Troubleshooting Frog preamp

[DIY Bass]

I have built this

https://frogpedals.com/wp-content/uploads/2016/07/Frogtubepreamp2.0Schem.jpg

Currently it is put together but not boxed.  I have added a master volume to the output.  Anyway, with a function generator on the input, and a headphone amp on the output, there is a horrible noise from the output.  Turn on the function generator and the noise goes away.  So far so good.  If I touch the volume knob (not the extra master volume I added - the one in the schematic soldered to the board) the noise comes back.  It doesn't happen with any of the tome pots, just the volume.  Only while I am touching it.  If I ground the pot shaft them I can touch it with no problem.  I know that the obvious solution is to box it.  Just wondering before I do that if that sort of touch sensitivity is normal/expected, or if there is something I should be checking before I box it.

Thanks

[GibsonGM]

1. Double-check all solder connections, especially around that pot. If it isn't grounded, 'bad noise' is expected. There are other reasons a finger can make noises happen (or stop them), like capacitive coupling, but first be confident of all connections (esp grounds).  On all wiring, jacks and so on.  Something is oscillating (obviously) - gotta find out why.   Run in/out and power leads away from each other as much as practical so they don't interact.   Move them a bit with a pencil while oscillating to see if anything changes or disappears.

What voltage is your function generator set at? Something reasonable like .5v or 1v?

I would remove the 'clipping section' until it is working, then see if you want it.  As this is a tube pre, diode clipping seems a bit...counter-intuitive, but it's your build...     I'd like to see some power filtering before your B+ hits the first triode, too.

2. Not really enough info to tell you 'hey, go here and do this'.  We'd need some pics of the front and back of the board.  Is it soldered, or on breadboard (which is very capacitive and cause problems).  Show any mods! Important.  Need some voltages - power supply in, plate, grid, cathode, heaters...esp. want to know how that charge pump is doing   Don't zap yourself!  Ask if you need directions on safe work with tubes.

Do you have an audio probe?  Be aware - there is usually a cap built into a probe, and it must withstand the supply voltage to use it at tube voltages!!  I used one rated 600v.  Wait on probing for instructions if you are not familiar with or comfortable doing it.  There are places ~200V in that preamp.

For the 'data' we need, go here and copy/paste back here:  https://www.diystompboxes.com/smfforum/index.php?topic=29816.0 

 

[DIY Bass]

1. Double-check all solder connections, especially around that pot. If it isn't grounded, 'bad noise' is expected. There are other reasons a finger can make noises happen (or stop them), like capacitive coupling, but first be confident of all connections (esp grounds).  On all wiring, jacks and so on.  Something is oscillating (obviously) - gotta find out why.  Run in/out and power leads away from each other as much as practical so they don't interact.  Move them a bit with a pencil while oscillating to see if anything changes or disappears.

I have checked (and redone) the solder on that pot.  I have tried to make sure the external wiring is away from the board and each other as much as possible.

What voltage is your function generator set at? Something reasonable like .5v or 1v?

It's set to 0.1v, but it doesn't even have to be running.  As long as the power is on the noise goes away unless I touch the volume pot.  The noise is the same as when you plug in a guitar lead to the input but leave it open to the air.  I am assuming that turning on power to the function generator is just stopping the "open lead" problem, but I could be in error about that assumption.

I would remove the 'clipping section' until it is working, then see if you want it.  As this is a tube pre, diode clipping seems a bit...counter-intuitive, but it's your build...    I'd like to see some power filtering before your B+ hits the first triode, too.

Yeah, it seemed a little odd to me too.  I soldered the diodes in, but the switch is on/off/on.  I expect I will leave it in the off position to be honest.  Just added it in so I don't have to bother about it later.

2. Not really enough info to tell you 'hey, go here and do this'.  We'd need some pics of the front and back of the board.  Is it soldered, or on breadboard (which is very capacitive and cause problems).  Show any mods! Important.  Need some voltages - power supply in, plate, grid, cathode, heaters...esp. want to know how that charge pump is doing   Don't zap yourself!  Ask if you need directions on safe work with tubes.

I used the PCB sold by the designer.  No mods except addng a 1M log master volume to the output.  I will take photos but that will be tomorrow.  The B+ on the test point measures 187V.  The heater measures 6V.  I am using a 6N2P valve.  I can measure the others tomorrow.

Do you have an audio probe?  Be aware - there is usually a cap built into a probe, and it must withstand the supply voltage to use it at tube voltages!!  I used one rated 600v.  Wait on probing for instructions if you are not familiar with or comfortable doing it.  There are places ~200V in that preamp.

I do have an audio probe but I think the cap in it is only 50V ceramic.  I will have to cobble together something with a higher voltage cap before I can audio probe successfully

[antonis]

I do have an audio probe but I think the cap in it is only 50V ceramic.  I will have to cobble together something with a higher voltage cap before I can audio probe successfully

It should be rated at B+ voltage, at least..

[GibsonGM]

Hmm.   The input jack sleeve is connected ground, right?  Ensure that pot has a good ground (with a DMM).  I have to ask  Place a jumper from the gnd lug to actual gnd if you have to.  Check every single ground.   


Could this be something with the signal generator?  I'd plug a guitar in and see what happens...also, thru an amp placed some distance away.  I like to eliminate all the external BS as much as I can before I think there is a problem.  (I've had trouble w/headphone amps esp. the kind I DIY lol).

I've had this kind of thing happen and it was a break INSIDE a wire going to a pot, solder connections were good.  I try to aggravate things if I can - make it do the noise, wiggle the wires (do not shock yourself). 

It may be oscillating above your hearing, and your finger changes the frequency by its capacitance.  We won't go there yet tho, let's rule out basic things.

If you have to buy a cap for the probe, just get one rated very high, say 600V, so it is useful in most amps.  Don't worry if it is small in value, like .47u or .68u...we're not looking for sound quality, just signal 

[DIY Bass]

OK, I have some photos.  When I get a chance I will plug into a guitar and amp and film a video of what is going on. I wiggled all the wires without any buzzing.  Somewhere I have a small packet of 250V ceramic caps.  I just need to find them tomorrow and redo my audio probe.  If that fails, I do have a small oscilloscope that can take 40V, which means that with a 10x probe it will do 400V, so I should be able to see things even if I can't hear them.

[GibsonGM]

Can't tell from the pic, but is the vol. pot wired correctly? It LOOKS to my eye that the center lug (the wiper) is not connected, and that 2 wires are going to the left lug.  Output should be from the center, one end to circuit, other to ground.  Sorry if I just can't see it. 

Can't really see the solder joints, the pic's fuzzy, I'm sure it's not your fault - but there could be something happening in there we can't see...these things are almost always build errors.

Try it with a guitar. Any circuit that adds a decent amount of gain is going to be sensitive to external junk.  Your hand is a big capacitor and EASILY can have an effect that is then amplified.   If it still does it, verify solid electrical connections with your DMM, continuity.    Really, an audio probe (tho you need one) is likely to just show a little 'brrrraaap' at the pot, then a big one at the plate   

[DIY Bass]

I think I have it sorted.  Reflowed and added extra solder to the valve socket and it seems to be behaving a lot better.  There is still some noise, but you have to be a fair way above unity volume to really hear it, so that seems fairly normal to me.  I intend running it around unity in any case.  It will be running into a DI box and I don't want to kill anything in the PA.

In the process I did manage to find a 3.3nF 630V capacitor, so I will definitely be upgrading my audio probe as well.

Thank you all.

[GibsonGM]

Glad it worked out!  It's almost always some kind of build error that gets us.   

The effect of your hand on the pot should also be decreased when it's inside a grounded enclosure.

Try 3.3n; if it seems too small you can always get a larger value one later.