5e3 Build Issues. Hum and Clipping/popping

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5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Sat May 02, 2015 5:21 pm

Ok, I have tinkered with tube amps and have a decent understanding of them for a novice (thanks to this forum along the way), but this amp build is about to drive me up the wall.

The Problem:
There is a considerable amount of hum early and the amp seems to jump up in volume way too fast then goes into an almost clipping sound at about 4 on either volume. The tone control affects the pitch of the popping and turning either volume effects the frequency of the popping. With each pop v4 shows a small blue flash towards the base of the tube. If i pull v2 the popping stops, if i pull v1 the popping frequency speeds up and goes up in pitch. Ive swapped the power tubes back and forth and it still flashes on v4.

Info: Mojotone 5e3 kit with multi-impedance OT and selector switch added.
I have checked and double checked my wiring and compared it to several different diagrams to make sure there were no errors from Mojotone. Everything seems to check out, but a second set of eyes never hurts.
I have moved grounding wires multiple different places and it has had no effect on my problem so ignore all the stray looking wires, as I am at my wits end trying to figure this out and am basically moving stuff around hoping to hit the magic combo.

One thing i am concerned about is the wiring of the OT as it is hard to find good info on a 5e3 with the selector but here is a link to the OT in question http://www.mojotone.com/transformers/Fender/Output/MT-FENDER-MOJO768sp.pdf. Should i have a ground wire the the output jacks?

here is a link to some of the build pics as it sits. Again ignore the untidiness i have been trying a lot of different stuff.
http://s79.photobucket.com/user/twiceta ... e3%20Build

Here is a pic to show how i currently have the grounding. the earth from the 3 prong and the red/yellow are on one bolt of the PT, the B+ ground and artificial CT go to another bolt on the PT.
Image

Thanks for any help in advance and for taking the time to read my troubles. I hope it is something simple I am over looking.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby PeterS on Sat May 02, 2015 7:37 pm

Which one is V4?

The popping sounds like you might have something charging up then discharging, like there's a leaky coupling cap or something like that. Try checking the voltage on the volume controls (should be zero on the side that connects to tube grids)...
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Sun May 03, 2015 2:38 pm

v4 is the power tube closest to the rectifier tube.

I'm not showing any voltage, AC or DC, at either of the volumes or tone pot. I did however find that if i put my voltmeter on pin 7, or any where the lead goes including the leg of one of the coupling caps the noise stops until the volume is pushed more. Not sure if that is relivent.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Sun May 03, 2015 4:07 pm

well this morning out of shear frustration i replaced all the coupling caps with mallorys of equal value and still the same problem. :POd:

Tell me something could this problem be at a filter cap? I don't have any on hand but can order them just in case. Any way to test filter caps?
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby PeterS on Sun May 03, 2015 4:30 pm

Could be a filter cap but i wouldn't say that was the most likely culprit.

Can you measure all 5 of the cathode voltages and report them here?

Have you tried swapping the 12ax7? With the split-load phase inverter circuit the cathode is at a high voltage and the popping might indicate you have an intermittent short between cathode and heater.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby Unit_1 on Sun May 03, 2015 4:58 pm

PeterS wrote:Could be a filter cap but i wouldn't say that was the most likely culprit.

Can you measure all 5 of the cathode voltages and report them here?

Have you tried swapping the 12ax7? With the split-load phase inverter circuit the cathode is at a high voltage and the popping might indicate you have an intermittent short between cathode and heater.


in which case it might be as easy as just spreading the tabs on the socket to move them further away from each other

I really hate it when you plug in a new build and there's a problem!!! like you're all ready to rock and roll after all that work and...... WHOOPS! back to the bench..... :POd:
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby PeterS on Sun May 03, 2015 7:55 pm

spreading the tabs on the socket


Good point. Try looking at the chassis in the dark, you may see tiny sparks...
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Sun May 03, 2015 8:50 pm

ok my cathode readings are as follows. 6v6:19vdc 12ax7: pin 3, 1.2vdc, pin 8, 45vdc 12ay7: pin 3 and 8 (conncted) 1.7vdc.

I had the amp clipping in the dark and all i can see flashing is the power tubes.
I also spread the tabs out on the preamp tubes with no luck.

I did find that the voltages of pin 8 on the 12ax7 drops when the clipping starts then returns to normal when the volume turned down below the clipping level.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby PeterS on Sun May 03, 2015 9:49 pm

Hmm, those voltages are all good...

Does it make the popping noise when there's no signal? Can you meter each cathode and see which ones make the meter move when a pop occurs?

You might need an analogue meter...
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Sun May 03, 2015 10:14 pm

yes this clipping is with nothing plugged into the amps input but does the same thing with a guitar plugged in.

The only voltage drop i saw while it was popping was on pin 8 of the 12ax7 it went from 42vdc down to about 6vdc.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby CHall1525 on Tue May 12, 2015 10:21 pm

Ok, got back to poking around a little. I did find that B+2 would drop voltage and B+3 would jump up in voltage when the amp was popping. Not sure what to think of that. I have been reflowing joints and rechecking my wiring since last post and have had no change.

Any other ideas? could this be a PT or OT problem, or is it strictly in the wiring or failed component.
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Re: 5e3 Build Issues. Hum and Clipping/popping

Postby Unit_1 on Wed May 13, 2015 1:02 am

It does kinda sound like a leaky cap. I had one in a Hiwatt DR504 clone I made and it *really* went whacko

started off with a slow popping, then faster, then popcorn then like a klaxon and then one tube red plated and was ruined before I could shut it off. sure enough, testing showed 475v of DC voltage *on the wrong side* of the cap!

and it was a brand new TAD EL34 too.... <snif snif> fortunately it wasn't expensive. I would have freaked out if it was something NOS...
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