Champ Micro rebuild

Hyakuya

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Hi all

I decided a while ago to rebuild my Champ Micro EF80 into a smaller chassis and change it from a desktop type layout to the normal cabinet mounted arrangement.

So, all done, checked over, started up and working but from being whisper quiet I now have a loud hum through the speaker and seeking ideas on possible avenues to explore.... (maybe I left the words behind in the old chassis.... 🤣 )

Info so far (all without the guitar plugged in but amp connected to speaker):

Voltages:
voltages.png


The EZ81 has the HV from the PT going to pins 6 and 8 with diodes from there to 1 and 7, hence the 4 VAC readings.

Turing the volume has no impact on the hum at any point

EZ81 + EF80 + 12AY7 = hum
EZ81 + EF80 = hum
EZ81 = no hum

Here's the gut shot:

IMG_20220501_142327.jpg
You may notice that the filament CT isn't connected to anyting, (I picked that up and it is now.. doh!)

Switch on the right is the 3 way boost (disconnected in this picture)
Switch on the left is feedback (5F1 / off / JTM45)

PT:

IMG_20220501_172613.jpg

Output & feedback switch:

IMG_20220501_172033.jpg

One thing I notice while taking voltage readings is that with the com connected to ground the speaker crackles when I touch the capacitor or resistor connections to pin 1 of the 12AY7 or either the input or tube side of the 3.3k grid stopper (input jack or pin 2 of the 12AY7). I suspect that's to be expected but maybe not.

Any ideas ?

Thanks in advance....
 

andrewRneumann

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More curiosity than anything… what arrangement allows you to use a valve rectifier and a PT with no HT center tap? You have a virtual center tap with some diodes?
 

Len058

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Not an expert but the EF80 is quite close to the rectifier. How close is the OT to the PT and how are they oriented, 90 degrees?
 

King Fan

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First thing — is it 50 or 100Hz? Or something else? This info basically sends us on one of two much shorter hum hunts. Note 50 Hz is well below open E 6th string, while 100 is around 3rd fret — G.

Second — the nominal layout and all components are the same? If not, what changed? If so, we almost certainly have a wiring problem, right? Worth measuring resistance to ground from all grids and cathodes to compare to expected values from schematic.

Your smart volume and tube removal tests will help localize it — but the hum frequency is now even more useful.
 

Hyakuya

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Not an expert but the EF80 is quite close to the rectifier. How close is the OT to the PT and how are they oriented, 90 degrees?
That is what worries me most at the moment, they are quite close and much closer than they were before.

OT and PT are at 90 degrees and not too close, I think. I'll put a picture up when I have time and try to work out what frequency the hum is.
 

Len058

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Looking at the pic's it looks like they're not at 90 degrees of each other. Can you rotate the OT 90 degrees?

And does a guitar signal come through and does the volume change the guitar volume. To be clear, I'm just trying to narrow it down. Always check the easiest stuff first.
 

Hyakuya

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First thing — is it 50 or 100Hz? Or something else? This info basically sends us on one of two much shorter hum hunts. Note 50 Hz is well below open E 6th string, while 100 is around 3rd fret — G.

Second — the nominal layout and all components are the same? If not, what changed? If so, we almost certainly have a wiring problem, right? Worth measuring resistance to ground from all grids and cathodes to compare to expected values from schematic.

Your smart volume and tube removal tests will help localize it — but the hum frequency is now even more useful.
Going by your guide, I think it's 100 Hz...

All grid and cathode resistance to ground measure good. I did a general continuity test from everything connected to ground and all good there also. Chopstick test didn't bring anything up either.

The real changes from old to new are the boost and NFB switches moving to the sides and the tubes going from top to back... I'll find a picture and post it.
 
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Hyakuya

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Old layout...

2554.jpg

The switches are both together at the front and the EZ81 was in front of the PT. EF80 and 12AY7 at the rear with the output jacks between them.
 

Hyakuya

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Question regarding the hybrid bridge HV. If you look at the picture above you can see the diodes connected to the 275V and 0V taps on the PT which then connect to the negative end of the first filter cap.

This works fine but while I was continuity testing I find that there is no continuity over this connection. So MM connected to negtive of first filter cap and 0V PT tap has no continuity.

It occurs to me that this may be because of the diodes but I wanted to check...

This is the section from Valve Wizard detailing the arrangement.
1651508860335.png
 

Hyakuya

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Looking at the pic's it looks like they're not at 90 degrees of each other. Can you rotate the OT 90 degrees?

And does a guitar signal come through and does the volume change the guitar volume. To be clear, I'm just trying to narrow it down. Always check the easiest stuff first.
This is the same arrangement I had before, though they are slightly closer, and that was whisper quiet....

Before:
2552.jpg

To move the OT I have to remove the tag board which you can imagine I'm fairly reluctant to do.

Guitar signal comes through ok and volume works as expected.
 

andrewRneumann

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Question regarding the hybrid bridge HV. If you look at the picture above you can see the diodes connected to the 275V and 0V taps on the PT which then connect to the negative end of the first filter cap.

This works fine but while I was continuity testing I find that there is no continuity over this connection. So MM connected to negtive of first filter cap and 0V PT tap has no continuity.

It occurs to me that this may be because of the diodes but I wanted to check...

This is the section from Valve Wizard detailing the arrangement.
View attachment 979287

You are probably correct. I can’t think of a way you could have miswired the circuit and not a) short the PT b) stop current flowing completely c) stop current on one side of the rectifier. Since your voltages are even both sides at 151vac, and your DC looks right, it leads me to believe the hy-bridge is working just fine. But yeah triple check everything.
 

Len058

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I'm sorry but if I look at the pic's I'm sure the orientation of the transformers changed. Look at the British flag. The long side of the flag was at 90 degrees to the long side of the OT. Now it's parallel and closer.

But I think there's help from a more knowledgeable person, so I'll follow this in silence from now on.
 

Hyakuya

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I'm sorry but if I look at the pic's I'm sure the orientation of the transformers changed. Look at the British flag. The long side of the flag was at 90 degrees to the long side of the OT. Now it's parallel and closer.

But I think there's help from a more knowledgeable person, so I'll follow this in silence from now on.
You're right, I hadn't noticed... I've turned the PT 90 degrees in the new build...
 




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