The Infamous Murder One Amp - Problem Solving

[drdn0]

Howdy,

Just finished off a Murder One amp with a couple of changes. I'd say we're like...99% of the way there. Everything sounds incredible and works as it should, except I get some bad oscillation until I start playing after about a minute of it being on - before that minute, everything works perfectly and is dead quiet even with everything dimed, but after a minute or so oscillation starts. Oscillation is purely tied to volume controls - changing gain makes zero difference to it, nor does changing anything around the input, but rolling the volume down stops the oscillation entirely.

Could it be some sort of heat build up with the LT1054 regulator? Schematic included. It's fairly close to the original, with a few small changes. I notice I did miss the coupling cap before the volume controls which I'll butcher in later on.

[ElectricDruid]

Definitely get those coupling caps in there. Whether that will fix the oscillation issue I couldn't say, but without those caps, you've got the B+ voltage across the pots, so the pots are going to sound very scratchy and are also disipating some current as heat, which will shorten their life significantly. Lucky they're 1M pots or they'd be toast already.

[drdn0]

Definitely get those coupling caps in there. Whether that will fix the oscillation issue I couldn't say, but without those caps, you've got the B+ voltage across the pots, so the pots are going to sound very scratchy and are also disipating some current as heat, which will shorten their life significantly. Lucky they're 1M pots or they'd be toast already.

Fwiw, I only missed the coupling cap after R8. All the others are in there

[antonis]

You also missed a coupling cap right after R8..!! 

As it is, G1 is biased at level strongly dependent to Volume pot setting..

P.S.
I'd also try the version with bias switch (selection between LT1054 -Vout and GND)

[drdn0]

You also missed a coupling cap right after R8..!! 

As it is, G1 is biased at level strongly dependent to Volume pot setting..

P.S.
I'd also try the version with bias switch (selection between LT1054 -Vout and GND)

Sorry, that's the one I missed. Misread the schem looking on my phone.

Only that cap is missing, all others are there

[antonis]

Shouldn't you place that cap there..??

[drdn0]

The issue is 100% around my power supply. The -out is only giving me -3v output, and if I change the pentode bias to a 1M resistor to ground instead of the attempted -6v bias the noise entirely goes away. Even with it disconnected, it's only giving me -4v output which seems incredibly low.

Pin 1: 11.8v
2: 7.8v
3: GND
4: -3.6v
5: -3.6v
6: 2.4v
7: 1.4v
8: 11.7v

[bluebunny]

Motorboating crops up often with the Murder One.  It did on mine.  I don't recall the fix, but search for "Murder One" and you'll find a thread (or threads) where this is addressed.

[drdn0]

Motorboating crops up often with the Murder One.  It did on mine.  I don't recall the fix, but search for "Murder One" and you'll find a thread (or threads) where this is addressed.

Most of the issues I'd seen others have was with using a 7660S/MAX1044, which I'd hoped to fix by using the LT1054 (much higher switching frequency and much higher current output).

I'll do some more digging though. It's super quiet without using the inverted output

[drdn0]

More messing around - changed the charge pump cap to 10uf Nichicon low ESR, then to a 10uf solid tantalum. Inverted output remained at -4v, even with no load.

My thoughts are it's faulty/dodgy.

[Chillums]

Without the coupling cap after R8 the bias voltage will go to ground through the volume pot, no?

[antonis]

Lower than GND, in case of R9 connected to -VOUT..
(strongly dependent on VOL pot setting..)

[drdn0]

And again, it's been fixed and made no difference. Yes, voltage/current would go through the volume pot. It's such an insignificant amount that it would only cause issues with the tube bias; not put too much draw on the LT1054.

The issue is with the LT1054's inverting voltage functioning not working. Even disconnected it's not producing anywhere near the voltage it's meant to be.

[drdn0]

Well...I fixed it.

Looking at datasheets again, I realised that the original schematic is missing a cap for the negative output to ground. Added a 10uf cap in and voila - instantly brough the -ve rail up to 11.6v and all the noise instantly stopped. How the 7660S works without this I am not sure as it's also in all of its datasheets. This brought the grid down to -11.4v, which is obviously a mile off from where we need. I changed R9 from 100k to 1M, then finally to 10M and I've now got -5.9v on the grid which is close enough to the -6v.

Everything works now and sounds great, but I don't really have any clean headroom - even with minimum gain there's still a bit of distortion, but rolling down the guitars volume knob cleaned the rest of it up. It's also fairly dark sounding and a bit farty, and decreasing the coupling caps to 10nf didn't knock all of it out. I'm wondering if I need to add a input cap and reduce the grid stoppers a bit to brighten things up.