Why does my treble boost "motorboat"?

[gtone]

Don't get me wrong - I like "motor boating" as much as the next guy, but this kind of motor boating gets old fast...

Picked up a nice little RangeMaster clone a while back, Ge PNP transistor, 9V battery powered only.  Sounds great with 2 of my amps, but get a "motorboat" hum/oscillation with a third amp that increases as I wick up the boost.  As I get to 75-80% boost level, the oscillation starts to drown out the guitar's signal coming thru the amp.  Crying shame too, as this amp's circuit really sounds dynamite with other TB's.

So what causes this?  Any simple fixes, short of not using it with that amp?

[mrjamiam]

Sounds like the problem is likely with the amp rather than the pedal.  I was researching the pulsing, motorboating kind of problem recently, and most of the discussion I found seemed to center on power supply capacitors going bad.  If the amp in question is on the old side, that would IMO be more suggestion that it's a bad (due to age) cap.

[Jakeboy]

Agreed. You have likely already troubleshot the issue to the amp. A RM is a simple circuit. If it works fine with all other amps, then it would seem that the amp in question is the culprit. 

It is very, very odd though that other TBs work ok with the amp....if they are also RM clones one would think the amp would act up with them too..

i would keep running combos between the amp and TBs to definitively nail down which is the problem.

[gtone]

No problems with the amp boys - works perfectly without the pedal and also with any other drive, distortion or fuzz pedal I have.  Also sounds very tight with a low noise floor (negligible hum), no microphony or squeals, etc and the tubes are relatively new.  I'm 99% sure it's an issue with the pedal.

[mrjamiam]

1 hour ago, gtone said:

No problems with the amp boys - works perfectly without the pedal and also with any other drive, distortion or fuzz pedal I have.  Also sounds very tight with a low noise floor (negligible hum), no microphony or squeals, etc and the tubes are relatively new.  I'm 99% sure it's an issue with the pedal.

I'd say it's more like you have a problem with the amp+pedal combination.  You said that your amp doesn't have problems with other pedals, and your pedal doesn't have problems with other amps, but these two in particular together cause a problem.  So it's a system problem, and the question is whether something is wrong with one or the other of the components, or whether they just don't play nicely together as a system.

I think we are all operating with too little data to really solve the problem.  In general, the pedal's job is to boost the signal into the amp and drive the tubes harder.  If your RangeMaster clone does that to a greater degree than your other pedals, that could be the difference as to why it happens with that pedal and not others.  If we don't measure, we don't know for sure, we just have a symptom.

An oscillation can only occur in a circuit with "memory", i.e. with a component like a capacitor or an inductor in it.  Unfortunately, that doesn't guarantee that the problem is the capacitor or inductor; a changed resistor value will change the frequency response of the circuit too.  But resistors apparently don't fail that way very much.  Inductors are constructed like transformers, and don't fail all that much either.  Capacitors, at least electrolytic ones, do fail more often than the other components, from what I understand.

So, you could have a component starting to fail in the amp.  Or, you could have a situation where the pedal+amp system operates at a point that wasn't foreseen by the amp designer and isn't going to work for you.

[gtone]

Think the key might be that original Rangemaster circuit was NPN and this is PNP version.  Builder might be able to shed some light on how he altered circuit to accommodate PNP transistor, that is - if I could get ahold of him.  I'll do some more digging and see what I can come up with...

 

Edited to add that the original circuit looked to be PNP, not NPN.  

[Jakeboy]

But how odd that it works with your other amps. Are the other amps similar to the one with the problem?

[gtone]

Issue appears using it in front  of the Vintage 47 Supreme, a Valco Supro variant that has sounded good with all my previous Ge TB's (Scott's Crispy Cream, AM Beano Boost, SGFX Rock Machine).  In all fairness, the underlying tone of the effect sounds frickin' awesome, but the motorboat hum is a complete bummer.  It's noticeable at 25% boost, mildly annoying at 50% level and completely unusable by the time you hit 75% (oscillation is basically as loud as guitar signal with modest touch/attack on strings, if you hit harder, the signal is more prominent than the oscillation).

No such issues using that TB with the Winfield Cyclone (TB15 variant) or my '66 Bassman head.  Haven't tried it yet with my Garnet Rebel head as it's over at a rehearsal space. 

Gonna try a couple of more things when I get the chance.  An SHO clone between the amp and the TB should be able to see if it's an impedance issue between the amp and the TB.  Still trying to contact the builder to ask him about the issue also.

[HSB0531]

 

Here's a link that mentions Germanium circuits and oscillation:

http://www.muzique.com/lab/fuzzface.htm

[gtone]

^^^

Awesome possum - thanks for posting that!  

[HSB0531]

On 4/9/2016 at 0:57 PM, gtone said:

Think the key might be that original Rangemaster circuit was NPN and this is PNP version.  Builder might be able to shed some light on how he altered circuit to accommodate PNP transistor, that is - if I could get ahold of him.  I'll do some more digging and see what I can come up with...

 

Edited to add that the original circuit looked to be PNP, not NPN.  

Here's a link to a forum that's discussing the NPN to PNP circuit mods:

http://www.diystompboxes.com/smfforum/index.php?topic=50136.0

[HSB0531]

On 4/9/2016 at 0:08 AM, gtone said:

Don't get me wrong - I like "motor boating" as much as the next guy, but this kind of motor boating gets old fast...

 

Me too

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