New Crack Build Voltage Problem [resolved]

[jminassi]

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

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).

[jminassi]

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.

[jminassi]

Doc,

The reading between pin B7 and B8 is 0.1 ohm.

PB,

I set the chassis plate on the base right side up, plugged it in, and both tubes were glowing after a few minutes.  However, when I removed the chassis from the wood base and turned it on its side, only the 6080 glowed.  The socket appears to be installed correctly and I can assure you that the plastic keyway is fully intact.  The wires going to the 9 pin socket appear to be securely soldered in place.  The LEDs are not lit.

I measured the voltages and they are way off as follows:

1.  205.1 V
2.  205.5 V
3.    96.6 V
4.  205.3 V
5.  197.2 V
6.       .3 mV
7.       .3 mV
8.        0 V
9.    92.6 V
10.   92.7 V

I disconnected the test lead and put the chassis back into the wood base.  The 12AU7 is not glowing.  The 6080 glows when first turned on and then goes dark after about 30 seconds.

Let me know what you think.  I do appreciate your help.

John

[Paul Birkeland]

I set the chassis plate on the base right side up, plugged it in, and both tubes were glowing after a few minutes.  However, when I removed the chassis from the wood base and turned it on its side, only the 6080 glowed. 

A loose solder joint will do that.

Since you have tubes lighting and not lighting, there is still an issue with the green twisted wires and their connections.

[jminassi]

Ok, it was a loose solder joint just like PB said.  Tubes are glowing.  LEDs are glowing.  All voltages look good except Terminal 7.   I have floating millivolts where I should have 90-115 V.

Any idea what I should be looking at?

John

[Deke609]

Double check that your meter is set to measure DC volts (parallel dashed and straight lines), not AC.

cheers, Derek

[jminassi]

Hi Derek,

Thanks for getting back to me.  I am definitely  on DC.  All the other voltages measure correctly.

Here's a picture of my wiring of the octal tube.

Best,

John

[Deke609]

Hmm. Just a shot in the dark. With the amp unplugged and cooled down, what resistance (ohms) do you get between terminals 7 and 3, when touching the probes to the terminal lugs themselves (not the leads of the 3K resistor)? 

[jminassi]

Yikes, what did I do?  Everything was working except for one voltage reading.  I plugged the Crack in and I saw a small flash where the UF4007 rectifiers are.  I shut it down and looked for shorts.   Not finding anything I powered back up.  The 270 ohm 5 watt wire wound resistor between 15U and 21U gets very, very hot.  I shut it down.

Is it possible one or more of the rectifiers failed?  I did initially have the polarity of one of them reversed.

Help!

John

[jminassi]

Deke,

I get 2.971k ohms between lugs 3 and 7 with amp cooled down and unplugged.  That reading is done by just touching the lugs.

John

[Deke609]

Post lots of detailed pics - overhead and from different sides.

If the first 270 ohm 5W resistor is getting super hot, but not the second one, then that seems to point to a short immediately after the first one - my bet would be between T15 and one of (a) chassis, (b) T16, or (c) T14.

A rectifier diode shouldn't be affected by being installed backwards - unless you ran a lot of current through it - but I think the fuse would prevent that.

cheers, Derek

[Paul Birkeland]

Look for leads poking through that should have been trimmed off, but are now touching the chassis.  Trim them off. 

Consider also posting some fresh build photos.