champ 5f1 clone -- sudden loss of volume

[cys]

I think I may have hexed my recent champ clone acquisition by posting about it, here: https://ampgarage.com/forum/viewtopic.php?f=8&t=35994

It sounded fantastic for about one hour of playing. Then, after a couple of odd muffled static noises, it lost about 90% of its volume.


Here's what I've cataloged/done so far:

-- The fuse did not blow.

-- Nothing detectable happened to any of the tubes. I have a Hickok tester and tube tracer to verify this. Still, I changed out all the tubes without any effect.

-- The volume control works. It takes the guitar signal from inaudible at 2-3 to weakly audible at 10. The tone coming out at volume 10 actually sounds fine, despite its being just a little bit beyond barely audible in volume.

-- Reflowed all the existing solder joints. I completely redid a few along the way. There are still a few "cluster" joints that I only reflowed and didn't redo, but I did check continuity in those cases. Per the link to the other thread above, the gut shot shows the build is creative but could be cleaner. I looked past the solder work and just purchased the amp for it's sound, which didn't disappoint for the 60 minutes I got to play it before it mostly died. No change.

-- Checked all the resistor values. I checked all the capacitor values. Found a coupling cap with a high esr for V1a and replaced that. I got nervous about the 16uf filter cap for B+ and swapped that out for a 22uf cap I had on hand. No change.

-- Checked the voltages on all the tubes sockets. They seem fine.

-- Checked continuity on power tube pins to socket pins by loosening the tube in the socket enough to get a gap I could get a probe tip into and all were fine (power tube is a focus per below info).

-- 6V6 plate current is 32mA.

-- Looked at the signal path with an oscilloscope using a 1KHz signal. The signal looks as it should to my eye anyway all the way to the 6V6 grid. The signal looks fine at the 6V6 grid, and after that it "vanishes." The only notable feature on the plate of the 6V6 is the sawtooth of the rectified DC power (per above I replaced the main filter cap just to cover my bases there).

-- I desoldered the output transformer leads from the amp circuit for the following measurements. The output transformer primary resistance is 18.0 ohms. The OT secondary resistance is 0.84 ohms. Neither the primary nor the secondary shows continuity to ground. The OT turns ratio is found to be 40. This was determined by shorting the primary leads to the heater supply and measuring the output across the secondary.


I tend to think the issue is the OT despite the test results. I think the insulation has only partially been worn through.

Any input appreciated. Thanks.

.
edit: fixed a typo

[nworbetan]

At a glance I can't see whether there's negative feedback on the V1B cathode, but it's there on the 5F1 schematic.

Did you accidentally reverse the OT polarity and turn the NFB into positive feedback?

On second thought, that idea doesn't seem consistent with the amp sounding great for an hour before it suddenly stopped.

[cys]

Everything I did to the inside of the amp posted above was only after the volume spontaneously diminished in an effort to find out what happened and fix it. There is a negative feed back resistor, and I was careful not to swap the leads during testing to find out what caused the volume issue. Thanks.

[nworbetan]

The second lowest hanging fruit that I don't see a mention of is whether you've tried a different speaker yet?

[cys]

Yes, I've tried a different speaker. I've also tried different guitars, and a different input and output cable. I also replaced the output jack with a new one. Thanks.

[nworbetan]

My next thought is "-- Checked the voltages on all the tubes sockets. They seem fine."

Can you be more numeric about how fine they are?

[cys]

Here's what V1a and V1b look like:

V1a:

IMG_1679.jpg

.
V1b at low volume:

IMG_1684.jpg

.
edit: reposted the first trace as it some letters superimposed on it somehow in the posting process.

[nworbetan]

And the quiescent dc voltages on the pins of the power tube?

I'm specifically thinking of the screen grid, which on a lot of amps is within a few volts of the plate, but on a 5F1 it should be ~50Vdc (ballpark number) below the plate.

But including all the other dc voltages on the other elements will paint a more complete picture of what is or isn't happening as intended.

[cys]

Pin voltages "in process"... just wanted to double check a few values before posting them, which I have done now. I think the 12AX7 ones are moot based on the scope traces, but I'll posted them with the 6V6 data as well.

12Ax7
1) 125V
2)
3) 1.19V
4) 3.3V
5) 3.3V
6) 187V
7)
8. 1.64V
9) 3.3V


6V6
1)
2) 3.3V
3) 398V
4) 345V
5)
6)
7) 3.3V
8. 26.1V

[nworbetan]

Is there no voltage on the 6V6 cathode?

[cys]

I just noticed that. In the process of doubling checking that one.

[cys]

6V6 cathode is 26.1V (adding it in above -- must have hit some flux first time through).

[nworbetan]

That's probably as much process of elimination I'll be able to help with.

It looks like the OT might be up next for troubleshooting? I've read about using a neon bulb to check for shorted turns that don't always show up when testing with a meter. Hopefully someone else will have better input to go from here.

[cys]

Thanks for pitching in on this. I purchased a new OT a few days ago, but it doesn't show until Saturday, or so. I figure on further testing the old OT by swapping in a new one. In the meantime if someone sees another path to getting this amp back up and running, please chime in.

[nworbetan]

Before I give up completely though, 32ma through the cathode resistor at 26.1 volts, by ohm's law, is a 815 ohm cathode resistor, is that what the two Xicon ceramic blocks are, in series?

If so, it's a little bit on the cold biased side, but doesn't seem catastrophic.