Continuing saga of plexi clone build with harsh distortion

Strictly for conversations sake and without a technical explanation.....

Your PI is not symmetrical as photos show and the more you drive it the more you may notice. This could be the result of the tube itself or either the plate load or tail network resistor imbalance. Don't go out on a goose chase over it. It appears normal to my eyes. I'll be watching hoping you get this sorted out. I applaud your efforts and determination!

 

I can't speak to the weird notch, but I'm tuned in to learn more on that.

But I think what you're seeing from the phase inverter is normal. Textbooks usually show full out-of-phase sine waves in their pictures because that's simple, but when an output tube clips at the grid, that clamps the plate voltage in its phase inverter tube for the duration. Pretty sure that's why you're only seeing partial waveforms in your oscilloscope. Someone correct me if I'm off-base there!

 

I wouldn't know. I don't know anything about using a scope. And blocking distortion i'm still learning about. I was just trying to bring levity.

 

Hey do try increasing your grid stoppers to 50-100k. Its a technique for taming blocking distortion but also helps with oscillations. Its worth a try if not just to eliminate the possibility. You've tried just about everything else so you never know.

It gonna be a couple of days before I can scope my amp again sorry. I was playing around with some ideas on the power section and a tube went kaboom boom so I need to diagnose and get to the bottom of that.

 

I have the same frustration with the FFT on my USB scope. Usually just fiddle around with ALL the settings until I get something that looks alright. One thing that helps is turning on "averaging" and setting it to maximum number of samples. It'll take several seconds for it to fill in the trace so you need to be using a repetitive signal (function generator).

 

Yes increase the stoppers on the power tubes. Something between 50-100k. Its a bit of a long shot, but like i said you've tried everything else at this point.

Definitely dont mess with the resistors on you CF and PI, that'll stop them working.

Can you point me to a clip of the sound you're trying to fix again? I thought I saw it before but cant find it now.

 

This has been going on for so long I can't remember what you have and haven't tried, but what are you bias settings? I wonder if a warmer bias might help...

 

I doubt it's going to end up being what solves your issue but just for the sake of trying everything if I were you I would try a much colder bias setting, around 50% let's say, just to see if it makes any difference.

There was a thread recently about a 6g3 experiencing blocking distortion with the tremolo on because of a very large negative bias voltage. That was an extreme situation that was only manifesting because of the bias vary tremolo swinging the bias voltage even more negative when it was on but it's a thought and since you've eliminated all the obvious culprits on this one a long time ago we might as well take a swing at the distant possibilities.

 

My mind keeps taking me back to the o-scope with the 50k ultra high frequency hash. The grid stoppers and snubber caps should have had an effect if large enough values were used. So far no luck though.

Listening to the clips again makes me wonder if a filter cap could be allowing a ghost note situation happen. It does have that trailing double note thing going on. Several items were changed when this was rebuilt but some of the filter caps are the same. As Nickfl said swinging at distant possibilities.

 

Are you hearing this noise at a more reasonable volume setting? With both channel volumes on 10, I would expect some unpleasant distortion and maybe some ghosting from this circuit which is perfectly normal. Mine is rip roaring loud by 5 on the volume dial. Are you running it through an attenuator to be able to play it this loudly?

 

That's kinda the nature of these old four hole style circuits. They were meant to be run at arena volume levels to get that great overdriven tone. Unfortunately that is too loud for most of us at home. Running a 5000pf bright cap on the bright channel's volume pot does give the amp a bit more hair but it also gets loud very early on the volume knob. A bypass cap on the v2a cathode will also make the amp a bit gainier at lower volumes. You could try subbing in a pair of 6v6's and running a 16 ohm cab with the amp set on 8 ohm. It will give you slightly less volume but it's still gonna be loud. A ppimv will also help but it does change the amps sound and many folks don't like it.

It's strange that a fuzz pedal doesn't sound good at lower volumes though. Did you build it to the split cathode lead spec or the shared cathode bass spec?