Lost left channel

[ritchiepurple]

I built my crack and had it working for about a month then the left channel went noticeably quieter than the right channe, i added some solder to some dodgey looking joints and it started working perfectly again until now. The left channel just completely cut out whilst i was listening, i added solder to as many joints as i felt would help but still get zero sound from the left channel. I took voltages and there are a few that are way off (everything checked out when i first built the amp only now are they off)

Terminal 5= 153.4 when it should be 90
terminal 9 = 153.6 when it should be 90
A1 = 154.2 when it should be 90
B6 = 153.5 when i should be 100

Both LED's light up fine. I haven't tried alternative tubes for it as i currently dont own any to swap in to see if that would solve it so any help on troubleshooting would be greatly appreciated. Ive searched the other threads for potential answers but I've had no luck sadly!


Many thanks
Callum

 

[Paul Joppa]

If the LEDs are glowing, then the 22K resistor (T4 to T5 and A1) is probably shorted out. Look for stray wires or excess solder blob size.

 

[Laudanum]

And I'll post this again because, I guess, it's my pet peeve ... Get yourself some spare tubes.  Even if they are New production or cheap NOS.  6AS7 and/or 6080 and 12AU7 types can be had for pretty cheap.  Not the prized tubes but usable spares that will get you by if you have a tube go down.  Im not an expert like a lot of these guys here but from experience with tube gear, you really need to have a couple spares on hand.  Even if it isnt related to your problem now, there will come a time when you will be happy you have some spare tubes.

 

[ritchiepurple]

resistance between t4 and t5 is still 22k ohms so dont think its shorted out and all soldering around that area looks neat. ive got some spare tubes on order so ill see of they do anything when they arrive but they wont be here for a while so any other suggestions?
Thanks very much for the responses and apologies for not checking tubes yet.

Callum

 

[Paul Joppa]

resistance between t4 and t5 is still 22k ohms so dont think its shorted out and all soldering around that area looks neat....

OK, here's my reasoning - maybe it can help you find the problem, by figuring out which assumptions are wrong?

Since the amp worked before, it must have been wired correctly. If the amp is wired correctly, then a glowing LED at A8 indicates the second triode (A6 plate, A7 grid, A8 cathode) is conducting reasonable current (nominally 4mA or so). If there is reasonable current, it can only flow to the plate (A6) through the resistor T4 to T5. If there is reasonable current through that resistor, and it has a 22K resistance, then there will be a voltage drop across that resistor, and the voltage at A6 will be closer to 90 than 150v.

A6 is also connected to the output grid at B1, so the output cathode at B3 will be at least as high a voltage as the grid B1. You measured both at around 150v.

 

[Doc B]

Also, when one measures a resistor in circuit one should measure by touching the probe tips to the terminals that the resistor is soldered to, rather than the resistor leads themselves. That way you are also verifying the continuity of the solder joints at each end of the resistor.

 

[Grainger49]

Also, when one measures a resistor in circuit one should measure by touching the probe tips to the terminals that the resistor is soldered to, rather than the resistor leads themselves. That way you are also verifying the continuity of the solder joints at each end of the resistor.

That is the voice of experience speaking.

 

[Lar]

Also, when one measures a resistor in circuit one should measure by touching the probe tips to the terminals that the resistor is soldered to, rather than the resistor leads themselves. That way you are also verifying the continuity of the solder joints at each end of the resistor.

That is the voice of experience speaking.

I just thought it was Doc. 

 

[ritchiepurple]

Also, when one measures a resistor in circuit one should measure by touching the probe tips to the terminals that the resistor is soldered to, rather than the resistor leads themselves. That way you are also verifying the continuity of the solder joints at each end of the resistor.

thanks for the tip i think i do usually do that but i made sure that was the case this time.

as im just a student and this is my first diy endeavour i dont have any spare resistors so i tried swapping the 22k resistors round to see if it made a difference and i dono if this was the case before but i just noticed that the led attached to A3 is now not lit, it may not have been lit before but i think it was though i can not be certain maybe this would explain all my problems? would i need a new led or maybe this would indicate a problem with something else I've done?

 

[Paul Joppa]

OK, progress! If the LED is not glowing then there is no current through that half of the driver tube. This could be because the tube is bad, or that half of the heater is not working (bad connection at A4 or A5), or the current flow is broken (bad connection T4-T5-A6-A8-LED-ground. You can look at the filaments to see if both sections have glowing heaters, and trace the rest of the circuit my measuring voltages at the above points.

 

[ritchiepurple]

looking at the 12au7 tube the filament closest to the front is light up fairly well but the one at the back is a lot dimmer, i wouldnt say not lit at all but alot dimmer for sure would this suggest bad tube?

Thank you very much for the support it really is appreciated!