Tweed Champ clone problem - nasty buzzing and white noise sound

[famous beagle]

Hey y'all,

I just finished my first amp build using the Weber 5F1 kit (tweed Champ).

I got the kit finished and fired it up last night for the first time. It seemed to be going well at first, but when I cranked it up to about 10 or so, I started getting a nasty buzzing sound. I’ve attached an mp3 file that demonstrates this. The buzzing sound comes in at about 0:21; it’ll be fairly obvious when you hear it.

Then I just noticed a new thing this morning. When I turn it all the way up to 11.5 or 12, a nasty white noise-type of sound happens whether I’m playing or not (or plugged in at all). It goes away immediately when I back the volume back down to 10 or so. I’ve attached another mp3 file demonstrating this. I’m slowly turning up the volume (no cord is plugged into the amp). The farty noise kicks in at about 11, and then the white noise is about 11.5. As you hear, it goes away once I bring the volume down again.

The only thing I've tried so far is trading out the 12AX7 with another one, and I still get the same result.

Any ideas on what this could be? Thanks for reading!

[cgates56]

bypass cap

are you running a bypass cap on the cathode of the preamp tube

[famous beagle]

are you running a bypass cap on the cathode of the preamp tube

Yes I am. I followed this layout exactly.

[cgates56]

voltages

send me voltage readings on the preamp and power tubes when set to max volume and I will compare to a champ I build here

[Prairie Dawg]

That second one sounds like me after two beers and a bologna sandwich. All kidding aside I'm think you've got some oscillation going on. Lead dress and shielding usually takes care of it. Pull the chassis out where you can see everything. Very carefully get your fingers close to anything you think may be suspicious. You can also take an AM transistor radio (remember them?) tune it between stations and use the antenna as a wand and if it is oscillation you will hear white noise through the radio. I had the same thing on a 5C1 I hand built and what cured it was taking about thirty turns of wire around a pencil, slipping it over the wires from the jacks to the grids and grounding one end of it.

[JHow]

I use RG174 on the grid circuits for same application as Prairie Dawg is describing. You could try that too. Just ground one end of the shielding.

[famous beagle]

That second one sounds like me after two beers and a bologna sandwich. All kidding aside I'm think you've got some oscillation going on. Lead dress and shielding usually takes care of it. Pull the chassis out where you can see everything. Very carefully get your fingers close to anything you think may be suspicious. You can also take an AM transistor radio (remember them?) tune it between stations and use the antenna as a wand and if it is oscillation you will hear white noise through the radio. I had the same thing on a 5C1 I hand built and what cured it was taking about thirty turns of wire around a pencil, slipping it over the wires from the jacks to the grids and grounding one end of it.

It's so funny you should mention that! Last night, I was reading everything I could on amp troubleshooting to see what I could find. In a book by Gerald Weber (of Kendrick Amps), he describes the problem of parasitic oscillation. And I was thinking, "That very well could be it." I was definitely not very happy with my dressing in this amp (it was my first build), and I actually thought about just redoing the whole thing (except for the component board) with the knowledge I gained from building this thing. The dang chassis is so small in this amp!

So, just to make sure I understand (I'm still a bit of a newbie):

Prairie Dog, you're just using standard, hookup (single-conductor) wire, wrapping it around the wire that runs from the two input resistors (68K in my amp) to pin 2 (grid) of the 12AX7, and soldering one end of this coiled wire to ground, correct?

JHow: You're saying that, instead of running hookup wire from the resistors to the grid on pin 2, you actually use shielded cable (kind of like instrument cable?). At one end of the cable you attach the shield to ground. Right?

So, this is just for the first half of the 12AX7, correct? Y'all don't do this on the other side, right?

Thanks!

[JHow]

...JHow: You're saying that, instead of running hookup wire from the resistors to the grid on pin 2, you actually use shielded cable (kind of like instrument cable?). At one end of the cable you attach the shield to ground. Right?

Yes. RG174 is a small shielded cable. You can get it at hoffman or AES, I think, in small amounts.

So, this is just for the first half of the 12AX7, correct? Y'all don't do this on the other side, right?

If by "other side" you mean the second triode (pin 7), actually, I sometimes shield that, too. Do the same thing, just run RG 174 from the middle of the pot to the second grid pin. Ground one side of the shield. I run it straight across the chassis (not under the board, as in the original layout).

[Prairie Dawg]

I only did that because I didn't have any shielded wire handy, and it seemed easier to slip over a capacitor like the 5C1 has in the input. It worked pretty well, too. Before that I could play the amp like a theramin. The second one I built I just went ahead and did it that way from the get go.

[famous beagle]

Thanks a lot y'all. I think I'm going to try the shielded cable idea, because I have some here. Hopefully I'll be able to get around to it this weekend, and I'll let you know how it goes.