Oscillation in Princeton Reverb build Hoffman layout

[beasleybodyshop]

Welp. Almost there.

I tried to wrap up my PR build tonight. Hooked everything up, and as soon as the recto heats up....

loud low frequency oscillation kicks in. really low. vibrates everything in my room. granted i am testing it through a 2X12 dumble style cab. When I turn the intensity knob, the oscillation fluctuates like the tremolo. Even moving the speed knob causes the oscillation to fluctuate faster. Turning the reverb knob does change the oscillation tone as well.

When I pull v3, the oscillation goes away. When i pull v1/v2 and put v3 back in, oscillation returns. This leads me to believe its in the reverb/tremolo circuit. When I move the grid wires on V3, the oscillation changes a little, but not enough to go away. Attached are some gut shots.

Anyone have any pointers? I have moved the wires running to the intensity knob, and moved the grids around on V4. Doesnt make this problem go away - i have resoldered every joint on the board, Plate voltages are close to fender specs, bias circuit seems ok....what am i missing?

[xtian]

V3 is half reverb recovery, and half final gain stage before the PI. V4a is the tremolo oscillator.

The wire running to the Speed control was most sensitive to placement in my build. I had some nasty thumping at first, but just had to move this wire a bit.

I'm thinking if you pull V3 and the problem vanishes, it's probably not the tremolo. Find out what happens if you ground the grid of V3a, the reverb recovery. Problem stop? What about if you ground the grid of V3b?

[beasleybodyshop]

V3 is half reverb recovery, and half final gain stage before the PI. V4a is the tremolo oscillator.

The wire running to the Speed control was most sensitive to placement in my build. I had some nasty thumping at first, but just had to move this wire a bit.

I'm thinking if you pull V3 and the problem vanishes, it's probably not the tremolo. Find out what happens if you ground the grid of V3a, the reverb recovery. Problem stop? What about if you ground the grid of V3b?

grounding v3a changes nothing, grounding v3b changes the oscillation from a low freq to a high pitched sound.

[xtian]

Have you tried swapping the OT secondaries, blue and brown? That, or lift the NFB wire to see if the NFB is the cause.

[beasleybodyshop]

Have you tried swapping the OT secondaries, blue and brown? That, or lift the NFB wire to see if the NFB is the cause.

Lifting the NFB does make the noise go away.

[beasleybodyshop]

however i get almost no audible sound from the amp when i disconnect the NFB. However when I turn up the reverb control, i get low audible hiss, and when i dime the reverb control i get a low volume signal.
UPDATE - swapping the OT leads on the power tube sockets has removed the low freq hum. Now it just has low output. Fix one problem, the next one pops up!

now my plate voltages are really low. im getting around 50v on V1, V2 seems to be fine, but V3 and V4 are like almost half of what they were before. Huh?

[Stevem]

Swapping the OT leads should not have done a thing to the B+ voltage, did the wires go back on pin 3 of each output tube?
What is the B+ on the red center tap of the OT, between 420 and 460?
You should have the PT and or a power supply dropping resistor getting very hot with this issue.
If you get the voltage good its easy to check the preamp atleast thru to the reverb drive tuibe.
The pan is a 8 ohm input so you can hook up a speaker to the secondary of the reverb pan drive OT and listen if all up to that point is fine sounding thru the speaker than proceed on down stream to the next tube.
To pin down where a oscilation may be starting up at take a section of wire and solder to the plate of the tube sectioin in question, insulate the other end and then rap this wire 3 to 4 times around that tube sections grid wire.
This will phase cancile some or all of the oscilation if its originating from that area and atleast let you pin down what section to focus on.

[beasleybodyshop]

Swapping the OT leads should not have done a thing to the B+ voltage, did the wires go back on pin 3 of each output tube?
What is the B+ on the red center tap of the OT, between 420 and 460?
You should have the PT and or a power supply dropping resistor getting very hot with this issue.
If you get the voltage good its easy to check the preamp atleast thru to the reverb drive tuibe.
The pan is a 8 ohm input so you can hook up a speaker to the secondary of the reverb pan drive OT and listen if all up to that point is fine sounding thru the speaker than proceed on down stream to the next tube.
To pin down where a oscilation may be starting up at take a section of wire and solder to the plate of the tube sectioin in question, insulate the other end and then rap this wire 3 to 4 times around that tube sections grid wire.
This will phase cancile some or all of the oscilation if its originating from that area and atleast let you pin down what section to focus on.

OT CT voltage is approx. 380 volts. Im not getting the oscillation any more, actually the amp sounds really good - super clean, yet even when i crank up the amp to 10 it doesn't distort or get very loud. Both reverb and trem are working - but with no footswitch in, the tremolo works. Your supposed to have to activate the tremolo sound with a footswitch right?

[Milkmansound]

Not on a Princeton - the footswitch shunts the cap network to the grid of V4a to ground and kills oscillation.

[xtian]

Right. I just learned this, too. Fender's bias-wiggling trem's footswitch needs to be open to work, and the optical-bug trem's footswitch needs to be closed to work.

[beasleybodyshop]

Thanks guys, that clears that up. Going to swap the OT leads again and measure voltage. I will pull the power tubes and see if that changes voltages as well.

[beasleybodyshop]

So, let me be the first to mention that I am a complete dummy.

I kept pulling my hair out over this issue. I was so confused as to why my plate voltages on one side the plate resistor was so low compared to the B+ side...

Turns out my V1 tube was a 12AU7, not 12AX7. This was why i could turn it all the way up and get low volume/absolutely no breakup. It was one from my vintage collection, and the lettering was barely audible. I had inadvertently stuck it into my 12AX7 pile some time before. D'oh!

Now, amp is louder. My reverb is lush sounding, and surprisingly quiet. However my tremolo intensity is...well...not very intense sounding. Doesn't sound very drastic even at high settings. I double checked and swapped several 12AX7s into V4, with no change...any thoughts?

[xtian]

However my tremolo intensity is...well...not very intense sounding. Doesn't sound very drastic even at high settings. I double checked and swapped several 12AX7s into V4, with no change...any thoughts?

First thought is, that's the way it works. Definitely not as "deep" as optical tremolo.

[JazzGuitarGimp]

But it should be more "lush" sounding than the "choppy" nature of the optical vibe.

[beasleybodyshop]

However my tremolo intensity is...well...not very intense sounding. Doesn't sound very drastic even at high settings. I double checked and swapped several 12AX7s into V4, with no change...any thoughts?

First thought is, that's the way it works. Definitely not as "deep" as optical tremolo.

I agree. After looking at several youtube videos, my tremolo isnt any better or worse.

Safe to say this amp is good to go! Thanks again everyone. And a big thanks to you, Xtian.