Need some troubleshooting help on tube amp

[JohnW63]

I have a Carvin Vintage 16 guitar amp and I could use some help figuring out what has suddenly failed. Is this the place to start this conversation ?

 

[vinylkid58]

Is this the place to start this conversation ?

Yes. If you could be a little more specific about the failure, I'm sure someone will come by to help you out.

jeff

 

[rayma]

Check the power supply voltage at C29 first, if the amp turns on.
https://www.thetubestore.com/lib/thetubestore/schematics/Carvin/Carvin-Vintage16-Schematic.pdf

 

[JohnW63]

I've been trying to get help in other places and gathering data.



The amp showed no symptoms before it failed. I don't gig at all.It's just a practice amp for me. It goes with me for lessons. So, it doesn't get banged around. I've upgraded to some tubes a few years past, from TheTubeShop.


The symptom was that I turned it on and it started humming and got louder, the a bit of a pop and then much louder. It would do this every time I turned it on. After watching a video on fixing these amps I resoldered the tube socket pins. It does not exhibit the loud hum or pop, but it does hum and it didn't do that before. It does not amplify much at all and the sound is distorted.



Here are some measurements:


Red lines from power transformer : 290vac
Green lines 6.13vac

At R36 = 399 vdc
At R35 = 355 vdc
At R34 = 320 vdc
D4/D5 to D6/D7 = cold 270vac, warm = 260vac

V5
( volts to chassis ground )
Pin 1 = 0vdc
Pin 2 = 0.004vdc
Pin 3 = 0vdc
Pin 4 = 2.6vac
Pin 5 = 2.58vac
Pin 6 = 0vdc
Pin 7 = 0vdc
Pin 8 = 0vdc
Pin 9 = 2.8vac
V4
( Volts to chassis ground )
Pin 1 = 0vdc
Pin 2 = 0.94vdc
Pin 3 = 0vdc
Pin 4 = 2.56vac
Pin 5 = 2.56vac
Pin 6 = 0vdc
Pin 7 = 0vdc
Pin 8 = 0vdc
Pin 9 = 0.12vac


A link to the schematic:
https://www.thetubestore.com/lib/thetubestore/schematics/Carvin/Carvin-Vintage16-Schematic.pdf

 

[JohnW63]

C29 is a big cap that covers it feet. I'll have to remove the board and flip it over to check that. However, if you look above I do list the voltages at resistors near it and down the line of test points.

 

[rayma]

There's no HV getting to pin 7 of the output tubes. Or the screens, pin 9.
Check connections between the output tubes and the output transformer.
The OT red/blue/brn leads all should have 360VDC wrt ground.

Check switch S1A and S1B. If bad or in the wrong position, could be a problem.

 

[JohnW63]

Sorry, I missed a proper measurement.


I have 390vdc on V5 pin 9 and 377 vdc on V4 at pin 9 ,

 

[JohnW63]

Red line = 380vdc
Brown = 377 vdc
Blue = 377 vdc


I sprayed the switch for the 5w or 15w with DeOxit electronics cleaner and moved it up and down a bit to make sure it was clean. I did a quick ohm check on the switch and is seems to check out. The center pin gets switched to one end or the other as I slide the switch. Works that way on both sides of the switch.

 

[JohnW63]

Measured the plate voltages on the preamp 12AX7 tubes


Testing voltage on the 12AX7 tubes. Schematic says 230V. I'm not getting that.

V3:
Pin 1 = 103 vdc
Pin 6 = 203 vdc

V2:
Pin 1 = 156 vdc
Pin 6 = 209 vdc

V1:
Pin 1 = 170 vdc
Pin 6 = 149 vdc

 

[JohnW63]

Grid Voltage on the output EL84s:



V5:
pin 2 = 1.04 vdc

V4:
pin 2 = 0.63 vdc