Princeton Reverb

[Valvehead]

Im working on a Fender Princeton Reverb.
The problem is a loud , low freq. hum when the verb is turned up. The hum is not there with the pot on 0 . As i turn it up , the hum increases. The reverb DOES work.

Ive tested the tank, its got 200 ohms on one jack and 1.5 ohms on the other.

Ive swapped out the rca cables.

all grounds check ok. The amp was re-capped about 7 years ago.

The voltages look normal .

ive swapped out all tubes

any ideas ?

[Enzo]

We are talking a low WOOOOO, not the same hum as a bad ground, right?

That would be the reverb pan feeding back acoustically. You can verify this by noting that the hum is not immediate when you turn up the reverb, it takes a few seconds to build up. And dismount the pan and set it outside the amp cabinet and see if it now behaves.

For good reverb health:

1. Reveb pan NEVER srewed down tight to the wood. The pan should float on those spongy grommets, and the screws only in enough to keep the pan from flying around loose.

2. Put the pan in one of those vinyl reverb pan bags, that's why they make them.

3. Make sure the cables are in the right jacks.

4. The innards are two or three long springs stretched end to end on a subassembly. Make sure none of them are broken free. The subassembly is in turn supported at the corners by four short springs. Make sure all four are in place. One comes loose, the innards can sit on the floor instead of floating.

5. The top surface of the metal pan is a large flat expanse of sheet metal. It can resonate and vibrate. Get some of that dense foam rubber weatherstripping. The kind you might run down the edge of a garage door. The stuff I have in mind is about an inch wide and 3/8 thick, but the dimension is not critical. The stuff has a peel-off adhesive back. Run a stripe of that down the center of the flat surface. It will dampen vibration in the metal. Car makers do that inside large body panels to reduce sheet metal noise.

6. If ther is room, just move the pan around. SOmetimes moving it over just an inch is enough to change the accoustics.

7. SOmetimes turning the pan upside down helps.

8. SOmetimes you just need a new pan, dammit.

[Valvehead]

"We are talking a low WOOOOO, not the same hum as a bad ground, right? "

Yes ... a low freq. hum --Not a buzz.

the hum is there the instant i crank up the verb pot.

it does it with the pan in the amp , pan out on the bench , pan in or out of the bag , different rca cables , etc...

[thumbs]

Is there any way you could swap out the pan with one that's known to be good?

[Valvehead]

yes- i have a new one in an Orange sitting here. I guess its worth trying

[Bruce / Mission Amps]

Chances are the Orange tank will not work with this circuit.... check the schematics for any differences... like one is solid state and the other is all vacuum tube... stuff like that.

Check to see if you have a lot of AC ripple on lug 1 and 6 of the 12AT7 reverb driver tube....

[Valvehead]

tried the orange tank --worked ok--still has loud hum.


AC ripple , pin 1 or 6 to ground is about 0.1 mv ac

from 1 to 6 - the meter goes nuts-cant get a reading

[Valvehead]

Today :
replaced some caps in the verb circuit ,
the .02 following v1b
the .003
500pf
10pf
both 25uf cath caps

all resistors check ok

checked shielded wire grounds

still cant find the problem . Could it possibly the reverb transformer causing the hum ?

[Bruce / Mission Amps]

Today :
replaced some caps in the verb circuit ,
the .02 following v1b
the .003
500pf
10pf
both 25uf cath caps

all resistors check ok

checked shielded wire grounds

still cant find the problem . Could it possibly the reverb transformer causing the hum ?

1 to 6 isn't what you want to do.
Bad reverb tranny, .... anything is possible but probably not what's wrong...
is the tank mounted with the RCA plugs forward.. at the speaker baffle board?
Another thing to try is making sure the RCA jacks and the 220K grid load resistor are well grounded on the chassis.
Sometimes moving the cathode bypass cap on the reverb recover triode, (lug 3) over to the jack helps and isolating the filter node of the reverb section alone to an independent one can cure some hum too.

[Valvehead]

bruce ,

the tank has been in the amp and also on the bench--hum doesnt change wherever i put it.

tried the moving cathode ground = no change


grounds on Rca jacks and 220k are good

[jmaf]

When you unplug the tank, does the ground on the input rca jack have condutivity to the ground on the output jack? they should be isolated, the input one should not conduct to the tank chassis.

one trick that solved this for me just last week, was reducing the coupling cap, after the return triode, to 100 pF only. I was using 1n and it passed a lot of 60 hz hum which is natural from the single coil transducer of the tank.

[Valvehead]

"When you unplug the tank, does the ground on the input rca jack have condutivity to the ground on the output jack? they should be isolated, the input one should not conduct to the tank chassis. "


yes--it grounds to the out jack . well , isolating the ground on the rca in jack KILLS the hum--but also kills the reverb . tried moving the ground - still hums

its all factory --the jack is bolted right to the chassis.




"one trick that solved this for me just last week, was reducing the coupling cap, after the return triode, to 100 pF only. I was using 1n and it passed a lot of 60 hz hum which is natural from the single coil transducer of the tank. "

where is this 1n ? i dont see it

[Bruce / Mission Amps]

He might be talking about the 3.3nF / .0033uF cap driving the reverb pot, commonly found in Fender Reverb combos.

[Valvehead]

Today : Tried the things mentioned above .

Also i tried a NEW reverb transformer

and i swapped out the rev pot and all the tubes again .

nothing changed .

about ready to throw in the towel on this one .....

[Valvehead]

any last thoughts?