Princeton Reverb Build Hum

[aabbs20]

Hey all,

I just finished up a high powered Princeton build and it sounds killer but has more hum than I'd like. I've tried everything I can think of and have gone through the troubleshooting steps on here and on the geofex and Rob Robinette pages.  All voltages are as expected and the power tube bias is right where it should be.

The hum does not change with any of the controls. It goes away completely with V4 pulled, mostly goes away with V3 pulled.  Grounding out the 470k, 3.3m, 10pf connection eliminates it.  It is mostly gone grounding out the other side of the 3.3m/10pf. 

I've tried chopsticking everywhere, different tubes in all positions, tested all filter caps/clipped in an additional cap, tried grounding the "D" power line cap to the preamp bus, installed a humdinger pot, and all with no results.  I've built a number of amps like this and never had this problem and I'm totally stumped. 

Anyone else seen something like this before and have another trick up your sleeve?

[nandrewjackson]

Are you using all carbon resistors,  metal film resistors,  etc. ?


SS rectifier   , tube rectifier?


Is it point to point, or on a board?


Are you using any shielded signal wires?

[aabbs20]

mostly metal film resistors with a few carbon comps.

tube rectifier

on a board

shielded wires going to the grids on V1A and V1B, grounded on 1 side only.

[aabbs20]

I've made some progress identifying the issue but I'm still unsure of how to fix it. I used a RTA to diagnose the frequency of the hum and while there is a little 60hz in there, the offending hum is definitely 120.  It is not super loud, like a missing center tap ground would be, it just hums more than I'd like.

My recent discovery happened when I was probing various ground points for what must have been the 10th time.  I had a lead clipped to the ground bus and was probing the other ground connections when I heard the hum get quite a bit louder all of a sudden. In fact, any time I moved the end of the lead close to the 10pf/3.3m/470k area the hum got a bunch louder. Same thing if it was near the grid pin of V3B or the grid wire.  I loosened the board and made sure the under-board wires were in the right places and they are.  I tried swapping in a shielded grid wire but that did not solve the problem either.

Seems like some kind of ground loop going on and that area is picking up the hum.  Not sure how to proceed from here but I'm happy there has been some progress. 

[Leevi]

Check the negative bias voltage that there is no ripple. Add an extra filter stage there if needed.
/Leevi

[66Strat]

Check the cables going to/from the reverb tank. Make sure that the shield is only attached to ground of the RCA connector at one end of each cable and that the grounded end connects to the amp chassis.

[aabbs20]

Thanks for the suggestions.

I've tried different sets of reverb cables and briefly disconnecting the reverb tank. The hum is still there.

I've also tried a couple different brand new bias filter caps (47uf) with no results. 

[shooter]

when I fixed broke things for a living, I would us the brute force method in cases where time vs money was in play.

so for $22 bucks (does not include tank cost) and 1 hr time, I'd just re-build it new.  My assumption, it worked fine, was designed fine, If I make it exactly like before, I'll be climbing rocks in 5 hrs 

[aabbs20]

Yeah....if I had done that from the start I could have re-wired the whole thing twice by now. 

[aabbs20]

more pics

[aabbs20]

couple more

[Latole]

You write:
"The hum does not change with any of the controls. It goes away completely with V4 pulled, mostly goes away with V3 pulled"

Look the issue is between volume control and beforeV4. A wiring dress ??
1- What do you call a Powered Princeton Reverb ?
2- Wich circuit do you use ? aa1164 ?
3-Can you show picture of you chassis ?

[aabbs20]

High Powered PR...bigger iron (Allen TP25 and TO35) and 6L6 power tubes.  Mostly 1164 preamp with some mods.  No Trem, NFB switch added, pentode/triode switch added, dwell control added.  Reverb is the pot on the far left, dwell is next to it. 

I've already taken the amp back to stock once to make sure my mods were not the issue...they don't seem to be.

Just uploaded pics. 

[Latole]

Great pictures thank's.

I did not see where the negative buss bar ( all along the board, close to pots ) is connected to chassis ground

[Latole]

No schematic with your mods ?

[sluckey]

I did not see where the negative buss bar ( all along the board, close to pots ) is connected to chassis ground

It's connected to the input jack ground lug which is mechanically fastened to the chassis.

[Latole]

Are you sure it is at chassis ground ? Did you read it with meter ?
I like more a bolt and lock washer thru the chassis

[aabbs20]

Pretty sure it's making a good ground connection there...I've used this type of grounding many times in the past with no issues and I've measured and re-done that connection a couple times. 

No schematic of the mods but I can draw one if need be. 

One more thing...

In addition to that area (3.3m/10pf/470k) acting like an antenna, if I tap the ground bus near there I hear crackles. 

Seems like a grounding or loop issue but I'm not sure how to find it.

[Latole]

You look to have experience in building amps, ground could be ok.
Your mods could or could not be the issue, seeing a schematic may help.

[Latole]

  if I tap the ground bus near there I hear crackles. 

 

It is not bad solder or bad ground on ground bus ?

[shooter]

It is not bad solder or bad ground on ground bus ?

It is, h's still thinking though 

[Latole]

In my opinion, you do not need to dream on the original circuit, remove your mods. But simply revise what you have done and so that we can help you provide us with a shematic with your mods.

On the other hand, repairing a new amp design remotely without having it in front of you is not easy

[Latole]

Your amp come from here ? ;

https://robrobinette.com/AA1164_Princeton_Reverb.htm



 

[aabbs20]

It is not bad solder or bad ground on ground bus ?

It is, h's still thinking though 

Seems like that would be it...that was my first thought too.  I've tried re-soldering all of those connections and just re-did the entire ground bus. The hum persists. 

I tried moving the ground point to a screw/lug attached to the chassis as well...same issue. 

I've built a bunch of Princeton type amps and I often use Rob's website for mod ideas and schematics/layouts. Some mods do come from there. 

[Latole]

I'll disconnect NFB from the phase inverter side. Or any mod between volume pot and V4

Can you do a schematic ? Not a layout.
Without schematic I'm blind

[aabbs20]

I've disconnected all mods and the problem persists.  I probably should have just started over by now. 

Preamp schematic here. I apologize for the poor penmanship...

[Latole]

Great , thank you. I wish to see output power tubes
I will disconnect here and see about hum;

[aabbs20]

I can draw the power tubes a little later. 

Disconnecting the reverb circuit didn't change much, if anything it made the hum a little worse.

[Latole]

Your incomplete schematic is a exact copy of part of PR B1270 from CBS
I show here;
You could have issue with lead dress or you make some building error.
Or issue is the schematic we don't see

[Latole]

I can draw the power tubes a little later. 

Disconnecting the reverb circuit didn't change much, if anything it made the hum a little worse.

You don't need to do sketch if you copy Fender / CBS PR B1270 I have this schematic.

[aabbs20]

Yeah...the power tubes are pretty much the same except they're 6L6s.

This amp has the tremolo omitted so that part of the schematic is not relevant here.

Thanks for your help.

[Latole]

Yeah...the power tubes are pretty much the same except they're 6L6s.

This amp has the tremolo omitted so that part of the schematic is not relevant here.

Thanks for your help.

I mean power supply ;

If you do a copy of a B1270 , we don't need your scketch.
Issue:
building error,
Solder,
Some bad wire dress

[Leevi]

Just try temporarily an extra filter stage before B+ I.e add a cap ~22u-33u and ~50R resistor there.
/Leevi

[Latole]

Disconnecting the reverb circuit didn't change much, if anything it made the hum a little worse.

It makes no sense if it induces even more hum.

Aren't the wires of the output transformer inverted?
The 6L6s are pretty well matched?
Are 12XX7 good? Out of all doubt?

Read AC voltage ( yes AC not DC ) a each tube plate : report

Without having the amp in front of me, I don't see how to help you any more. Good luck

[Latole]

Did you build Hoffman PR no tremolo ?

https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb_NoTremolo.pdf

[Latole]

Schematic;

[aabbs20]

Thanks Leevi...I did try an extra filter cap in various places with no results.

I've spent a ton of time hunting this down and want to give it a rest. I'm going to play the amp as is for a while and re-visit the hum situation at a later date...it does sound amazing.

Thanks for the help everyone.

[aabbs20]

Success!  I decided to revisit this hum issue yesterday and finally found the problem. I used the RTA app on my phone to measure the offending frequency (60hz) at the speaker and changed V4. Sure enough, the hum dropped by about 1.5db.  Very slight and my ears hadn't noticed it when swapping out the tubes before.  I did the same thing for the other three preamp tubes and each time the hum decreased by 1.5-2 db.  By the time all four were changed the hum was mitigated.  In my previous troubleshooting I only swapped out one preamp tube at a time, never all 4 together. 

The humming tubes are all TADs, three 12ax7s and a 12at7. The amp is now running on JJs and is much quieter.  Anyone want to buy some very lightly used TAD preamp tubes???

[shooter]

The humming tubes

how many hours are on them?  I've found with new tubes once I prove they work, I just burn 'em in for 10-20hrs Then start "using" them

[aabbs20]

Not many

The humming tubes

how many hours are on them?  I've found with new tubes once I prove they work, I just burn 'em in for 10-20hrs Then start "using" them

Probably 5-6?  Will they hum less once they're burned in?

[shooter]

I only know EH tubes and mostly the AU7, they seem to be cranky as hell before 10hrs, nothing "stays the same".  by 10 hrs they seem to resign themselves to becoming what my amps like 
hum typically isn't my issue, it's normally erratic voltage/current