Another Princeton with volume pot hiss AA1164 original 66

[Ed_Chambley]

Don't know why this is happening, but I got another one and this one is different.  Still there is a place where there is no hiss.  This one is original and in good shape.  All the pot check out, replaced tubes with known good tubes.  Has been modded to fixed bias.  Pull tube v1 and hiss remains.  Pull reverb tube and hiss remains.  V3 and V4 silences the amp.  The amp has been maintained and has fairly new orange drops and all metal film resistors.  Used a listening amp and can hear the beginning of the hiss at plate on v4b, but the 56k resistor checked out.  I replaced it anyway.  Gets worse when disconnect the NFB.  All the caps seem fine and the bias cap is not leaking.  I replaced the resistors in the B+ as they looked like 1 watt so I put in 3 watt. 1K and 2 18k.  Tried swapping OT leads and squealed like a pig. Re-flowed solder joints and traced wiring.  Break the input ground and the amp will oscillate.   Owner said the volume pot is not the same in relation to output.  There is no volume until the pot is turned about 1/4, but pot tracks correctly if checked with a MM.  What else is there that could cause this? 

[worth]

Well. although I like to bias at 70 % plate dissapation, this will produce more audible hiss than a lower bias setting. Setting the bias a bit colder should reduce this a bit.

[Ed_Chambley]

Well. although I like to bias at 70 % plate dissapation, this will produce more audible hiss than a lower bias setting. Setting the bias a bit colder should reduce this a bit.

With the volume off there is a hiss as you increase the volume the hiss will disappear at just over half turn and reappear at 3/4 turn.  Not a constant hiss, but the hiss is loud with the volume at zero.

[HotBluePlates]

If pulling V3 ends the hiss, at least some/much of the hiss is in/before V3.

So it sounds almost like hiss from the normal channel could be cancelling some of the hiss in the reverb circuit. I don't know how this is happening. I have, however, noticed Fender reverb circuits tend to create a lot of hiss. Try keeping the reverb pot all the way off to see if it changes the behavior.

Also, if unhooking the feedback loop increases hiss, it either does that because there's a small amount of additional gain in the output section without feedback, or the hiss in the feedback loop was offsetting another source of hiss.

There will likely be more than one source of noise. Pull V3 and listen all over the amp. Cure what hiss you can find. Replace V3, remove V2, repeat. And repeat with V1.

[phsyconoodler]

Has been modded to fixed bias

They are fixed bias stock.
  Hiss can be plate resistors.Cheaper quality metal film resistors or ones that have been overheated on installation can hiss like crazy.
 Reacheck the wiring.Di you do the changes or did someone esle? There really is no reason other than the changes,really.

[Ed_Chambley]

Has been modded to fixed bias

They are fixed bias stock.
  Hiss can be plate resistors.Cheaper quality metal film resistors or ones that have been overheated on installation can hiss like crazy.
 Reacheck the wiring.Di you do the changes or did someone esle? There really is no reason other than the changes,really.

Yes, there is a bias pot. I guess I should say modded to adjustable fixed bias.  I just ordered some 100k and a few other resistors 1 watt, vishay. Never had a noise problem using them.  No, I have not done any work on this amp.  I am planning on just reworking the resistors.  I don't know anything that could cause it.

[Ed_Chambley]

If pulling V3 ends the hiss, at least some/much of the hiss is in/before V3.

So it sounds almost like hiss from the normal channel could be cancelling some of the hiss in the reverb circuit. I don't know how this is happening. I have, however, noticed Fender reverb circuits tend to create a lot of hiss. Try keeping the reverb pot all the way off to see if it changes the behavior.

Also, if unhooking the feedback loop increases hiss, it either does that because there's a small amount of additional gain in the output section without feedback, or the hiss in the feedback loop was offsetting another source of hiss.

There will likely be more than one source of noise. Pull V3 and listen all over the amp. Cure what hiss you can find. Replace V3, remove V2, repeat. And repeat with V1.

I did give it a try with the reverb pot off.  No change.  The noise probably is in a few areas as you mentioned.  I am thinking the largest problem is around v3.  Since it has been basically restored, it may have a couple of wrong values.  Haven't had time to check for this closely. Troubleshooting is harder than building or more tedious.  I will start at 3 and see.
Thanks

[HotBluePlates]

Not that you want to hear yet more possibilities, but Platefire once had a hissy amp cured by changing the cathode resistor rather than the plate resistor.

[Ed_Chambley]

Not that you want to hear yet more possibilities, but Platefire once had a hissy amp cured by changing the cathode resistor rather than the plate resistor.

I am already sort of expecting this.  Touching one of them makes the amp much quieter.  Found it when I was re-flowing.  This amp is sort of strange.  It looks like a different person put the caps in and another put the resistors in.  I think he has been chasing this problem for a while.  He did not say so, but I assume he would not want me to know that.  He was worried about the cost so I offered to buy it.  That derailed his concern.  These amps are quite valuable now and it has everything original except caps and resistors.  The old eyelet board has seen better days and may be holding humidity which is another wrench.  It's not like I cannot start from scratch with a new eyelet board, but I think it is not necessary.  My old SR's have warped boards and sound great with old CC resistors and blue tube caps.  Never really had a problem with noise in a AB763.  Always been tubes and filter caps.

[Ed_Chambley]

I have not been able to trace this down.  Replaced all the plate resistors and think since it affected by volume pot, seems like the problem would be prior.

What could possibly make the amp get quieter as you turn it up, the begin to get noise again.  There is a section on the volume where it is dead quiet.  When the amp is hissing touching the wire from the volume pot does nothing, but when it is in the quiet zone chopsticking the wire you can hear the stick touch the wire.  Sort of a bump.

I used a listening amp and can now only hear the noise at the plate of the power tubes, but not on the screens.  You can hear some signal if you touch the probe to the chassis, but it is very faint. 

I made the box version of a listening amp as well, but grounding the 1 meg pot to the same bolt where the chassis connection is makes the pot not work at full turn?  I do not know why.  I made it so I could use a louder amp.  I cannot hear anything coming from v1a.

Any input would be appreciated.  Since my next move will be replacing each part until I cure the problem.  The only place I can hear it clearly is at the first node in the power supply.  I have never had a filter cap do this and was wondering if it could?

[phsyconoodler]

Use sheilded wire on the volume pot.

[plexi50]

Dougs listening device has been a god send for me since i built one a few months back. It's like having an xray machine to diagnose a patient. A must have/

[Ed_Chambley]

Use sheilded wire on the volume pot.

Will do.  I am willing to try anything at this point.  If that doesn't get it, I am just going to rebuild it.

[HotBluePlates]

Break the input ground and the amp will oscillate.

This makes me worry. I owned a number of Princetons and never had one so obviously unstable. Does it look like a lot of "techs" have been inside it?

Anyway, I still have a hunch the reverb circuit is hissing (which you hear with the Volume at 0), and when you turn up the volume some the preamp hiss reaches a point that it cancels reverb hiss (minimum hiss setting of Volume control), until the preamp hiss is louder and dominates (rising hiss at higher Volume levels).

I've had vintage Fenders where there was nothing obviously wrong, but which I could not get completely quiet. I've made new builds (and heard Tubenit's builds in person) where you wouldn't know the amp is on except for the pilot light (until someone plays a power chord).

[DummyLoad]

i had a super reverb of the same era with a similar problem - turned out to be fish paper eyelet board was becoming conductive. after about a 20-30 min warm-up hiss became tolerable.
richard and & i replaced every component in the reverb circuit to no avail - we finally checked the eyelet board. that amp i had spent 20+ years in a garage in hot & humid south texas.
measure DCV from several open areas on the eyelet board to ground... you'll know if that's your problem.

usually it's a resistor. with the amp on and speaker loaded, work your from the preamp onward with a DMM; usually the culprit resistor the hiss gets better or worse when you probe it. cathode resistors can be suspect as well, hell, just about any resistor for that matter.

--DL

[Ed_Chambley]

Break the input ground and the amp will oscillate.

This makes me worry. I owned a number of Princetons and never had one so obviously unstable. Does it look like a lot of "techs" have been inside it?

Anyway, I still have a hunch the reverb circuit is hissing (which you hear with the Volume at 0), and when you turn up the volume some the preamp hiss reaches a point that it cancels reverb hiss (minimum hiss setting of Volume control), until the preamp hiss is louder and dominates (rising hiss at higher Volume levels).

I've had vintage Fenders where there was nothing obviously wrong, but which I could not get completely quiet. I've made new builds (and heard Tubenit's builds in person) where you wouldn't know the amp is on except for the pilot light (until someone plays a power chord).

Yes, lots of different looking work inside the amp.
The volume pot hiss does not get louder and cover up another hiss, what it does is gets dead quiet when the volume pot between 500 and 700 ohms or around this.  Like you said, you do not even know it is on till you hit a chord.

[Willabe]

The volume pot hiss does not get louder and cover up another hiss, what it does is gets dead quiet when the volume pot between 500 and 700 ohms or around this.

I think you might have missed/missunderstood what HBP was saying.    

You might have 2 sources of hiss that are 180deg. out of phase that sound/seem like 1 source of hiss. As you change the settings at some point they cancle each other out more than at other settings.

Since you have a volume pot and not a switch on an R with the exact value to cause full cancelation it becomes more or less hiss as you turn the pot. Like tuning in a radio, closer, closer, oops, to far.    

I think that's what HBP was saying?


                    Brad      
 

[HotBluePlates]

Yeah, that's what I was thinking.

I hope DL's not right that you need to replace the fiberboard. Then again, a replacement fiberboard is probably the least invasive way to fix this (as odd as that sounds). It would probably be a less-obvious repair than any other way to correct the noise.

I think I had somewhat similar issues with hiss in my old Fender amps. I tried replaced plate load resistors, other simple stuff. Never did have a listening amp at that time, and never got rid of all the noise.

If you just unsolder the wires running away from the board, replacement oughta be pretty easy. Might be fun, regardless of the outcome.

[Ed_Chambley]

Yeah, that's what I was thinking.

I hope DL's not right that you need to replace the fiberboard. Then again, a replacement fiberboard is probably the least invasive way to fix this (as odd as that sounds). It would probably be a less-obvious repair than any other way to correct the noise.

I think I had somewhat similar issues with hiss in my old Fender amps. I tried replaced plate load resistors, other simple stuff. Never did have a listening amp at that time, and never got rid of all the noise.

If you just unsolder the wires running away from the board, replacement oughta be pretty easy. Might be fun, regardless of the outcome.

Already begun this process as shielding the volume pot did nothing.  The board is assembled, I am just waiting on the owner.  He is concerned with value.  I asked what he thought it was worth hissing so bad.  He said he would think it over.  People are funny.  I told him he could keep the old board and I would put it back in any time he wanted the hiss again. He doesn't like my humor, but that is understandable.  I think he has had this amp in a few shops and has grown weary.  I know the feeling.

I did not get any volts from the fiberboard due to moisture.