Weber Java build - some oscillation

[Lindz]

I just finished a Weber Java build. Sounds pretty good but I am experiencing some oscillation issues and thought with all the Wreck guys here I might get some help chasing it down

1. it squeals even at low volumes with the presence dialed up high but if I turn down the presence somewhat and turn it down more as I turn up the regular volume I can keep the squealing at bay. I can run it completely dimed with presence off and it sounds great so all is not bad

2. Oddly with the master volume dimed it squeals way less at all volumes vs if I use other ranges on the master even if the regular volume is not that high - it seems to squeal a lot in the mid range of the master pot and not much down low or nearly wide open - any ideas here?

For instance if I dime the regular volume, it squeals with the master down low but once I get the master about 6 or 7 it will stop squealing (but blows my head off volume wise - ha ha). Does this even with the presence off. Seems pretty weird to me

Seems like I have something haywire with the feedback loop or phase inverter perhaps?

I've read all the Java posts over at Weber and many oscillation threads here but hope a few of you knowledgeable types can chime in with some ideas that might help get it a little more under control.

So far all I have done is move some wires around and add a "Jade mod" .001uf cap v2 pin 6 to ground as suggested over in the Weber forums - improved the squealing a bit but still pretty out of control at many settings.

pic of my build below. I used shielded wire for the master volume, input jacks, and presence wiring

any help would be greatly appreciated

Thanks

[Reeltarded]

Just to clarify, this is while mounted with a plate as shield?

[bluesfendermanblues]

What amp design is this? - a Weber Java? Would help if you added a link to schematic or layout.

Anyway, it sounds like a grounding error somewhere - Check that all grounding is done according to the schemo and that shielded wires are not grounded at both ends.

[Structo]

Swap your OT primary wires to stop the squealing.
If they aren't long enough now, swap the wires feeding the grids.

[cbass]

Whats with the BBQ

[TUBEDUDE]

A Derogatory term for Weber products.

[xtian]

Swap your OT primary wires to stop the squealing.
If they aren't long enough now, swap the wires feeding the grids.

+1

[Lindz]

I edited to remove the BBQ comments.. I thought it was funny and in fact on some other forums I posted on a while back the text "Weber" is actually automatically changed to BBQ which I thought was funny though not necessarily derogatory. Ceriatone was changed there as well...

But.. my intention was not to be derogatory even if some here may have their issues with them. So far my experience with this build is good, it shipped promptly, was inexpensive, sounds good and I more or less was hoping some of the gurus here might be able to help me chase down the oscillation I have going.

Shooting my mouth off here unintentionally as it was might result in no help on their forum so I want to make sure I don't do that as that was far from my intent

their layout/schem are attached for anyone who might want to dig in. The theory is that the layout is loosely based on a Trainwreck express so those familiar with debugging those amps might have some insight on taming this build

thanks!

[M Fowler]

In all Express builds the presence can be a real PITA so lead dress is important.

[Lindz]

Tried swapping OT primaries and it just howled as soon as it turned it on so I think I have them right

Next I tried disconnecting the presence altogether but I get no sound from the amp at all - does that sound right? Hook everything back up and I get sound but I have the squealies as I describe.

I had thought disconnecting it would simply take the feedback loop out of the circuit but the amp should still work otherwise?? - Perhaps I have some connections incorrect somewhere in the master volume and/or presence wiring but so far cannot find anything. Can anyone think of something obvious to look for with this symptom?

Next I tried some different wire for the presence circuit, this time normal hook up wire twisted for the leads instead of the shielded wire in case somehow the shielding was shorting against the signal lead somewhere in those original wires. Same squeal

This has me perplexed because the amp works and sounds great if I set the dials in certain configurations especially with the volumes dimed and presence off or nearly off, but I can't do something simple like play quietly with low volume and master volume or it squeals like a stuck pig

Its funny but I can get the oscillation to behave almost like a tone generator.. when I dial up or down the volume controls the pitch changes from lower tones gradually going up to microphonic and higher pitches, but at various dial settings I can kill the squeal and it actually sounds really good when I play through it.

Any thoughts?

[TUBEDUDE]

Since the pitch is changing, you may have the feedback throughout the range if adjustment. The "squeal" may be changing from infrasonic through ultrasonic, just being noticable when dialed into the audible range with the volume control. Do you have an oscilloscope?

[martin manning]

I think you have some kind of lead dress issue. Your photo is not too clear, but it looks to me like you have the 47k feedback resistor mounted on the impedance selector switch instead of on the board. If so I would redo it like the layout- run a wire from the impedance selector (OT 16-ohm tap) to the board, and run the 47k from there to the far end of the 22k PI tail, and a wire to the presence pot. Keep the wires on the chassis surface as much as possible. Re your sound dying when you disconnected the feedback, I don't know where you broke the connection, but you probably disconnected the PI's ground along with it.

[M Fowler]

Probably removed the PI tail resistor from ground when presence removed.

[Lindz]

No I do not have a scope and to be honest that is above my current knowledge - this is 3rd build and I am learning my electronics as I go

I am becoming pretty addicted to building and do feel my lack of knowledge frustrating so I am likely to buy and learn to use the right tools if I keep it up

Martin - I did move the 47 k resistor to the board and rewired the entire presence circuit with regular hook up wire in case I had the original shielded wire jacket touching the lead or melted through from a hot soldering iron at one of the connection points on that run. Same result

That said it is in the air above the board and is a long run due to the layout of the board and position of the presence control. Perhaps I should carefully redo it with shielded cable again?

In fact this whole layout has REALLY long runs for many wires though I tried to keep things close to the chassis etc where I could - I can certainly redo it better but was hoping I could tame it to a tolerable level without ripping out the whole board and redoing all the leads

Do you honestly think my lead dress could be SO bad that it literally cannot function without pretty intense squealing with the master and volume controls down low? There is not a whole lot of gain - i.e. controls at 1 or 2 sort of thing. I thought I must have something else wrong to cause as much squealing as I am getting

Perhaps it is my grounding? If you look you can see 2 ground points - one by the input jacks and one by the PT. As I did not have any heavy gauge wire on hand for a buss bar, I instead ran the preamp grounds individually from the pots and board to a chassis point on the lower right of the input jacks - I thought that was a reasonable compromise though would be happy to get some helpful criticism if it is not

Did the same for the filter caps and power section to a point on one of the PT mounting posts - if you look you can see the 2 copper fittings I used on the 2 corners of the amp. The presence ground is at the PT ground point. Essentially 2 star points

Amp seems reasonably quiet hum wise though I had not thought I could introduce this high pitched oscillation through my ground scheme - again my ignorance if that is possible

Thanks again for any input

[Zippy]

Did you use the "alternate" V1 wiring?

That is the Express way - not the extra stage (cathode follower) as shown in the schematic.