BM Double-stop OD Problem

[mdroberts1243]

O.k. this is gonna hurt (your ears).

I've got a problem with the OD of my BM100W... it manifests itself as a loud bass note when you play double-stops above the 9th fret. "Like you've got a bass player in complete sync with you."

I expect to hear some of the Intermodulation between the two notes... it is a standard product, but just not this much!

I've played quite a bit with varying presence and the negative feedback... I don't think it is in the PI/Power stage. It is accentuated by the speaker... I haven't done the work to see if it is being produced in the speaker, but I did note it was much worse with the RW&B compared to a Cannabis Rex.

I suspect it is being accentuated/produced in the OD stage. I'm looking for suggestions to tone it down. My build is based on schematic 11, but I have rolled off the bass a bit entering the OD (not shown on the schematic).

I've provided two clips, about 20sec that were chosen to show the problem in all its gory glory. One clip is the guitar signal recorded directly... the 'dry' clip. I can reproduce the problem easily by just feeding the dry clip into the amplifier. The other is the BM100, playing into a C-Rex & RWB simultaneously, with the C-Rex speaker close-mic'd with SM57... the 'wet' clip.

OD & PAB engaged (~6dB PAB). The OD trimmer is set pretty low (~24k), the Drive was up around 6.8 (dial marking). HRM trimmers: Treble 11:00, Mid & Bass around 9:30.

Thanks in advance,

[Fischerman]

Hi Mark,
That sounds pretty much like mine is doing. I tried cutting bass...to a point that I would consider extreme...and it didn't get rid of it. I notice you have cut bass in the CL stages quite a bit...I cut a little there (1uF Ck on V1a and a .022uF coupler after CL2) and tried massively cutting it at the OD entrance. No dice. I have no cure.

However, just this past weekend after cutting so much bass I tried the mid-boost (which usually makes it way too bassy) and it was an improvement over no mid-boost (PAB ON in both cases). It didn't seem to lessen the IM/whatever-it-is in as much as everything else was increased and the IM-stuff wasn't as prevalent in the tone. I guess my point is that if you can't eliminate/reduce it enough...maybe try masking it/covering it up such it's relative level isn't so offensive.

I just got frustrated with it and put it away for a while...I felt like playing...and enjoying it...so out came the Marshall/Fender amps.

[Structo]

Boy that would be annoying.
Hope you can get it sorted out!
Good luck.

[mdroberts1243]

Hi Mark,
That sounds pretty much like mine is doing. I tried cutting bass...to a point that I would consider extreme...and it didn't get rid of it. I notice you have cut bass in the CL stages quite a bit...I cut a little there (1uF Ck on V1a and a .022uF coupler after CL2) and tried massively cutting it at the OD entrance. No dice. I have no cure.

However, just this past weekend after cutting so much bass I tried the mid-boost (which usually makes it way too bassy) and it was an improvement over no mid-boost (PAB ON in both cases). It didn't seem to lessen the IM/whatever-it-is in as much as everything else was increased and the IM-stuff wasn't as prevalent in the tone. I guess my point is that if you can't eliminate/reduce it enough...maybe try masking it/covering it up such it's relative level isn't so offensive.

I just got frustrated with it and put it away for a while...I felt like playing...and enjoying it...so out came the Marshall/Fender amps.

Speakers/cabinets seem to make quite a bit of difference... what is the speaker/cabinet you are using?

[dogears]

mdr,

I am confused as to why you built your BM around some unknown internet schematic. As you know, I had one here where I measured and documented. Why don't you follow that one instead of messing with possibly incorrect stuff.....

[Fischerman]

I have tried the following:
2x12 open back with Scumback H75
Mojo BW 4x12 with 2 G12H30 and 2 Weber C1230
Marshall JCM900 with 2 CL80 and 2 V30
2x12 Bluesbreaker-type cab with Eminence RWB
Marshall 4x10 with orig. G10L-35s
Carvin 1x12 with the stock Eminence-made 'American Series' Carvin speaker

They all sound different but I can hear the 'problems' pretty much equally in all of them.

[Bob-I]

That's the strangest thing I've heard. It sounds like a LF oscillation to me, but why it would be triggered with these double stops I have no idea.

I'd start chasing it down with a scope to see if you can find the origin. Since it's reproducable you should be able to find it fairly easily, then you can narrow your troubleshooting focus to that area of the ckt.

Good luck,.

[skyboltone]

Weird man. Cap way out of spec? Cap leaking?

[odourboy]

I spent some time today puzzling over this. It's a weird one. There's an intermodulation 'sidetone' in the original 'dry' guitar clip at a frequency that appears to be the difference of the frequencies of the double stops being played. I had a look at the waveform on with a wave editor and I can see a lower frequency envelope that's around the 100Hz range (+/-). If you listen really closely, you can hear it.

I played that dry clip like it was a guitar through two of my amps and four different speakers and I can reproduce, though to a lesser extent, the effect that Mark has demonstrated. My HRM BM is a worse offender than my non-HRM. My RW&B brings it out way more than my 12-65. So there is something in Mark's amp and/or cab that is really accentuating that sidetone bigtime.

[BobW]

It's possible the amp is marginally unstable, and a change in the feedback circuit may help reduce the problem. Try a different tap on the OT or experiment with the Feedback resistors.

[tsl602000]

mdr,

I am confused as to why you built your BM around some unknown internet schematic. As you know, I had one here where I measured and documented. Why don't you follow that one instead of messing with possibly incorrect stuff.....

Now this is confusing me.... The posted schematic is the one posted in the Bluesmaster topic, right?

[mdroberts1243]

mdr,

I am confused as to why you built your BM around some unknown internet schematic. As you know, I had one here where I measured and documented. Why don't you follow that one instead of messing with possibly incorrect stuff.....

Now this is confusing me.... The posted schematic is the one posted in the Bluesmaster topic, right?

TSL,

Scott is referring to the genesis of that schematic... I cobbled it together by going through all the postings on the forum and with some additional input from forum members... I could get a good handle on the pi/pa from the postings but wasn't able to get a good handle on the plate/cathode/coupling values (or at least a definitive set of those values). There were rumoured values and etheral schematics floating around that suggested the values departed from the 'norm' for HRM... best summed up by the spreadsheet that ampdoc1 posted.

Today, I am still not aware of a fully documented BM, the only pics I've seen had blue goo.

For my build, reflected in schematic 11, I put in spring clips at critical places and then tried about seven different combinations of values... I settled on the 'sunnyland' rumoured values from the spreadsheet, making the obvious (with hindsight) corrections to the plate/cathode values which were flip-flopped on each tube.

I used the sunnyland values because they contributed to cleaner cleans (there is a much wider range of input levels accepted) and also tended to offer a wider range of compression in the potentiometer rotation before hard clipping sets in.

I would love to do a version of the schematic that has a real, definitive set of values from a real live dumble bluesmaster... send the values to me! I am a strong believer in having reference schematics (like #124) for a basis of discussion and tweeking.

[mdroberts1243]

I got side-tracked on this debug but tonight I was able to throw together one of those 'amp stethoscope' devices and walk my way through the amplifier... this is a great tool, but you've heard that many times before in other postings!

The low harmonic from the guitar double-stop decays quickly so I rigged up my synthesizer and used the harmonica sound to produce a double-stop on the keyboard that would sustain the low harmonic indefinitely with the foot pedal.

There is no evidence of the low harmonic until the plate of OD2... the grid of OD2 is completely clean. The low harmonic is in full bloom on the plate of OD2 though!

My 'sunnyland' OD2 values are 150k/2.2k/4.7uF... IIRC the 'standard' HRM would be 120k/1.8k/1.0uF.

Started playing with the bypass cap... no cap and you barely hear the low harmonic, 0.1 is very good too, 0.68uf seems to be a good compromise, 1.0uF doesn't sound much better than the 4.7uF, but might be subtly better... I'm wondering if the 1.0uF with the slightly lower plate/cathode might rolloff the bass sufficiently to put this to rest... something to try in the morning...

[dogears]

Interesting observations...

Fwiw, although the BM I had here was gooed, I did thoroughly measure everything.

[mdroberts1243]

Interesting observations...

Fwiw, although the BM I had here was gooed, I did thoroughly measure everything.

Scott,
Did you ever document the complete set of values in a posting somewhere? I'll make a 'reference' version of the BM schematic if you'd like to provide them.
I'm thinking that the 1uF is important as the bypass cap on OD2 now... I just finished a simple little simulation that varies the bypass cap only... see attachment. I didn't simulate the pre-OD or HRM Tonestack, just the two triodes, so this isn't the actual response curve of the amplifier, just a 'relative' set of curves for various bypass values.