Solderin' time!

Misterg

EdGrip said:

Now. Pin 7 of V2 connects to the previous stage (and the reverb) via a 3.3M resistor and 10pf capacitor in parallel. I was poking about in the amp with a stick to see if anything responded oddly, and I found that poking this capacitor, resistor, and the wire which connects them to pin 7, resulted in my poking/scraping being quite loudly amplified. No other components behaved this way. I'll investigate further today.

Some capacitors can be quite microphonic, but the fact that you got the same effect on the resistor & leads might indicate that there's a bad solder joint on one of these parts, or maybe that the resistor is cracked.

It does sound like the problem is microphonic, though - you could try pressing on these parts with the proverbial chop-stick when the oscillation is happening to see if it makes any difference. (You could do the same, but holding each valve in turn with a thick cloth to see if any of them are causing the problem...)

Good luck...:)

guitargeek62

EdGrip said:

Just found this: Some dude's Princeton Reverb kit build... :O

I've gotta make one of those some day - although I'd want it to have silverface cosmetics like yours!

EdGrip

Silverface is best face. Fact.

Right, so I finally got round to doing some investigatin'. Tested that NFB resistor - it's fine, about 6.3k. 

Re-flowed all the joints around that grid pin 7 on V2, to no avail. So I just desoldered the wire from pin 7 and lifted it... the problem goes. Have ordered another 10pF cap and 3M3 resistors - I'm going to use 1/2W metal film as it's easier and this little junction and signal-carrying resistor seems to have been problematic for others too.

 - Ed

EdGrip

Might as well shield that lead from the resistor/cap junction over to pin 7 while I'm at it - where do you chaps get shielded hookup wire from?

EdGrip

In fact, rather than buy 3m3 1/2W resistors I will just get three 10M metal film resistors from the pedal building box and screw them up in a ball.

Misterg

EdGrip said:

...where do you chaps get shielded hookup wire from?

I bought a reel of it....

If you're stuck, I can stick some in the post.

EdGrip

That's very kind of you, sir - I've just this minute bought some from BitsBox. They're very good, BitsBox.

Thanks, though!

EdGrip

I cancelled the BitsBox order - turned out I already had some shielded single-core.

You know how it is. You've been through the wire and cable drawer, you've been through the pedal parts drawer, you've even been through the batteries and stuff drawer just in case, y'know.

And then you think, Oh! I haven't looked in the aircraft control column drawer! Duh!

So the amp will have Alan Turing t0nez AND Civil Aviation Authority t0nez. Bargain! (Aircraft wiring helps preserve the highs)

Cacofonix

Does it have a Bigglesby?

EdGrip

No, but it does help the amp maintain composure during divebombs.

Cacofonix

Makes it sound rudder lovely.

Edit: and you can coax better performance from it.

I'll get me coat.

EdGrip

Right.

Apologies for the mess - I've just been substituting parts and poking stuff around to see what affects the problem and how.

See the silver mica cap and the 3m3 *resistor* in parallel? At the right end of that is the new, black shielded wire that goes to pin 7 of V2. At the left end is the wire that goes round the corner to the 500pF and .02uF caps.

Also coming from the shielded wire is the 470k *resistor*, and from the other end of that the white wire which goes to the wiper of the reverb pot. Here is what we know:

 - Replacing all of these components has not solved the problem.

 - Lifting the white wire at the left end does not solve the problem.

 - Lifting the black shielded wire (to V2 pin 7) at the right stops the problem completely. 

 - Lifting the white wire to the wiper of the reverb knob stops the problem completely, but it also renders the amp eerily quiet and non-functional.  

 - The problem is the same whether or not the reverb tank is connected. 

 - Putting a pillow in front of the speaker stops the problem completely, suggesting that it's microphonic - although it occurs to me that damping the speaker might also be electrically damping the secondary of the OT, or something like that.

Hmm.

I'm a bit stumped. I will continue investigating, but any thoughts would be very welcome at this point.

Cheers,

 - Ed

EdGrip

Someone on the Vintageamps forum says: 

"You've got a bunch of good advice - just a word on the damping the speaker with pillows - all you're doing there is preventing the speaker from reproducing the low frequency note. It's still there."

It's an interesting thought, but when I whip the pillow away, the amp takes a moment or two to notice the change and start making the noise again.

EdGrip

Someone on Vintageamps thinks the noise is most likely symptomatic of a bad filter cap/filter caps. I know they got a bit of a roasting (literally) thanks to the miscalculated custom PT. They got too hot to touch, poor lambs. I will remove them, and temporarily recommission the old cap can, which is still in situ. If that solves it, I'll order some new F+T caps.

ICBM

EdGrip said:

Someone on Vintageamps thinks the noise is most likely symptomatic of a bad filter cap/filter caps. I know they got a bit of a roasting (literally) thanks to the miscalculated custom PT. They got too hot to touch, poor lambs.

Yow… you didn't mention that.

Yes, they're knackered if they got that hot.

EdGrip

ICBM said:

Yow… you didn't mention that.

I know. *facepalm*. In hindsight it should have been the first thing I looked at. I think it's because it was fine when I had my initial little jam through the amp after I powered it up - it was only when I took it round Pete's that it started happening, so I think I decided that it was because something had got knocked or shaken in transit. Sorry.

EdGrip

Right. I pulled the filter caps out and wired back up to the cap can. The amp is STILL doing the foghorn thing. I just.... GRRARRGHHGH

Misterg

EdGrip said:

 - Lifting the white wire at the left end does not solve the problem.

 - Lifting the black shielded wire (to V2 pin 7) at the right stops the problem completely. 

 - Lifting the white wire to the wiper of the reverb knob stops the problem completely, but it also renders the amp eerily quiet and non-functional.  

 - The problem is the same whether or not the reverb tank is connected. 

 - Putting a pillow in front of the speaker stops the problem completely, suggesting that it's microphonic - although it occurs to me that damping the speaker might also be electrically damping the secondary of the OT, or something like that.

Something doesn't ring true about that...^

Lifting the white wire at the left end should kill everything except the reverb signal. Do you still get dry signal through? If you disconnect the reverb tank, do you still get signal through? With reverb on zero??

Lifting the black wire (to pin 7 V2) should kill *ALL* the signal and render the amp non-functional. If you're still getting signal through, it has got to mean a wiring error of some sort...

Lifting the white wire to the reverb pot should kill just the reverb signal, but it also removes the ground reference for V2a grid (there are capacitors in all the other connections), so likely this valve will run away and become saturated due to grid leak current. Likely this is why the amp appears 'eerily quiet'. You could try grounding the other end of this white wire instead, but (if all is correct) this is the same as turning the reverb pot to zero.

Are you disconnecting the input or output of the reverb tank (or both?) - if you have just been disconnecting the input, it it possible that the tank itself is responsible for the microphonic feedback.

I'm looking at this schematic, btw:

http://www.spearfoot.net/sfpr/princeton_reverb_aa1164_schem.gif

EdGrip

That's the one - I usually use the GZ34 schematic because the scan is clearer and everything but the rectifier is the same.

I don't know if I still get signal through when I disconnect the white wire before the cap and resistor - the oscillation remained, so I didn't bother to try playing guitar though the amp. I would imagine that the wet reverb would still get through.

Lifting the black wire to pin 7 does render the amp non-functional for amplifying - it just leaves the familiar buzz of an unplugged guitar lead, and it also stops the oscillation. This is why I think that the oscillation is somewhere before this point, as if it were OT or filter cap related, it would still do it regardless of what the preamp was doing. At least, that's my intuition, but I don't know a lot. It would be helpful if the symptoms I've described DO narrow down the search to the pre-pin-7 area.

I have tried it with the tank connected (both leads) and disconnected (both leads), and there is no difference. It might just be a lead in an unhelpful place or something. I don't know. My head hurts. 

Thanks, again, for all your help with this.

juansolo

EdGrip said:

Just found this: Some dude's Princeton Reverb kit build... :O

:-o MOTHER OF GOD! :-o