Strange "splatty/swirly" distortion on Plexi build

[Jarrodthebobo]

Hi everyone!

This amp has been buggin me for quite a bit now; I finished building it a few years ago but honestly never got around to playing it enough to really start 'listening' to it.

Last week however, I noticed that there was always this odd "splat" sound like a misbiased fuzz pedal when I really hit the strings.

Turning down the mids and treble pot and boosting the bass makes the sound very prevelent. The included video starts with all knobs but the bass knob turned down so ya'll can here what I mean. I tweak some of the tone knobs throughout the video.

Initially I thought it was the poor wiring situation of the PI since I had originally situated it far away from the main board leading to very long leads having to fly over to it; so I drilled in a new spot for the PI tube and shortened all the wires up. While this stopped some oscillations I was hearing when I turned the MV up to max, the weird splat remained.

I've also tried add a 220k resistor after the treble wiper of the tone network but that made no change other than rolling off some highs and making it seem a bit dull.

The MV I have in the amp is a Rich/LarMar PPIMV and I ALSO thought this could be a culprit and cleaned up the wiring but no dice there either. Maxing out the MV also does not have any effect on this odd splat. In the clips above, I DO have the amp dimed and running into an attenuator.

I have a snubbing cap across the first triode stage along with a 100pf cap on the PI.

The PA of the amp is cathode biased instead of the typical fixed bias (for no reason other than the fact I've never heard of a cathode biased marshall before). 250ohms on the shared cathode resistor to the two el34s. 3300uf cap bypassing the resistor. 405v to the plates, about 395v to the screens. Last time I checked the tubes were bias'd at around 25ish watts.

Any advice with dealing with this would be apreciated! At this point I'm really at a loss for what's causing this issue!

Thanks,

-Jarrod

[tubeswell]

schematic handy?

[Willabe]

In the clips above, I DO have the amp dimed and running into an attenuator.

Does the 'splatty/swirly' sound happen without the attenuator hooked up?

[Jarrodthebobo]

In the clips above, I DO have the amp dimed and running into an attenuator.

Does the 'splatty/swirly' sound happen without the attenuator hooked up?

Yes it does

[Jarrodthebobo]

schematic handy?

Unfortunately none that are exactly identical to the circuit I'm running since its a bit of a frankenstein of things. I'd guess the closest would have to be the SV20 marshall has been putting out as of recent.

[pdf64]

schematic handy?

Unfortunately none that are exactly identical to the circuit I'm running since its a bit of a frankenstein of things. I'd guess the closest would have to be the SV20 marshall has been putting out as of recent.

It could be sketched out, take a photo of that, upload it to a hosting site, and provide a link.

Or edit the SV20 schematic (my understanding is that SV20 HT voltage is about 300V in high power mode).

[Jarrodthebobo]

schematic handy?

Unfortunately none that are exactly identical to the circuit I'm running since its a bit of a frankenstein of things. I'd guess the closest would have to be the SV20 marshall has been putting out as of recent.

It could be sketched out, take a photo of that, upload it to a hosting site, and provide a link.

Or edit the SV20 schematic (my understanding is that SV20 HT voltage is about 300V in high power mode).

I can give that an attempt after work today, although I may have found a bandaid solution that's finally worked; upping the grid stoppers on the Power tubes to 47k and adding a 100pf plate snubbed to the first triode has all but completely removed the odd distortions.

To me this indicates I have some parasitic oscillation occurring somewhere within the circuit that I should probably trace out but alas, I don't have an oscilloscope handy to make thay job easier on myself.

It's very strange that such a (relatively) low gain circuit such as this one is having such an issue. I feel like my lead dressing is "ok" enough for the most part... no parallel runs and 90 degree angles when wires cross over one another...

I may just rewire all of my leads since I initially used solid core for everything in order to try and make my runs neater but it seems like that's not a very common building tactic...

[elams1894]

It sounds like a bung tube perhaps. Have you tried swapping the positions, or replacing the tubes before anything else?

[Jarrodthebobo]

It sounds like a bung tube perhaps. Have you tried swapping the positions, or replacing the tubes before anything else?

Im going to be honest; I actually haven't! Every time I've ever had an amp related issue from a diy build the tubes have never once been the cause so I've pretty much swore off even checking em.

I'll give them a roll when I get home!

[tubeswell]

found a bandaid solution that's finally worked; upping the grid stoppers on the Power tubes to 47k

Without seeing your schematic, I'm guessing that you had some imbalance in the PI causing problems with your output stage when cranked.

On amps with LTP inverters (and I'm only guessing here because you haven't posted your schematic, but you said 'plexi'), hot biasing of the LTP can cause imbalance in the push-pull duty cycle - whereby the inverting LTP output spends more time positive than negative (due to hot biasing) and the non-inverting LTP output is the opposite (i.e., more time negative than positive)*. This can lead to unequal blocking distortion in each output tube etc. The fact that 47k grid stoppers has brought this under control would seem to bear out this hunch, because that is one of the solutions for amps where this happens. Another remedy is to bias the LTP more moderately. If it's a 12AX7 inverter, I would use a 820R or 1k bias resistor (and not 470R) on the LTP.


Or if you want to stick with 470R (if that's what you have, but I can't tell because you haven't posted a schematic, so I'm going out on a limb here and could be wildly barking up the wrong tree - if so please feel free to correct me or confirm my hunch by posting your schematic), then maybe swap to a 12AT7 tube.

*This, and other amazing stuff, is covered in more detail in Merlin's designing guitar preamps book (2nd edition)

[Jarrodthebobo]

found a bandaid solution that's finally worked; upping the grid stoppers on the Power tubes to 47k

Without seeing your schematic, I'm guessing that you had some imbalance in the PI causing problems with your output stage when cranked.

On amps with LTP inverters (and I'm only guessing here because you haven't posted your schematic, but you said 'plexi'), hot biasing of the LTP can cause imbalance in the push-pull duty cycle - whereby the inverting LTP output spends more time positive than negative (due to hot biasing) and the non-inverting LTP output is the opposite (i.e., more time negative than positive)*. This can lead to unequal blocking distortion in each output tube etc. The fact that 47k grid stoppers has brought this under control would seem to bear out this hunch, because that is one of the solutions for amps where this happens. Another remedy is to bias the LTP more moderately. If it's a 12AX7 inverter, I would use a 820R or 1k bias resistor (and not 470R) on the LTP.


Or if you want to stick with 470R (if that's what you have, but I can't tell because you haven't posted a schematic, so I'm going out on a limb here and could be wildly barking up the wrong tree - if so please feel free to correct me or confirm my hunch by posting your schematic), then maybe swap to a 12AT7 tube.

*This, and other amazing stuff, is covered in more detail in Merlin's designing guitar preamps book (2nd edition)

It was the PI tube!

I rolled another random 12ax7 and the issue went away completley. I've even removed the 47k grid stoppers and brought them back to the 10k I originally was running in the amp and it remained stable.

Thanks for the help! I don't think I would have ever actually bothered to replace the tubes!

[tubeswell]

With normal variances in tube manufacturing tolerances, you might find some tubes behave differently to others of the same type when subjected to more-extreme operating conditions. You may have stumbled on a particular tube that works better in that PI slot. Or not. We didn’t really get into what was causing the issue yet.

[Jarrodthebobo]

With normal variances in tube manufacturing tolerances, you might find some tubes behave differently to others of the same type when subjected to more-extreme operating conditions. You may have stumbled on a particular tube that works better in that PI slot. Or not. We didn’t really get into what was causing the issue yet.

Just as a note as well: the issue was happening whether the MV was low OR cranked. I'm still willing to bet there is some sort of parasitic oscillations sneaking in somewhere into the circuit where my wiring may not be as neat as it should or my solder joints not as pretty as they should be.

For the time being however, I'm no longer experiencing the issue.