Blocking distortion, grid stoppers, Google and still I'm a bit puzzled

[Hallingamp]

Hello everybody.
I haven't posted here for ages, but I have to tell you this.

I have built a stereo guitar amp, basically two champs (5F1) in one chassis. Not easy to find much info about stereo amps, but I figured I could get away with just one PSU. Long story short, the amp worked but when I turned up the volume I got this really ugly crackeling sound, but on one side only. When plugging in to only one of the sides it was gone. Left or right side - didn't matter. One side at a time sounded good, but both sides at the same time it was crackeling again.

So I split the PSU in two for the preamp thinking it was due to unwanted coupling between the sides. And the problem got even worse! Ugly crackeling on both sides now. I googled away and found out it was probably blocking distortion. Lots of solutions for that, so I tried the simplest one. A 6k8 resistor on each 6V6's input grid and the crackeling is gone. Amp sounds very good, problem solved.

BUT - I don't really understand why. I kinda understand blocking distortion, but why I got it on only one side and only when I plugged into both sides is beyond my grasp.

Can anyone explain this?

Regards

Jan

[Chuck H]

I kinda understand blocking distortion, but why I got it on only one side and only when I plugged into both sides is beyond my grasp.

Can anyone explain this?

Regards

Jan

Sure. It wasn't blocking distortion. If it had been the splitting the PSU wouldn't have made any difference. Nor would one side the other operate independantly without the problem.

The problem you were having was most likely some parasitic oscillation. Very often amps that don't have grid stoppers can suffer from this. The crackle was probably spiking voltages due to a positive feedback loop between the amps output and some earlier stage. This positive feedback loop isn't a part of the circuit. It's caused by radiant fields inside the amp. Layout and grounding in ambitious builds (such as a stereo amp) becomes ultra critical to avoid problems with component and lead proximity. Search the forum and you'll learn more than you ever wanted to know about grid resistors.

[Hallingamp]

Thank you Chuck H.

I didn't think of that since I thought parasitic oscillation would sound more like squeeling. I've never experienced any of this in any of my other builds (5), but then again I have always used grid stoppers. (I have at one point wired a negative feedback loop positive, and I think our cat lost a few lives listening to that oscillation...)

Thanks a lot. I feel a little wiser now.

Best regards.

Jan

[Gingertube]

Jan,
Chuck H is (as usual) almost certainly correct. The reason you couldn't hear a "sqeel" is that parasitic oscillation is usually up in the 100's of kilohertz range. Even your dog would'nt hear it. What you hear is the secondary effects of that oscillation not the oscillation itself. The Output Tubes are working flat out ("like a lizard drinking" as we say in Oz) and power supply draw is at maximum.
There is an old tube mans saying -
"Tubes are radio frequency devices, if you don't feed them radio frequency they will make their own". Always use grid stop resistors on the output tube grids.
For Solid State guys, MOSFETS do exactly the same thing and the cure is exactly the same thing except with mosfets we call them gate stoppers instead of grid stoppers.
Cheers,
Ian

[woodyc]

Jan,
Chuck H is (as usual) almost certainly correct. The reason you couldn't hear a "sqeel" is that parasitic oscillation is usually up in the 100's of kilohertz range. Even your dog would'nt hear it. What you hear is the secondary effects of that oscillation not the oscillation itself...

I think the reason for the crackling is that the conditions for oscillation only occur during a part of the signal waveform, and the oscillation is somewhat fickle -- by which I mean that even with a constant signal, the oscillation amplitude and duration will change from one cycle to the next. So what you get is a series of irregular pulses added to your music signal, hence the crackling.

Here's an example of a crackling parasitic in a Pro Reverb:

[exclamationmark]

Oscillation can be a rather strange creature. I once ran a wire too close to something that it shouldn't have, and oscillation only occurred on a couple strings when they were played REALLY lightly. It didn't do anything out of the ordinary when the strings were muted or played normally, which made it a nightmare to troubleshoot.

[Guitarist]

I was laughing imagining all the cel phones and radio stations going bezerk when you plugged in your amp.

Glad you fixed it, you certainly seem to know your way around electronics.