Princeton Reverb Distortion issue.

[Anthon]

I solved the hum issue from the previous post, thanks to some advices. It's practically hum free now, even at the max volume. Some hum still coming from the reverb, but it doesn't bother me.

But there is still couple of nasty things going on:

When I adjust the volume pot, it makes a scratchy click sound. I've noticed that if I go slowly, there is no clicking - but if you change it fast enough you can clearly hear it. I suppose it's not the pot, otherwise the speed would not matter?
Also, when the amp is pushed into distortion, it makes nasty scratchy sounds. Not always, but half of the time.
I suspect these problems are related.

I tried swapping preamp tube, but the problem is still there.

Does it ring any bells?

 

[JohnRoberts]

Look for a leaky (leaking DC current not spoo) in series with level pot.

JR

 

[Anthon]

Look for a leaky (leaking DC current not spoo) in series with level pot.

JR

Leaky cap? Before the volume pot, there are 3 tone control caps. I measured DC voltage after them referenced to ground, but it gives 0 volts DC. Could it come from somewhere else?

 

[andyfromdenver]

Just to add, I recently serviced a Proreverb that had a really horrible distortion.  sounded correct and good on 2-3, but turning volume pot up had a scratchy-ness that never cleaned up with maintenance.  The pot showed no physical damaged, but was bad.  Upon removing it, it read way out of spec and a new pot cured all.
One may not suspect the pots, and look for other things like DC leak, especially when the harder solution is to remove the pot, but, it's worth a try for your problem.

and  to add, I didn't see your other thread, but when you mention hum in the reverb, have you tried moving the tank, or rotating it 180 degrees?

 

[Anthon]

Just to add, I recently serviced a Proreverb that had a really horrible distortion.  sounded correct and good on 2-3, but turning volume pot up had a scratchy-ness that never cleaned up with maintenance.  The pot showed no physical damaged, but was bad.  Upon removing it, it read way out of spec and a new pot cured all.
One may not suspect the pots, and look for other things like DC leak, especially when the harder solution is to remove the pot, but, it's worth a try for your problem.

and  to add, I didn't see your other thread, but when you mention hum in the reverb, have you tried moving the tank, or rotating it 180 degrees?

Okay, I'll give it a shot. Getting desperate. I really should fix my old scope, it would make troubleshooting so much easier.

Yes, the hum issue was partially bad grounding. I made a grounding bus which went to the input jack, and connected all grounds from the turret board to it and it went away.
And partially reverb tank sitting right under the power transformer on my work bench. I moved it away, and it's much better.
It is still there, but not noticable when you play, so I guess it's good enough.
The amp itself, with reverb level turned down, makes virtually no hum even on the highest volume.

EDIT: I've replaced the pot, but it didn't help 

 

[CJ]

what kind of pan is it? some have different grounding on the jacks, like both grounded, input only grounded, or output only grounded,

as for the the other problem, try using a pencil to mover wires around while the amp is making noise, could be a lead dress problem that is causing ultrasonic osc.

make sure output xfmr has the blu and brn wires going to the right tube socket, and that the FB is going to the right place on the phase inverter, maybe , maybe try disconnecting the FB resistor and see what happens, you may like the sound better with no FB.

 

[andyfromdenver]

Okay, I'll give it a shot. Getting desperate. I really should fix my old scope, it would make troubleshooting so much easier.

Yes, the hum issue was partially bad grounding. I made a grounding bus which went to the input jack, and connected all grounds from the turret board to it and it went away.
And partially reverb tank sitting right under the power transformer on my work bench. I moved it away, and it's much better.
It is still there, but not noticable when you play, so I guess it's good enough.
The amp itself, with reverb level turned down, makes virtually no hum even on the highest volume.

EDIT: I've replaced the pot, but it didn't help 

Anthon! sorry I didn't know this was a new build...I wouldn't have so quickly suggested the pot.  poop.  I have posted re: before, but I had a build going for a long time with no problems, then when I sold it it came back after a couple months with some bad distortion and problems.  After close scrutiny, I had wrapped a connection (turret board) and forgotten to solder it, it was a cathode resistor in the preamp and after a year my wrap gave out and was intermittent.  I agree that chopsticking could reveal a bad joint.  I have helped others with their builds in the past and in one instance they had a shielded lead connecting to the signal and it didn't reveal until we moved stuff with chopsticks.
Best wishes with the Gremlin!
Andy

 

[Tubetec]

Bear in mind there are little critters ,spider mites that take up home in amps ,they like all the rest of us have to poop ,if they happen to poop on the tracks of your pots you can get slight discontinuities , a shot of iso will de-cak it.
When de-caking pots I prefer not to allow the cleaning solution get into the bearing /shaft area which has its own special grease ,it wont affect the workings of the pot electrically ,but it may well give a poor feel to it ,before hitting it with cleaner try sweeping the pot back and forth lots of times it may well clear the trouble ,if it doesnt sparingly apply a shot of iso and set the amp on the bench so it doesnt run into the bearing surfaces and wind the pot up and down . Leaky caps will cause dc across your pots and that will result in noise as you move it. . Sometimes pots can just go funky ,I did a job on a guys hi spec bass amp recently sealed pots ,so no spider crap ,yet still noisey as fook ,I had to lever the pot casing open to get some iso inside ,short term it worked and I had to tell the customer that  it was a common fault which many people were complaining about ,you have to be slightly diplomatic in these situations , but holding back the truth of the matter only makes you look incompetant if the trouble resurects itself .Its never fun to be the harbinger of doom and have to tell someone they have been sold a dud and see their  face sink into the ground when the bit of kit they pinned all their hopes on and payed out top dollar for has issues .

Sounds like youve made the best of the grounding situation Anth ,in my experience theres always going to be some fuzzy buzzy on the return from the tank no matter what you do ,its just the laws of physics coming into play, unless you want your sound awash with reverb it shouldnt matter too much .

 

[Anthon]

what kind of pan is it? some have different grounding on the jacks, like both grounded, input only grounded, or output only grounded,

as for the the other problem, try using a pencil to mover wires around while the amp is making noise, could be a lead dress problem that is causing ultrasonic osc.

make sure output xfmr has the blu and brn wires going to the right tube socket, and that the FB is going to the right place on the phase inverter, maybe , maybe try disconnecting the FB resistor and see what happens, you may like the sound better with no FB.

It's output grounded.

The OT wiring is okay - it doesn't sound like oscillation. More like a bad contact. The distortion itself is fine, but it has loud scratchiness/crackling artifact added, sometimes.
It's hard to replicate distortion problem now. I adjusted the speaker connectors on my cab. Perhaps it was just a bad speaker connection? Usually when I assume something, I'm wrong - I must know it for sure.

I moved/tapped wires and solder joint, nothing seems to make a noise.

 

[CJ]

you can try wiggling the preamp tubes around in the sockets, 

 

[CJ]

just did some work on my PR, put in an OPT that has a 4 ohm tap so we can run an extension cab,

this amp has three things to get us a step up on the new kids on the scene with their AC 30s and Blues Juniors>

1) 12AY7a preamp tube (V1)
2) Cathode Bias
3) Tweed DeLuxe gapped OPT

so what we have is kind of like a Tweed DeLuxe with reverb and Vibrato and a crispier tone stack.

We also have a 100K Audio Taper for the Reverb pot as the stock linear has resolution that leaves much to be desired.

there is also a switch on the feedback line to get a little more kick if desired.

and running a 5U4 rect instead of the 5Y3  mainly to avoid breakdowns.

here is the AC and DC voltage chart, plate voltage on the 12AY7a is lower than what you would have with the stock 12AX7a, >

 

[Anthon]

Interesting.
I've also built a Deluxe - I like the way it saturates more, I guess it's because it is cathode biased. Also a NFB, but I actually modded it with Champ-type NFB, and I like it more that way.
But the spring reverb on PR sound awesomes.

So I guess it's the best of both world. I will build a Tweed-style amp with a reverb eventually.

However,  the problem of volume knob clicking persists, and I have no idea why it does that.
When I turn it past 1 o'clock at moderate speed it just makes a clicky-pop sound (I tried replacing the pot, preamp tube, tone caps, no difference). Other than that it makes no noises.

How could I narrow down the problem? Perhaps bypassing the tone stack, to see if it changes anything?
I assume it comes from the preamp section - but can it come from somewhere else?

 

[CJ]

unsolder  the wire that goes to the top of the vol pot and see if i sill does it,

check ground of vol pot wire.

 

[Anthon]

unsolder  the wire that goes to the top of the vol pot and see if i sill does it,

check ground of vol pot wire.

I disconnected the wire coming from the treble control. (only ground and output to the next stage remains on the volume pot), but it still makes these clicky sounds.
So I guess it doesn't come from the preamp/input, but somewhere else? The pot itself is fine, I already tried replacing it.

EDIT:
Would it be a good idea to skip the stage before the pot, and send it directly to the next stage, and see if it the volume pot makes clicks then? To further isolate the problem.

 

[MagnetoSound]

Yes. Try everything until you find it.

 

[Timjag]

What valves are you using? New 12ay7s are pretty terrible and now ones are, well, old.

I've had no end of crackly ones straight out of the box. The socket is another killer for crackling. 

Of course I frown on guitar players destroying black plate 6072a tubes in guitar amps ;-)

 

[Anthon]

What valves are you using? New 12ay7s are pretty terrible and now ones are, well, old.

I've had no end of crackly ones straight out of the box. The socket is another killer for crackling. 

Of course I frown on guitar players destroying black plate 6072a tubes in guitar amps ;-)

I use JJ.  I tried swapping all tubes,

Now I've noticed something that I don't think is normal.
After the coupling cap of the third stage I read 18VDC. As far as I know the grid of the phase inverter must not get DC currents.

I already tried swapping tubes and replacing the cap - what else can cause this DC? Or is this normal?

 

[MagnetoSound]

It's supposed to be there. Elevated 'fixed' bias derived from the cathode, this path also provides a degree of positive feedback for the phase inverter, perhaps to boost the gain slightly as it is a single stage PI, a fairly uncommon arrangement in a Fender amp.

 

[Anthon]

It's supposed to be there. Elevated 'fixed' bias derived from the cathode, this path also provides a degree of positive feedback for the phase inverter, perhaps to boost the gain slightly as it is a single stage PI, a fairly uncommon arrangement in a Fender amp.

Darn, I thought I was getting somewhere 

 

[Tubetec]

I think thats normal Anth, you have a split load phase invertor ,basically that means the load is split between anode and cathode ,as you can see the grid is fed from a tapped resistor off the cathode ,the upper resistor drops a volt or two so the grid is slightly below where the cathode sits . Bear in mind also you  probably arent reading the true volts off the grid due to the loading effect of the meter .JJ tubes seem to be rated highly there ,I have a buddy who lives in Belgium and he swears by the JJ's too.