Princeton Reverb Help

[Paul-in-KC]

Okay - here are the voltages:

Mains = 124 VAC

Recto pins 4&6 = 337 VAC; pin 8 = +412 VDC

V5 & V6 pins 3 = +407 VDC; pins 4 = +395 VDC

V4a [here's the weird stuff] pin 1 something around +260 VDC, BUT with
some AC around 80 V ???; pin3 some AC around 0.6 / 0.7;

V4b pin 6 = +194 VDC; pin 8 = +59 VDC

V1a pin 1 = +155 VDC; pin 3 = +1.4 VDC

V1b pin 6 = +159 VDC; pin 8 = +1.4 VDC

Any thoughts about where the AC is coming from???

[martin manning]

You might have some ultrasonic oscillation going on there.

[Stevem]

Martin may be on to something, take your volt meter and set it for ac volts and hook it up across the amps speaker output, then plug a guitar in and turn down it volume control, then crank everything on the amp wide open, if you than read 1.5 vac or more the amp is oscilating !

[ToneMerc]

You might have some ultrasonic oscillation going on there.

Yes it could be a parasitic; here's where the VTVM and O'scope comes in handy. The O'scope would really help looking for them in the ultrasonic range.

TM

[sluckey]

V4a [here's the weird stuff] pin 1 something around +260 VDC, BUT with some AC around 80 V ???; pin3 some AC around 0.6 / 0.7;

... Any thoughts about where the AC is coming from???

What you are seeing is quite normal. V4a is the tremolo oscillator. There will be a healthy low freq. AC sine wave on the plate if the oscillator is enabled. My Fluke 87V is capable of measuring AC voltage and frequency of that trem signal. Probably a lot of other DMMs can also.

If you disable the trem with a footswitch or by shorting the trem footswitch jack to ground, that 80VAC should go away.

[ToneMerc]

V4a [here's the weird stuff] pin 1 something around +260 VDC, BUT with some AC around 80 V ???; pin3 some AC around 0.6 / 0.7;

... Any thoughts about where the AC is coming from???

What you are seeing is quite normal. V4a is the tremolo oscillator. There will be a healthy low freq. AC sine wave on the plate if the oscillator is enabled. My Fluke 87V is capable of measuring AC voltage and frequency of that trem signal. Probably a lot of other DMMs can also.

If you disable the trem with a footswitch or by shorting the trem footswitch jack to ground, that 80VAC should go away.

Excellent point hidden in plain sight, that is the ultimate oscillator and would explain that condition at V4 if it ceases when grounded out.

I still think from what the OP described orginally, the amp is relatively normal. The PR is not a Vibrolux or Deluxe Reverb.

TM

[Paul-in-KC]

Ah - I was wondering if it (the AC) might be related to the tremolo/vibrato.

I'll check it out after I get home.


Thanks very much to all that take the time to post. It is much appreciated.

I'll report on what I see (and hear) after "turning off" the vibrato.

-Paul

[sluckey]

I still think from what the OP described orginally, the amp is relatively normal. The PR is not a Vibrolux or Deluxe Reverb.

I agree.

[xtian]

Good point. Paul, do you have a sine wave generator? You could put this to rest if you measure the clean output wattage. I get about 11 watts clean out of my PR build with 366v on plates, and 14 watts clean from my simplified blackface build with 336 on the plates of 6V6 pair.

[Milkmansound]

sometimes its something simple - like the output jack is still nearly shorted when the cable is plugged in.

Pretty sure Mojo kits ship with Chinese input and output jacks - I would visually inspect the shorting output jack

a Princeton has a cathodyne PI so you can't screw up the 470ohm resistor like you can on a long tail PI. (anyone who says they have never made that mistake is probably lying)

[Paul-in-KC]

Good point. Paul, do you have a sine wave generator? You could put this to rest if you measure the clean output wattage. I get about 11 watts clean out of my PR build with 366v on plates, and 14 watts clean from my simplified blackface build with 336 on the plates of 6V6 pair.

xtian, I do have a sine wave generator (because I figured I'd need one sooner or later). It's an old tube model, but it works. I have generated a signal and looked at it on my oscilloscope.

Please hold the laughter, but - how would I measure the clean output wattage using an sine wave generator?

-Paul

[xtian]

How would I measure the clean output wattage using an sine wave generator?

Put 1kHz, 1v p-p (peak-to-peak) at the input (this part is not critical, as long as you're not driving V1 into distortion)

Forgot to mention dummy load! You will not want to listen to the sine wave at full volume!

Connect o-scope to speaker output/dummy load. You should see your 1kHz sine on the scope.

Slowly turn up the volume on the amp until your sine wave starts to squish/flatten/distort/square off. Make a note of the voltage of the sine wave. In my Princeton, it was about 27v p-p.

To find RMS voltage, multiply p-p voltage by 0.3535.

To find wattage, square the RMS, and divide by the load.

So in my case, it was (27 x 0.3535)^2 / 8 = 11.4 watts.

[Paul-in-KC]

How would I measure the clean output wattage using an sine wave generator?

Put 1kHz, 1v p-p (peak-to-peak) at the input (this part is not critical, as long as you're not driving V1 into distortion)

Forgot to mention dummy load! You will not want to listen to the sine wave at full volume!

Connect o-scope to speaker output/dummy load. You should see your 1kHz sine on the scope.

Slowly turn up the volume on the amp until your sine wave starts to squish/flatten/distort/square off. Make a note of the voltage of the sine wave. In my Princeton, it was about 27v p-p.

To find RMS voltage, multiply p-p voltage by 0.3535.

To find wattage, square the RMS, and divide by the load.

So in my case, it was (27 x 0.3535)^2 / 8 = 11.4 watts.

Okay - can do. I also have a dummy-load box.

Thanks so much for all of the tutoring.

[Paul-in-KC]

All,

Well - indeed the AC on V4 was from the vibrato.

I haven't measured the clean output yet, but did also (partially) find my lack of volume.

While testing input jack #2 (which I hadn't previously), I found the "expected" volume/output.

So, something in jack #1 is either dumping signal to ground, or has some resistance. I need to investigate that more closely to see exactly what's going on.

Other than that, I'm pretty sure this is a normal healthy PR AA1164.

Thank you fellas for your input and advice.

xtian - I'll post the numbers I get for "clean" power out after I measure.

[sluckey]

So, something in jack #1 is either dumping signal to ground, or has some resistance. I need to investigate that more closely to see exactly what's going on.

The #1 jack (has the 1MΩ resistor) is considerably louder than the #2 jack by design.