Shield the BOTTOM of the Reverb Pan? -SFDR

JJman

Member
Messages
994
My '71 SFDR starting buzzing a little as the Reverb is turned up. It does not matter what environments/outlet the amp is connected to. The buzz is on the grid of the post-pan tube. Unplugging the pan on that side eliminated the buzz (and the reverb of course.) Unplugging the driver side does not stop the buzz. It is not very loud but I’d like to eliminate this new issue.

The cables are good and all connections check out inside and outside the pan. Tube changes did not help. Removing the recovery tube stops the buzz. Removing the driver tube does not help. The level of reverb has not changed and is “normal.” I’ve isolated the problem as being somewhere in the pan, I think. One rca jack, on the pan, is grounded to the pan and the other is isolated, by design. I tried grounding the isolated one, but this did not help.

Moving the pan out of the cab reduced the buzz. The bottom of the pan has just the wood/cardboard screwed on to it which I assume is stock. Has anyone ever heard of the need to shield the bottom of the pan (which sits on the bottom of the cab?) Any other ideas?
 

rcboals

Silver Supporting Member
Messages
339
Interesting have you tried another reverb pan? Borrow one from a buddy if you don't have another amp with the same reverb pan.
 

ClinchFX

Platinum Supporting Member
Messages
878
Some time in the dim past (more than 15 years ago), I recall a reverb pan that had a broken connection between the ground of one of the sockets and the metal of the pan. The reverb still worked, because the coils were still connected, but the ungrounded pan wasn't doing much shielding. I can't remember what kind of amp it was, but it's worth checking.

I hope this helps.

Peter.
 

alltone

Member
Messages
291
I have seen cardboard covers with foil ...If the tank is mounted open face down..this is correct placement and should have sufficient shielding from above. Try turning the tank around (if lead length will permit),open side down. Normally ,tank input is on the right closest to the output transformer.. Solid state dimmers for lighting that are on the same A.C. branch line as your amp will cause buzzing as rev. is turned up..Just guesses????
 

RedRock

Member
Messages
3,639
Sometimes the the reverb recovery triode, V4a, needs its own separate cathode
resistor and by-pass cap, soldered to the same place the 220k ground resistor is
soldered, which is to a ground tab on the RCA reverb return jack. In other words, you
have to separate the 2 cathode pins on V4, change the mix triode (V4b) cathode
resistor to 1.5k with its own 25/25 cathode bypass cap across the eyelet board
(connected to pin 8 of V4), and install a 1.5k cathode resistor with a 25/25
bypass cap from pin 3 of V4 to the RCA jack ground tab. Also connected to this
tab is the 220k ground resistor and the ground wire from the reverb transformer.
The general rule is that a triode's input grid resistor and its cathode resistor
should be grounded at the same spot. If they are not, a buzz will often result.
Fender went to the above method in the mid-seventies I believe. This may or
may not be related to your problem, but it often will silence a buzzy reverb
where pin 3 and pin 8 of V4 originally share the same 820 ohm cathode resistor.
 

JJman

Member
Messages
994
Hey Redrock

Is the root of the problem you are describing from the fact that they are sharing a cathode resistor/cap, or from the fact that the 220k grid-to-ground resistor is physically mounted on the other side of the chassis? Would moving the 220k to the other side of the chassis address the problem? I could use a shielded wire to fly the lead across the board and ground the 220k at the same ground point as the cathode caps/resistors. This way I could move just one part and not have to change and add others.

What do you think?
 

rcboals

Silver Supporting Member
Messages
339
If I were betting, I'd bet heavily that it's the pan gone bad and a new pan will solve the buzz. I've screwed around and tried to repair a bad pan a couple times during my life and it is nearly impossible, always ended up getting a new pan to solve the problem. Far as a reverb pan shielding goes, every old Blackface or Silver Face I've owned has been in a bag with a piece of cardboard taped to the bottom of the pan.
Good luck to you with killing the buzzzzzz.
 

RedRock

Member
Messages
3,639
Hey Redrock

Is the root of the problem you are describing from the fact that they are sharing a cathode resistor/cap, or from the fact that the 220k grid-to-ground resistor is physically mounted on the other side of the chassis? Would moving the 220k to the other side of the chassis address the problem? I could use a shielded wire to fly the lead across the board and ground the 220k at the same ground point as the cathode caps/resistors. This way I could move just one part and not have to change and add others.

What do you think?

___________________________________________________________

It's worth trying. I would solder one end of the 220k resistor
to pin 2 of V4 and run a wire from its other end to the point
across the eyelet board where the 820 resistor is soldered to
ground. I don't think this wire needs to be shielded because
it represents ground from the point of its connection to ground.
You can also try the method you described and see if there is
a difference in buzz as compared to soldering the 220k directly
to pin 2 of V4. As i said, it may or may not help, depending
on the individual situation. I have eliminated reverb buzz about
20 times in the last 10 years with this method. The problem
was not caused by cathode resistor sharing, but rather by
the fact that the 220k resistor was not grounded to the same
point as the cathode resistor connected to pin 3. Good luck.
 

RedRock

Member
Messages
3,639
If the discussed circuit change is of no help, I would replace both RCA reverb send
and reverb return jacks; then the cables; then the tank.
 

rockon1

Platinum Supporting Member
Messages
13,696
Before I messed with the amp or amp circuit I'd check the tank and cables with a multimeter. Easy stuff first. Digging into the amp before doing so is putting the cart before the horse IMO. Bob
 

RedRock

Member
Messages
3,639
Good idea, but if you have an intermittent on the cable shield or tip or jack,
it occasionally will read good on the meter.
 




Trending Topics

Top Bottom