Peavey Delta Blues buzzing

[WholeToneMusic]

I have been working on a Peavey Delta Blues with the 2x10 combo cabinet and it seems to buzz on top of certain notes being played. I have checked all the caps and the voltages are in the ballpark with the schematic. I have gone through the amp several times with no luck, I did notice that the PCB was un-supported near the Power transformer connections and when I pushed with a chopstick the buzz had the potential of getting intermittently louder when the board flexed.I cut the molex connectors and hard wired the PT leads onto the board for a stronger connection. I staggered the adjacent wiring to keep it more isolated, and then double checked the connections with the schematic. Would there be parastic oscillation from the parallel traces on the board when flexed? I have heard longer grid runs can cause the parasitic oscillation. I have tried changing out the preamp and power amp tubes with no luck. I noticed the metal tube protector panel was loose so I tightened it up. I re-tensioned the tube sockets. I checked all the wire jumpers on the board. I am at a loss here. Any suggestions would be much appreciated.

Also when shutting it off the amp pops, which wasn't noticeable before, The pop seems louder when the boost is engaged, This is a typical issue with the non-standby amps, Is there anything to do to minimize it with a suppression cap?

[Alex R]

I know you said you checked the caps but they are the prime suspect in oscillation problems and somtimes cap probs don't show up on testers - if it's possible to get the amp making the noise with access to the chassis, my method is to take a cap say 22 or 33uF, clip the -ve end to a grounded lead, and touch the +ve end of the each preamp filter in turn to see if the noise goes away; if it does, change that cap.

[WholeToneMusic]

Thanks

Hello Alex,

Thanks for the tip, I will give it a try and let you know the outcome. I have heard that the bias supply can be subject to problems also, Could I check out all caps with this method?

[Alex R]

- opposite polarity for the bias caps acourse, always match the polarity of the cap you're paralleling. Sometimes weak bias caps can cause hum by injecting ripple with the bias voltage at the power tube grids - you could try paralleling them with a good cap, polarities aligned with the old one don't forget, and see if you can remove the hum, but as you can see that kind of hum with the scope anyhow I've never done it.

It's not an infallible method, because caps might for instance be shorting and it won't change that, but I find that if you've got interstage feedback problems it can sometimes be due to an underperforming filter cap on a preamp stage, and if you bolster that cap by bypassing it you can suck out the feedback and thus troubleshoot cap problems that are hard to see/hear any other way. It's something I use just to sort out motorboating and howling, but if you can recreate your parasitic noise it could help with that too, I thought.

You'll get a spark, and you'll then have a charged-up cap in your hand that can give you a jolt, which you should discharge with care and a resistor (I use a 20uF 500v with a 2watt 220K across it).

[Enzo]

Before you get inside doing stuff, play those notes to get the buzz happening - or connect a geberator to the inoput and dial it in. Now have someone in back grasp and hold the power tubes. If they are too hot, use a rag or gloves. You may have the dreaded EL84 tube rattle. if grasping the tubes stops the buzz, you do.

If so, they are not necessarily bad tubes. SOme EL84s rattle right out of the box. It is the innards rattling against the inside of the glass. The EL84 is not a mechanically robust tube.

Electrical buzz is usually unrealted to mechanical buzzing. Disconnect the internal speakers and play the amp through external speakers across the room. ANy better? If so, it is mechanical.

The circuit boards are not glitchy nor unstable, and flexing the board will not change the spacial relationships between traces any meaningful amount. If the design was flawed, they ALL would do this.

[WholeToneMusic]

Thanks for the info

Alex,
Sounds good, I built a tester with an alligator clip on the Negative side of the cap( I couldn't find a black alligator clip.) and i hooked a wire to the positive end and wrapped it around a chopstick to keep me isolated from the circuit.
I have included a pic. I have a 5 watt resistor that i use to bleed the voltage from amps that I will use to discharge it, thanks for the safety tips, safety pays in many ways!!
Enzo, I have tried trading out the EL84's with no luck, I will use my trust Weber grilling glove and try what you mentioned about holding the tubes. I wil try your ideas out on monday and really appreciate all of your help Alex and Enzo.

Best Regards,
Brian

[Alex R]

That looks a whole lot safer than the poky little thing I use, congratulations. Enzo's right though, check for an acoustic problem first. Often if it's acoustic/microphonic, it's the same note you get ringing after you play, as somthing is resonating somewhere.

[WholeToneMusic]

Acoustic

Hello Alex,

I have a feeling it is an electronic buzz, I went over all of the acoustic issues before opening it up last time. I will try the Tubes once again when the customer brings the amp over this afternoon before opening it up. Thanks again for both of your input on this. Much appreciated.

Best Regards,
Brian

[WholeToneMusic]

Results

The owner and I went through a series of tests today. First we tried what Enzo had recommended with the power tubes and this did help to a degree the customer still wasn't happy with the degree of the results. I disonnected the stock speakers and plugged in another one and the buzz (rattling) was still there on the lower notes. I then proceeded to take the amp chassis out of the cabinet and we hooked it up for phase 2. While he was playing the amp i put the 22uF 500v cap in parallel with the other caps and still did not notice a difference with the rattling. I think I can rule out acoustic rattling now having the chassis out of the cabinet. All the voltages are dead on and the bias is right on with the schematic. Any other ideas for the Ghost in the machine? The customer also notices that the amp breaks into feedback alot easier. Which I think is a plus. The amp is much more responsive.

[Alex R]

Last time I had this kind of issue was an old Marshall combo. Sounded like t was coming from the amp, but actually it was the handle. Try another speaker cab to make sure it's not acoustic, I'd say.

[WholeToneMusic]

cabinet

I am testing the chassis out of the cabinet with a different speaker that i know is good. The sound is more prevalent in the bass to mid notes, It's a fizzy sound that starts to break into oscillation slowly. I went through and double checked all the ground connections, jumper wires, Grid resistor values, power resistor values. no luck so far. I hope i have some hair left after I finish with this amp.

[Alex R]

You tried swapping in preamp valves I guess.

[WholeToneMusic]

preamp tubes

Yes, I tried that to begin with. I replaced two coupling caps and that helped a little bit. I noticed one of the 10"s voice coil had a slight rub, when i disconnected that speaker the oscillation was still there.