PNP Rangemaster + 1044 voltage inverter oscillates in effects loop

[slashandburn]

Hey all, hope you're well. I'm back looking for advice.

Ive sold a PNP Rangemaster a few months ago and the buyer was happy but has just run into a problem. For whatever reason they have decided recently decided to run it in the effects loop of a Vox AC30 and found that it oscillates when turned past 10 o'clock.

My first thought was that it's likely the TC1044 voltage inverter causing the issue but since it only happens in the effects loop I'm starting to think there's a chance they're trying to run this pedal a bit like the old "volume pedal in the effects loop" trick which would make the think input shielding is the more likely cause.

Any ideas? Seems a strange place to put a Rangemaster to me.

[idy]

Yes, strange. The whole point of an R-Master is the "antique" impedance relation between pickup and transistor.

And effects loop is generally higher level than instrument level... people do things like "pad" the signal down a bit and then re-amplify before the return.

[PRR]

> a strange place to put a Rangemaster to me.

"Ah, to do that you need our 'Rangemanger FX', designed to sit in an effect loop. Only $75 more than our traditional Rangemaster!"

[Rob Strand]

For whatever reason they have decided recently decided to run it in the effects loop of a Vox AC30 and found that it oscillates when turned past 10 o'clock.

When there's a threshold of oscillation related to volume or gain it's usually a true oscillation problem.   Any whine from the converter would be expected to be present at all times and just get louder when you crank something.

Given it doesn't oscillate normally with a guitar you would expect wiring in the pedal is OK.   The rangemaster  only has mild gain anyway.   It's more likely to be related to the amps wiring.  Inside the amp, the FX in is probably wired too close to the amps input circuitry.  When you put in the rangemaster the extra gain is enough to cause oscillation.   Oscillation involves the amp and the pedal.

The way around that would be to add some small caps to ground on the input and/or output of the rangemaster.   There's no guarantee it will work.  Maybe the caps need to be so large it affects the tone.   A dodgy way you to do a test is with long leads to the Rangemaster - the cables will add the capacitance!

So that's probably the most likely scenario.

It's possible the output impedance of the AC30's FX out is high and the pedal is oscillating on it's own.   I suspect that's less likely since the rangemaster gain is fairly low and the guitar impedance is pretty high anyway - the difference is though the guitar always has a long cable. [The inverting gain also helps against input to prevent oscillation but it doesnt make it immune to oscillation due to long emitter lead wires.]

[kaycee]

I have no idea why someone would want to put a Rangemaster in an effects loop? Guitar, Rangemaster, amp and stick the other effects (modulation) in the loop.

[slashandburn]

Thanks everyone. (Particularly Rob, really helpful!)

I have no idea why someone would want to put a Rangemaster in an effects loop? Guitar, Rangemaster, amp and stick the other effects (modulation) in the loop.

Same here, honestly just thought it would better idea to get some other opinions before going straight in with "why would you put it in the effects loop if you were already happy with it running into the preamp?" I'm guessing either they're trying to run it like a volume pedal in that "faux attenuator" type role, or maybe they're losing a bit of high end with whatever other effects they have in the loop and thought sticking the treble booster in there might solve that. Pure conjecture though.

Even if my main amp did have an effects loop it's likely not something I'd ever have thought to test with a Rangemaster.

It's not a massive issue really, was just looking for possible explanations before resorting to just bluntly telling them that the obvious solution is just to take it out of the effects loop.

Cheers
Iain

[idy]

The only way to go beyond spit-balling and hand-waving is to experiment.
Try other types of booster in same place.
Try tiny caps in the RM.
Try making an NPN Rangemaster.
Try removing the charge pump and kludging a battery in directly...

[slashandburn]

Yeah all good ideas. Try to recreate the issue and then take.ot frommthere. Only problem with any of that is that none of my own amps have an effects loop! 

I do have a mate with an AC30, he's just a bit of a nightmare to get hold of.

[slashandburn]

> a strange place to put a Rangemaster to me.

"Ah, to do that you need our 'Rangemanger FX', designed to sit in an effect loop. Only $75 more than our traditional Rangemaster!"

Actually huge thanks for this Paul, I initially took this as a fairly blunt and humourous response but as usual you've hit the nail on head in the most succinct way.

I'm not sure the AC30 ever had an effects loop until recently. It's certainly not what the Rangemaster was ever designed to be used for.

Just to help me sleep at night I'll try to recreate the issue and then see if ditching the inverter solves it. Any "fixes" beyond that is surely no longer really a Dallas Rangemaster.

Thanks again folks. Really appreciate the replies.

[PRR]

The Rangemaster input overload is 10mV-20mV.

The output level of most FX loops at normal "loud" is 100mV-400mV. So it will be overloaded almost all the time.

The gain of a Rangemaster is up to 32dB @10kHz.

FX loops are made for "unity gain" devices. Reverb, delay, chorus, echo, EQ. While these things may get set for a small gain, 32dB (1:40) is huge gain. Most guitar amps run not-far below internal oscillation. Because they have a lot of gain (already) in a not huge box, and because it adds a little zing on the sound. If your microphone PA system is somewhat shy of howl, and you bump-up 32dB, it's sure to scream. (and turn the knob way down, it may stop.)

So yeah, you can use a broomstick for a car clutch-shaft, but it may not be happy.

http://www.geofex.com/Article_Folders/Rangemaster/atboost.pdf