The Amp Garage

I had tried moving the nfb wire, to the pot and to the selector. Chopsticked it in different spots, etc... I didn’t observe any change with the bias. Actually, with low presence settings, the amp goes back to normal. With presence on 0, the bias current goes up. If I disconnect the nfb, the bias is ...

Nope, Mullard xf2 but put some ruby tubes after I discovered the issue. It still does it after a significant warm up period.

I’m at a loss...

Cleaned the board with alcohol, underneath as well. No change...

If it’s an oscillation, where could it be coming from?

I’ve checked all the grounds, replaced the nfb wires so they are far from everything, tried changing all the caps, measured all resistors. No change. I changed the presence pot, same... I grounded the preamp and the issue is still there, so it comes from the power stage. If I lift the nfb resistor, ...

All right back to the black flag! I forgot about it... So I changed the Pi caps (all 3 .1uf), no change. I changed the bias caps, no change. Changed the .022uf in the Pi too and the presence cap. As soon as I turn the presence pot, the bias lowers significantly ( around 10ma). I also found out the s...

Success!!! I noticed I had grounded the bleeding resistor from the first cap near the power tubes. I removed it and soldered another one across the cap this time. Most grounds looked good but redid them just in case. I also moved the mains ground to the heaters ct. I’m not 100% sure what the issue w...

Measured resistance to ground everywhere and everything is between 0.1 and 0.2 ohms. As I mentioned before, continuity is good everywhere. Checked the inputs, all is good. Most of the hum goes away with V1 but not all. It does with V2 though.

I’ll reflow the joints and report back.

Thanks Phil. I want to confirm something: if the grounding scheme was an issue, I would have had the hum from the start, right? So assuming it’s not a design issue,since the hum is new, I guess it would be a mechanical one and the main culprit in that case would be a bad ground connection? Checked l...

Ok, so I’ve determined it’s a 60hz hum (played on youtube, exact same pitch). If it was a ground loop, wouldn’t it have been present before? The scheme I used is pretty similar to the one in the layout of Charlie Wilson in the files section. I checked the continuity in the power section, everything ...

Thanks! How would I check for rectified hum? Continuity for ground is good everywhere. Chopsticking doesn’t seem to indicate anything. Also, how could I test if it’s the elytics? Just unsolder and measure them? Hum issues are super common but in over a decade of building amps, it’s a first. My build...

So, for no apparent reason, my TD started to hum like crazy. Not a buzz, it’s a low pitched, muffled sounding hum. It’s only present with the volume turned up, on either channels, with or without anything plugged in. I built that amp a few years ago, never had that issue. With V1 removed, the hum is...

I thought about decreasing the first cap to a lower value, I think I had tried that but it mostly took low end away...

On these types of amps, why does adding a cap across the volume pot doesn’t do anything?

Hi, I hope everyone is safe and sound in those strange times we live in. Over the years, I built several tweed and brown combos (5e3s, 6g2, 5d3, tweedledee or even a Marshall 18w). I’m not sure why, but all of them sound dark... the tone knob is always maxed out. I’m more of a plexi player, so me ea...

NFG, it sounds like our issue was similar. My 6G3 had original its speaker and preamp tubes (1 RCA and 2 TungSol). The amp turned out to be ok for the most part. I upped the filtering a tad and installed grid stoppers on the power tubes to minimize high end oscillation. It helped a bit but not much....

+1 for TAD GC or WGC (more mids).

Remember they are rated for 30W pd.