Help With Kit, only one LED lights on Tube

[Bruce65]

Hello Audio geeks,

Old Guy built a kit for Son, Found a few solder joints missing solder, Plugged in, only one channel. Lifted Headphone jack one 16th of an inch out, and hear the same volume in both ears. Looking at unit's guts see only one LED lit on tube socket.

Resistance Checks, but Voltages are: 1-141vdc, 2-160vdc, 4-160vdc, 5-79vdc, 6-varies 0-12vdc, 7-141vdc, 9-100vdc,,,any suggestions?

[Paul Birkeland]

I'm assuming this is a Crack kit (I can move the post elsewhere if it's not).

Do both halves of the 12AU7 glow? 

[Bruce65]

I'm sorry, it is a Crack Kit, I have the speedball option, but do not want to install it before the regular kit works. yes I believe both elements on the 12au7 are lit, proving the element heaters?? gee I should know this stuff, actually old enough to study tube circuits. I have a 12ax7 that I have spare/new for my Vox acoustic amp, interchangeable but higher gain.
The thing sounds awesome to me but I have to lift the headphone jack about 1/16th inch or less to hear both. Son thinks that puts it in Mono, but gain is not less when I do this. Thanks for the help! Bruce

[Paul Birkeland]

The thing sounds awesome to me but I have to lift the headphone jack about 1/16th inch or less to hear both. Son thinks that puts it in Mono, but gain is not less when I do this. Thanks for the help! Bruce

This does indeed put it in mono.  It shorts the tip and ring together. 

I'm guessing one channel is weak or not working?

[Bruce65]

Correct,

Could it be the Preamp Tube? I can try my 12ax7 with the Volume turned down first. Are both biasing led's supposed to lite? One does not. could that be the problem, and is there an led I can get locally to replace the one that is not lit?

[Paul Birkeland]

It's somewhat uncommon that the LED is actually bad, but builders tend to see that it's not lighting, and they rewet the solder joints to the point where the LED does fail.

I would start by resoldering the joints on either side of each 22K resistor (terminals 1/2 and 4/5), as well as the wire that goes from 1 to A6.

If your voltmeter has a diode checker, you can start by checking the diode that lights up, then check the diode that isn't lighting up.

You could also put the 12AX7 into the socket (remove the 6080 though).  I think the LED will light, but the voltages and terminals 1 and 5 will be very high (hence my recommendation to remove the 6080).

[Bruce65]

Thank you so much,,,I will leave it here as I will not be home to try your suggestions until 12Midnight EST. Thanks again, and I will get back when I have something more to add. Are LED's supposed to be lit or not? One post says they are not supposed to be lit.

Bruce

[Paul Birkeland]

When you use the diode testing function of your meter, you'll get low impedance when you have the red/black probes in one direction, and high impedance in the other.  When you are measuring the low impedance, the diode may or may not light depending on the meter.  (When measuring the LED's on the Speedball boards, getting them to light up with a meter becomes a bit tougher)

[Bruce65]

So are Both LED's on the 12AU7 lit or not?

I would think that if one is lit and not the other that is a sure sign that one Channel is Broken.

Bruce

[Paul Birkeland]

When the Crack is powered on and running, both LED's will light up.