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Old 2nd November 2014, 11:58 PM   #1
diyAudio Member
 
Join Date: Jan 2012
Location: Minneapolis/Saint Paul,MN
Default Amp Build Not Working Correctly....

Hey,

I finally revisited a project I started long ago and finished it up....but, alas, it is not working correctly. It would make a great noise generator, but unfortunately thats not what I am looking for. It also produces only very low volume.

here is a video of what it is doing:

https://www.youtube.com/watch?v=M8H9...ature=youtu.be

and here is the layout I built off of with the edited turret board that I used:

Click the image to open in full size.

and the edited turret layout:
Click the image to open in full size.

any help on where to start troubleshooting would be great.

thanks

-david
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Old 3rd November 2014, 12:42 AM   #2
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Join Date: Mar 2007
Location: Kansas
Without gut shots it's almost impossible to troubleshoot an amp over the internet, especially a homebrew amp where any mistakes would likely be in the actual construction rather than the schematic. That being said, double-check all your solder joints, especially those on the cathodes. You'd be surprised how easy it is to miss one and run around in circles chasing problems. Also, check for any accidental short-circuited signal wires, and if you're using shielded wire, make sure none of the strands of shielding accidentally got soldered/ are touching the core. Some varieties are easy to melt and short out if you're not really careful during construction- ask me how I know
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Old 3rd November 2014, 01:17 AM   #3
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here are a bunch of gut shots, I did find a couple visually suspect solder points around the first preamp tube, Ill reflow those and see if it helps the volume issue. I am unsure if this will help the oscillating though.

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Old 3rd November 2014, 02:47 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by bradberry00 View Post
I finally revisited a project I started long ago and finished it up....but, alas, it is not working correctly.
It also produces only very low volume. any help on where to start troubleshooting would be great.
How about a schematic too?
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Old 3rd November 2014, 03:52 AM   #5
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courtesy of the Orange Amp Field Guide (The Orange Amp Field Guide)

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slight modifications were made also...

Last edited by bradberry00; 3rd November 2014 at 03:56 AM.
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Old 3rd November 2014, 03:55 AM   #6
Poacher is offline Poacher  Canada
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Hey there,

Have you tried reversing the output transformer leads? That might stop the oscillation.
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Old 3rd November 2014, 04:01 AM   #7
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thats first on my list when i get this on the bench. Ill need to sort out the low volume issue as well...
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Old 3rd November 2014, 01:48 PM   #8
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I would check voltages all over the place first, B+ to tubes, cathodes, grids, then check signal with a scope (with a Sig gen providing a known sign wave). Make sure everything needing grounding is grounded.
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Old 3rd November 2014, 07:03 PM   #9
JMFahey is offline JMFahey  Argentina
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Ouch !!!!

Pity because it looks VERY well made

That's not transformer oscillation but poor grounding/layout/wiring paths.

Didn't look joint by joint, too busy now, but in general:

1) you're not the only one .... FWIW all of my first amps oscillated/buzzed/hummed one way or the other, although I followed schematics religiously, wired everything "Hiwatt type" with perfect looking 90 degree bends on solid wire, shielded even the filament wiring, etc. ... now I wire an amp "sloppily" with desk lamp zip cord, do not use shielded wire (or maybe a couple inches, from input jack to first tube/fet/op amp) ... and they never oscillate !!!!!
I mean, unless I do something gross such as forgetting to solder some ground

So in a way "the hand of the cook" , call it experience, is a factor.

Without getting into fine detail comparing pictures and drawn layout, in general if you follow an original layout such as those classically provided with Fenders, or what accompanies, say, Ceriatone kits, that job has already been done ... but on home designed layouts ... unless you are already experienced it may prove tricky.

Worst is that you may have done things the right way ... but a wire passing here instead of there, 1 inch away, may be tha cause of instability.

As a general purpose troubleshooting guide, work backwards, that is start with the power amp, make it stable, you'd really need a scope because sometimes the waveform sounds almost clean, yet part of it is unstable and briefly oscillates, all you know is it sounds funny.

Once that's working, work backwards towards the input.

In that particular amp, I distrust very much the input FAC, that rotary switch at such a sensible point provides a big antenn a to catch junk,what were they thinking?

And anyway that's a **** poor point to place it, it should be between at least after one gain stage (maybe just before the volume contrtol), I never saw your version with it straight at the input, and unbuffered.

Try going from the input jack straight to the first tube grid, without the FAC, using shielded wire.

Also float the input jack, meaning it does not touch the chassis by itself, so ground "comes" through the shield straight from where the first tube cathode resistor is grounded.

Post results.
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Old 22nd November 2014, 05:23 AM   #10
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That much oscillation could certainly be the feedback loop oscillating, perhaps due to the polarity of the primary connections to the output transformer making the feedback connection opposite phase of what it should be, making an oscillator instead. There's no easy way I know of to check, except trying it both ways I guess? You could probably just disconnect the negative feedback wire to start with. Did you check the relative resistances to figure out the sequence of each successive tap in the primary? If so, then there's only 2 possible connection schemes and if it doesn't squeal with the NF open then you'll have to try the other connection scheme, the only one left. I'd definitely start there. Come to think of it I have a rapaired amp I need to fire up and find out whether I got it right or wrong myself LOL.
Fahey is much more experienced than I am...but...did you have any way to know which way the NF tap would have the correct phase? Probably not, so that's one thing you already know could be wrong. And the symptom would be...oscillation. If it still oscillates with the negative feedback disconnected, then you can start pulling tubes to isolate where it comes from.

Last edited by cyclecamper; 22nd November 2014 at 05:31 AM.
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