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Princeton Reverb Hum...stumped

Discussion in 'Amp Tech Center' started by tube-works, Feb 20, 2014.

  1. tube-works

    tube-works TDPRI Member

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    1966 Princeton Reverb all original no modifications. All voltages within spec. Bias is at 29mA with 435 on the plates. Amp sounds good, reverb works fine (other than hum described below), and vibrato functions correctly.
    To Date:
    Recapped with 4 x [email protected] cap can from AES
    Replaced all electrolytics in circuit
    Bias board mod per Hoffman Amps
    New Tubes
    Cleaned pots, jacks, and tube sockets
    Installed 3 prong grounded power cord
    Problem:
    Excessive Hum when powered on….Hum not affected by Volume, Bass, or Treble pots, Reverb pot acts as volume for hum. Reverb tank cables not plugged in so tank and cables are not the issue. All RCA jacks as factory with no apparent issues. All lead dress and circuitry as factory and chopsticking has no effect.
    Troubleshooting to No Effect: I.e. Hum remains constant
    Re-floated all solder joints and grounds
    Unhooked reverb pot
    Unhooked reverb transformer
    Swapped in another 4 x [email protected] cap can to eliminate power supply problem associated with a bad cap can
    Swapped known good tubes (two different sets)
    Biased colder
    Troubleshooting with an effect
    Plugged in Reverb footswitch. With switch engaged- hum is reduced
    Other Potentials
    Before recapping and fixing the bias I played two gigs with the amp…it was humming but I just figured it was the caps or tubes….on the second gig I noticed that the PT was pretty hot….Could the PT be internally shorted to some degree…all voltages seem good on the primary and secondary
     
  2. muchxs

    muchxs Doctor of Teleocity

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    Oh, crap. Sounds like a fun one.

    Several things I'd check:

    It sounds like you re- flowed your cap can grounds twice. I notice I can sometimes lift the entire blob of solder off the cap can ground leaving a clean spot on the chassis. I use a Weapon of Mass Destruction for chassis grounds I call "Big Smoky". It's a giant soldering iron that was once used to solder countless chassis grounds on the assembly line over at Western Electric.

    Make sure the cap can has a solid ground. I'd jumper the can to the chassis and see if that made a difference.

    That's the thing with these old amps... they use the "ground of convenience" method. Everything on one side of the circuit board grounds to the brass strip under the pots. If the amp has been damp or stored in a damp environment the grounds get touchy.

    I've been replacing a lot of input jacks and the star washers underneath them lately. The input jack needs to be nice and tight. The star washer needs to bite into the grounding plate to get a good ground.
     
    Last edited: Feb 21, 2014
  3. roberts67

    roberts67 TDPRI Member

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    Hello!

    I think you are getting good advice! Let us know if you get it fixed!
     
  4. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Check the continuity to ground and correct value resistance in both of the two 100 ohm resistors in the filament circuit, which are located near the jewel lamp assembly.
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

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    Have you tried it with the reverb cables to the tank unlugged ?

    Now that I look again maybe you did.

    How about some good gut shots ?

    Did it hum before you started working on it ?

    Those new tubes are you sure they're good ? (new in box don't mean good)
     
  6. muchxs

    muchxs Doctor of Teleocity

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    I think CBG is on to something. I've done so many custom builds lately with a grounded center tap on the filaments I forgot all about the 100 ohm resistors. I deleted them on one Princeton and went with a grounded center tap. The other one had the dreaded four piece 100 ohm resistors... that's each resistor burned in half and dangling by its leads.
     
  7. kranz

    kranz Tele-Meister Silver Supporter

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    I had a hum in a Vibroking that bugged me for a long time. It turned out to be the input jack, which once replaced eliminated the hum.
     
  8. DavidP

    DavidP Friend of Leo's

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    Is the cap can original?

    If its been replaced or you reflowed the solder tabs on the original, I'd check the tab connection grounds -- I had bad hum after a so-called local tech replaced a cap can and didn't use a 'big Bertha' soldering iron -- the solder joints looked solid but the hum disappeared when I reflowed them with a 100W iron. You need a lot of heat to flow the solder onto the chassis and the 100W does it in seconds! I picked up a used 100W one and use it exclusively for chassis soldering jobs.
    If that doesn't work, I dunno...
     
  9. Andy B

    Andy B Friend of Leo's Silver Supporter

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    +1 100 ohm filament resistors. Also grounds on reverb tank & foot switch RCA jacks as well as input jacks.
     
  10. songsmith1950

    songsmith1950 TDPRI Member

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    A wild card guess here. Did you replace the filter in the fixed bias circuit. That can also go bad or the rectifier itself can become a little leaky and it can put a hum into the sound.sd Usually the hum will get louder as the unit heats more and more, but that could change your bias voltage if it is dropping volts there. That could cause your output trans to run hot.
    Just my thought.
     
  11. muchxs

    muchxs Doctor of Teleocity

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    '60s Princetons both reverb and non- reverb use the same power transformer as a Champ or Vibro Champ. It's a 70ma transformer. For the benefit of those who don't know what that means it means there is 70ma available on the high voltage. In the Princeton Reverb the secondaries are 340v-0-340v at the rectifier. Supply voltage to the 6V6 plates is indicated on the AA964 schematic as 420v. We refer to the AA964 schematic because it shows a GZ34 rectifier. Most AA1164 schematics show a 5U4G. The schematics aren't gospel. The schematics don't precisely match the physical reality inside the amp.

    Both schematics show the 125P1B power transformer. The physical reality is most '60s Princeton Reverbs used the 22772 power transformers, same power transformer as found in Champs and Vibro Champs. I've seen a lot of Champs and Vibro Champs with cooked power transformers over the years starting back in the '70s when they were nearly new. Yup, I was poking around in amps back in the '70s.

    Very Important Transformer Information:

    The 22772 transformer must be used with a rectifier with a 2 amp filament or it will run hot. Rectifier choices with an original 22772 are 5Y3, 5V4 and 5AR4/GZ34 even though the 5AR4 sends the plate voltage way high. The 420v shown on the schematic is "way high" for 6V6 tubes. Do not use a 5U4G with an original 22772 transformer! Not in a Champ, not in a Vibro Champ, not in a Princeton, not in a Princeton Reverb. It will run hot.

    The 125P1B "export" transformer has 100ma available on the high voltage. It will tolerate a 3 amp rectifier such as a 5U4G.

    This likely has little to do with the OP's hum problem. It's likely why the PT runs hot.


    Tubes for noobs 'cuz there may be a few reading:

    If we were to generate a useful although slightly random number we may note that the 6V6 plate voltage is roughly 1.3 times the supply voltage to each GZ34 plate. 340v x 1.3 = 420v at the 6V6 plates. In the "Keep your fingers out of it!" department you may notice that there's 680v from one lug to the other at the rectifier. That's typical in tube amps. There is usually 300+ VAC above ground waiting to give you a whack and 400+ VDC available in mumerous locations.

    Your results may vary. Schematic says 420v at the plates while the OP's actual measured supply voltage is 435 volts. That's because the supply voltage at the wall has gradually crept upwards. It was 115v in the mid- '50s, 117v in the mid- '60s and 120v+, now.
     
  12. andyfromdenver

    andyfromdenver Friend of Leo's

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    I've posted this in Shock bros.
    Here is what Michael Clark showed my friend who showed me for my PR to make it studio quiet.
    Disconnect the filter cap from the can that runs to the pres and install a new cap right from the 2 x 100k junction to the pre cathode ground.
    It will look a little funky, but should help.

    Make sure your dropping string is unchanged, just relocate the cap to the pre side to separate it.

    Make sense??

    This was their last ditch effort to remove hum when all caps were replaced and there was still an issue. It absotively worked for my amp, but yours may have some type of cumulative issue.

    I'll try and find the thread and link it cause there were some pics (may have gone away to archive hx).
     
  13. tube-works

    tube-works TDPRI Member

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    Thanks for all the replies fellas...
    Some notes....
    I used a 150watt iron to float the grounds...I bought it years ago after I had grounding issues trying to use a regular Weller to float the grounds...

    I removed the input jacks and gave them a thorough cleaning and checked the ground connections at the star ground behing each of them....

    Rectifier is a 5AR4

    The PT on this amp is stock and has a center tap for the heater filaments (so no 100ohm resistors)...a poster on the Weber site suggested that I check to make sure the AC voltages on each leg of the filaments were equal...they were not..3.0VAC and 5.1VAC respectively....I have been suspecting this PT since it had gotten so hot previously...I suspect it may have an internal short ....Thoughts???
     
  14. jhundt

    jhundt Doctor of Teleocity

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    those heater voltages are not right; the two together should add up to around 6.3 and yours are at 8.1 VAC. AND quite out of balance.

    The PT is stock and has a heater CT? Hmmm, maybe you should disconnect the CT and try the two 100-ohm resistors.

    OR - try adding the "hum balance" control. It is a 100-ohm linear resistor wired between the two legs of the filament winding, the center terminal to ground. This allows you to balance the heater and reduce hum. Check the PRII schematic, or one of the other Rivera-era amps.

    I'm still wondering about that 8 volts though... what do you measure at the actual tube heater connections?
     
  15. tube-works

    tube-works TDPRI Member

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    Hi jhundt! I measured each leg of the heaters filaments to ground at the pilot light which resulted in the voltages above...I double checked on V1 at the end of the circuit which (no suprise) resulted in the same values....all the heater lead dress is stock and in good shape....I checked the pilot light connections and its chassis ground ...all good there....

    PT is stock and has the CT...I'm gonna get some 100ohm resistors from home and lift the CT to see if that helps...

    Also, the PT has a slight physical vibration to it at the same freq as the hum....
     
  16. tube-works

    tube-works TDPRI Member

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    Update, I just remeasured the heater fialments....
    Filament 1 to ground + 3.0VAC
    Filament 2 to ground + 5.1VAC
    From one side of pilot light lugs to the other = 6.57VAC
    STRANGE
     
  17. jhundt

    jhundt Doctor of Teleocity

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    hoo-boy! now ya got me! We're gonna have to think about this one for a while. Maybe one of our amp wizards will see something that we are missing.

    My only suggestion now is to try the hum balance pot suggested in my last post, and see if you can dial the hum out.

    you might also try removing all tubes and measuring the heater voltages; then replacing one tube at a time and measuring those voltages to see if they change. Maybe you have a tube that works fine but has a faulty heater?

    I'm working this angle because, for some reason, Rivera decided to add a hum-balance control in the 80s. It was never featured on a Fender amp before, or since. I have never used the one on my PRII. BUT - for some reason they put it on there for a while... and Fender isn't known for putting useless things on their amps...
     
  18. Cleeve

    Cleeve Tele-Holic

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    Is the hum gone if the reverb control is all the way down?
    Maybe the bypass cap on the reverb recovery cathode is shorted or bad?
     
  19. tube-works

    tube-works TDPRI Member

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    Just for kicks and giggles I unsoldered the heater CT...no effect on the hum connected or not...also when I press down on the pilot light assembly the hum changes intensity....I measure heater voltage on all tubes and they are the same throughout as above posted voltages. I disconnected the heaters from the pilot light and connected them topgether (sans pilot light assembly) to see if it was a problem with the pilot light assembley...this had no effect...
     
  20. tube-works

    tube-works TDPRI Member

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    hi Clint...Hum is just quieted by the reverb control...it's still audible even at 0...thanks!
     
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