This article is about a nice player grade 1965 "Fender Electric Instrument Company" Vibrochamp that came across the bench. However, this applies to many other amps out there.
This amp came to me in reasonably good condition. Speaker was replaced with an 8 ohm (I installed an early 70's 3.2 ohm Jensen) and the output transformer is long gone and was replaced with a classy Mercury Magnetics Tone Clone. It had been re-capped reasonably well and this included the signal caps. I like pristine amps but I kinda like ones like this a bit more. I don't feel bad about making improvements in such an amp, and I know it will not sit in a museum after it leaves, it will be used in a living room or someones studio as it was meant to be. These amps are no longer cheap so I do my best to make them as functional as possible.
So the trouble was even after a re-cap it still had a good deal of hum. That's actually not all that unusual on one of these and it's challenging to get the hum out of a class A single ended amp to begin with. The trouble with these lies in the filament line. To cut cost on this little student amp Fender grounded one end of the filament line along with one side of the filament in each tube. This creates a wide and kind of stupid path and eliminates the hum reducing effects of having a twisted filament line as you see in anything from the Princeton on up to the Twin.
This is a pretty easy thing to fix. An experienced tech can knock it out in about a half hour.
So first thing locate the green wire that is going to ground near the filter caps. That is one side of the 6.3VAC filament line. Un-solder it or simply cut it. It's soldered to the same point as the center tap on the high voltage line (red with yellow stripe):
Next unsolder the tab on your pilot light that is connected to the housing which is ground:
Take the green wire that you disconnected from ground and solder it to the tab you have liberated from the pilot light:
Now add the wire you will be using for the other side of the filament. I use high quality cloth heavy gauge wire for this. It's nice to work with and it looks classy. Some day someone will open this amp up and I'd rather have it look good when they do! Here's a link to the good stuff....
Added wire:
Next disconnect the ground wires attached to pin 7 on the 6V6 and pin 9 on both 12AX7 tubes:
Remove them:
Now add the green wire and carefully twist it like you see in bigger amps. You want it to float above the tube sockets and drop down over them.
Now for the all important ground reference! The original Fender power transformer has a center tapped filament. Woo hoo! You'll likely see an extra wire that is green / yellow just taped off with electrical tape. You will need to extend it likely. Do that, add heat shrink tubing for safety and simply solder it to ground:
Now this is one method of creating ground reference. It's actually not my preferred method. I like using a 100 ohm pot across the filaments with the center tap either going to ground or to pin 8 (cathode) in the output tube. You can really dial it in this way. But if that's not an option you can also just use 2x 100 ohm resistors across the filament to ground or to the cathode. The advantage is safety. If your 6V6 plate shorts to the filament and your fuse doesn't pop, you'll burn up those 2 resistors rather than burn up your 6.3v heater winding, thus destroying the power transformer. Better to burn up a couple bucks over a transformer that cost a C note or better and making your hip amp even more "player grade" than it is.
As an example here's what it looks like, 100 ohm resistors going from pin 2 and 7 to pin 8:
This is an arrangement you see in a lot of old hi-fi amplifiers. My own hi fi amps have this arrangement as well.
Now, will this eliminate the hum? In my experience, no. But it will greatly reduce the hum. Can the hum be eliminated? It likely can by converting the 6.3VAC to DC with a bridge rectifier and voltage regulation. But why bother. It's a guitar amp that was never meant to be perfect.
Now I have done this mod (described in this article, not DC conversion!) in push pull amps. One is the very well built and classy Guild 66J. Nearly silent. The other of note is the not well built and trashy Silvertone 1484. A customer pushed me and pushed me years ago and lo and behold, this made him happy. Quietest Silvertone on the planets. And if you are going to buy a Guild 66J (very under-rated!) buy this one:
It's one I had the pleasure of working on. Brand new expensive filter can and quiet filament line.....
One other thing of note. I did replace the cathode bias resistor as it was burnt up real good..... I like to use a 5 watter myself. These old Champs run very hot. You can experiment with the value on this resistor. Try going up to 750 ohms or better. This may, and I haven't tried this myself, reduce some hum as well. The 6V6 cooks in a typical Champ. The later 70's ones in my experience can really burn through tubes. If this is a problem for you that is worth having a look.
If you notice I always separate the cathode bias resistor from the capacitor. This is to reduce heat getting on that cap. With the standard 2-3 watt resistor people like to use it gets very hot. I've seen it melt that cap literally. Again with some 70's amps the little white cap is almost always melted! I also always go with a 50V or better cap in this position. Just from experience, if it's a hot Champ a 25V cap may blow up, literally! And if you do experiment with a higher value resistor please note the voltage will rise on that cap.
Once you are done remember always use a Variac or a current limiter to test your amp. Remove the 5Y3 rectifier and make sure your filament line is good before adding the high voltage. Tubes should light up as well as the #47 pilot light.
And I cannot stress enough, safety first. Always double check your work as you go along. I just had a 70's Princeton with so many errors in it, big errors. Go slow and if you don't feel comfortable doing this kind of thing, pay someone with a good reputation to take care of you! Your amp wasn't cheap and it's worth it!
J
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