IM distortion

[Fischerman]

In my homebrew ODS (Skyliner/HRM/BM PI) I'm getting what I think is IM distortion (or maybe I should say that I think I'm getting too much of it). Doing certain double-stops or bending one note while holding another causes me to have a sort lower harmonic that really mucks with the tone. I assume this is always present to some degree but I think I'm getting too much of it. PAB is ON BTW.

I'm also getting some 'splashy' distortion for lack of a better word. It makes higher notes or bright chords sound harsh and mostly occurs on loud notes or chords and double-stops. It may be the same 'problem' as above...just a slightly different symptom.

I've tried a few things but none of them seemed to address this (or these) problem(s)...they just changed the voicing a little or changed the gain a little.

Some things I tried other than the obv adjustment of OD trimmer and HRM controls and tube swapping:
Lowered CL2 Ck to 1uF. Nope, back to 5uF.
Lowered OD2 Ck to 1uF. Nope, back to 5uF.
Lowered OD1 coupler to .001uF. Thinner but not better, back to .002uF.
Lowered CL2 coupler to .022uF...this one I kept.
Added an additional .0047uF coupler for the OD (before the low gain network). This one I kept as well...seemed to clean up the lows.
Increased the grid stopper on OD2 from 150K up...the tone got more and more 'muffled' as I increased it...it's at 200k now but I'm thinking this is just a band-aid for the 'real' problem. I also have a treble bleed off of the OD Master...again...band-aid to cover up the 'real' problem whatever it is.


Any ideas where to look? I have an o'scope and a sig gen but don't really know what to look for. Also, I have to run my HRM bass control pretty low...all the way down is too thin but anything above the early, quick bass ramp-up is too much. Current schemo below including my latest voltage measurements...and the band-aids.

[heisthl]

Play with increasing the feedback in the output section - you can reduce this to almost nothing with the presence turned up.

[Fischerman]

Thanks heisthl. Right now I'm at 100K from the 8ohm tap (there is no 4 ohm tap) and that's with the BM PI/Presence. I was thinking it must be in the preamp since it does this at all volume levels. And the D'lator didn't seem to affect it either. But I usually run the Presence on 0 so I'll try it.

EDIT: I just thought...I haven't really tried pushing the clean channel hard and cranking the amp to see what I get...I'll try that this evening. Whether or not this (or these) problem(s) still remain should tell me something.

[ayan]

Thanks heisthl. Right now I'm at 100K from the 8ohm tap (there is no 4 ohm tap) and that's with the BM PI/Presence. I was thinking it must be in the preamp since it does this at all volume levels. And the D'lator didn't seem to affect it either. But I usually run the Presence on 0 so I'll try it.

EDIT: I just thought...I haven't really tried pushing the clean channel hard and cranking the amp to see what I get...I'll try that this evening. Whether or not this (or these) problem(s) still remain should tell me something.

If you're using an 8-ohm tap, you already have about 40% more negative feedback than if you used the 4-ohm one, so your amp should be inherently smoother because of that. Your schematic looks for the most part fine, I wonder if your iron and voltages may be contributing to the effect? The Skyliner/HRm with 22K trimmer, as you built, is a low gain affair that tends to sound pretty smooth and cleanish. What are your settings on the amp, by the way?

Gil

[Fischerman]

Gil,
My settings are all over the place. This is all with Fender knobs...so my 1 = 0 on any other knob. I'll try CL vol @ 4 and Drive @ 7 and then try CL vol on 7 with Drive on 4...and all points between. Usually the CL vol doesn't go much below half and the Drive is anywhere from 3 to 8. I'm trying all sorts of settings. One thing I have noticed is that if I run the CL vol lower and the Drive higher...the tone is brighter and not as fat than if I run the CL vol higher and the Drive lower (for similar levels of OD).

EDIT: OD trimmer is at about 28k from ground.

I'm using a MM 50W Hiwatt OT that was brand new. Was using EH 6CA7s but switched to JJ KT77s just this past weekend (and one of the quad was VERY microphonic out of the box ). I thought my voltages were correct. I have about 435vdc on the power tube plates and my dropping string is 3K-15K-10K with the D'lator supply on it's own separate string from the screen node.

[ayan]

I'm using a MM 50W Hiwatt OT that was brand new. Was using EH 6CA7s but switched to JJ KT77s just this past weekend (and one of the quad was VERY microphonic out of the box ). I thought my voltages were correct. I have about 435vdc on the power tube plates and my dropping string is 3K-15K-10K with the D'lator supply on it's own separate string from the screen node.

Hmmm... nothing sounds out of the ordinary. Have you tried plugging your preamp out into another amp, to see if you can isolate what you don't like about the tone to either the amps's pre or power amp? That's a thought.

Also, try lifting the tone stack on the HRM on the middle control. Listen to the amp that way, you can even hang a 100K resistor across your 1 Meg OD master volume. Now, if THAT doesn't sound sweet, I am not sure it will unless you start replacing components, trying other tubes, etc.

I would probably revert back to the "classic" PI, if you haven't tried it with that amp yet. The classic PI should make the sound smoother.

Gil

[Fischerman]

Thanks Gil. I had been kicking around the idea of trying the Fender-ish value PI. Haven't tried it into another power amp but I can easily do that...and will. FWIW, I really like the Clean and at least at clean-ish settings it seems to sound 'right'. I'll also try lifting the HRM and paralleling the Lead Master, maybe I should have tried a non-HRM first.

I'm also gonna build one of those amp stethoscopes discussed around here recently...see if I can't blow something up.

[ayan]

Thanks Gil. I had been kicking around the idea of trying the Fender-ish value PI. Haven't tried it into another power amp but I can easily do that...and will. FWIW, I really like the Clean and at least at clean-ish settings it seems to sound 'right'. I'll also try lifting the HRM and paralleling the Lead Master, maybe I should have tried a non-HRM first.

I'm also gonna build one of those amp stethoscopes discussed around here recently...see if I can't blow something up.

I currently have a BM w/HRM, in addition to two non HRMs, and had HRMs a long time ago. The overdrive of the non HRM is definitely sweeter to my ears. Of all the variations, the 220K/150K non HRM with a .047uF midrange cap and a deep switch is the sweetest/smoothest in overdrive. Happens to be the one amp that sounds great with the largest number of guitars. But, it is somewhat compressed sounding, so if you're looking for more of a "crunch" amp, probably not the best candidate. Now, the 100K plate non HRM is more touch sensitive and 3D sounding, with better cleans, but does not stay as smooth as the 220K/150K amp with high gain levels. Hope this helps, somewhat!

Gil

[heisthl]

Make sure your first filter cap is actually working - If only the screen node on is being filtered it will sound like that too.

[Fischerman]

Thanks Gil and heisthl. This time of year is so hectic and I had to work over yesterday so no progress.

heisthl, I'll check that. My filtering is slightly different...for example; I used 2 each Sprague Atom 30uF/500v in parallel for the OT center tap node and one 20uF/500v for the screens, the rest are mostly 22uF/500v except for a coule of 33uFs (PI and FX loop nodes). I was using up all the surplus caps I had.

Gil,
I'm what most would call 'a Marshall guy'. The ideal for me is Fender BF clean, Marshall crunch, rodded-Marshall lead. From what I had read...I figured the HRM would suit me better (although I never expected it to out-Marshall a Marshall). However, now that I've built it I think my reasoning was wrong. I can just use a Marshall or rodded-Marshall (my fave) for Marshall tones. I probably should have just went for the smoothest, most compressed OD from the get go. IOW, I probably should have gone for the most non-Marshall tone. Maybe if it's an easy enough conversion (and possibly even if it isn't) I'll try the non-HRM. My HRM pots are panel mounted anyway.

Just last night I broke out RF's Mystic Mile for a listen. Man, he got some killer tones on that record. Does anyone know what he used?

[Bob-I]

My filtering is slightly different...for example; I used 2 each Sprague Atom 30uF/500v in parallel for the OT center tap node

I'd double check this node. Make sure you have proper grounding. I'd also run a 470k to ground across this node. The balance resistors using in the "totem pole" arrangement can affect the amp too.

Just last night I broke out RF's Mystic Mile for a listen. Man, he got some killer tones on that record. Does anyone know what he used?

That's the "gold standard" of Dumble tones IMHO. Great tone and great playing!!

[Fischerman]

Thanks Bob. I have a 330K resistor from B+ to ground in parallel with the 2x30uFs. Then I have the 150K FET simulator resistor on the V1 node. The OT center tap node and the screen node use the same ground wire and ground near the PT. All other filters ground to preamp ground buss. There is no hum (well...none significant)...the amp is very quiet regarding noise/hum. I'll check it though.

[Bob-I]

The OT center tap node and the screen node use the same ground wire and ground near the PT.

I'd seperate these. Ground the OT CT near the PT, the screen near the tube sockets.

[mlp-mx6]

I concur with separating these. I had a problem with "ghost notes" in one amp (My TDS-style). The power plate current was likely modulating the screen voltage, which showed up in the speakers as ghost notes when cranked. Separating the grounds solved this issue.

[Fischerman]

I'll consider it...I've seen other unexplainable, strange things happen when changing a ground point. I once had an amp go into major squealing oscillation when I added a small value cap from signal to ground. But this happens at all volumes...if anything it improves when I crank the amp.

I certainly want to track this down but I definitely don't want to get into the dog chasing it's own tail thing by just changing things willy-nilly. I've used a lot of restraint to not mod this thing to hell and back like I often do...only to have to make a new board because the old one looks so bad and ultimately go back to 'stock' or very close to it. I already did that to 3 previous homebrews.