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Scott LK-48B

Thanks Paul,
I'll try the lytics I have first,,, may not make a difference... Might as well change the Rks while I'm in there, as I only have radial caps and wires are too short, so I can piggy back em on the Rs!
 
The main reason for relocating the cathode resistor and cathode bypass capacitor is because it was easier. Those resistors usually were still within tolerance but would get ruined removing them etc. Also, by doing so, the other option was to use a JJ Electrolytic capacitor and clamp, 22-22µFd @ 500 VDC capacitor. They are not the same cosmetically but sure made life easier.
 
yep,,, I agree, there are many ways to skin this cat,,, I want to use parts I have already, as I build/rebuild amps for something to do, gave a few away,,, but have no desire to sell em!!!
So working on them is teh fun for me,,,, Time doesn't matter, as I'm retired, so stuffing caps instead of buying em, is a no brainer!!!! Plus I still need a couple of expensive parts for this guy,,, output tubes and one OPT, which seem to be pretty costly for this amp,,,
 
In answer to one of the above comments, yes, HH Scott equipment does use an aluminum chassis. I too have had a noise that was caused by a riveted terminal strip, the ground lug that had gone high resistance. But it's an easy fix. Drill out the rivet and replace it with a screw, star washer and nut. Very easy fix.

Scott amplifiers are very straight forward and not too complicated. Also as mentioned above, isolate the issue as best you can. Issues that occur to both channels sure do point to the common denominator... the Power Supply. But never overlook the obvious. You will find the problem.
 
kvflyer said:
HH Scott equipment does use an aluminum chassis. I too have had a noise that was caused by a riveted terminal strip, the ground lug that had gone high resistance. But it's an easy fix. Drill out the rivet and replace it with a screw, star washer and nut.
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This is timely as I found something similar while I was just replacing the 6U8 cathode caps/resistors... Seems some one before me tried to solder teh shield of a shielded cable to one of the aluminum rivets holding the tag strip I was working on!!! There was a blob of solder on teh shield lead and a burn mark where teh "soldering" happened,,, I drilled out teh rivet, and bolted the tab down securely after cleaning the area with a little wire brush on a Foredom,,, Resoldered the shield to a polished ground/mounting tab on the lug strip,, Got rid of a pesky occasional hum!! Basically what kyflyer explained above...
I would have never saw it til I removed teh old cathode caps/resistors,,
They came out good, and teh "amp" is gettin better as we move fwd,,,
Pic of teh K caps/resistors,,, tag strip in question is on teh L side,,,
Got Floyd and The Dead screaming thru teh Scott pre amp and 829B amp for teh output stage,,,,
Keeps me pluggin away when I make changes that work, and can listen to it while its getting rebuilt!!!
 

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Found I had enough .022s to replace 12AX7 coupling caps except for one, that I need... Got I the pre amp section still connected to the 829B for power,,, Its good teh pilot light works as I couldn't tell it was running....
Think I'll listen for a while!!!
 

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I found the one cap I need to finish the preamp stage on a L/R table last night,, how and when it got there IDK,, but I soldered it in first thing this morning!!!
Output stage next,,, gotta find a a set of 6BQ5s to use as "proof of concept" to test OPTs,,,
















 
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gadget73 said:
'twas. John actually found them, so I definitely owe him a thanks for spotting them.
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Your welcome, 11 years later!!!!
I just found this post while searching for a schem for a TA-600 so I can compare your OPTs to see if they can work in the Scott, missing one OPT, I got there last Spring,,, Scott OPTs are hens teeth it seems!!!
 
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Hi Larry
I dont have any (tube) Fishers,,, just the OPTs from Gadgets,,, I just got a Scott LK48b,, w/ one TX and was hoping to use teh Fishers in it until I find the right one... But my amp ha 6U8s instead of teh 7247 or 7199s teh Fisher schems show,,, and the Fishers only run the 7189s at 370Vp,,, vs 410 for teh Scott...
It may not work out unless I run em at a lower voltage,,, not sure what to do about the preamp/PI stage tho,,
I got the preamp/tone sections working well, using a power amp for output stage for now...
Thanks for the links!!!
 
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I believe the parts carcass is a 20xxxx range but I don't know that the transformers changed any. They should be a fine match for an EL84 Scott amp, though they may be too tall to fit under the lid. The pair I have from an LK72 are that same low but thick sort of proportions that the LK48 ones I've seen use.

I wouldn't be too concerned about the voltage, though you will probably need to re-tune the feedback.
 
gadget73 said:
Finally I get to make a legit post in this forum. I picked up a pair of Fisher TA-600 receivers over the weekend. One is a rusty mess of a parts rig that is now missing the output transformers, but the other one is actually in pretty doggone good shape. Its actually better after some cleaning. Needs the standard re-cap and all that, but it is workable at the moment.

Any special tips or tricks for the 600? I was thinking of maybe bumping the filter cap values, but probably not very much. I was thinking 47uf per section. I may also change that free-hanging pair of ceramic caps to a chassis mount unit. I dislike components flopping around loose like that.

Oh, something odd, mine doesn't have a tube lineup that matches the schematic. This one is serial number 59028. I'm using the 50000-59999 schematic. That schematic calls for both FM tuner tubes to be a 6AQ8/ECC85. Mine uses one 6AQ8 and one 6DJ8/ECC88. Thats from the silkscreen on the chassis, not just what was installed. Any ideas about docs on that change ?


and since people love pics, a couple before and after cleaning pics. The improvement on the faceplate really surprised me.
Click to expand...
That cleaned up really nice.
 
gadget73 said:
I believe the parts carcass is a 20xxxx range but I don't know that the transformers changed any. They should be a fine match for an EL84 Scott amp, though they may be too tall to fit under the lid. The pair I have from an LK72 are that same low but thick sort of proportions that the LK48 ones I've seen use.

I wouldn't be too concerned about the voltage, though you will probably need to re-tune the feedback.
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I was going to try to move this to my build thread,,, as this thread is 11 years old,,, But I got some replies on it here so I dont know how that would work,,,,
Anyway,,, as far as I can see from the info Larry posted,, these Fisher OPTs are the same on all the schems,, (T 720 )... Also, mounting height doesnt matter as I could put "lay down bells" on them and just mount em flat, because they would fit teh same foot print/mount holes the Scotts do, so I can work on the output stage... My intention is to try to find a proper Scott OPT to make it right,,, Would even consider swapping the well tested Fisher pair for a Scott in like condition!!
Thanks for replying!!!!
 
Hi Larry
You mean merge this whole Fisher 600 thread with my Scott build thread? I was thinking of putting a link in my Scott build thread to the pertinent posts in this F600 thread,, but not sure that is OK either??
Either way is fine with me,, I just dont know how to ask the question!!!!
 
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Finally got two matched pair of 6BQ5s from a buddy this weekend,,, I'll try them on variac, so I don't burn em up!!! They max at 300Vp compared to 410Vp (per schem) for the 7189s IIRC... Gotta watch Screen V also,,,
 
its plate to cathode voltage so its not quite as bad as it seems but they did push these things hard.

its been claimed that there isn't actually any difference between a 6bq5 and a 7189 other than a price tag but who knows.
 
s-petersen said:
Hopefully it works well!
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Thanks Scott,,,
I hope so too, I will treat em gently,,, the biggest test is if I got the spkr wiring to teh terminals wired correctly,, as one terminal was missing when I got it and the wires were all hacked off,,,, I had teh wrong build manual when I first got this amp, and the spkr wiring was different from the proper manual... I re did it and double checked, hope it is right!!!
 
gadget73 said:
its plate to cathode voltage so its not quite as bad as it seems but they did push these things hard.

its been claimed that there isn't actually any difference between a 6bq5 and a 7189 other than a price tag but who knows.
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Yep,,, the "push em hard" sounds familiar as Fender ran 6V6s to way past design readings,,,, I'm not as used to 6BQ5s tho,,, so variac, ammeter and DBT will be my friends here!!! May have to use a SS rectifier, as the 5R4 may draw too much htr current for the Scott PT with the output tubes included tho...
You were very correct saying kit builds may have never been wired correctly,,,, and the first one I had (WA-P2) wasn't!
Stay tuned we'll see good, bad, or ugly!!!
 
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