Input tube grid pin squeals if approached with a chopstick?

[w2cfx]

Hey all, I've been building a tube amp using a lot of information from this forum. I don't have a schematic as it has changed a lot in the process, but I don't think this is much of a schematic thing. Basically it is relatively high gain and I've used a bunch of snubbers etc to prevent oscillation. I finally have it working fine, having stubbornly refused to ask for help that probably would have got it working ages ago because I wanted to do it myself... Also too lazy to redraw my schematic and a little shy about my ugly lead dress... It's mostly functional, short as possible or shielded with one end grounded. It's generally quiet enough for me.

However, I was playing around with it today and it started oscillating out of nowhere. Opened it back up, and noticed that if I so much as approach the grid pin of v1a (6n2p, I've checked that I've wired it correctly) with my wooden skewer, it begins to squeal like crazy - that is, even if I'm just *close* to it with a wooden object, not touching, it starts to oscillate (gets worse if I get closer/touch it). The tube doesn't appear to be microphonic (if I tap it without wiggling, nothing, swapping the tube doesn't fix it). Grid stop resistor is right on the pin, if I touch the other side of it, nothing. Just the pin side. The resistor *seems* mechanically fine, though I could try swapping it out. I've reflowed the joint, and fiddled with pushing the pin around, but I'm mostly concerned with it screaming if I go near it.

Any idea what would cause this? Heck, is it normal with a vaguely SLO style gain channel? I can move wires around it to improve it slightly, but at the end of the day I'm a bit concerned by just how reactive it is...

Hoping one of you very clever folks has some advice for me.

[tubeswell]

Pickin’ up some chopstick vibes. If you hold the chopstick differently, does it change pitch or go away?


Nah, just messin’

[w2cfx]

I've actually played around with that - it is pretty much just volume of the oscillation that changes based on how close the skewer/chopstick is. Not a very good theremin, unfortunately 😂

[sluckey]

You didn't give us anything to work with! A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp.

What resistance does V1A grid measure to chassis?

High gain amps don't like ugly lead dress.

[w2cfx]

Sorry, I'll try and get a schematic and voltages together when I get a chance. Figured it might be worth asking before getting everything together with how odd the issue seems to be  (like, how is it reacting to the skewer being nearby and not contacting anything ‍) in case it had been encountered before ahah.

V1A grid measures 1.1Meg to chassis with a cable inserted in the jack and 100k without - correct for the 100k grid resistor and 1meg resistor from input to ground I guess. The oscillation in reaction to a chopstick barely touching it is much less audible without a cable in, which makes sense.

It is the same whether I grip the chopstick directly or with insulated pliers, though.

The lead dress is ugly enough to be shy about, but seems mostly effective, this being the only part of the circuit where even vigorous chopsticking does anything audible.

Edit: I've attached a rough schematic of the first few bits of the gain channel (with 12ax7 instead of 6n2p, mind). I hope it's enough to provide some insight - but it could be wrong as I've just whipped a lot of it up now. I guess it's also worth noting that there aren't really any other issues with the amp aside from this, like I'm super happy with how it sounds and behaves... Just this one weird behaviour! If I really get stuck I might just silicone the pin in place so it can't shift at all and hope it doesn't hate the silicone! Or fashion a little shield for it 

[tubeswell]

Ditch C25, C16, C9, C31, C33 and C34 to start with (i.e. try it without plate bypass caps, especially the ones going direct to B+ rail.)

[w2cfx]

Ditch C25, C16, C9, C31, C33 and C34 to start with (i.e. try it without plate bypass caps, especially the ones going direct to B+ rail.)

Removing even just c31 makes the oscillation occur very loudly even when the chopstick is not near it. Putting the chopstick near it after this makes the oscillation pitch drop. Removing c25 (and reconnecting c31) made it appear faintly without touching.


I put them back in and then swapped out the grid stop resistor for a 68k one - the oscillation now only triggers when I touch the body of the resistor with the chopstick, and if I prod it, I can hear it through the speaker, as if the resistor itself is microphonic? It's a 0.6w metal film.


Edit: I have added a 2.2Meg resistor to ground between the ferrite bead/sleeve (which is around shielded coax) and the grid stop resistor. The resistor retains some level of microphony but no longer squeals? Could it be related to the ferrite? It's a pain in the behind to remove and frankly I'm happy to leave it where it stands now, but I haven't checked what effect the resulting input impedance might have on effects before it.

[tubeswell]

Running plate bypass caps that connect to the B+ rail is not an optimal solution because of the tendency for electrolytic filter caps to not provide perfect signal shunting from B+ to ground (and plate bypass caps are ‘in series at AC’ with the filter caps). It’s preferable to run plate bypass direct to ground (if you need them at all). But You’ve not only got them on all stages to the B+ rail, you’ve also got them from plate to cathode. This is a crazy double-up. If one of the cathode bypass caps is suboptimal, it could cause all sorts of strange feedback.

[w2cfx]

Running plate bypass caps that connect to the B+ rail is not an optimal solution because of the tendency for electrolytic filter caps to not provide perfect signal shunting from B+ to ground (and plate bypass caps are ‘in series at AC’ with the filter caps). It’s preferable to run plate bypass direct to ground (if you need them at all). But You’ve not only got them on all stages to the B+ rail, you’ve also got them from plate to cathode. This is a crazy double-up. If one of the cathode bypass caps is suboptimal, it could cause all sorts of strange feedback.

Yeah I just kind of... Added them progressively as individual suggestions I had seen for reducing noise and each one seemed to help a little bit, until the noise was mostly gone aside from this one bit of weirdness. I will try an individual larger plate bypass to ground on v1a tomorrow and see how it does!

Thanks for all of the advice by the way - I've also removed C34, given that it's plate to an unbypassed cathode...

[bmccowan]

but I don't think this is much of a schematic thing

I suggest that you rethink that. I appreciate the desire to DIY. But there comes a time when it is smart to consider it a learning experience completed and go back and get it right.
I assume you have a workable chassis, good iron, and a bunch of other good parts. Everything you need to build an amp to be proud, rather than "shy"
of. Folks here will help you build a great amp - without the Band-Aids. But as others say, it starts with a willingness to share your schematics and progress photos.

[nandrewjackson]

Is it built with turret board, PCBoard, terminal strips, modeling clay?
Maybe I missed something,  but I don't see that info.

[w2cfx]

but I don't think this is much of a schematic thing

I suggest that you rethink that. I appreciate the desire to DIY. But there comes a time when it is smart to consider it a learning experience completed and go back and get it right.
I assume you have a workable chassis, good iron, and a bunch of other good parts. Everything you need to build an amp to be proud, rather than "shy"
of. Folks here will help you build a great amp - without the Band-Aids. But as others say, it starts with a willingness to share your schematics and progress photos.

Thanks for this, I needed to hear it. Yeah I have those, drilled a big Hammond enclosure myself, made some mistakes in doing so but it's okay enough, I have a nice solder station and good quality components (the transformers are a slight question - they work and test fine but they are ancient Australian things with limited documentation and next to no recorded uses, took forever to find the specs, I have no idea where they would stand quality wise). I will get some stuff together and post it when I get a chance. I'm proud of how it sounds even with it being a bit hacky, but if I can get a result I can be proud of aesthetically as well I guess I really should. I did intend for this to remove the need for me to buy another amp again, after all.

Back to work this week though, so this sits as it is for a little while unfortunately.

@ nandrewjackson: turret board, it does have 2 relays on an entirely separate transformer (as my 5v tap on a separate 6.3v secondary of my pt added a crazy amounts of noise no matter what I did, and I had one lying around from an old project), that are on Vero. As much as possible is wired directly on the sockets to keep leads short. The number of reworks has burnt some insulation on the leads where any exposed wire has just been taped up pending me replacing them and tidying up everything once I've ironed out the bugs and finished the schematic/layout. I guess I assumed from the nature of the issue (oscillation when approached, not even touched) that there would be a more general answer so I didn't get things together beforehand, sorry.

[HotBluePlates]

... The lead dress is ugly ...

Edit: I've attached a rough schematic of the first few bits of the gain channel ...

Wow...  I would start by looking at your power supply.  Do you really have all of the preamp fed from a single filter cap ("D" in the schematic)?

Next I would be removing at least C25, C31, C13, C28, R23, C22, C16, C33, C9, C34, C5 and C35.

You know what oscillation is?  An output late in the circuit that is in-phase with an input earlier in the circuit, with a bunch of gain in-between.  And every one of those caps is shifting phase, probably enough that there's no telling which wire may be next to another wire that's the right-phase to howl.  That the wiring is messy enough to be embarassing just means there is compounded risk of squeal.

It might be wise to start with 3 gain stages and get a stable, working circuit with fewer bolted-on circuit elements.  Then if you can get that running without oscillation, you can add a gain stage & adjust.  Then work from there (although 5 gain stages may require some experience to get running well).

[w2cfx]

... The lead dress is ugly ...

Edit: I've attached a rough schematic of the first few bits of the gain channel ...

Wow...  I would start by looking at your power supply.  Do you really have all of the preamp fed from a single filter cap ("D" in the schematic)?

Next I would be removing at least C25, C31, C13, C28, R23, C22, C16, C33, C9, C34, C5 and C35.

You know what oscillation is?  An output late in the circuit that is in-phase with an input earlier in the circuit, with a bunch of gain in-between.  And every one of those caps is shifting phase, probably enough that there's no telling which wire may be next to another wire that's the right-phase to howl.  That the wiring is messy enough to be embarassing just means there is compounded risk of squeal.

It might be wise to start with 3 gain stages and get a stable, working circuit with fewer bolted-on circuit elements.  Then if you can get that running without oscillation, you can add a gain stage & adjust.  Then work from there (although 5 gain stages may require some experience to get running well).

Thing is despite the horrors it does work - I've got it down to this one bit of oddness despite the amount of gain and after adding the 2.2M resistor it's pretty much under control. As such, I'm not super keen on taking it apart entirely when it works well enough, even if it could be a bit better. Risk vs reward isn't there...

[PRR]

Then don't play anywhere that serves with chopsticks.

[w2cfx]

Then don't play anywhere that serves with chopsticks.

Unfortunately my wife is Malaysian so that includes my house! That said PRR, I owe you a huge thanks for teaching me a substantial amount of what I've learnt about building pedals over the years. The number of issues I've found answers to in your posts over the years is astounding.

As a general update, I've taken some advice from the thread, run supply voltages from different caps, removed a bunch of the capacitors, and I do have a just sliiiightly higher noise floor but a lot more treble to work with which is a good result. The 2.2M resistor did fix the bulk of the sensitivity anyway, giving me a lot more wiggle room.