Solved - Origin 20 mod hum issue

lysander

Wondering if the amp wizards can help:
I modded my origin 20 according to this:
https://headfirstamplification.com/amp-mods

and am getting an issue with high frequency oscillations which are audible not only through the speaker / load but also acoustically.
It seems to only happen with the gain and master high, the boost weirdly makes it slightly better and the tilt shifts the oscillation frequency.
When scaling down the power it mostly disappears.
 

I went through everything a couple of times and can’t see any obvious issues.
Any suggestions on how to proceed ? I guess the most obvious place to start is the modified feedback with fixed depth, but it will probably be tricky to debug.

ICBM

Put the NFB back to stock and see if it goes away.

lysander

Thanks I will try that.

lysander

So I took out the [220k // 4n7] fixed resonance that was in series with the 100k NFB resistor and just left the 100k as per stock circuit and it seems to be better, and there’s enough bass as it is so I don’t really need it.
However I think the main issue is actually hum, very very loud hum that gets distorted.
I was testing with the headcab off so there wasn’t a full faraday cage but I doubt it’s the issue.

Some other oddity is that the pull boost actually appears to function backward ( ie pulled is off ).
I had replaced a relay that I had damaged with the iron while desoldering so must be related but I used the exact same part so it’s surprising.
I’m scratching my head a bit about the hum however.

The modded sound is otherwise awesome so I hope I manage to sort it out, any suggestions much appreciated !
There are pics of the before and after boards on the link, mine looks exactly the same.

lysander

Tried swapping all the tubes to no avail.

lysander

And it’s dead quiet with V1 out so at least this locates the issue a bit. 

lysander

I’ve just realised that the V2a ( actually V2b in this case due to the
repurposing of the fx loop ) plate resistor that is supposed to go from 220k to 100k was missing from the BOM and instructions, and thus from my mod.
I would be surprised if it’s the whole story but I’ve ordered some and will retry after swap.

lysander

So, through a process of elimination it seems that the most likely issue is a grounding problem, but having tested all the obvious ground points on the first two gain stages they all show very low resistance to the chassis.
Still having crazy loud 100hz hum that responds to gain and master. I changed all the power supply / filtering caps so it’s not that.

I have a high voltage diff probe and a scope, how would you guys go about debugging this ?

ICBM

If it’s affected by the gain control it must be in the first valve stage or the input jack network.

Check the valve pin voltages first.

lysander

Thanks, yeah I agree it’s definitely something in V1.

lysander

Finally solved it - the boost relay and possibly some leftovers of the stock boost circuit were causing it.
The modded boost was only adding a cathode bypass cap to the cold clipper triode so I hardwired it on and removed the relay and associated circuitry.
The amp sounds insanely good now - it’s basically a low power hot rodded JCM 800.
Was a long road but totally worth it.