More SFSR questions

[sledge57]

I'm getting ready to do some work on my SFSR, I've had it for about a year now.

It's (I believe) a 73 or 74 based on the info I'm able to find online, and the fact that I'm 300 miles from the amp so I can't look at date codes yet.

What I'm curious about is when Fender starting putting 3 wire grounded cords in these amps. I have a 73 Vibro Champ also that has it's original 3 wire plug (the red plug they used) and my SR has the same cord/plug.

However my SR also has the ground switch. I can't see any reason they would put a ground switch and a 3 wire cord in the amp but I know strange stuff happened back then (it was the 70's and as a teenager in the 70's, I can say that decade was fuzzy to say the least )

I suppose the answer could be the 3 wire cord was added later but it looks original, did Fender make those cords available separately?

I don't seem to be able to find the answer elsewhere yet, I did see a mention that the AA1069 circuit had a grounded cable but the schematics I found for that circuit still show a 2 wire cord...

So when I get home I want to make sure this switch and cap are disconnected (if not already)

Is the switch useable for any of the mods for the SR's or should I just remove it?

Thanks

[Wally]

That switch in reality is a polarity switch. I believe your amp was originally outfitted witht he 3-waypower cord. Even with the 3-way power cord, a polarity switch is of use to some folks. I take it out of the circuit unless the owner wants it left in. IT allows for a polarity correction in certain situations.
(ONE would always want to know what the power source wiring was as always. Those little circuit testers are a necessity, imho.)
IF you remove the AC power wiring from the switch, the switch or the hole can be used for whatever you find useful, right?

[sledge57]

Originally Posted by Wally

That switch in reality is a polarity switch. I believe your amp was originally outfitted witht he 3-waypower cord. Even with the 3-way power cord, a polarity switch is of use to some folks. I take it out of the circuit unless the owner wants it left in. IT allows for a polarity correction in certain situations.
(ONE would always want to know what the power source wiring was as always. Those little circuit testers are a necessity, imho.)
IF you remove the AC power wiring from the switch, the switch or the hole can be used for whatever you find useful, right?

Well the amp will never leave my basement, except to go in for service, or if I sell it, so the switch isn't needed.

Yes it can be used for whatever I find useful, question is, which mods might that be? Just looking for ideas. I have no use for the volume a SR is capable of.

I already have a switched pot coming to replace the intensity pot for the tremolo disconnect and I'm going to pull V1.

I'm considering the NFB disconnect mod for the switch, just not sure.

First thing though is to get it tuned up......

Then the modding starts.

Thanks for the reply.

[BobbyZ]

MY SR is that vintage 3 prong cord and polarity switch. Other than a recap and the blackface mod I did 10 years ago it's stock. Myself I like it that way but I can see where doing something with the normal channel would be interesting.
Also you could go Fezz on it and put JJ6v6s, a 5v4 rectifier in it and run it on two speakers.

[E5RSY]

If the accessory outlet on the back is a 3-holer/grounded, then the power cable was originally, too.

[SoK66]

Fender started adding 3-prong power cords in late 1970. The tipoff as to whether it came from the factory that way is the ground switch. If it's a 3-way, then the 3-prong is original. The original cords had a red plug, which had a bad habit of separating from the conduit. Many have been replaced as a result, so your cord may have been replaced.

The 3-way ground was a practical feature. Most wall plugs had not been converted to 3-wire at that time. So, the system allowed the player to use a 2-prong adapter and use the full left & right positions of the ground switch like the old 2-way switches. However, if the amp was plugged into a 3-way outlet the center position on the ground switch was used, which disables the switch. In reality, what most guys did was just cut off the third (ground) prong on the plug. Another reason why many of the original cords have been replaced by now.

[Justinvs]

I left my ground switch on just for that reason. Still lots of the old style wall-plugs around here .

[Wally]

Sledge57, I can't say that I have ever used that hole for anything. I use the ext speaker jack hole for a pot to adjust the resistance in the negative feedback loop. I like versatility. I leave the Vibrato channel alone on these two channel FEnders. YMMV.
I like to separate the cathode connection between V1 and V2. INstall a 1.5k bypass resistor in the position where that 820ohm is on the shared bypass. Use the 820 and another 25mfd/25v cap to bypass that V1, pin 8 cathode. This biases that triode 'hotter'. I hten change the tone stack values to yield more mids...and sometimes change the slope resistor to affect the mid freq curve.
Thsi type of approach gives you the classic Fender reverb channel and another channel that is more tweedy in its tone and gain. So, you have two amps ...instead of two channels that aare much alike when in stock form.Siple, reversible mods that make a big difference.
I suppose you could use the 'ground' switch hole for some approach to the output section....lowering voltage?, pentode to triode mod?, fixed to cathode bias....lots of options, I suppose.

[sledge57]

Thanks for the replies. It does have a 3 way ground switch and the red plug (coming apart as usual) I figured it was grounded from birth...

Wally I really like the idea you have there for a Tweed/Reverb kinda deal on the two channels.

Would you by chance have a schematic showing exactly what you mean?

That would seem to be the ideal mod for me.

Thanks again...

[Wally]

Sledge, I pretty much gave the info needed to do that channel separation thing. I didn't give tone stack values because those can be 'personalized' depending on the guitar....but changing the .1 bass cap and the .047 mid cap to .022's is one way to go. .033mfd for the bass might work if you only use single coils. AS I said, personal choice thing. I use silver micas for the treble caps...same value as stock. The slope resistor change to a 56K changes that mid curve some.
NEgative feedback loop....I put a 50K pot in series with the 820ohm resistor on the top of the NFB loop. Big changes there....go from stock clean to a much hotter amp.
If this is not enough info, then I may be inclined to think that you need a tech to make these changes. Also, I never do this work on an amp like yours that doesn't have fresh strong electrolytics in it. Meaning...if the amp doesn't sound good befroe the changes because of weak e-caps, it won't sound good afterwards, either. 40 year old e-caps aren't doing what they are supposed to be doing, ime.

[sledge57]

Originally Posted by Wally

Sledge, I pretty much gave the info needed to do that channel separation thing. I didn't give tone stack values because those can be 'personalized' depending on the guitar....but changing the .1 bass cap and the .047 mid cap to .022's is one way to go. .033mfd for the bass might work if you only use single coils. AS I said, personal choice thing. I use silver micas for the treble caps...same value as stock. The slope resistor change to a 56K changes that mid curve some.
NEgative feedback loop....I put a 50K pot in series with the 820ohm resistor on the top of the NFB loop. Big changes there....go from stock clean to a much hotter amp.
If this is not enough info, then I may be inclined to think that you need a tech to make these changes. Also, I never do this work on an amp like yours that doesn't have fresh strong electrolytics in it. Meaning...if the amp doesn't sound good before the changes because of weak e-caps, it won't sound good afterwards, either. 40 year old e-caps aren't doing what they are supposed to be doing, ime.

I've done a few repairs and mods to a couple smaller class A amps, (BJR , Vibro Champ) but haven't been in a bigger AB amp, not sure I will in this one either. I have a great tech to take it to in the area and he's considered "The" guy for vintage Fender amps around here.

I just need to know exactly what I want done and I really like your ideas.

Definitely going to re cap it and go through everything.

I was thinking of selling it before but I'm committed to this amp now (especially after buying all those speakers ) So it's time to get it done.

Thanks again.

Doug