Bad squealing in OR120 build.

[antieatingactivist]

Hi. I just completed my first non-kit based build. Its a Orange OR120 clone that I build on a blank chassis, layed out and fabricated all of the turret boards. After fixing all the careless errors, I have it making sound. Too bad it is awful squealing sound. Basically, with the volume knob all the way down, it has a pretty intense hum. It could be related, but also could just be some sort of ground loop that I can figure out later. If I bump the volume up just a tad, squealing starts at full volume. I have eliminated the elusive backward OPT leads as a cause. If I remove the powertubes and scope various places on the amp, including the power tube grid pins, the waves look very nice and respond to volume controls as they should. With the power tubes in, everything is ugly. I've been over the schematic a bunch of times and cannot find any errors. I would be grateful if someone could give me a suggestion.

Garrett

[antieatingactivist]

This is the last picture I took, just incase anyone is curious.

[loudthud]

Do you have a link to a schematic? The symptoms suggest some parasitic feedback around the volume control. Either a lead dress or a bad ground. Are any leads from the output tubes routed close to anything in the preamp?

[antieatingactivist]

Do you have a link to a schematic? The symptoms suggest some parasitic feedback around the volume control. Either a lead dress or a bad ground. Are any leads from the output tubes routed close to anything in the preamp?

http://www.orangefieldguide.com/OFG_...20schem_72.gif

I used the bias circuit from the 74 model as it is adjustable. Which leads should I be extra careful to keep away from others?

[loudthud]

Can you post any kind of layout? It looks like you have something like a Hiwatt arrangement with the output transformer on the end with the input jacks. This puts the biggest signals in the amp near the smallest. That's an invitation to unintended feedback and/or oscillation.

The plate leads for the output tubes have very large voltages so they need special attention. If they are too close to the preamp, they will have to be moved or shielded. The schematic shows no grounding of unused input jacks. This makes them susceptible to picking up unwanted signals. Wires in the tone control circuits are high impedance so they are also susceptible to picking up signals. It may be necessary to shield some wires in the preamp.

Usually in cases like this if you move one of the wires that is broadcasting or receiving the signal, the frequency of the oscillation will change. Use a chop stick or other insulated tool to poke around and move wires until you find which one affect the oscillation. Be carefull, this is dangerous.

[antieatingactivist]

I wish I was so organized to have a layout that I could show you. For better or worse, this is all sort of a lay it out as I go with lots of scribbled paper to try to keep track or what I'm doing. Anyways, I lifted the board up and played around with the wires with a drum stick. Poking around at the plate leads seems to the only think that really makes any difference. It seems pretty arbitrary what happens when I move them around though. One thing that I am seeing that might have been a design flaw on my part is that the leads come out from the chassis under the main board near the input section. Is that bad news?

Here are some facts:

Pulling the output tubes yields no squealing or noise at all points on the amp based on oscilloscope readings.

Pulling the input tube makes the completely quiet, no hum, no squealing.

Is it safe to say that something in the output section is interfering with something in the input section, and that everything in between is fine, or are there other things to consider?

[loudthud]

One thing that I am seeing that might have been a design flaw on my part is that the leads come out from the chassis under the main board near the input section. Is that bad news?

Yes, that's bad news. Are the wires twisted together? If that doesn't do it they will need to be shielded. Options are a braided copper wire shield (if you can find one), copper tape or some kind of sheet metal. The shield needs to be grounded to the chassis. Would it be possible to route the plate wires above the chassis for some distance before they go through the chassis?

Is it safe to say that something in the output section is interfering with something in the input section, and that everything in between is fine, or are there other things to consider?

I think that's ok until there is evidence to the contrary. One thing you should check. Is there a center tap to the filament suppy? Each side should measure about 3.2VAC to ground. If it doesn't that would explain the hum and quite possibly the oscillation.

[antieatingactivist]

Yes, that's bad news. Are the wires twisted together? If that doesn't do it they will need to be shielded. Options are a braided copper wire shield (if you can find one), copper tape or some kind of sheet metal. The shield needs to be grounded to the chassis. Would it be possible to route the plate wires above the chassis for some distance before they go through the chassis?



I think that's ok until there is evidence to the contrary. One thing you should check. Is there a center tap to the filament suppy? Each side should measure about 3.2VAC to ground. If it doesn't that would explain the hum and quite possibly the oscillation.

It seems fixed now. I relocated the hole that the OPT leads come through the chassis, and twisted them. This helped quite a bit. I think what did it was using a shielded wire going from the first grid to the resistor after the input jack. I'm sort of kicking myself for not doing that. So far everything sounds ok. The one thing that seems to be causing trouble is the feedback wire. If I run it with everything else it causes crazy crackling and bad sound. Right now I just temporarily routed it on top of everything and it seems to be ok. Is there something specific that this wire should be kept away from?

[loudthud]

The one thing that seems to be causing trouble is the feedback wire. If I run it with everything else it causes crazy crackling and bad sound. Right now I just temporarily routed it on top of everything and it seems to be ok. Is there something specific that this wire should be kept away from?

Other than a loose connection or broken wire (hidden inside the insulation) it might be a marginally stable power amp. Try moving the feedback to the 8 ohm tap. Can you see anything that looks like an oscillation that accompanies the noise on your scope?

[antieatingactivist]

So the last mistake that cleared everything up was a ground wire that I ran from the screen supply caps all the way to my star ground at the other end of the chassis. Turns running it under the board made it angry. The feedback wire didn't cause any problems after that. I'm tickled at how good it sounds now. Thanks for your help.