LED A3 to Center Pin not lit

[egildernew]

I built the crack several months ago. Everything seemed to operate fine. I could detect a slight channel imbalance. I replaced the stock potentimeter with a valab 23 stepped attenuator. I also used a 6F8G to 12AU7 tube adapter to run a Tung-Sol USA 6F8G/VT-99.

I noticed the A3 to Centerpin LED was not lighting. I do not know when it stopped working. I just got a speed ball upgrade delivered and would love to install but I am concerned about tracking down the cause of the LED failure. I reinstalled the stock potentiometer and tubes. The A3 LED is still dead.

My current voltages
Terminal: Measured (Expected)
1: 83.5 (50-100)
2: 176.1 (170)
3: 0 (.0001)
4: 176.1 (170)
5: 61.65 (50-100)
6: 0 (0)
7: 109.9 (90-115)
8: 0 (.001)
9: 96.5 (90-115)
10: 0 (0)

My current resistances
Terminal: Resistance
3: 0.1
6: 0.1
7: 3.043
8: 0
9: 2.976
10: 0.1
12: 0.1
14: 0
20: 0
22: 0
B3: 3.043
B6: 2.974
Right RCA: 93
Left RCA: 97.3

Any ideas?
Thank you guys for your time!

 

[Paul Birkeland]

Can you carefully measure the DC voltage at A3?

 

[egildernew]

.937V

 

[Paul Birkeland]

I'd be interested in seeing photos of the wiring around the 9 pin socket if possible. 

 

[egildernew]

First timer

 

[Paul Birkeland]

What DC voltage do you get at A2?

 

[egildernew]

0V
I'm not running anything through it right now.

 

[Paul Birkeland]

I suppose it's also worth it to check A1.

62V plate and 0.9V cathode is about 2mA of plate current on the 12AU7 triode curves, but you're drawing 5mA.

Did replacing the pot end up curing your channel imbalance?  Were the voltages looking OK when you built the kit? (And did both socket LEDs illuminate?)

 

[egildernew]

A1 is 0V

Replacing the pot helped with the channel imbalance.

When it was built both LEDs lit. All the voltages checked out.

 

[Paul Birkeland]

Try pulling the 6080 and powering up the amp with just the 12AU7 to see if that changes things much (just measure voltages on terminals 1-5 and note which socket LEDs light).

You are correct that installing the Speedball isn't going to resolve whatever is going on here.

 

[egildernew]

Voltages with 6080 Pulled
1: 98.4
2: 228.4
3: 0
4: 228.3
5: 75.3
A1: 74.8
A2: 0
A3 1.123
A4 0

Voltages with 6080 in
1: 83.5
2: 176.1
3: 0
4: 176.1
5: 75.3
A1: 61.57
A2: 0
A3: .927
A4: 0

A3 LED still not lit, other LED lit
I am happy with how it sounds. I am just worried something might burn up.

 

[Doc B]

Pull the 12AU7 tube out of the socket. Set your meter to continuity test, or to a very low, like 200ohms, resistance test range.

Measure the resistance of the pins on the 12AU7 tube. Measure from pin 9 to pin 4. It should be very low, just a few ohms.

Then measure the resistance from pin 9 to pin 5. Again the resistance should be very low.

This will verify that the heater in both halves of the dual triode tube is good.

 

[egildernew]

12AU7 Continuity test
Pin 9 to 4 : 5.7 ohm
Pin 9 to 5:  5.7 ohm

I can see both heaters light up when using the 12AU7. I have an ECC82 and when it is run the A3 led remains dark.

Evan

 

[Paul Birkeland]

So you have multiple 12AU7 tubes that exhibit this same issue?

Does your meter have a diode test setting?

 

[egildernew]

I have two 12AU7 tubes, an ECC82, and a 6F8G to 12AU7 tube. They all result in a unlit A3 LED.

I have a Fluke 117. I am not home but I think it has a diode test setting.

 

[Paul Birkeland]

You should be able to test the red diodes with your meter on the diode setting (With the amp off).  The meter should give a voltage reading with the probes one way and nothing with the probes the other way.  Let us know how they measure.

-PB

 

[egildernew]

The diode shows voltage both ways!
.240 DC V negative probe to marked side of LED
.623 DC V positive probe to marked side of LED

It was my understanding electrons should only flow one direction through a diode. Is my problem as simple as a broken LED?

Evan

 

[Paul Birkeland]

Although I rarely say this, I would recommend replacing them both.  If you have an adjustable soldering iron, turn it up all the way when you solder the new LEDs in.  If you don't have an adjustable iron, then get a 45+ watt handheld iron.  The LEDs are OK being exposed to the ultra high heat, provided it isn't applied for all that much time.  It's much worse to use low heat for a much longer period of time.

 

[egildernew]

PB,

I was caught up with work and school but I finally installed the replacement LEDs. This solved the problem. The crack is humming. I installed the Speedball. Its working great!. I love this amp.

Evan

 

[Paul Birkeland]

The crack is humming.

Hopefully figuratively!