First Post

[joesnuffey]

I’m a new lurker here, looking to learn as much as possible. I’ve built a few amps so far this year, and honestly never cared about the nuances of amps until I decided to expand my hobby / possible next career into amp building and design. I’m retired from the military (lost a leg in the process), and have the fortune of being able to chase my dreams because of it.

I started a few years ago, making guitars, after talking with a Chicago blues guitarist who I consider a friend. One thing led to another and I am now trying to make boutique guitars that are visually and sonically matched to their corresponding amps.

I decided to venture down the kitless TW build on my 4th build, and have an Express “clone” that is respectable, but suffers from parasitic oscillation when the presence is turned past 4:00. I’m certain at this point, that I’ll eventually find it, but would like to speed up the process. Based on your collective experiences, what would be likely culprits to cause parasitic oscillations, that are controllable via presence. V1 grids are run through RG-174, as is the NFB line. Other than not being able to dime every control at the same time, it’s amazing.

Also, this build has me planning a Rocket build. In terms of new tubes, what would be your top 2 choices of new production EL84?

-Brian

[xtian]

Welcome to ampgarage! Regarding oscillation, lead dress is the most likely culprit. Can you post photos of your build?

There are only three manufacturers of tubes these days: Russia, China, Czechoslovakia. I prefer JJs.

[dorrisant]

Lead dress may be your culprit. That said, pics would tell the story. And welcome!!

[pompeiisneaks]

As a veteran myself, welcome! Another possible related issue affected by presense could be the OT leads on the power tubes may be in phase instead of inverting, so they create positive feedback. You could try swapping the OT leads and if it goes away that was the problem, if the sound becomes worse or has massive oscillations/howl it's not.

~Phil

[joesnuffey]

Thanks for the kind welcome. Today, I played with NFB resistor value, and increased the first grid stopper to 68k, after noticing that I couldn’t induce oscillation if I removed V1. I also noticed that when a guitar is plugged in, it’s becomes much less inclined to oscillate.

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[dorrisant]

Maybe try moving (with a chopstick) the orange wire that feeds the treble pot from the silver mica cap on the board. Get the amp to oscillate as is. Then try pushing it away from the other yellow and green tone stack wires. Make sure the yellow and green wires run against the chassis as much as possible. If you have more volume pot left to turn up, do so and repeat the chopstick process again. You should be able to remove nearly all of the oscillation. If you can't get it to completely go away, try moving the wire that runs from the output jack away from any other wire that might carry signal. This last part might not make much difference since you used coax for this run. Also, confirm that the shield of the coax is soldered to that ground lug... I can't really "see" the connection from the pic.

This pic is not a real Wreck, but you can get a better idea from it. Find pictures of Francesca here in the Trainwreck files section for how Ken wired it. Look at how that treble wire stays up high and away from the other tone stack wires.

[Stephen1966]

Welcome to ampgarage! Regarding oscillation, lead dress is the most likely culprit. Can you post photos of your build?

There are only three manufacturers of tubes these days: Russia, China, Czechoslovakia. I prefer JJs.

Just a small point, but I see it often: Czechoslovakia hasn't existed as a country since December 31st, 1992 where it was split into the Czech Republic (nowadays also known as Czechia!) and the Slovak Republic. JJ (Jan Jurco) Electronic was founded in 1993 and is based in the Slovak Republic.

[ViperDoc]

Hey joesnuffey, welcome.

I thought I'd ask the experts about your power tube socket and 1R bias measurement resistor arrangement. Your 1R resistors have long exposed leads that run incredibly close to the chassis before being soldered to a flat grounding tab that is then bolted to the socket. Maybe bend those tabs away from the chassis to eliminate unintended contacts with chassis. You might also consider mounting those 1R resistors directly onto your bias jacks, but it looks like it could work as is.

On my Express build, I ended up eliminating the diodes on the power tube sockets because I found they were noisy. Maybe lift one end on those and see what you think.