No sound from left channel after working without issues earlier in the day.

[Colonl_Charisma]

Hi all,

Seems as if I have a new problem haha. My amp had been working great since installing the 33K and 75K resistors a few days ago. This morning and into the early afternoon my amp was working without issue and sounding great. I turned it off because I left my house for a few hours. When I got back home I turned the amp on and let it warm for about 15 min, but now when I play music I am only getting sound from the right channel.

Nothing odd occurred when or after I turned it on. The tubes are burning evenly, LEDs are all lit, and I checked my voltages and resistances and they all checked out. I even went back and reflowed my joints on the volume pot, but that did not seem to help. I can confirm that the 33K and 75K resistors are where they are supposed to be on the pot.

 

[Colonl_Charisma]

I should also note that my amp was working fine prior to the volume pot modification. It was just loud haha.

 

[Deke609]

You should probably redo the resistance and voltage checks just to rule out a whole bunch of stuff.

If that all checks out, I'd trace the signal wiring from the rca jacks to the pot and on to (a) ground and (b) the first tube (12AU7), and reflow every connection along that chain.

I'd also scrutinize the wiring at the headphone jack.

And post pics.

cheers and good luck, Derek

 

[Paul Birkeland]

You could also post photos and we might be able to spot something.

It could be an issue with the additional resistors at the pot, or there could be something else that's just now causing problems.

 

[Colonl_Charisma]

Thanks guys! I’ll post pics along with my resistance and voltage measurements a little later today.

 

[Colonl_Charisma]

Resistance + Voltage check:

T1= *, 83.5 V
T2= *, 182.1 V
T3= 0 ohms, 0.5 V
T4= *, 182.2 V
T5= *, 81.9 V
T6= 0 ohms, 0.4 V
T7= 2.93K ohms, 109 V
T8= 0 ohms, 0.5 V
T9= 2.95K ohms, 108.6 V
T10= 0 ohms, 0.4 V
T12= 0 ohms,
T13= *,
T14= 0 ohms,
T20= 0 ohms
T22= 0 ohms
B3= 2.92K ohms,
B6= 2.95K ohms,
RCA Jack L= 107.3K ohms,
RCA Jack R= 99K ohms
Ground Tab = 0 ohms

 

[Paul Birkeland]

How about the 9 pin socket?

If you had said you had no sound from the right channel, I would say that the insulation is melted on the red wire going from the volume pot to the 9 pin socket, and it might be shorting out to where the black wires connect to the volume pot, but I don't see any issues there for the left channel.

 

[Colonl_Charisma]

I'm having some issue uploading photos. I modified my previous message with my 9 pin socket pics.

 

[Paul Birkeland]

I would temporarily solder in a jumper between the upper and lower middle lugs of the volume pot, then let us know if that changes the behavior of the amp.

If it doesn't, remove that jumper and install it between 6U and 10U, then let us know if that changes anything.

 

[Colonl_Charisma]

Will do! What kind of wire should use for the jumper?

 

[Paul Birkeland]

If you have anything leftover from the kit, that will work fine.

 

[Colonl_Charisma]

I definitely do. Should it be the bare buss wire or can I use any of the insulated wire?

 

[Paul Birkeland]

I would use insulated wire.  You really don't want the jumper going between 6 and 10 to touch anything other than those two terminals.

 

[Colonl_Charisma]

Cool! Just installed the jumper between the U and L middle lugs of the volume pot. I’m going to do my resistance and voltage checks then let you know what happens.

 

[Paul Birkeland]

No, don't do the resistance and voltage checks, listen to the amp to see if you have both channels.

 

[Colonl_Charisma]

Sorry, I did do the res and vol checks lol. It definitely worked! Both channels are working, and the amp sounds amazing again.

 

[Paul Birkeland]

Sorry, I did do the res and vol checks lol. It definitely worked! Both channels are working, and the amp sounds amazing again.

No, that's not to fix the amp, it just makes the amp into a mono amp.

Do both channels work with just the jumper on the volume pot?  (this needs to be on without the jumper between 6 and 10)

 

[Colonl_Charisma]

Gotcha! Yes, both channels work with the jumper on the U and L middle lugs of the volume pot. I have not installed any other jumper cables.

 

[Paul Birkeland]

That tells you that the problem is between the RCA jacks and the volume pot, which eliminates 90% of the amp.

It is actually possible to destroy the pot if lots of solder gets into it (and the flux with it).  You can remove that jumper, then measure...

What's the DC resistance between the outer top lugs of the pot?

What's the DC resistance between center pin and shell of the left RCA jack?

What's the DC resistance between the left RCA jack center pin and the upper outer lug on the volume pot?

 

[Colonl_Charisma]

I hope that I have not destroyed the volume pot haha.

Resistance of left upper lug of vol pot = 32.6 ohms
Resistance of right upper lug of vol pot = 00.1 ohms

Resistance between center pin and shell of left RCA jack = 107.2 ohms (I think I measured this correctly)

Resistance of left RCA jack center pin and upper outer lug of vol pot = 107.2 ohms (again, I believe I measured the correct thing)