1968 Princeton Reverb Squealing Tremolo

[rifflicks]

Hello...could someone please give me a hint at where I should begin looking for a problem with squealing Tremolo....It's in a 1968 Princeton Reverb.
With the volume and speed knobs turned completely CCW (down) the amp will start to put out a Loud, Pulsating, High Pitched Squeal if I turn the Intensity knob past 4.

[eleventeen]

It's probably something pretty simple in the interaction between the bias supply and the tremolo circuit.

Do you have the footswitch? Can you plug it in and disable the tremolo? Can achieve the same result if you have an RCA cable and plug it into the trem footswitch jack on the rear of the chassis and short out the loose end of the plug.

Is there any change in the appearance of the output tubes when the squealing starts? Does the trem freq control affect the freq of the pulsating squeal?

Sounds to me like something is way off in the (simple) bias supply and it *could* be grinding your output tubes in a way that would shorten their lives.

Regardless of anything you're able or unable to find out....it needs to go onto the bench, and the problem is probably pretty simple....but will require a meter and the schematic (available in the schem section, probably the same ckt as an AA1164 Princeton, or close enough) and the ability to troubleshoot.

[rifflicks]

Thank you 11teen for the quick response....I"ll give your suggestions a try.

[floyd]

It's probably one of your preamp tubes .. try switching V1 with V4

[tubeswell]

At a guess without seeing any gutshot pics, lead dress could well have something to do with the squealing problem. Is this a vintage PR or modified?

[rifflicks]

Sam....When I started this project, I replaced the Power and Output transformers, the Speaker, Capacitors and Tubes. Everything sounds really good except the squeal in the tremolo....I'm still working on eleventeen's suggestions.

[rifflicks]

Tubeswell I guess I don't know what lead dress is.
 it squeals with the volume off until I turn the Frequency control about 3/4 and the squeal stops.
I am going to look for a schematic with voltages on it in the morning and put a meter on it.

[floyd]

"new tubes" are sometimes "bad" meaning microphonic.. try switching the 12AX7 tubes anyway.

[eleventeen]

Another (entirely real) possibility is that your new output tranny is opposite-phased relative to the stock one; this means the negative feedback is now positive feedback, and the amp will oscillate at one or more or all freqs. This is especially true if it's a Mercury or some other non-Fender brand. You imply this is the case because you didn't say "my OT opened up or shorted out", eg; you replaced a working tranny in an effort to improve the amp's performance. In effect, then, you have a brand new build and you have that issue of OT phasing.  It cannot be predicted, you just have to hook the bugger up and see if she feeds or not. More often, the symptom is what's called "motorboating" and tends to be very LOW in frequency (sounds like a motorboat engine)...and generally occurs at almost ALL volume settings....but it can selectively oscillate as well.... The point is, given that you have now revealed that you swapped out the output tranny, you cannot rule this possibility out without going thru the exercise of reversing the 6V6 plate leads on the OT. Most likely, you just swap which tubes the brown and blue wires to the OT go to. Swap those leads around. Don't bother making broadcast-grade solder connections when you are testing this out...just make temporary tack solder connections. Generally, you will know right away which way is right and which is wrong. Once you find that, go back and ruggedize the connections.

[tubeswell]

Ah I see its been modified. 'Lead dress' is how you lay out the amp (and 'dress' the leads). If you changed the layout (including how the leads were laid out/curled around each other/organised) from stock, you could introduce unwanted EMI coupling between components and/or leads. Take a look at the stock PR layout and then work out what is different to that from yours. I'd start by looking at the part of the amp where you have the tremolo circuit and if/where this has leads etc that are running close to other parts of the circuit in the amp's main signal path, particularly any (sensitive) grid signal wires/leads.

[rifflicks]

Eleventeen.... the original power transformer was bad (smoking) so I changed them both....when I first fired it up after the transformer swap (ClassiTone transformers) I had the "motorboating" and  I reversed the output primaries and it went away....so I did learn what that sounded like....
Tubeswell....I did move some wires differently for convenience soldering in the orange drops in place f the original capacitors...I did take a picture of the original wiring...I'll double check that and put things back the way they were.
I sure do appreciate the help.

[tubeswell]

Remember that wire can be an inductor if it is within close enough proximity to an electro-magnetic field. When current changes direction in a wire (e.g.: like alternating current under signal conditions), it induces an electro-motive force (EMF) outside the wire  that moves in a circum-polar direction around the outside of the wire. This EMF changes direction corresponding to the changes in direction of current within the wire. If another wire is within close enough proximity to the first wire when the 1st wire is experiencing EMF, then that will induce corresponding changes in current into the 2nd wire. This will occur if the 2 wires are laid out parallel to one another and close enough to one another. If these two wires relate to different parts of the circuit then you can get (unwanted) positive feedback produced within the circuit (which is what causes the squealing).

So it pays to carefully consider how you lay out your signal wires in relation to each other and/or to other signal leads (like cap or resistor leads etc). Minimising wire lengths, having good spaces between various wires, and (where wires have to be close to each other) having wires crossing each other perpendicularly, can help eliminate stray sources of unwanted coupling in this regard.

Of course with some types of wires (for example, like AC pairs of heater wires where the AC in each wire is in equal but opposing phase to the other wire in the pair) it is actually desirable to twist the pairs together so that any resulting EMF is cancelled out, thus eliminating potential sources of hum. It depends on what purpose the wire(s) in question are intended to function for.

[eleventeen]

OK, if you've checked for m-boating, fine. I had to bring it up. Now you know what it is, so at least you've learned something. Generally---it is not selective---the circuit will oscillate obnoxiously, all the time.

Just thinking about this now, you fixed the m-boating by reversing the SPEAKER leads. OK, theoretically that achieves the same phase reversal as flipping the primary leads...BUT I would do this by reversing the primary, HV-to-tube-plate leads. Perchance...is there an internal ground connection inside your output tranny?

[drive-by suggestions follow]

I lean towards lead dress, then, along w/tubeswell, assuming you've done everything else wire-for-wire. Get your wires (except for the up-in-the-air-heater wires if that's what you have) close to the sheet metal of the chassis. Pay attention to any wires that go to the preamp tubes, pins 2 and 7. Those are your grids, those are what's gonna be sensitive.

By the way...1968 is when lots of Fenders went from the older type of wiring to the newer type. Older = green heater wire up in the air; piushback insulation, yellow wires from the board to the tube sockets. Newer = "spaghetti" wiring with plastic-insulated wires coiled around bundles of other wires like a hangman's noose. Which do you have?

Is the power transformer supposed to be a "drop-in" replacement?

Pull the tremolo tube. Still squeal?

[rifflicks]

I found the problem.....lead dress....I put the wires that I moved to make soldering the new caps easier back to their original positions, and no more oscillation ......I really want to thank you guys for your help and the quick responses....I really learned a lot too. Thanks Again!