Scott 299a Type 1: Silicon or Selenium???

[pilotputz]

I picked up an early Scott 299a this weekend.
I'm just confident enough to attempt a resto and right away I have a question!!
I've done some looking around on the web and there's talk about replacing the selenium rectifier with a silicon bridge. But it looks to me like it IS already a silicone rectifier.
Am I wrong about that?
It is also recommended that you change out the first resistor after the rectifier, changing it from a 10 ohm @ 2 watts to a 33 ohm @ 5 watts.
If you're replacing silicon for silicon is this really necessary?
Any advice would surely be appreciated!!

 

[jaymanaa]

If it's the flat square metal rectifier it ain't silicon, but a radio shack full bridge will fit in that case. Those resistor values might be fine, sort of depends on wall voltage. I put a pot in for the heaters, and then do two pots for balance and two pots for bias.

 

[pilotputz]

Hey jaymanaa.
Yeah. It's the little metal box.
It's not what I'm used to selenium rectifiers looking like!
Anyway, I replaced it with an NTE 5310.

Here's a few pics.

The amp's been worked on already: The rear can cap has been removed and replaced and a ground has been screwed to the chassis. I'm not sure I think that's right...

 

[pilotputz]

OK
Here's some pics!

 

[jaymanaa]

Looks good so far, Scott had what I consider to a fairly unimpressive grounding scheme, so I'd say that screw is probably ok. Have you powered it up any at all?

 

[jaymanaa]

Oh, you probably noticed the little wafers when you pulled out the old rectifer. If I'm correct, those are layers of selenium.

 

[NOSValves]

Looks good so far, Scott had what I consider to a fairly unimpressive grounding scheme, so I'd say that screw is probably ok. Have you powered it up any at all?

:scratch2: well when working properly they are about the quietest vintage integrated including phono section known to man so the grounding must work pretty well....What is gods name is wrong with star grounding? The only models I have not been impressed with were the very last models to be released after 1966 or so where they used random riveted terminal strips connections all over the chassis.

 

[unknown]

The rectifier in these was(is) selenium) which is not silicone. Looks like you have at least replaced that (which would not give you your negative bias that's needed) and its good you have considered the dropping resistor after. Now Onto to testing voltages!
Mike

 

[pilotputz]

Hey guys!!
I have fired it up slowly on my variac.
All tubes light but there is a substantial hum at about 80v.
Both channels put out music but one channel sounds a little weaker than the other.
One thing that I've learned over the years is that Pyramid caps should always be replaced and this thing is chock full of 'em!!

 

[jaymanaa]

:scratch2: well when working properly they are about the quietest vintage integrated including phono section known to man so the grounding must work pretty well....What is gods name is wrong with star grounding? The only models I have not been impressed with were the very last models to be released after 1966 or so where they used random riveted terminal strips connections all over the chassis.

I've never seen a star grounded scott and neither have you.

 

[jaymanaa]

Hey guys!!
I have fired it up slowly on my variac.
All tubes light but there is a substantial hum at about 80v.
Both channels put out music but one channel sounds a little weaker than the other.
One thing that I've learned over the years is that Pyramid caps should always be replaced and this thing is chock full of 'em!!

Yeh, she just needs a good going over.:yes:

 

[kvflyer]

Here is an example of how I rebuild the bias/filament filter. Small, 105 degree capacitors from Mouser that are inexpensive and very thin. Even though this capacitor used 75 uFD at 75 WVDC capacitors, I use 100 uFD at 100 WVDC in every can. Later Scott amps sometimes used the 100 uFD capacitors instead. After doing a few, it only takes about an hour to do one. To me, it is a more finished looking product and you don't have to find a place to jam components under the chassis of an already cramped Scott amplifier. The 340B and 380 are even more cramped.

Wires from the capacitors are brought out next to the lug and soldered to the lug prior to re-insertion to the wafer. The only thing that you have to do is to file a very small notch for the common positive lead that passes through the wafer and is soldered to one of the lugs of the steel ring.

 

[pilotputz]

Hey kvflyer!
That's pretty darn clever.
Thanks for the advice!!

 

[kvflyer]

One thing that may help the next owner of our amplifiers is to put a paper tag or note inside of the chassis letting the next owner know that capacitors have been "stuffed". It would be a shame to tear one open only to find new parts inside. I have not found an effective way to stuff the American Radionic Ceracaps yet since they just don't lend themselves to dis-assembly. But wax paper capacitors are easy to rebuild. However, Scott amps of this vintage didn't use the wax coated ones. Brown Pyramid was an early brand but they are molded (and leaky).

I have not stuffed the bias supply yet but plan to do it later. The Radio Shack bridge rectifier is just too easy to use. It has a hole that can be used with a screw, nut and washer mounted in one of the Siemens mounting holes.

Here is my early generation Scott 299 that was my first Scott rebuild. I still need to replace those two .02 uFD disc capacitors in the preamp section. Not a lot of room for Orange Drops. You can also see the replaced bias bridge and the 5-Watt resistor that replaced the first one after the bridge.

 

[pilotputz]

kvflyer, you sure got that amp looking clean!
I intend to use Orange Drops too.
I'm sure I'll have a few more questions once the parts arrive!!
Thanks again for the pics and the advice!!

 

[kvflyer]

The only problem with Orange Drops is their size. One has to be careful with placement especially with the .1µFD capacitors due to their size. And the selector switch is a real challenge. I take my time and remove all of the old component from the lugs before connecting the new component. I know it is a pain but I don't like splicing the leads. Not criticizing anyone else, it is just the way I like to do it. At times, you will break open the lug on a 9-pin tube socket. Most of the time, those lugs can be removed and replaced with one from a donor socket. I also stopped replacing the 1-watt 100KΩ plate resistors. I have found them to be spot on value and not noisy.

Here is that selector switch:

This is the same 299 as I received it. You can see that the coupling capacitors to the 7189 outputs had already been replaced. But those darn Pyramid paper capacitors. If you look closely between the 10-watt 15KΩ resistor and the 10-watt 2000Ω resistor, some of the wires are covered with masking tape. They had been burned by a soldering iron (I suppose) and "fixed" with masking tape. The wires were replaced one at a time by tacking the new wire, same color, to the old and fishing it through the chassis following the same path. What a pain!

And the bias/filament supply circuit. You can see that the first 1-watt 18Ω resistor has been replaced by a 5-watt 33Ω. That usually works but you can adjust that value as needed. I don't get into modifying circuits. I am not an engineer and these amplifiers were excellent when new and when brought back into spec, they just work and work and work...

The absolute key to success is to walk away if you get tired and frustrated. Tomorrow is another day and often, problems that seemed insurmountable just become easy when you get up the next day, have your coffee and a go at it... Really!

 

[thornev]

How did you attach the silicon rectifier so that it won't move around and cause a short?

 

[larryderouin]

4x40 1" Screw and nut for KV Flyer's. If it's one of the flat 4legged ones (radial legs) I usually JB Weld them to the side of the chassis, or use a 5 terminal "T"-strip and attach it to the T-Strip.

 

[kvflyer]

Hello Thorn. The bias supply has a hole in the center...

Update, I see that Larry is typing at the same time. Yes, I use a square silicon bridge rectifier with a hole in the center. I attach that to the chassis using a 6-32 machine screw, nut and washer. No new holes are drilled, I use one of the existing Siemens Selenium bridge holes. I will see if I can find a picture...

BTW, at one time, I tried to rebuild the Siemens bridge with individual 1N4007 diodes. Too much work!

 

[kvflyer]