Princeton Reverb 75 with odd decay noise/distortion.

[caesparza]

Hi,

Got a situation with this Princeton Reverb from 1975. The client brought it to me after recording the attached audio.
I proceded with a recap (PS and Bypass caps), since some of them drifted in value. I suggested to him to install Screen & Grid resistors for the power tubes (told him all the benefits from it).
The owner wants the least amount of modifications to the amp, so He didn't approve that.

The situation improved a lot, but you can still listen some of that odd resonance with certain notes. I can determine if it is some sort of vibration on the cab or maybe the speaker.
I have tried other speakers and cabs with different results, but still you can hear a faint resonance on some notes, specially low A (6th string 5 fret) to E (open 6th string).

I tried looking the signal with the scope on the different grids and the output. Everything looks normal, the only thing I notice is that the signal tends to clip sooner on that lower registers.

I checked coupling caps too and none of them reveals problems. Could this behavior be related to the Screen/Grid resistors?

Any suggestions would be gladly taken.

[Jon Snell]

I would suspect the output valves or their bias point. Gently tap on the valves, they may make a vibrating noise.
The lower the register, the less output you will get due to coupling capacitor values and the type of output transformer used.

Did you mean the Fender 75? Please show the chassis number for identification. Thanks

[stokes]

Plate resistor in the pre?

[One10]

Noise sounds almost like an electical issue to me, but to rule out the most common problems, maybe try any, or all of the following before diving deeper into the amp:

Can you:
1). Check to see if the speaker coil leads rub, touch, or vibrate against the speaker cone?

2.) Press on the baffle board in various areas, holding fingers against it, while someone plays a few note on a guitar and see if the buzz stops?

3.) Turn amp upside (180˚) down and see if the buzz stops while playing a few notes?

4.) See if dust cap is loose, separated (even just a tiny bit can cause buzzing).

5.) Loose parts, small 'debris' anywhere in amp, behind the speaker cone & (inbetween) the speaker basket.

6.) Remove speaker from cab and play it in 'free air', is buzz now gone?

7.) Take removed speaker and place face up on a sturdy, flat surface, or table and then gently press cone near voice coil with fingers spread out evenly let cone return to resting position. Did you feel, or hear a scraping, dragging noise as you pushed down, or when releasing cone?

[caesparza]

Hi One10,

I checked the amp with another speaker and the result is the same, the sound is present. Thanks for the suggestions.


I decided to trace the signal. And this is the first thing I found.

I'm feeding 150mV to the amp. Pin 7 of V1B has a perfect sine wave (it means that there are no problems at the tone stack), but the plate of that triode (pin 7) has the output attached in the image.
I tried different 12AX7, 100K plate resistor and a 0.022 coupling cap, in each case the result is the same.

What should I check next. Could it be a problem in the socket?

[Helmholtz]

Did you test for microphonic tubes?

[caesparza]

Yes Sir.

[Tom Phillips]

...I'm feeding 150mV to the amp. Pin 7 of V1B has a perfect sine wave (it means that there are no problems at the tone stack), but the plate of that triode (pin 7) has the output attached in the image.
I tried different 12AX7, 100K plate resistor and a 0.022 coupling cap, in each case the result is the same. What should I check next. Could it be a problem in the socket?

Check the V1B cathode circuit components and wiring. Those components are involved in the bias setting of V1B.
Does the waveform clean up if you reduce the 150mV input test signal?

[g1]

Does the waveform clean up if you reduce the 150mV input test signal?

Also, the setting of the volume control has not been stated, which is right before V1B stage. AC voltages for grid and plate are needed for the scope image to have any reference.

[Tom Phillips]

Also, the setting of the volume control has not been stated, which is right before V1B stage. AC voltages for grid and plate are needed for the scope image to have any reference.

Right. If you turn down the volume control and the signal at the V1B plate cleans up, that would be a sign of that tube stage being overloaded. signals shaped like the one you showed in post #5 are not unusual in Fender amps when driven into overload. The setting of the tone controls will affect the waveform too.

[The Dude]

I have ALMOST always found that noise to be a symptom of bad filter/decoupling caps. I know you said you already recapped the amp, but perhaps there is a connection or grounding problem with the caps? I'd double check connections. Also, scope supplies to make sure filtering is adequate. There's also the possibility that one or more of the replacement caps is bad, especially if they were sitting on a shelf for a while.

[caesparza]

At lower volumes the scope shows a proper sine wave, when the volume goes around 4-5, the sort of clipping starts.

At 10 volume I can get a proper sine wave if I reduce the output of the generator to 12.1 mVAC.

Since I can get a proper sine wave at pin 7, that means I could conclude that the tone stack (caps, resistors in this circuit) are not to blame for the problem. Am I right?

[Tom Phillips]

At lower volumes the scope shows a proper sine wave, when the volume goes around 4-5, the sort of clipping starts. At 10 volume I can get a proper sine wave if I reduce the output of the generator to 12.1 mVAC.
Since I can get a proper sine wave at pin 7, that means I could conclude that the tone stack (caps, resistors in this circuit) are not to blame for the problem. Am I right?

Based on my experience, I'd say that what you are describing is within the range of normal behavior for a Fender amp circuit and it's probably not the cause of the bad sound we heard in the audio sample that you attached to your post #1.
The method I use to track down where the signal starts to sound bad is to use a signal tracer so I can listen to a sample of the signal at any point in the circuit.

[xtian]

Quick troubleshooting: disconnect the reverb tank. Does that make the issue vanish?

[caesparza]

Did it. The same.