Tweed Champ clone - problem

[famous beagle]

Hey y'all,

I just finished a build of a Tweed Champ (5F1), and I fired it up last night for the first time. Seemed to be going well, but when I got to about 10 on the volume (out of 12), I got a nasty buzzy tone. (Definitely not the natural tube distortion!)

I was wondering if anyone had an idea of what it might be. Here's a sample of the noise. The volume was probably at about 6 or 7 at the start of this, and I gradually turn it up. When the nastiness kicks in, at around 0:21, the volume was at about 10 or so.

Any ideas would be greatly appreciated. Thanks!

 

[elbandito]

I'm on a cellphone, so I haven't listened to the track but have you checked the input capacitor? That's the first thing I'd check. In assuming you're decent at soldering. I made a similar amp, so when I get a chance, I'll have a listen and see if I can help. Is this from a kit?

 

[elbandito]

Oh, if you haven't already, check across the sockets for the power amp tube. Seems to me that since the sound doesn't happen until you crank it, that it could have to do with the power amp section.

 

[famous beagle]

Oh, if you haven't already, check across the sockets for the power amp tube. Seems to me that since the sound doesn't happen until you crank it, that it could have to do with the power amp section.

Yes this is from a kit by Weber. The only things I subbed were two 100K carbon comps for the plate load resistors.

I'm still pretty green when it comes to this; this is the first amp I've completed (I'm working on another as well). So what exactly do you mean by "check" the input capacitor and the power tube sockets? Do you just mean to check the solder connections?

And yes I'm a decent solderer (I would think you have to be to build an amp).

Thanks!

 

[famous beagle]

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. It goes away immediately when I back the volume back down to 10 or so.

???

 

[famous beagle]

sound file

Here's an mp3 demonstrating the new problem. No cord is plugged into the amp at all. This is just me slowly raising the volume. The farty sound kicks in at about 11 or so, and the white noise kicks in at about 11.5. It goes away immediately as I lower the volume.

 

[ranjam]

Is this a kit? Or your own 'assembly' of a circuit you found online? It sounds like you just have too much preamp gain, or the 6V6 isn't biased right. Are you copying a 5E1 or a 5F1? The 5F1 is a 'better' design, but both utilize a 470-ohm resistor for bias. Is the first 12AX7 gain stage cathode bypassed? Remove the cap and try again. Is the 6V6 cathode resistor bypassed? Remove the capacitor and try again.
You'll get it. You may have to play with the gain, or try a lower coupling cap from the 12AX7 to the 6V6. The 5F1 shows a .022uF, but try a .01uF, and you may reduce the low end gain enough that your farting goes away.

 

[ranjam]

Oops. I see it is a kit. Still, try my ideas, and you will possibly be rewarded with a great sounding amplifier.

 

[famous beagle]

Oops. I see it is a kit. Still, try my ideas, and you will possibly be rewarded with a great sounding amplifier.

Thanks so much for the suggestions ... but ... shouldn't the values listed on the kit work, considering it's an exact replica of the original Fender circuit?

I'm not saying that your ideas aren't valid, and I'll certainly give them a shot if I can't find out what's wrong, but those ideas seem more like a mod when I'm just trying to get the original design to work properly.

 

[ranjam]

I may be wrong (as I often am), but that is what it sounds like to me. Sure, the original values should work fine, but I have no idea what they send for tubes. You just never know how the 'modern' tubes will react to those old circuits that ran tubes on the edge. You know Deluxes would run the 6V6's at well over +400VDC. Champs I don't know, but if history teaches us anything, it's to never assume the original design will always work.
Start with the basics; check your wiring carefully, maybe move your lead layout around a little, and then if all else fails, try different tubes. Maybe try different tubes first. If you have, try a 12AY7 instead of a 12AX7. If the farting goes away, it likely is a gain/biasing issue for that particular 6V6 tube. Try anything you can think of, and eliminate wiring, tubes, or other bad parts as the culprit. That only leaves the design.

 

[famous beagle]

After reading up in just about every book that I have, I'm 95% sure that I have some parasitic oscillation going on because of a sloppy lead dress (which you mention above as well). I've heard that if the wire from the grid resistors to the grid pin is too long, you can have this problem. I had no idea about this, and I specifically remember when wiring the eyelet board saying to myself, "Just cut the wires extra long---you can always trim them down later." I think I may have forgotten to trim some of them down though.

So, I'm gonna look into that this weekend hopefully and see what I can find.

Thanks so much for help! I had no idea there was more to amp-building than following the schematic!

 

[drumslinger]

Are you seeing 30-33ma on the 6v6? Some 6v6 tubes can handle up to 14watts like the JJ's so you can bump up the bias to 38ma. Have you tried swapping out tubes ( preamp and power tube)? Like Ranjam advised, try a .01uf to help tighten up the bottom.

 

[ranjam]

You aren't tightening up the bottom so much as trying to eliminate blocking distortion. Blocking Distortion. And that's what it kinda sorta sounds like, on my actually half-decent computer speakers.

 

[famous beagle]

You aren't tightening up the bottom so much as trying to eliminate blocking distortion. Blocking Distortion. And that's what it kinda sorta sounds like, on my actually half-decent computer speakers.

Thanks for the suggestion! I'm going to try taming a parasitic oscillation first, and if that doesn't work, I'll try your suggestion.

 

[Thurgood]

Hey Beagle, did you get it working? Yeah that nasty noise sounds to me like oscillation but could be a blocking issue but I am no expert but in my limited experience blocking ends up sounding "blatty" fwiw. I did see a post the other day (shame on me for not saving it) that talked about adjusting the 6v6 bias as a common more modern cure for some issues with the 5f1 circuit. Did you look over at the music-electronics forum, they are helpful and there are lots of posts. You may even try thre 5e3 builders subforum as most of the regulars do lots of other amps too. Your 5f1 sounds pretty good UNTIL..... Best of luck. Now I go back to tweaking my 5e3. As that is what saturdays are for. Be well all.