"Superfly" - submini tube version of Doug H's Firefly

[cab42]

I'll try that. Unfortunately it looks like I shouldn't have bought my wire from Amazon,

I read this on my phone without glasses as “Unfortunately it looks like I shouldn't have bought my wife from Amazon” 

[Ben N]

I read this on my phone without glasses as “Unfortunately it looks like I shouldn't have bought my wife from Amazon” 

Everyone should take this to heart. Words to live by.

[GibsonGM]

"Vibration"...I took apart an old computer that was left in the barn for a few years. Re-used the nice small-gauge stranded wire.  During testing, the fuzz I made, which sounded great otherwise, showed symptoms of intermittents.  I debugged it XYZ ways, and could only come up with corrosion of the wire caused by being stored in a damp environment all that time, and perhaps from the shock of pulling on it while deconstructing.   Once I replaced all the offboard wiring, it was fine.
What a nasty experience that was. 

[jma38]

Anything loose could be part of it, yes, if good contact is coming and going. Even a bad cable can cause that (tip, move them around, make sure that's not it.  Use 1 guitar cord direct to the firefly, make the signal chain simple).  You can use the eraser end of a pencil to 'move wires and things' around to see if something is loose.

Does it do it both with the tube screamer in the signal chain, and also with it not there (completely removed)?

Any way you can make a recording/video of the issue in action?

Here's the video. The volume dropoff happens at around 1:26: https://www.youtube.com/watch?v=RKTRihsxvOU&ab_channel=JonathanSilasBull

[GibsonGM]

Ooo, sum ting not right for sure.  I'm picking up almost a rectified sound, the 'octave-like' tone.  Then, of course something happens that dumps the volume.   

A couple of things come to mind...something strange with the bias...maybe a polarized electro cap in the signal path backwards...a missing pulldown resistor somewhere....bad bias in the power section (?) causing that cutout (R values...).  Lack of grid stoppers maybe.   Does it cut out like that if left alone (not playing; no input)?  Does it come back in a bit if you stop playing?

- can you affect this dropout by using the eraser end of a pencil as formerly mentioned, to move wires/sockets a bit, to see if something is loose?

If nothing seems loose, I'd try to get it to happen again, and audio probe through the preamp (triode) signal path from last preamp stage back toward input, see if there is a spot the sound seems normal while it's cutting out. 

BE CAREFUL, respect the voltages where you are probing, and be sure everything you touch is fully insulated and the cap in the probe is of higher working voltage than the B+.  It's helpful to have something inputting a 'noise' while you probe so YOU don't have to....a test oscillator or even tunes from a cell phone or something.   It's awkward to try to pluck strings while you probe, esp. with high voltage there...

[jma38]

I'll try that. Unfortunately it looks like I shouldn't have bought my wire from Amazon,

I read this on my phone without glasses as “Unfortunately it looks like I shouldn't have bought my wife from Amazon” 

 

[jma38]

Quick question for those who have built the Superfly: Do you put thermal paste on the IRF740 before screwing it to the heatsink? Would it do anything?

[thomasha]

Yes, you have to.
The roughness of the surfaces create small gaps with air between parts, which drastically affect the heat flux.
The paste fill these gaps, and has a better conductivity than air. 

On the other hand, I haven't used a heatsink in my superfly. The current flowing through the mosfet is low enough. It gets hot, but not very hot.

[rankot]

Quick question for those who have built the Superfly: Do you put thermal paste on the IRF740 before screwing it to the heatsink? Would it do anything?

You can even try to use MOSFET with lower Rds(on) than IRF740. Lower the resistance, lower the dissipated heat.

[jma38]

So today I applied thermal paste to the IRF and got it secured to a heatsink. A problem I'm running into is when I try to adjust the center tap to 185v my multimeter shows around 193v and slowly goes down until it hits 15v, the lowering in volume is directly correlated with how low the voltage drops. Not really sure how to approach this bug.

[thomasha]

Well, good to know it's SMPS related. Is anything getting too hot?

- It could also be a saturated inductor? How much current can it supply and how high is its resistance?
- it could also be a problem with the transistor which controls the voltage...
- It could be the mosfet getting too hot (in one of my builds, using a different SMPS it would drop from 250v to 100v when hot)
- It could be a bad/leaky reservoir cap (the large electrolytic one, C6, that should be rated for more than 300v)
- It could be a short demanding too much current from the SMPS
- It could be a bad high voltage cap too (C8)

To test the SMPS alone you need to unsolder R11 and the transformers centre tap.
With them disconnected test the voltage and see if it still drops after some time. If not, the mistake is somewhere else.
If it still drops check the reservoir cap.

If you could measure some voltages would also help. Like:
- Both sider of R10, VR1, and Q1

Maybe if you replace the caps and the transistor it works.

[jma38]

Okay so the IRF is getting really hot. I turned the amp on for 30 seconds then turned it off and touched the heatsink and it wouldn't have burned me but it was really hot. What could be the cause of this?

[rankot]

Make sure you don't put too much pressure on IRF! So keep NE555 at approx 50% duty cycle, not more than that. I've had problems with SMPS in the past putting it to more than 90%, which will not allow MOSFET to cool.

[jma38]

Make sure you don't put too much pressure on IRF! So keep NE555 at approx 50% duty cycle, not more than that. I've had problems with SMPS in the past putting it to more than 90%, which will not allow MOSFET to cool.

With further tweaking it's running stably at 126v. Not what Rick recommended in the original layout but my Superfly doesn't have any dropouts now and the IRF isn't getting hot at that voltage and the amp is plenty loud.