70wt SFSR advice needed

[Chris Scott]

Hey everybody,

Just got a very beat, but all orig. late 70's SR. It's the 70wt model with SS rectification and MV. What I'd LIKE to do is BF the thing, yet I'm wondering if it's even worth doing, what with all the bells and whistles and huge transformers the thing has.

I have yet to locate a schematic for it- what model # is this thing, anyway? It's got a hum balance, output tube balance and a line out, in addition to the rest of the normal SF features. All the caps on the board are dark blue- can't remember if these are crap or not- if they're worth keeping I'd check them for value and leakage, but if they're junk, you'll be saving me the trouble!

I'm going to replace all the electrolytics as a matter of course, as well as a set of Jensen Neo's, as the speakers are tired and this thing will get used pretty hard. So any suggestions regarding any possible component swapping will help a lot, as I'm putting an order together as we speak....

Any tips/links will be hugely appreciated!
Chris

 

[slider313]

I wouldn't even consider a blackface conversion on that amp. It would be too much work and you wouldn't really be able to get it to sound anything like a blackface. Service it, put some good tubes in it and use it for what it is.

 

[AL30]

I'm sure slider313 hates me about now but...

I'm actually in the process of doing that myself. Although I've been "in the process" for several months so I don't anticipate anything getting finished too soon.

But, I wouldn't worry about the schem (it's pretty ugly) but I think it's at The Fender Amp Field Guide. There is also a blackface schem and layout which is what I'm using.

http://www.ampwares.com/ffg/

Here's my plan of (in)action. I ordered a new board from Watts Tube Audio.

http://www.turretboards.com/

I have enough parts lying around from other projects to complete most of the board this way. I'm probably going to leave the Filter Cap board as is (but you can order one from the above link as well) and I don't think I'll be moving the bias board or installing a new one but again, you can get a new one if need be.

The back panel will stay and I'll just pick up some plastic plugs for the empty holes (Ace Hardware).

New Transformers - I'm still undecided about what transformers to put in but Weber seems to have a nice assortment. I'm sure Mercury's are great but I can't afford them. Heyboer is also a consideration.

What to do with the MV? I'm leaving it. I've got a SF Quad Reverb with a MV and it's very useful. And since it's a push/pull I'll need to find something to do with the extra feature.

You'll need to drill a hole for a rectifer tube. Make sure you order an extra tube socket. Watts Audio has these as well - I ordered the Belton sockets.

Since this is a project for me (and I only paid $50 for the amp ) I'm going to rebuild channel one to something else. Still undecided - Marshall? DrZ? EF86?

And I'll be cleaning up the rat's nest Fender was calling "wiring".

It should be fun. Good luck.

AL

 

[Chris Scott]

Thanks you guys, and AL30- good luck, you've got quite a project there!

I should have made it clear that my goal is NOT to "Blackface" this amp in the strict sense- WAY too much work, imo. My plan is to just clean things up a bit, maybe lose what appears to be a fair bit of junk they stuck in there to correct some lead dress issues, convert to adj. bias, BF PI, etc.

Aside from new tubes and electrolytics, my first thought was to leave it alone. Yet as I was able to MARKEDLY improve a Twin that came from the same era, I can't help but to wonder if maybe this thing can be helped out a bit as well.

I've never seen all this other crap they stuck in here, so I just thought I'd post in the off chance that someone out there has some experience with this particular version of an old friend......

I'm listening

 

[slorinczi]

I'm in the same boat: I have my bandmate's 70W SFSR on the bench and I'm trying to do some gentle mods. Replaced electrolytics and phase inverter coupling caps (they no longer match, if they ever did), converted to adjustable bias, and did the simple BF PI conversion.

Have I gone too far? I'm really not trying to make this into a different amp, just make it a little friendlier sounding. Thoughts, input, rages, whatever....

Thanks,
Seth

 

[VikingAmps]

I have had some success with building a screen supply and removing the UL taps on these amps. The other changes are pretty simple resistor cap changes to blackface it up but the plate voltage is really too high. Bringing it down on the preamp/PI helps and a PPIMV can really help as well.

 

[slorinczi]

Thanks, I still can't decide whether or not to go for it. It's not my amp, but I have free license to do whatever to make it sound better. The owner certainly doesn't want or need 70W....

Another poster suggesting feeding the screens (through 470 ohm 2-5W resistors) from the 500V source located above the standby switch in the partial schemo below. Sound about right?

Also, sorry what is PPIMV?

Thanks much,
Seth

 

[JJman]

Thanks, I still can't decide whether or not to go for it. It's not my amp, but I have free license to do whatever to make it sound better. The owner certainly doesn't want or need 70W....

Another poster suggesting feeding the screens (through 470 ohm 2-5W resistors) from the 500V source located above the standby switch in the partial schemo below. Sound about right?

Also, sorry what is PPIMV?

Thanks much,
Seth

I would tap it off the junction on the other side of the 2700ohm 10wt resistor in the schematic. The 500v is the same node as the plates (thru the OP transformer.)

PPIMV= Post phase inverter master volume.

 

[VikingAmps]

I'd be careful running the screens from the other side of the 2.7K resistor, besides pulling the voltage down quite a bit there for the rest of the preamp you stand a chance of overheating that resistor. I'd run separate power resistors or a choke and filter it.

 

[jay42]

The most common advice I've tracked on these is to simply disconnect the negative feedback and see what you think of it then. This is supposed to help noticeably. Not sure what Ed Jahns was thinking, having UL and negative feedback.

 

[slorinczi]

Thanks for all the tips gents...to be honest I'm 99% sure I'm going to keep it simple and not mess with the UL. Again, the amp IS what it is, right?

RE: NFB. Is there a "best" or "proper" place to disconnect? Honestly I've never messed much with it. (It IS the tap off the "GRN" OT secondary in the schemo above, correct?)

Thanks,
Seth

 

[jay42]

Not having one in front of me, I would guess that there's a short wire to the Line Out, then another wire to the 820 ohm||0.01uF. Remove that wire and save it.

 

[slorinczi]

Sounds about right, thanks Jay!

 

[JJman]

I'd be careful running the screens from the other side of the 2.7K resistor, besides pulling the voltage down quite a bit there for the rest of the preamp you stand a chance of overheating that resistor. I'd run separate power resistors or a choke and filter it.

With screens at around 5ma each, I'm wondering about the numbers.

extra v-drop=2700 x .010 = 27
extra watts = 27 x .010 = 0.27

So the preamp/PI voltages would go down about 27 volts and the 10watt 2700 ohm resistor would be asked to dissipate an extra 0.27 watts. A choke would make more sense I guess if practical. Filter adequacy may also be a concern?

 

[slorinczi]

The amp is up and running, and sounding pretty good overall (though it IS a late-era Silverface; dime the preamp for saturation and you just want to go hide under a rock, it's a terrible sound!).

Jay, I think we corresponded about where to disconnect the NFB; I used the now-disconnected ground switch to interrupt the NFB line from the "line out" jack to the 820ohm resistor etc., but when I throw the switch to disconnect, the amp loses all output. What am I doing wrong here?

I also note that the grn/yellow OT secondary CT meets the 820ohm/.01 junction (this is not called out in the schemo below, is it just a given?). I suspect that's the culprit but I'm still a little dim on CT and their functions....

Thanks,
Seth

 

[jay42]

I finally found the schematic in AH Pittman...that should work. The one you're posting looks more like the 135W circuit.

Check the connections at the master volume push.pull vs. the schematic. Are you sure you interrupted the right wire?

 

[slorinczi]

Well, I suppose "right wire" is subjective! I interrupted what I thought was the NFB line, the one coming off the line out jack back to the master volume....

As for the schemo, I'm pretty sure it's the correct one.... This is a 2 x 6L6 amp, and so far everything in the amp has matched the schemo I posted....