5F6A + 'blocking' distortion & Grid Stopper trials

[tubeswell]

Okay everything else is going pretty sweetly now (reverb and trem etc) except for this slight intermittent temporary slight buzzing/crackling that happens when I hit a note on the geetar. The buzzing/crackling fades as the note begins to die off - it is there at whatever volume but it does get louder as I turn the vol up. I think this is blocking distortion, but I'm not sure where its coming from.

The current schematic is attached. In summary;

1) I have put 3k9 stoppers on the 6L6GC grids. What is the maximum size I should be able to get away with here before it kills the tone?

2) I also have put a 12k stopper on the reverb bypass stage (fixed bias CF) grid (not sure if this is the right thing to do, or even it it would be enough to do anything, but it didn't seem to do much except faintly reduce some high end hiss), and a 15k stopper on the DC pair input grid (ditto), and I lowered the input cap to the LTP to 500pF (which cut out a lot of bass and made the TMB controls less bass-rangey, but) which didn't seem to cure the intermittent buzzing - just made it a bit more trebly.

[pdf64]

From your description I'm prettly sure it's not blocking distortion.
Guesses at what it might be are
1/ a bad connection somewhere - could be something not soldered properly, poor contact on tube socket etc.
2/ ultra-sonic oscillation of some sort, maybe parasetic riding/relying on the signal.
3/ bad speaker - voice coil rub can have this sort of effect.
4/ bad tube.
To chase these down, 1st things 1st
1/ try the chassis through a different speaker.
2/ try swapping out tubes.
3/ put a scope on the output and feed in a steady signal from a sig gen at various levels, and carefully examine the trace.
4/ give everything a wiggle to check for looseness, reflow solder joints, power up and chopstick tap test on board, components, wires, pots etc.
Good luck - Peter.

[tubeswell]

Thanks Peter

After I posted this I tried swapping the 0.1uF coupling caps for 22nF mustard caps in front of the output stage. I also increased the output tube grid stoppers to 4k7. I can now get it reasonably loud without the problem occurring (Fingers crossed - touch wood) - I'm beginning to like this amp. I think I'm ready to try to set down some soundbytes.

[jrfrond]

With true blocking distortion issues, you either need to reduce the value of the bias divider resistors (typically 220K) or reduce the value of the coupling caps (which you did). Because you went from .1 to .022, you reduced the RC time constant of the circuit and the smaller cap discharges quicker, so you may have, in fact, had a blocking distortion issue. Blocking distortion is caused when the cap cannot discharge quickly enough and drives the power tube grids positive. You can reduce the values of the grid stoppers back to 1.5K. Also, if you feel that your bass response has suffered, you can incrementally try higher values like .033, .047, .056, .068. Remember, this only happens when the amp is overdriven and allowed to sustain. That is your acid test.

[JoeM]

Hmmm, I'm not sure it's blocking distortion that occured. I've only got 220 ohm stoppers on my 5F6A clone and dont have any issues. (Though mine doesnt have reverb or trem). I dont crank this thing (almost always play clean), so maybe in changing your components you fixed a flaky solder connection. Just a thought.

[tubeswell]

Hi Joe M - I don't think it was a bad connection because I had soldered a few plate resistors in an out initially (thinking it was those causing the problem) and I always go the distortion until I changed to 22nF caps.

Having said that I just put an extra 22nF cap in parallel with each cap (making them up to .044uF) and it sounds quite a bit fuller without having the problem. It actually sounds very ice now (if I may say so myself)

[JoeM]

tubeswell,

I'm not quite clear on this. Did the buzzing/crackling you describe happen when the amp was played clean or pushed?

[tubeswell]

It was happening clean and more-so on bassy notes

[Merlinb]

When I have encountered the problem of a crackling sound which blooms and decays as the note decays, it was always due to a bad solder joint. Not sure if this is the same problem you're having, however.

[pdf64]

My understanding and experience of blocking distortion was that it is exclusive to overdriven circuits? Can't see how it would occur on a clean signal.

[tubeswell]

Well okay you guys you might've been right. But the sound was a temporary buzzing straight after each note and it faded before the note faded. It wasn't a rattle and it wasn't coil rubbing (I tried different speakers). It was a consistent intermittent short term buzz (like a bit of harmonic flab) and it isn't there any more. What does this mean? I dunno now.

[Gaz]

I'm sure you've switched preamps tubes, but I once had a very peculiar problem that sounds like what your describing that was cured by swapping tubes. I thought I'd mention it because it was an intermittent problem, and the sound would disappear before the note faded.

[tubeswell]

Yep I did swap the pre-amp tubes. Hang on - I'll try putting some of them back in to see if that was it.

[tubeswell]

Bingo

Yep after chasing my tail round and round, I found out that one of the pre-amp tubes (a NOS Marconi) was buzzing on each note. This seems to have been accentuated by the charge on the coupling caps though, because I also bypassed the grid stoppers on the output tubes to double-check and a slight oscillation came back. So I have put 1k5 grid stoppers back in (and removed that bad tube) and the amp is running very sweetly now.

This amp has been a valuable lesson in troubleshooting multiple problems. I think its time I invested in a scope to eliminate some of the guesswork in future builds.

[unparalleled]

Yep after chasing my tail round and round, I found out that one of the pre-amp tubes (a NOS Marconi) was buzzing on each note. This seems to have been accentuated by the charge on the coupling caps though, because I also bypassed the grid stoppers on the output tubes to double-check and a slight oscillation came back. So I have put 1k5 grid stoppers back in (and removed that bad tube) and the amp is running very sweetly now.

This amp has been a valuable lesson in troubleshooting multiple problems. I think its time I invested in a scope to eliminate some of the guesswork in future builds.

I would use the Marconi for a Tremolo oscillator.