5 watt SE guitar amp head with sidetone buzzing problem when strings are strummed

[rock soderstrom]

I have modded a China 5 watt SE guitar amp head and I have a constant buzzing background noise sound (when I strum the strings) that I can't get under control. It sounds like something is vibrating with the strings, but it's not a mechanical problem with the speaker or tubes. I've swapped them all to isolate the problem, it does seem to be coming from the amp and I'm running out of ideas now.

Does anyone know the problem or have similar experiences. What could be responsible?

The circuit now is a really good sounding hot Mini Z clone (but with Master Pot) but the problem was already there with the donor amp. Tubes are 1x 12AX7 and 1x EL84.

 

[rock soderstrom]

This is the circuit at the moment

 

[CJ]

Sounds like oscillation, try moving wires unless it is on a PC board, your gain went up with the deleted tone circuit, you could try a divider to attended the signal. Maybe a NFB resistor from the xfmr secondary,

 

[rock soderstrom]

Sounds like oscillation, try moving wires unless it is on a PC board,

I think you're right, I've already opened up all the wiring harnesses but I can't influence the behavior. I think the problem originates on the circuit board.

your gain went up with the deleted tone circuit

That's true, but the problem already existed in the original version. By the way, the tone stack had very little effect, sounded terrible and choked the amp. That was a very good decision to take it out. The voltage divider in front of the gain pot had originally also swallowed a lot of gain.

you could try a divider to attended the signal. Maybe a NFB resistor from the xfmr secondary,

I'm afraid it will change the character of the amp too much, I'm very happy how it sounds now, really great, wild and super loud for 5 watts. This is not my first 5 watt amp, but I really like it now - good recipe!

I'm going to try a grid stopper before V1b, see what that does and I'll contact the transformer primary to the 240V winding to get the secondary voltages down, they're all too high!

Maybe it has something to do with the poorly filtered DC heating?

 

[Alva Goldbook]

Well, there’s a few silly things happening in the circuit, but tell us what the noise sounds like. Go to YouTube and put in “60Hz sine wave”. If you can tune a guitar by ear you should be able to tell if this is the noise the amp is making. If it’s not 60Hz try the same procedure with 120Hz. If it’s neither and sound like a boat engine or a ambulance siren, let me know.

Some obvious bad things with the circuit.
1) no grid stopper on the power tube. Helps prevent oscillation. Value is not critical. 10k. 4k7. 1k. Even 100k. Just put something there.

2) no grid leak on power tube. Check data sheet for upper limit on this. If I knew the power tube being used I’d tell ya.

3) a resistor bypassing the tone stack? I mean, why?

4) A voltage divider on the B+ before the output transformer? Why? Just use the correct power transformer. Iron ain’t cheap. The tube might be able to handle the higher voltage. Dissipation is what matters more.

5) DC heaters with just a single filtering cap? That might be your culprit right there. On such a low gain build, DC heaters are unnecessary. AC heaters would be fine.

 

[ruffrecords]

It is also worth knowing if the level of the noise changes with the setting of the controls, particularly the one before the output tube.

Cheers

Ian

 

[rock soderstrom]

Well, there’s a few silly things happening in the circuit, but tell us what the noise sounds like. Go to YouTube and put in “60Hz sine wave”. If you can tune a guitar by ear you should be able to tell if this is the noise the amp is making. If it’s not 60Hz try the same procedure with 120Hz. If it’s neither and sound like a boat engine or a ambulance siren, let me know.

Yep, I can tune a guitar by ear, but I have no idea what this has to do with my (no solved) problem. I got no 60/120Hz problems because I am in a 50Hz country, but joking aside. There is/was no hum problem, I never mentioned that. It was whirring noise, similar to when parts of the housing vibrate loudly with the strings.

Some obvious bad things with the circuit.
1) no grid stopper on the power tube. Helps prevent oscillation. Value is not critical. 10k. 4k7. 1k. Even 100k. Just put something there.

There is a grid stopper in front of the EL84, 3.5K!

2) no grid leak on power tube. Check data sheet for upper limit on this. If I knew the power tube being used I’d tell ya.

It is clearly described in my first post which power tube is used (12AX7+EL84 inside), just read everything before you post.
There is a grid leak resistor now but that was not the problem.

3) a resistor bypassing the tone stack? I mean, why?

Nonsense, the resistor replaces the tone stack which I completely threw out because it had hardly any effect, sounded bad and stole a lot of level. But I've already written about that in my previous posts. Again, read first, then post.

4) A voltage divider on the B+ before the output transformer? Why? Just use the correct power transformer. Iron ain’t cheap. The tube might be able to handle the higher voltage. Dissipation is what matters more.

What voltage divider? What are you talking about?

5) DC heaters with just a single filtering cap? That might be your culprit right there. On such a low gain build, DC heaters are unnecessary. AC heaters would be fine.

Not my design, but I know where it comes from. This amp is a generic Chinese design that is sold under many brand names. Gretsch, Epiphone, Harley Benton and so on and the first models had the usual AC heater. There were problems with hum, according to internet forums, and the people in charge went with a (rather poorly implemented) DC heater. I wouldn't do it that way, none of my guitar amps have DC heating, I get them quiet without it.

The main problem with these amps is the design of the PCB, where local oscillation occurs due to crosstalk.

The amp sounds really good now and is loud as hell...

 

[rock soderstrom]

It is also worth knowing if the level of the noise changes with the setting of the controls, particularly the one before the output tube.

No, it doesn't change anything (apart from the overall volume). This is an attempt on a master volume so that the amp can also be played at moderate volumes (in rented apartments, for example). This also had no influence on the problem, as the pot was not even present at the beginning.

This works more or less at the start of the potentiometer's control travel, but from a certain position the potentiometer is then ineffective. There are better solutions for this, but it's not worth it for such a low budget amp. They were sold here for 89€ at Thomann.

 

[Khron]

4) A voltage divider on the B+ before the output transformer?

What voltage divider? What are you talking about?

Likely that 200 ohm / 220K combo, which has an... academic effect there. The 220k is likely more of a discharge resistor, than anything else (imho).

 

[rock soderstrom]

Likely that 200 ohm / 220K combo, which has an... academic effect there. The 220k is likely more of a discharge resistor, than anything else (imho).

Yep, just an ordinary bleeder resistor for the PSU caps, a good thing, no doubt.

 

[dai h.]

(I'm just throwing stuff out there, perhaps pictures and video (sound) would help the diagnosis?)

weird buzzing when you have signal (when you play)?

(I have no idea if this would matter in such a low power amp, but..) output transformer magneto-striction--something something. Sometimes when people obtain attenuators (the kind to reduce output going between the amp speaker output and speaker(s)), they freak out since they hear the OT producing sound (which was present previously but they just didn't hear it before since it was drowned out by the speaker(s)).

(Another one--just a WAG really) A problem is described where the stapled alu shield in the cab isn't making good contact with the chassis and causing weird noises:

 

[rock soderstrom]

Thanks Dai, for chiming in. As already written, it was definitely not a mechanical problem but oscillation. The PCB is the problem from my point of view, with some mods and trace hacks on the PCB I was able to solve it so that it is no longer annoying.

Is this now the best amp in the world? No, but honestly, I would have killed for this amp in the late 80s. Back then we only had the cheap transistor amps that sounded really bad. We're talking about a practice amp that's really fun now.

Now this amp really gets down to business, depending on the volume pot and the guitar you can go from a little clean to distorted to powerful overdrive, everything is possible. Surprisingly loud for 5 watts, but with the crude master pot solution and the gain control you can make a lot of noise even at lower volumes. Now the amp is much better than when it was shipped. Definitely.