SF Twin Re-Cap

[LedZepp007]

Ok, I know (and see) there are like 1 million similar posts on the forum. BUT, I finally had time to crack open the '73 SF TR that I bought a couple of weeks ago.

A couple of observations:

1) Even though I must have checked the filter caps 10 times with my multimeter, I was actually pretty nervous to begin working in the amp. The fear went away quickly. I didn't realize until after the fact that the SF Twins must have bleed resistors on the filter caps. I was registering 500V on one of the caps which rapidly dissapated no matter what manner I turned the amp off. The "bleed cable" that I made also worked.

2) Most of the caps inside are blistered. Most of the blue signal caps are OK, but the reddish drop signal caps are pretty much screwed. The filter caps are in poor shape.

I am currently blackfacing the circuit, but I figure I also need to replace most, if not all, of the caps. Is this a good idea? I hear that the blue caps in particular are worthwhile to keep if in good shape.

Also, the Vibrato doesn't work. Closer examination reveals that the opto-isolator looks to be pulled apart. I don't need a footswitch to activate the Vibrato, correct?

Thanks!

P.S. The MV sounds like dog poop (particularly the boost). Probably going to remove the boost, but put the MV post phase inverter/

[xtian]

Why do you think the "reddish drop signal caps are...screwed"? Generally, coupling and tone caps will last a lifetime, certainly longer than power filter, bias, and cathode e-caps.

True, you do not need a footswitch in order to hear the vibrato.

I don't think a PPIMV will do anything worthwhile in a Twin, but others may have better experience.

[sluckey]

I don't need a footswitch to activate the Vibrato, correct?

Incorrect. You will need the footswitch plugged in and the contacts of the VIB switch must be closed. Or you can put a ground strap on the VIB footswitch RCA jack on the amp. However you do it, the jack must be grounded to enable the tremolo oscillator.

[LedZepp007]

Why do you think the "reddish drop signal caps are...screwed"? Generally, coupling and tone caps will last a lifetime, certainly longer than power filter, bias, and cathode e-caps.

True, you do not need a footswitch in order to hear the vibrato.

I don't think a PPIMV will do anything worthwhile in a Twin, but others may have better experience.

I'm just guessing on the drop caps as they are almost all blistered. Better to just leave them in?

Thanks for the help!

[LedZepp007]

I don't need a footswitch to activate the Vibrato, correct?

Incorrect. You will need the footswitch plugged in and the contacts of the VIB switch must be closed. Or you can put a ground strap on the VIB footswitch RCA jack on the amp. However you do it, the jack must be grounded to enable the tremolo oscillator.

So as long as I can do it without buying a footswitch, I'll be happy. Thanks for the info.

[matt h]

(deleted)

[xtian]

I'm just guessing on the drop caps as they are almost all blistered.

Can you post a photo of the blistering?

[Richie]

to turn the trem on the SF, just find an old rca cable you don't use, and cut the cable leaving enough to strip and twist the wires together. you can solder and heat shrink, plug in to the footswitch jack... just to turn it on that will work. I usually keep one handy when someone does not have the footswitch. The older amps are just the opposite, they need the switch to turn them off..

[LedZepp007]

I'm just guessing on the drop caps as they are almost all blistered.

Can you post a photo of the blistering?

Sure thanks

[jjman]

The blisters on the coupling and trem caps look like the wax that is supposed to be there. I think 1973 was a peak time for that wax. The ends of those filters are bad for sure.

[xtian]

Good photos. Yes, what jjman said.

[Stevem]

Those blue tubular white lable caps are not bad sounding for what they are, and as long as they are not leaking any dc then I would leave them in or if you have a a spare 25 bucks or so on hand then get some film and oil type caps at least for the full vibrato channel.

In regards to getting a lower volume over drive tone that uses both the preamp and output stage and it's much sweeter type distortion, save up and get a weber mass type attenuator as pulling out two of the four output tubes still does not cut the volume in half, but running one speaker will with the issue of May be arcing over the OT and killing it by running a higher impedance load on its 4 ohm tap!

[pops]

I forget the value, but if you place a resistor network that is close to the resistance of the reverb tank and put it in place of your tank then the reverb knob becomes an overdrive control. You can get nasty at low volume with everything else stock. Use it when you want and plug the tank back in if you don't.

[LedZepp007]

Thanks for all the info everyone!

I will say this: I have really not enjoyed working with the eyelet board. In some cases it's taken multiple attempts to get a seemingly solid connection. Hopefully this thing works when I'm done with my modifications. If not, I may have more questions

That being said, I have had a lot of fun working on it, which is a good sign.

I will NOT be using eyelets when I build amps for myself.

[Stevem]

If you are not using at least a 30 watt iron with a medium tip then that's what will happen!