Bias Imbalance (leaky caps?!) sf princeton reverb

[psychbeat]

Heya-
this is my first post (been lurking)
anyways, my friend gave me a 75 princeton rev (FREE!)
that was pretty hacked up with a weird master volume
thingy wired to the trem intensity knob
I removed the wires and hooked the trem back
going by the schematic. the amp had a blown
fuse which I fixed only to have one of the sylvania
6v6s go on me. I stole another set from one of my
other amps and busted out the Bias Rite.
~400 on the plates with 26ma on the tube closest to
the tranny and 32ma on the one next to the recto....
oh I also did the "stokes mod" since the amp came with
a 12oxford in it.

anyways if I turn the trem intensity knob to 0 the bias gets
way hotter on the bias rite. especially on tube 1 (next to the recto)
like ~40ma + hot. the other tube only moves a little.

I ordered some vitamin t (cheesy name but I thought id try them)
coupling caps from mojo.

Ill keep the blue molded just in case


sorry for such a long post-
just wondering if anyone would know why
the trem would be affecting the bias balance so
much?

I plan on doing the paul c mod (with a 4m7 tho)and a bias pot too
as soon as the parts come + a slightly bigger mid res (8.2)
and possibly a toned down "Rick E" mod
sounding pretty dang jagged right now above 2.5 on the vol.

the amp was hacked already please dont flame

[überfuzz]

Try looking at a layout of the circuit, compare it with you wiring, AA1164 layout. When you've triple checked every thing get some readings on does power tubes.

[frankeg]

Welcome to the addiction... I've seen a few Princetons using the intensity pot as a MV. I agree with Uberfuzz, carefully check vib/trem and bias circuits. Check the neg dc bias going to the power tube grids from the intensity pot. Make sure you have equal bias on both grids. Let us know what you find.

[km6xz]

Until you find out the problem with the wiring, save the power tubes and do measurements without the tubes installed. Unless you know the caps are bad, it is easier to trouble shoot by keeping the number of variables to a minimum. Fix the amp before making changes. If the bias is even on the sockets, measure the resistance, and if you have the gear, the impedance of either side of the primary of the output transformer. A mismatch of plate loads could account for the difference in cathode current and rate of change with bias changes. So can having two tubes of different condition. If that is all OK and the tubes are reasonably close take a look further downstream.
Be sure to check the wave form of the Tremolo modulation. Distortion can account for average polarization of the modulation that would affect average reading of a bias tester. A scope is much better for spotting that than a current gadget. If your scope has a summing function, sample the pure bias on one channel, and the other channel the modulated bias and set the summing amp switch to sum A+B and swap the phase of one channel with its phase switch. That will cancel the DC bias component on the scope using DC coupling. That way you can have a resulting trace of the modulation to DC if there is a DC component in average modulation or distortion that shifts the duty cycle of the peaks and valleys.
To reduce the number of confusing circuit conditions, turn the intensity to 0 while tracking down the static power tube imbalance. The tremolo modulation will cause reading inaccuracies with any long period averaging meter like a bias tool or a DMM. A scope conveys information masked by sample period of digital meters. You may find that the modulation is throwing off your meter and that nothing is wrong there. Note if the speed control changes your readings much. A slow modulation rate could straddle the sampling rate of your tester and simulate a value quite different than is actually there. That is another reason I like real VTVMs like the servo controlled HP410C or better, the really old ones that can't be hurt by anything in an audio amp. They can drive a meter faster than its natural damping so the reading settles very fast and can follow variations to give a visual clue as to waveform of the composite signal.

[psychbeat]

thanks guys

Ill recheck the layout again when I get home
but am pretty sure everything looks right.
I loaned my DMM out so thats why Ive been
stuck using my little bias tool. Ive tried 2 sets
of matched tubes and both have a similar unbalance
and have been testing with the intensity off etc.

KM6-
wish I had a scope!
Ill bring it to a tech when I get some $
and after Ive done some tinkering (hopefully not
messing anything up too bad!)

[psychbeat]

FIXED!!!

AWESOME!!!

OK you guys were right- I rechecked the trem wiring and
there was a wiring mistake- probably mine but maybe left over
from when the previous owner had the crappy master volume
installed. The bias is balanced now and the trem sounds better.

Now I can get to the noisy reverb. I swapped pans and
tubes (driver and return) with no luck. My BF pro reverb blew a
reverb trans and it was a pretty easy fix so maybe next paycheck
Ill do a preemptive replacement...
anyways,
thanks again for the help!

[überfuzz]

Nice, the hunt for noise can be quite a challenge. How did the noise that still lingers in you amp sound? Turn the amp off and pull the reverb tube, how does it sound? Do the same with the pan.

[psychbeat]

with no pan or driver tube it still has a
60hz hum when I turn up the rev knob.
with the pan and driver it has a small
amount of white noise added. its not
terrible but I get nervous about reverb
trannys- when the one in my BF pro went
it was during a live show and it sparked +
dripped molten wax or plastic onto my
JBL D131.. Ill probably change it no
matter what even tho its probably
not the source of the noise.

any ideas?

[km6xz]

Don't change the transformer unless you know it is defective, leaving the old one in will not only be easier but since it has been reliable, which is something you have no knowledge of about a new one. The two most common failure points for a part is when it is new and when it has passed its design life period. Nothing from the symptoms suggests adding more ambiguity(replacing a perfectly good part) will correct the noise problem.
Noise is common in reverbs because there are so many access points where it can be introduced. The pickup coil can have low frequency hum magnetically induced, the high gain follower stage can develop a slight noise problem that is amplified by a high degree, the drive amp has its own entry points for noise but less likely.
Does the hum rise and fall with the reverb pot or does it remain at a constant level? Is it noise, hum(60 or 120 hz) or buzz? Each has a different method of being introduced into the signal path.

[MWJB]

When checking idle current on an amp with trem, always turn off the trem at the footswitch, rather than just turn the intensity down.

[frankeg]

Great! Glad you found it... Don't give up on the reverb noise, there is plenty of help available here. I'd leave the tranny alone as well for now but I would check the reverb pan and what some folks forget, the cables. I'd verify the pan is the correct unit (matching impedance). Is your driver a 12AT7? Does your reverb pot sound scratchy when adjusted? If all this is good, I'd check the grounds in the reverb circuit. Make sure you have good solder connections. The reverb pot is usually grounded to it's casing and the reverb tranny should have a good ground. Check your cathode grounds as well... It's a process of elimination but usually works well. Let us know.

[psychbeat]

cleaned the reverb pot out again

and now the hum is a bit better

so I went ahead and did the "paul c mod" subbing a 4m7
instead of the 2m2 per someone here's suggestion.
I already had done the "stokes mod"
I also swapped the mid resistor to an 8.2 (pretty subtle)

Im pretty pleased with the amp now!
the break up is a bit smoother than it was
before the paul mod.

one weird thing tho-
the pilot light is dimming with the Trem and its
got me nervous that somethings not right.
My other Fender amps (blackface pro and quad reverb)
obviously dont have the same kind of trem so maybe
its normal for the princeton light to slightly dim with the
trem?
I rarely use vibrato anyways...

still waiting on tubes and bias pot...

[Tom Phillips]

...one weird thing tho- the pilot light is dimming with the Trem and its got me nervous that somethings not right....

That is very unusual and definitely night right. It would take a huge load to cause the heater voltage to vary so much that you could see the pilot light dim.
Does the light dim steadily when the tremolo is turned on or does it pulse with the tremolo beat? Never saw this problem before.

[psychbeat]

subtle pilot pulsing with trem

yeah its pretty weird but really subtle
the trem got a bit deeper with the Paul C mod

no oscillations or hum tho - it sounds great!

I need to get it back to the practice studio n play
it loud for a while n see how it sounds with all
of the different speakers Ive got down there.
I think the D123 Ive got will be perfect!

hope Im not destroying the PT with this dimming...
Ill avoid the trem til I can get it to a tech.

also, I dont have a dummy plug in the RCA but the trem
is working anyways - probably form when the previous owner
had the cheesey master wired to it.

I actually like the dimming light - looks kool in the
dark

[Tom Phillips]

...also, I dont have a dummy plug in the RCA but the trem
is working anyways ...

That's normal. The trem circuit in the Princeton Reverb is designed to be ON when the FS jack is open circuit.