Parasitic Oscillation Questions

[Jhyatt2012]

If the leads from the B+ on the power supply are run too close to the underside of the pre amp board, could that cause parasitic oscillation?

[ToneMerc]

If the leads from the B+ on the power supply are run too close to the underside of the pre amp board, could that cause parasitic oscillation?

Everything is relative, but in general terms it should not. FWIW, there are plenty of eyelet and turret boards designs where the B+ wires are on the backside of the board.

Signal wires, under the right conditions/circuit combinations can make an oscillator and this where a scope comes in handy.

TM

[Jhyatt2012]

Thanks TM. That may answer my second question. I have the longer leads for the pre amp running along the top surface instead of under the board and am a little suspect of those.

[martin manning]

Early 60's Fender designs ran B+ wires above the board, typically elevated a bit.

[Jhyatt2012]

I may have happened on something in checking voltages...

With a guitar connected and the MV above 1, when I lay into a note really good, I get the oscillation. It sounds like a loud, nasty overdive. With the guitar connected, (and laying down on a table) I set the volume to a pretty high level, rang out a note to get the oscillation and started checking voltages. When I get to either of the 1M resistors in the PI, like magic, the oscillation disappears and I get a beautiful tone. I'm going to move the lead dress around some to see if that helps any.

Anybody have any thoughts?

[ToneMerc]

I may have happened on something in checking voltages...

With a guitar connected and the MV above 1, when I lay into a note really good, I get the oscillation. It sounds like a loud, nasty overdive. With the guitar connected, (and laying down on a table) I set the volume to a pretty high level, rang out a note to get the oscillation and started checking voltages. When I get to either of the 1M resistors in the PI, like magic, the oscillation disappears and I get a beautiful tone. I'm going to move the lead dress around some to see if that helps any.

Anybody have any thoughts?

You have gut pics so we can see the lead dress?

TM

[Structo]

You can use a non conductive probe, such as a wooden chopstick, to move wires around a bit to see if you are in the right area that needs attention.

Although sometimes probing with a voltmeter is enough to stabilize a unstable area.
You will learn about this as you progress.

Remember to only probe the active amp with one hand to avoid a lethal shock that goes from hand to hand, through your chest.

[Jhyatt2012]

Not a problem. This is a 183 base preamp with reverb and effects loop - both tube driven, and a standard (well, not precision anyway) power supply. The large orange drop you see at the right is to the effects loop. It's my second build so I felt really ambitious

[Structo]

Just an observation, most caps are microphonic to some extent.

Mounting them up high on the board like that can sometimes create some weirdness.

Not saying this is your problem but just in a sort of FYI type thing for future reference.

[67plexi]

Check the primaries on the output transformer sounds like you may have them backwards.
Pin #3 power tube.

Steve.

[xtian]

Your photos are from an iPhone 5, no? Dang. This just moved up a notch on my gift registry…

[Jhyatt2012]

Just an observation, most caps are microphonic to some extent.

Mounting them up high on the board like that can sometimes create some weirdness.

Not saying this is your problem but just in a sort of FYI type thing for future reference.

That's something I had considered. I might redo those just to rule that out.

[Jhyatt2012]

Check the primaries on the output transformer sounds like you may have them backwards.
Pin #3 power tube.

Steve.

Thanks for the observation, but no they are correct. I had tested both ways before trimming and doing the final solder. Made that mistake on my very first build and learned my lesson then!

[Jhyatt2012]

Your photos are from an iPhone 5, no? Dang. This just moved up a notch on my gift registry…

Yup. Love it! You wouldn't be disappointed at all.

[ToneMerc]

Do you have a schematic? From what you describe you only get an output when you reference the PI to ground via your VOM, that leaves me to believe you have a wiring or design issue?

TM