'78 Princeton(non-reverb) Humming

[cyborg_stew]

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.

[Helmholtz]

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.

[Leo_Gnardo]

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.

[xtian]

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.

[cyborg_stew]

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

[cyborg_stew]

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.

[cyborg_stew]

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.

[cyborg_stew]

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?

[Helmholtz]

Schematic?

[cyborg_stew]

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

[cyborg_stew]

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

[Justin Thomas]

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

[Dave H]

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?


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.

[Helmholtz]

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

[cyborg_stew]

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.