plugged the Crack into the wall.  The tubes are not glowing and neither LED on the 9 pin socket is lit.
I turned off all the lights to make sure.  I unplugged the Crack and removed the fuse.  It looked good and I tested it for continuity.  The fuse is good.

Any suggestions for where to start looking?  The Crack passed the glow test on page 31 of the manual when I was at that part of the build.

Any help would be appreciated.

John

 

[Paul Birkeland]

The Crack is no longer passing the glow test.  Nothing else matters until you can get the tubes glowing. 

The tube heaters (that glow) are fed with the green twisted pair of wires.  Since both tubes don't glow, either the green wires going from the power transformer to the B socket aren't well connected, or the power transformer isn't well connected to 120V wall power. 

You could recheck the power transformer voltage check on page 29 and let us know what you get.  I would also post some build photos while you're at it.

 

[jminassin]

Hi Paul,

The voltage between terminals 7 and 9 reads 6.39 V AC.  The voltage between terminals 11 and 12 reads
173.7 V AC.

So I suppose I’m left with the green twisted pair to check.

I don’t see an “attach” icon that would allow me to attach photos.  How do I do this?

Thank you for all your help,

John

 

[Deke609]

I don’t see an “attach” icon that would allow me to attach photos.  How do I do this?

Click "Attachments and other options" -- see attached screenshot.

cheers, Derek

 

[Paul Birkeland]

The green twisted pair of wires leaving the power transformer and going to the octal socket is 100000% the first problem to address.  At least one wire in that twisted pair is not properly connected.

 

[jminassin]

PB,

Transformer lug 7 and 9 have continuity with A4, A5 and A9.  Those transformer lugs also have continuity with B7 and B8.  Lugs 7 and 8 are effectively connected to each other.  Can you confirm this wiring is correct?

The system won't let me attach photos for my ipad even though they are .jpg files.  Is there a way to fix this?  Then the system would not let me reply via iPad.  So I am using my laptop.

John

 

[jminassin]

In my last post I meant to say that transformer lugs 7 and 9 are effectively connected (not 7 and 8).

Sorry for any confusion.

John

 

[Paul Birkeland]

The winding itself has continuity.  In the photos you sent me, B7 and B8 aren't well soldered.  The way to check these connections would be to measure the AC voltage between B7 and B8, which is likely to change when you poke it with your meter probes (assuming those connections are loose, the wires on your power transformer looked to be well soldered).

 

[jminassin]

PB,

I re-flowed all the lugs on the 6080 tube.  I read 6.24V AC between lugs B7 and B8.  The 6080 tube still does not glow.

Both sides of the 12 AU7 are now lit, but I can’t see the LEDs glowing.

Continuity of the twisted green pairs looks good and measured resistance of each green wire point-to-point is effectively zero ohms.

Any help would be appreciated,

John

PS:  I just noticed how many emails/day you deal with.  I don’t know how you do it.

 

[Doc B]

Have you tried measuring the continuity of the heater in the 6080? Pull the tube out and measure ohms from pin 7 to pin 8. Should be very low, like an ohm or so.

 

[Paul Birkeland]

The 6080 also takes some time to start glowing, so if you aren't leaving the amp powered up long enough, it may not have enough time to glow.  Another thing that can happen is sometimes the plastic keyway on the base breaks off, then you have to be careful not to insert the tube into the socket improperly.  Lastly the tube socket can be installed improperly (not the case based on the photos you sent me), and that will also cause glowing problems. 

Since the 6080 did pass a glow test originally, I would be betting on time or a broken keyway being part of the problem.

Since the 12AU7 is glowing, you can run the amp and check DC voltages at least.