Express-build - oscillation problems.

[jakehop]

Alright - it works now. Sort of.

It makes a lout buzzing squeal when I turn it on. My primary suspect is the placement of the tubes. They are between the potentiometers and the board, and I had to run some semi-long (10-12cm / 4-5") wires from the tube sockets to the board. This is pretty far from the ideal that the pictures have given me, so I am going to drill out some new holes in the chassis tomorrow and see if it helps to rearrange it.

If anyone has some input or wants to strengthen me in my theory, please let me know.

Jake

[skyboltone]

Alright - it works now. Sort of.

It makes a lout buzzing squeal when I turn it on. My primary suspect is the placement of the tubes. They are between the potentiometers and the board, and I had to run some semi-long (10-12cm / 4-5") wires from the tube sockets to the board. This is pretty far from the ideal that the pictures have given me, so I am going to drill out some new holes in the chassis tomorrow and see if it helps to rearrange it.

If anyone has some input or wants to strengthen me in my theory, please let me know.

Jake

Easy one Jake. Switch the two outside primary leads to go to the opposit tube. What happens is that it's a 50/50 chance with amps that have negative feed back if you'll get the squeel. Right now you have positive feed back, so just switch leads from one tube to the other and you're set.

[jakehop]

Easy one Jake. Switch the two outside primary leads to go to the opposit tube. What happens is that it's a 50/50 chance with amps that have negative feed back if you'll get the squeel. Right now you have positive feed back, so just switch leads from one tube to the other and you're set.

So far so good. I followed your directions, and now instead of a low thundering squeal, I've got a "static" noise with everything turned down. If I turn up either controls (vol, treb, mid, bass, pres) I get a loud, searing squeal. It should be noted, that with no tubes in the preamp, and a matching pair of EL34's in the power amp there was no crazy noises whatsoever.

I feel my preamp wiring is compromised due to the layout of the chassis (that I was going to change) and it makes good sense to me. With the leads switched from the OT I can now tap V1 and the amp goes into oscillation.

Don't you think that changing the layout of the chassis to match the Kelly would help? I mean, I've got 4-5" wires from the tubes to the board, due to the nature of the eyelet board I've used, and that was a thing I didn't feel good about when I built it. Still, I gave it a shot, as drilling holes in big pieces of metal isn't my favorite hobby, playing guitar is

Thanks for your kind help so far!

Jake

[skyboltone]

Yes, the amp is unstable. The "Francesca" layout and wiring is proven over and over. I've found that the pot wiring is just as important as the wiring from the tube sockets to the board. One of the genius's here (Gearhead?) came up with the idea of making a jig from wood or garolite to exactly the same dimensions as the front of the chassis. By mounting the pots and jack to this board and wiring it outside the chassis you are assured of duplicating this very important wiring layout. Secondly, some tubes are simply micro phonic and or prone to noises. Check by trying other tubes in the pre-amp before ripping it all out and starting over. The important thing on the socket wiring I think is that the wiring lays hard against the chassis for as much of it's length as feasible.

[jakehop]

Thanks again for your comments.

I plan to make new holes in the chassis (it's only three new that need to be made), and I'll try to follow the Fransesca-layout.

Thanks for the help so far - I'll update here when I've made further work.

Jake

[jakehop]

As most people in here knows, letting something rest on the shelve for some time, helps!

I took a long vacation, and started restoring two old Super Reverbs before I sat down with this one again. After I made the SR's sound amazing, with zero noise, I went back to this old guy, gutted the wiring and drilled new holes for the preamp tubes. With the new wiring in place, I fired it up and presto - it worked first time around!

The transformers I'm using are some old Drake's from a Carlsbro I gutted for the purpose. It has seperate bias widing that I'm using, with the bias-network from the Carlsbro amp (10K pot in series with diode, wired as a resistor, with 47k to ground of the tap) and it gives me everywhere from -56V to -20V, so I guess I should be able to play around with different tubes in this one, even though it sounds amazing with the old stock Siemens EL34's right now.

Thanks to everyone helping me out on this one!

Jake

[skyboltone]

Good plan and good work Jake. Congratulations.