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Thread: '78 Princeton(non-reverb) Humming

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    '78 Princeton(non-reverb) Humming

    My Princeton has recently developed a 60hz hum when powering up. The hum "surges" shortly after the amp is turned on, then it quiets down to being barely audible. It isn't affected by the volume or tone controls; however, the hum is pulsing and can be altered by turning the intensity knob. Knowing the little that I do about the tremolo circuit in these amps, I'm wondering if there is a problem in the bias circuit or possibly a coupling capacitor. The tremolo seems a bit weak as well. Any help would be greatly appreciated. I'm new to the tube amp world, and trying to learn as much as I can about them.

    Also, I've measured the bias a few times over the last 2 years and noticed a trend. The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise. Not sure if this has anything to do with the recent hum or not.

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    The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise.
    That's almost impossible, especially as your amp seems to work basically. OT DCR increases with wire temperature, though.
    Can you post DCR, idle currents, HT and plate voltage values ?

    And please check your meter battery.

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    Last edited by Helmholtz; 02-20-2020 at 10:29 PM.
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    3 things to look at, hmm, 1st, 2nd, & 3rd. In no particular order.

    1: Output tube bias different from one tube to the other. Not by just a couple mA, but you'd start to hear a hum if it went past say 10 mA.

    2: Failing hi voltage filter cap. Easy to test, just parallel a cap on each section, one at a time. What value? 20 uF more or less, 500V.

    3: Failing bias filter cap. Might as well replace it, the amp's 42 years old. Cheap insurance.

    You report a trend of OT primary resistance dropping over the course of 2 years. That is bizarre & I don't have any explanation. One thing to take into account, wire resistance depends on temperature. (Similar to guitar pickups.) The resistance will tend to be higher when the transformer is warmed up, and it will be at a minimum if measured before the amp's been warmed up, especially if it has been stored in a cold area.

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    I have met a few amps over the years that exhibit this behavior: A few seconds after turn on, a 60Hz hum arises, then fades to silence, and amp runs normally and quietly. This includes an amp I built from scratch last month--solid state recto, 22uF reservoir, two 12AX7 and two EL84 for about 12 watts. The hum comes and goes before the power tubes are conducting current, I think.

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    I wasn't aware of temperature affecting the resistance like that, but it makes perfect sense. Most likely explains the difference in the readings.

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    The power tubes are a matched pair, and the measure within 1mA of each other, so I know it's not a mismatch issue.

    Electrolytics have been replaced recently by the former owner. How recently? I'm not sure. The filter cap can has a new capacitor wired to it in the chassis, but I'm not sure if the whole can has been replaced or not. I'll check the date on it and post pics. Also I'll get the meter out and post the measurements that Helmholtz requested.

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    It seems that I can't figure out how to upload pictures. Lol... Anyway, the can appears to be original. It has Mallory stamped on it, and the numbers are 235-7712Y. Guessing that means the 12th week of '77. Could someone please confirm?

    I monitored the AC and DC voltage of the caps while feeding a guitar signal into the amp and turning the volume up. The ACV only rose from 1.4V to 2.5V or so, and the DCV hardly drops at all. I don't have any spare caps to parallel in as Leo suggested.

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    Last edited by cyborg_stew; 02-23-2020 at 08:33 PM.

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    One interesting thing I found in my probing is this: on pin 7 of V2 there is 29VDC. This doesn't seem right, as this is the grid, and the schematic doesn't have any voltage listed. I'm thinking the .022 coupling cap must be leaking from the plate of V1.

    Another thing is I can't get a steady reading on pin 1 of V2. Is that normal since it's part of the tremolo circuit? When I disengage the trem via the footswitch jack, the reading is a steady 261VDC.

    Also, should there be 83VDC on pin 8 of V2?

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    Schematic?

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    I could only find the non-reverb b1270 schematic from one that was being sold on reverb.com. I can't figure out how to upload pics on here, so here's the reverb schematic.

    http://www.ampwares.com/schematics/P...ERB_AB1270.pdf

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    I'm new to this stuff, so please bear with me. Here's some values.

    OT Resistance
    pin 3 of rectifier to plate of v4: 153.5ohms
    pin 3 of rectifier to plate of v3: 154.2ohms
    Voltage Drop
    pin 3 of rectifier to plate of v4: 3.21VDC
    pin 3 of rectifier to plate of v3: 3.13VDC
    Plate Voltage
    v4: 440
    v3: 440
    Current
    v4: 20.0mA
    v3: 20.2mA
    Dissipation
    v4: 9.20watts
    v3: 8.93watts

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    Keep posting & checking in as far as not being able to post pics. I think there's a trial period for new members. Just build a post count responsibly & you should be good. If it doesn't resolve in a few more posts, check back in. When you reply, select Go Advanced & then check for "Manage Attachments."

    As for schematics, the Princetons (Reverb & Non) didn't really suffer any drasric changes. The latest Princeton schem you can find will likely be more than adequate.

    Justin

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    Quote Originally Posted by cyborg_stew View Post
    The power tubes are a matched pair, and the measure within 1mA of each other, so I know it's not a mismatch issue.

    One interesting thing I found in my probing is this: on pin 7 of V2 there is 29VDC. This doesn't seem right, as this is the grid, and the schematic doesn't have any voltage listed. I'm thinking the .022 coupling cap must be leaking from the plate of V1.

    Another thing is I can't get a steady reading on pin 1 of V2. Is that normal since it's part of the tremolo circuit? When I disengage the trem via the footswitch jack, the reading is a steady 261VDC.

    Also, should there be 83VDC on pin 8 of V2?
    Is this the correct schematic?
    Click image for larger version. 

Name:	Fender_princeton_aa964_schematic-1.png 
Views:	22 
Size:	1.11 MB 
ID:	57102

    It could be that the power tubes warm up at different rates and you are getting single ended hum before the are both fully conducting. The hum pulsing with the Intensity knob is to be expected as its a bias wiggle trem.

    Pin 7 of V2 is a high impedance point and may not give a true voltage reading and it's a cathodyne PI circuit so the grid won't be 0V. It's better to measure at the cathode (pin 8) and compare the reading to the value on the schematic.

    I think the trem voltages are OK.

    Pin 8 of V2 voltage is higher than on the schematic but it could be OK. Disconnect the 0.022u grid coupling cap to see if that reduces pin 8 voltage. Also check pin 6 and node [B] voltages.

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    Last edited by Dave H; 02-24-2020 at 11:56 AM.

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    Does the amp use a GZ34 or a 5U4 rectifier? I wouldn't expect a hum surge with the slow heat-up GZ34.

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    Quote Originally Posted by Helmholtz View Post
    Does the amp use a GZ34 or a 5U4 rectifier? I wouldn't expect a hum surge with the slow heat-up GZ34.
    It's a 5u4GB rectifier.

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    The schematic is a B1270, but I was surprised to find that the earlier blackface AA964 is exactly the same. Makes me wonder what all the "blackfacing" hype is all about, especially for a Princeton.

    I didn't know that voltage on the grid was normal for this type of circuit. I still have alot to learn.

    And the power tube theory that Dave mentioned makes alotta sense to me. Maybe it is just a power tube problem. They're only a year old though, so I wouldn't expect them to behave that way. I don't push the amp too hard (usually play at lower volumes), and they're not biased much over 70%. Maybe there is a slight problem in the bias, tremolo, or power section that is causing them to wear out faster than they should?

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    Makes me wonder what all the "blackfacing" hype is all about, especially for a Princeton.
    Well the BF uses a GZ34.

    Can you post the B1270 circuit?

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    Click image for larger version. 

Name:	b1270schematic_edited_edited.jpg 
Views:	27 
Size:	719.6 KB 
ID:	57126

    This is a photo of one that was being sold on reverb.com. I couldn't find any online. The file name has "edited" in it, because I altered the photo, so it would print better.

    I didn't notice the rectifier was different in the AA964. lol. Still, the voltages and component values are the same as far as I can tell.

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    the voltages and component values are the same as far as I can tell.
    If B+ is the same the PT secondary voltage must be higher with the 5U4. The difference is the slow heat-up of the GZ34 (as opposed to the 5U4), which would prevent the turn-on hum surge.

    Also the BF had a lower value bias filter cap which allows the bias voltage to build up faster.

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    Could the problem be something as simple as a bad rectifier? From what I understood, rectifier tubes either work, or they don't.

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    They're only a year old though, so I wouldn't expect them to behave that way.
    ANY tube can fail at ANY time. New or twenty years old, doesn't matter. New is no guarantee of anything.

    rectifier tubes either work, or they don't.
    Well, two elements can short together. One side can quit, the heater can die. It can just get old and weak.

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    Quote Originally Posted by cyborg_stew View Post
    they're not biased much over 70%. Maybe there is a slight problem in the bias, tremolo, or power section that is causing them to wear out faster than they should?
    That sounds pretty hot for a Fender to me. Especially with that kind of trem. I'd think the factory set-up closer to 50%. I'm assuming you put a bias pot in place of the 27K? What kind of % dissipation do you get with bias pot set for 27K ?

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    Quote Originally Posted by cyborg_stew View Post
    Could the problem be something as simple as a bad rectifier? From what I understood, rectifier tubes either work, or they don't.
    Had a 5Y3 in an old 50's Silvertone amp that was faulty. I recapped the amp, replaced dropping resistors, and some plate resistors. I got rid of most of the bad noises I was hearing but I still checked the 5y3 on my tube tester on a hunch. All the short lights were flickering and it was obvious that tube was getting ready to die. Funny thing is that the tube was still working in the amp. Voltages were good and the amp sounded pretty good, although better with a new tube. So I guess they can work, not work and work too much.

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    Quote Originally Posted by g1 View Post
    That sounds pretty hot for a Fender to me. Especially with that kind of trem. I'd think the factory set-up closer to 50%. I'm assuming you put a bias pot in place of the 27K? What kind of % dissipation do you get with bias pot set for 27K ?
    That's a good question. I've never measured the resistance of the trim pot; just adjusted it to get the current to where I like it. I'll have to check.

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    Quote Originally Posted by Enzo View Post
    ANY tube can fail at ANY time. New or twenty years old, doesn't matter. New is no guarantee of anything.
    That's true. I'm just wondering if there is a problem somewhere in the circuit that's putting more stress on the tubes in some way, and maybe that problem is causing the amp to hum.

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    Well, I installed a CE cap can in place of the original, and the hum is completely gone now. However, I was checking B+ voltages at the can, and 2 out of the 4 capacitors are reading significantly low. (They're numbers 3 and 4 in the circuit. The first 2 are fine.) The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC.

    The voltage is normal (420VDC) after the 1k resistor. Then, after it passes thru the 18k it drops down to 241VDC. Much too low for this point in the circuit. I checked the resistor value, and it checks out at 17.8k.

    What could be causing this? I can't imagine that I did anything wrong installing the new can, and I have no guesses as to what's going on with the voltages. Any help would be greatly appreciated.

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    Quote Originally Posted by cyborg_stew View Post
    What could be causing this? I can't imagine that I did anything wrong installing the new can, and I have no guesses as to what's going on with the voltages. Any help would be greatly appreciated.
    Bad preamp tube? Bad section of the new filter cap?

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    There is another 18k resistor following the first 18k in the rail. What is the voltage drop across THAT resistor?

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    Quote Originally Posted by Chuck H View Post
    There is another 18k resistor following the first 18k in the rail. What is the voltage drop across THAT resistor?
    It drops all the way down to 76V after the last 18k resistor.

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    Quote Originally Posted by 52 Bill View Post
    Bad preamp tube? Bad section of the new filter cap?
    Yeah. It makes me think that one or 2 of the new caps is bad. I can't think of any other possibilities. But I'm new to this stuff, and I still have alot to learn.

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    Quote Originally Posted by cyborg_stew View Post
    Yeah. It makes me think that one or 2 of the new caps is bad. I can't think of any other possibilities. But I'm new to this stuff, and I still have alot to learn.
    Pull preamp tubes one after the other and monitor voltage at circuit point D.

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    Will do. I'll get back in the chassis tonight and check the voltages without the tubes.

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    Quote Originally Posted by cyborg_stew View Post
    Will do. I'll get back in the chassis tonight and check the voltages without the tubes.
    Sorry, I forgot to mention: Please first lift one end of each 18k resistor and measure resistance, if ok reconnect and make sure they are well soldered.

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    Anybody else notice the schematic is very likely incorrect? Typo!?!

    First, there's no node between the two 18k resistors. Why would they do that? And I just ran some simulations and the voltages don't add up at all. The voltage indicated for the preamp stages would be consistent with ONE 18k resistor and no additional node. But it's more likely that Fender ran the concertina/split load inverter from one node and then decoupled the preamp from the following 18k rail resistor. Which would mean the voltages for the preamp node are inaccurate and would be a bit lower than indicated on the schematic. In any case IT'S WRONG!!! And how most of these amps were actually wired is a mystery. Because people are very rote animals and it's probable that the schematic was copied by Fender themselves. So we're actually in dark territory here as far as what's right and wrong.

    With the exception that the current draw from the final stage is clearly huge by comparison to what it should be. About a hundred milliamps? So yeah, that's a lot for a preamp stage I think there must be an error there as well. Probably a bad can cap or it's connection.

    But I hope we can keep in mind through the course of this diagnosis that the schematic is clearly flawed and we don't know what original preamp voltages should actually be (other than speculation and simulation I suppose).

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    The numbers all work out for me? Roughly 3mA through both (18K) resistors for approx. 110V across 36K, 1mA used by each of 3 triodes on node B.

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