Parasitic Oscillation

[Luthierwnc]

Yeah, that's the one. It is the mahogany that's hogged-out though. Since I made it for myself I left the board completely bare with really minimal headstock abalone. Put a pair of WCR Crossroad pups in there to serve as my general purpose Gibson clone. Nice guitar, very light. If you split the difference in color on the two pictures you'll get an idea what it really looks like.

The more PRS looking one has a Rio Grande Texas bucker in the bridge and a Vintage Vibe P90 wound HB sized pup in the neck. Both are regular players as I have been thinning the herd a little. A USA Custom Strat that I shot with a zillion coats of Fiesta Red nitro and a franken-tele round out the gig-ready guitars. In the closet somewhere is a Steinberger clone I made about 10 years ago. Fits in the overhead on a plane.

[skyboltone]

Beautifull. I haven't even started the USAGC sprucecaster other than drool. Too many project right now. How do you play that 336 with no dots?

[Luthierwnc]

There are position markers on the side -- just not on the face. Usually I make classical guitars. Traditionally they don't have anything anywhere but most people want something at the 7th. Sterling silver wire is a good fit.

I made one for Warren Haynes this Christmas on behalf of Habitat for Humanity to thank him for hosting the Xmas Jam. That's me with the mic. sh

[gearhead]

Wow, very very cool!

BTW, how would you bypass the tonestack on an express? V1 Pin 4 to Pin 2 (disconnecting their wires) with some moderate coupling cap (.02)? And a resistor to ground after the cap as in V2 to emulate the vol pot? Or just run to the volume pot where the treble pot connects?

Conversely, how to bypass V1a? The V1 Pin 2 is uber-sensitive, so it might not be the tonestack. Can I just disconnect the V1 pin 2 from the volume pot, disconnect the pin 2 wire to V2 and directly connect the volume pot to that V2 pin 2? Or do I need to disconnect the V1a plate resistor too?

I did try the easy one you suggested, bypassing V2. No difference, other than it dropped in sensitivity (due to the lack of a gain stage.)

Right now, all I'm looking for is where the sensitivity is lost. All the first stage stuff is SUPER sensitive. After that it just gets lost somewhere.

BTW SB, yes I am using Belton sockets. Got them so long ago, had forgotten!

Thanks,

Dave

[Luthierwnc]

Yeah, you just go from one stage (usually after the coupler) to the grid of the next.

Sometimes I cheat. I'll put a known-good chassis side-by-side and jump different points on the amp. For example: pull the power and inverter tube in the good amp and use its preamp to drive the suspect amp (you have to pull V2 on your wreck). Is it weak? It is the power section. Strong like bull? It's the pre. Move forward or backward to isolate the guilty stage. It pays to write down the plate volts before and after. If one of them really tanks (read: increases the draw) without a corresponding increase in volume, you have to wonder why it is working so hard.

Be extra careful on safety precautions. It is easy to forget which one is which. If you don't already have them, solder a 220k/2w bleeder on the rail so you don't have to keep manually draining the caps. Use the same outlet to keep the grounds friendly.

Good luck, Skip

[austinb]

Don't know the history of this amp but if the oscillations go away when you disconnect the feedback then you have positive feedback instead of negitive feedback, swap the output transformer primary leads on the output tubes.

[gearhead]

Thanks, but been down that path already on day one. Experienced the Death Screech, so have already swapped the OT secondary wires (to the tubes).

I was looking for some above-audible, high pitch, power-robbing oscillation.

So far a nogo.

[islandamp]

I just had a similar problem with a Plexi I built. A very hi-pitched squeal, not too loud but annoying all the same. I sat the amp on a flat piece of doubled up aluminum foil to emulate a head box with bottom shielding and it vanished.

[gearhead]

There is no high-pitched squel, unfortunately. Sort of dod the same thing of emulating the box shielding also (except with a spare chassis).

I just had TWO tubes die on me, a preamp and a power tube. Took me 12 fuses to figure that out; thought it was related to some minor work had just done. Don't know if something shorted out, or there are destructive oscillilations.

[jjman]

Question:

I haven't spent much time here or on the TW side. Has anyone ever posted the definitive TW study with sine wave pictures at each point in the preamp circuit? Gearhead (and everyone else) might get a lot out of seeing what the millivoltage should be at the different stages.

Food for thought, sh

I hope to do that soon. It won't be "definitive" since mine is not a perfect clone, but I'm interested in seeing it myself and I need to justify the scope purchase. What it the gospel input voltage for the input sine wave?