Fender Super Reverb - ground switch/cap + 3-prong?

[VintageCharlie]

Fender Super Reverb - ground switch/cap + 3-prong?

Postby VintageCharlie » Wed Nov 20, 2013 4:52 pm
Hi folks,

Here's a buddies Super Reverb that has been converted to a 3-prong cable. Surprisingly it still has the ground switch and the associated "death cap". The amp is being used in conjunction with a step-down voltage transformer to run it over here in Europe. In this situation, does the ground switch still do anything at all, if the cap is still in there and connected to the chassis and switch? in which position should it be set? European plugs can be set into the wall in either direction - i have no idea how to make sure, which would be hot or neutral at the point where the step down transformer enters the wall socket.

Attached is a picture of the circuit. Any advice is highly appreciated, thanks!

[VintageCharlie]

oh, and would running it with two 8 ohm cabinets hooked up at the "speaker" and "ext speaker" outputs result in the amp seeing a 4 ohm load? And would this be safe, at least for test purposes to make sure the amp works, etc. Super Reverbs have an output impedance of 2 ohms.

[Randall]

FWIW, I would clip the death cap at the switch side, but leave it soldered to the chassis. I'm not really sure about your transformer question, but this way if the cap is leaky or someday did short it makes it a moot question. Plus, one never knows where the amp may someday end up and possibly plugged directly into a 120v 60 Hz supply, where if not clipped could possibly be dangerous.

Clip the cap. The "ground" switch will do nothing. All it did before was switch that cap between the hot or neutral side to chassis to filter out line noise. And I would be OK running two 8 ohm cabs at 4 ohms as you describe for test purposes. I would also put a meter on the cab speaker plug to verify less than a two ohm load to see if all four speakers are showing continuity. In that parallel config, it's not uncommon for one to open up and not be obvious.

[Reeltarded]

Simple to either bypass the switch or remove it completely from the circuit, one of these should be done!

The Super will be fine with a 4ohm load. 2ohm pan in the ass!!

[Structo]

Here

[VintageCharlie]

thanks a bunch, guys, you're fast! And thanks for the diagram.

ok, so again, as russians say, dla osobo tupih (for particularly) dumb people like myself, do i HAVE to lift the "death cap" from the switch to make sure the operation of the amp is safe in this particular case? Of course, it's not a big deal, i'm just wondering if it is really necessary?

If the answer is yes, then i could just unsolder the cap from the switch, leave it soldered to the chassis and isolate the free floating lead, right?

[VintageCharlie]

I forgot to mention that the amp is a stock super reverb chassis in a head format, so it does not have the built-in speakers, just the head. In this scenario, if i attach one 8ohm cab to the "speaker" output and one 8ohm cab to the "ext speaker" output, the amp sees 4ohms, right? So would this mismatch form the amps 2ohm output to the 4 ohm load (2x 8ohm cabs) be ok, at least for test purposes, without crankind the amp, etc.?

[Reeltarded]

It can crank with that load just fine as long as everything else is normal.

Unless something has been done to the output jacks 2x8 should equal 4 there. Better a bigger load than a lesser. I hate mismatch with a smaller load. That can be scary.

Since the amp is already modified just unwire the cap and switch. It's simple enough.

[xtian]

If the answer is yes, then i could just unsolder the cap from the switch, leave it soldered to the chassis and isolate the free floating lead, right?

Correct.

And yes, you can push a 4ohm load safely, even cranked, I would say.

The only time I ever ran into trouble, I was cranking my 50-watt build with Bassman OT (2 ohm tap) thru a buddy's 16-ohm Marshall cab. Did fine until we dimed it, then the amp shorted the plate to the heaters in one power tube, blowing out my 100R heater balance resistors. Exciting!

[VintageCharlie]

This forum is faster than the typical chatroom

Exciting indeed, but i would prefer to avoid this kind of excitement in first person
Good point on the output socket wiring. To make sure, i will make a picture of them and ask you guys again for advice, just to make sure they have not been modified and result in a bigger, not smaller load.

As for the ground switch/cap situation - apart from the 3 prong and it being rehoused, this BF SR is literally untouched - a real gem. Hence i'm nervous about doing too much. If lifting just one leg of the ground cap that goes to the switch does the trick, then i would prefer not to touch anything else.

[VintageCharlie]

Ok, so i lifted the ground cap - the lead that went to the switch. I bent the lead in a way that it won't touch anything else - it is free floating. i left everything else as was. (see attached pic)

and here are 2 pics of the outputs. It's hard to get all the wires into the shot, but i hope you can tell from them, if it looks original and if attaching 2x 8ohm cabs (4ohm load) on the 2ohm OT would be safe.

[VintageCharlie]

Can i now safely fire up the amp through my step-down attenuator (220v->120v) without worying about first-hand excitements?
The position of the ground switch is no irrelevant, since the ground cap has been lifted, correct?

[VintageCharlie]

oh, and here is a bit of porn. sorry, my proper porn camera is out of order and i'm left with my phone camera. but the pic is still better than most latvian "gentleman's magazine's" pictures

[Reeltarded]

haha, awesome!

It is paralleled but just to calm your nerves plug both cabs in and put a meter on the jacks.

[VintageCharlie]

ok, thanks man, will do!