120hz buzz, something fried?

[slamsho]

Firstly, I don't know much about amplifier electronics, but I'll have help if there's something to be done. I recently bought a single-ended Fender Champ-based p2p combo with some additions from someone who built it of quality parts. It came with a 6v6 and a EL84, of which one could be used at a time. Accidentally I put them both in and swithed on with volume at 0. Some rather loud hiss and buzz ensued so I hurried to turn the amp off, and realized my mistake. I took the EL84 out and tried it again: everything works as intended, except there's a (120hz?) buzz that goes up with the volume knob.

The problem is I don't know if it's been there from the beginning or if the aforementioned mistake damaged something and is causing this. Is this possible? Before I bought it, the seller said he had built it in a way to make it silent and suitable for studio use. (rectifiers?) It's around 4-6w so the buzz stands out quite clearly at normal playing volume.

Here's the buzz recorded:
https://soundcloud.com/skla-1/buzz

Any help/insight/clarification on the cause, troubleshoot or fix is greatly appreciated.

[Mick Bailey]

That's a pretty hard-edged buzz. From the clip, I'm hearing it almost disappears with the volume right down - can you confirm this is the case?

[slamsho]

I was actually moving the mic around a bit, hence the dip in the buzz volume. It's present at all levels (zero is almost quiet though) but does get louder when turning the volume up.

And yeah, it's kind of harsh. I wouldn't mind about a low hum, but this..

[Mick Bailey]

A few things to check;

Does it still buzz with the guitar unplugged? Also, remove the preamp tube - still buzz?

Sounds obvious, but make sure you're not close to low-voltage halogen lighting. It can put a serious amount of noise into an amp.

[slamsho]

I removed the preamp (12AX7) tube and tried the amp on. The noise was gone. Unplugging the guitar and turning lights and other devices off didn't do anything.

I noticed something else during past few days: when I play harder certain chords or power chords on clean, there's an ugly flabby distortion sound. With some initial googling it seems the preamp tube might be causing this (too?). I don't have a replacement yet to try and eliminate it as culprit. Considering the buzz went away after removing the tube, you think it's what causing this, and not, say, faulty/broken capacitors? What about the flabby distortion with certain hard chords?

Thanks.

[Mick Bailey]

Removing the preamp tube establishes that the power getting to your output tube is clean. SE amps are sensitive to ripple on the power supply and if it's noisy you'll still get a buzz/hum if the preamp tube is removed.

You could just be looking at a bad 12AX7 and swapping it out would be the easiest way to eliminate the possibility. I can't see how switching on the amp with both power tubes installed could have damaged the preamp - the current draw of the power amp is upstream of the preamp nodes.

Any further diagnosis requires checking the DC voltages to the tube and establishing if there's any AC riding on the plate supply.

A quick check is to set your meter on AC volts and measure if there's anything on pins 1 & 6 of the 12AX7 and report back what you find. You may get a few mV, but anything higher will cause noise. Bear in mind the AC will be riding on a DC level of over 200v so the usual caveats about working safely with tube amps apply.

Is your amp tube or SS rectified? A schematic would be useful.

As to the flabby distortion - best to fix the buzz first and then see if this is still a problem.

[slamsho]

Thanks for the reply. The power being upstream crossed my mind too, why I thought speaker damage as one possibility.

The amp is based on Vox AC4/Fender Champ, but has different preamp section with an Accutronics Digilog reverb, built in artificial load with on/off switch, and apparently all tuners affect each other, volume affects reverb etc. (if that's anyhow relevant).
Something in it is rectified, yes, but I have the info in different language, hard to translate with limited knowledge..

Don't have the schematic, but here's a photo of the innards if it's of any use:
http://i.imgur.com/JYCIVKo.jpg

I'm going to try out a new preamp tube once it arrives, then possibly another speaker, and report back.

[Mick Bailey]

The KBU4J bridge rectifier in the upper-right of the pic looks like its supplying your B+. No tube rectifier. There's another bridge rectifier just below it, positioned across the heater supply. It feeds the large vertically-mounted capacitor with an attached 3-pin regulator IC and this appears to be how the supply to the Digilog is being created.

Some interaction between controls may be part of the design and depending how the signal is bled off for the reverb this may explain why the volume control affects the reverb. It may be symptomatic of a grounding or other fault, but my guess is that there's going to be a fair bit of level change to the reverb input as I don't see anything to suggest otherwise.

The main thing to do is to establish that all the DC supplies are correct, and they're clean.

On the board, those three large capacitors to the right with their ceramic resistors form the supply nodes to each section of the amp. The left one of the three is the supply to the preamp. Measuring across that capacitor you'll get 200-something DC volts. You should see very little AC on that node, otherwise it will get amplified. My gut feeling is the fault lying somewhere else, but you have to check. It's important to never assume anything is OK when troubleshooting an amp. Measure it and know it's OK.

An important check is to make sure the voltage to the Digilog is correct, and it's also clean. Again, check the DC and also measure any AC riding on the supply. Because the bridge supplying it sits across the power tube heater supply it could have got damaged. Any fault with that part of the circuit will put noise into the amp.

[slamsho]

Yeah, interaction between controls is part of the design. Tested with new preamp tube and both symptoms are still present, so it's not that. The flabby ugly distortion seems to be stronger and happen easier when the reverb nears max.

Going to have to resort to outside help at this point, I will add a note based on what you wrote, especially regarding the digilog voltage. The amp itself is probably the best vintage style bedroom/studio amp I've ever played, with the 6v6 ruby, so I definitely want to get it fixed..