Loss of Power and Volume

[Led Hed]

Hi, I'm new to the forum, glad to be aboard. I have a head version of the 18W that I built from a TubeDepot kit. The amp has worked perfectly for several years but recently I'm experiencing lack of power and volume, at first it was intermittent but now a constant condition. I tried replacing the power tubes, and preamp tubes one at a time, but no change. I haven't tried replacing the EZ81 rectifier but my understanding is that rectifier tubes rarely fail. Possibly a bad power or output transformer? I'm pretty good at working inside amps but my level of knowledge on how to proceed with troubleshooting is somewhat lacking. Any thoughts?

[zaphod_phil]

It's even less likely to be a transformer issue. Check if either of them is running really hot. Check the amp's B+ voltage. If it's low, then your rectifier tube may be worn out.

[Led Hed]

The B+ voltage is right around 360V, which is pretty much exactly what it was when I first built and tested the amp. Any other thoughts or ideas of things I can check?

[zaphod_phil]

Did you check if either of the transformers is overheating? Can you take voltage readings at all the tube pins and compare against the values shown in the voltage chart? That often helps pinpoint the cause of the issue.

[Led Hed]

All tubes, pre and power both? And where do I find the voltage chart?

Thanks.

[JMPGuitars]

All tubes, pre and power both? And where do I find the voltage chart?

Thanks.

Pick the one that matches your specific build: viewtopic.php?f=25&t=24418

Also keep in mind that sometimes preamp/pi tubes get switched, so pay attention to which is which in your build.

Thanks,
Josh

[lavrgs]

Check for loose connections. I have an amp I built three years ago start to sound weak - I thought I would have to replace the tubes but as took voltage measurements and poked around I found a wire that was loose - which came off as I chopsticked the area. The voltage measurement give great insight into what is going on.

Good luck!

[Led Hed]

The voltage test results are below.
V1:
P1- 166
P3- 1.3
P6- 166
P8- 1.3

V2:
P1- 246
P2- 54
P3- 80
P6- 237
P7- 53
P8- 80

V3:
P1- 263
P3- 3.2
P6- 282
P8- 3.2

V4:
P2- 30
P3- 12
P7- 342
P9- 349

V5:
P2- 16
P3- 12
P7- 344
P9- 348

V6:
P1- 20
P3- 353
P7- 20

Any thoughts? Some of these look way out of spec, but as I mentioned the amp worked and sounded great for several years before recently starting to have issues...

[JMPGuitars]

Check for loose connections. I have an amp I built three years ago start to sound weak - I thought I would have to replace the tubes but as took voltage measurements and poked around I found a wire that was loose - which came off as I chopsticked the area. The voltage measurement give great insight into what is going on.

Excellent advice. And also why it's so important to make sure solder connections also have a good mechanical connection.

Any thoughts? Some of these look way out of spec, but as I mentioned the amp worked and sounded great for several years before recently starting to have issues...

Which version did you build? Is it the classic with the tremolo? If so, this would be the correct doc: files/JMPGuitars_18_Watt_Classic_Voltage_Test_Chart.pdf - fill out all values.

V1 and V2 look okay, but a little high.

V3 doesn't make any sense to me. Which build version did you make?

I'm assuming P2 on V4 and V5 is mV, not V. Otherwise V4 and V5 look messed up. Pin 7 should be 15 to 20V higher than Pin 9.

V6 P1 and P7 are AC Volts, and it looks like you measured them in DC? Try again with AC mode.

Thanks,
Josh

[lavrgs]

Is this a tremolo version? My V3 voltages were all over the map until I turned the tremolo off- all knobs to off.