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Filament Wiring? Pin to Pin Specific?

HotBluePlates

Member
Messages
11,889
... with respect to vintage Fender amps is there a thing about leaving the shield disconnected on one of the reverb tank cables?
Yes, but perhaps not the way you were thinking.

In every vintage Fender amp-with-reverb I've seen, the shield of the cable is connected to the shield/sleeve/ground connection of the RCA plug at both ends of the cable.

Now you might think, "That creates a ground loop, from the amp chassis through the cable to the tank's input, through the tank's chassis and out the cable at the tank's output, and back to the amp's chassis were the reverb returns into the amplifier!"​

Except the 5th digit of the Accutronics part number indicates which of the tank's jacks have a ground connection to the tank's chassis. In the standard Fender 4AB3C1B, the "C" indicates the Input jack on the tank is not connected to the tank's chassis, while the Output jack's ground is connected to the tank's chassis. This choice breaks the ground loop.

I removed some hum once on a 64 deluxe by re-wiring green heater wires to matching pins.
I tried paying attention to the "heater-wire phasing" before (in a blackface Fender amp, as it happens) and found it did nothing. I agree with @Malcolm Irving when he said if it did make a difference it would be for push-pull output tubes. However, I would prefer to simply use output tubes that don't leak hum from heater to cathode.

That said, I have also wired up amps where I used 2 different colored wires for the sides of the heater string, and had them run to the same pin # at the sockets. I think I did that because it looked neater.
 

PushedGlass

Member
Messages
734
Yes, but perhaps not the way you were thinking.

In every vintage Fender amp-with-reverb I've seen, the shield of the cable is connected to the shield/sleeve/ground connection of the RCA plug at both ends of the cable.

Now you might think, "That creates a ground loop, from the amp chassis through the cable to the tank's input, through the tank's chassis and out the cable at the tank's output, and back to the amp's chassis were the reverb returns into the amplifier!"​

Except the 5th digit of the Accutronics part number indicates which of the tank's jacks have a ground connection to the tank's chassis. In the standard Fender 4AB3C1B, the "C" indicates the Input jack on the tank is not connected to the tank's chassis, while the Output jack's ground is connected to the tank's chassis. This choice breaks the ground loop.
Thanks, HBP. When I replaced the original broken tank for my PR, I used a MOD 9AB3C1B, which I guess follows the Accutronics numbering scheme you describe. I made both cables, which are ridiculously heavily-shielded right down to the metal right-angle RCA plugs and I recall that I connected the shield at both ends of those cables. I do have some hum/buzz coming in when the reverb output is turned up. I figure I have two things to run down:
  1. Being a head conversion, the reverb tank is a lot closer to the amp chassis than it otherwise would have been. I suppose I could mount a strip of mu-metal (?) above it to capture flux.
  2. The end of the reverb tank that is closest to the PT is the one that connects to the REVERB INPUT jack in back of the amp; I ought to determine which way "INPUT" is in Fender-ese and see if I need to flip it around and reroute cables. I don't recall if I already sweated that detail when I built the conversion. As it is, that end of the tank is a little over 2" away from the PT.
 

Jeff Gehring

Gold Supporting Member
Messages
6,751
Reverb input = the INPUT of the REVERB TANK, which we know is a low impedance node, the input transducer having an impedance of 8 ohms or so. Since this is the low impedance node, you'd want that nearest the power transformer. You'd want the relatively higher impedance of the reverb tank output to be far away from the flux-slinging PT.

So, as you described your connections, you've got it how it should be.

One way I like to reduce hum and noise in the reverb in combo Fenders is to use insulated RCA jacks to replace the regular panel RCA jacks that are used in the stock amp. Then I run a dedicated ground wire to the cathode ground point of the reverb recovery tube. The kind of insulated RCA's I have will directly retrofit the 3/8" hole that the old RCA jacks were installed in.
 
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PushedGlass

Member
Messages
734
Reverb input = the INPUT of the REVERB TANK, which we know is a low impedance node, the input transducer having an impedance of 8 ohms or so. Since this is the low impedance node, you'd want that nearest the power transformer. You'd want the relatively higher impedance of the reverb tank output to be far away from the flux-slinging PT.

So, as you described your connections, you've got it how it should be.
Got it - so it seems like I'm okay on that score - at least that's not the very next thing to address. Rather...

One way I like to reduce hum and noise in the reverb in combo Fenders is to use insulated RCA jacks to replace the regular panel RCA jacks that are used in the stock amp. Then I run a dedicated ground wire to the cathode ground point of the reverb recovery tube. The kind of insulated RCA's I have will directly retrofit the 3/8" hole that the old RCA jacks were installed in.
Okay, so if I follow you here - you replace the reverb output jack, arranging it so that the solder tab is on the inside. Solder a wire from the output (incoming) jack's tab to right where the reverb recovery stage's (V3B in a PR) cathode resistor and bypass cap meet ground. What about the reverb output jack?
 

Jeff Gehring

Gold Supporting Member
Messages
6,751
The best way to convey it is with a picture. This is a Princeton Reverb build I did a couple years ago. I used the jacks in my link above for all the rear panel RCA jacks here. You will notice that all the RCA jacks are on insulating shoulder washers that prevent any contact between the jack shield and the chassis metal. Also note all the jack ground tabs are bussed together and run to the cathode ground point for the reverb driver on the parts board.

 






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