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Scott 299C variation? Electrolytic question

D

Diverted

Active Member
Hi all,

Getting ready to start a Scott 299C I just picked up. Sweet amp, can't wait to hear it. One question: This amp contains a four-section electrolytic, positive to ground, ID'd on the schematic as 100/100/100/100@75V.
However on the actual amp, the can is a 75/75/75/75uf@75v. Do any of you know of a variation of this amp that uses 75ufs to positive ground as opposed to the 100?
It's the original can, for sure. Just wondering if I should replace/stuff it with 75uf caps, or go with the 100uf noted on the schematic. Thanks!
 
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Ideally if there is a difference, you go with what is on the unit itself. But you CAN go up to 100uf caps. And make them 100V due to increases in the wall voltage from when it was originally built (110V) to todays 120 to 125V. It won't hurt anything either.
 
I just rebuilt my 299C and it uses the 75uf positive com can. Mine is the first version with the big output transformers. I know the later ones followed some of the 233 schematics. When in doubt use what's installed. I got the cap from New Old Sounds for around $25.
 
Diverted said:
Hi all,

Getting ready to start a Scott 299C I just picked up. Sweet amp, can't wait to hear it. One question: This amp contains a four-section electrolytic, positive to ground, ID'd on the schematic as 100/100/100/100@75V.
However on the actual amp, the can is a 75/75/75/75uf@75v. Do any of you know of a variation of this amp that uses 75ufs to positive ground as opposed to the 100?
It's the original can, for sure. Just wondering if I should replace/stuff it with 75uf caps, or go with the 100uf noted on the schematic. Thanks!
Click to expand...
If i am correct that cap is used for rectifying the fixed bias. There is a FWBR, full wave bridge rectifier, that rectifies the acv. There is more leeway in cap values with a SSR and you could probably double the value with no issues. But caps of 100uf x 100vdc are quite small and can easily fit under the chassis. You can disconnect the can cap but leave it in place. It would be much cheaper and easier to implement. You can also upgrade to something like Elna Cerafine or Nichicon Muse.
 
This 299C schematic in the AK database shows 4x75 uF @ 75 VDC, as you have installed in your unit:

http://akdatabase.com/AKview/albums/userpics/10007/HH Scott 299C Schematic D-299C-C1 Sub 4.pdf

Whenever I replace this can, I use 100 uF 100 VDC axials or radials, as Larry suggests, under the chassis and leave the original can (disconnected) in place for the look. As I recall, there was some variation in the 299C and 299D models throughout the production runs. This schematic may be closer to what you actually have, but you'll need to confirm against your unit itself.
 
There are at least 7 variations of the 299C. Subtle differences but basically they are the same. As mentioned, the 233 was the Chevrolet Biscayne version with less bells and whistles. (Actually, there is a version of the 233 which had 7868 output tubes instead of 7591 outputs... but I digress...)

Another approach is to open up that can and place Nichicon "Pencil" capacitors inside. Yes, it is the bias supply and the filament supply for the 12AX7 preamp tubes. And, it is a positive ground. Somewhat confusing I know.

If these capacitors will fit inside the can, you can use them.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22caaJbuRiMQkwhFgEk2nCr4=
 
Diverted said:
Hi all,

Getting ready to start a Scott 299C I just picked up. Sweet amp, can't wait to hear it. One question: This amp contains a four-section electrolytic, positive to ground, ID'd on the schematic as 100/100/100/100@75V.
However on the actual amp, the can is a 75/75/75/75uf@75v. Do any of you know of a variation of this amp that uses 75ufs to positive ground as opposed to the 100?
It's the original can, for sure. Just wondering if I should replace/stuff it with 75uf caps, or go with the 100uf noted on the schematic. Thanks!
Click to expand...
I dunno how much experience you have with rebuilding a tube amp but the question you asked indicates that you have not dealt with this issue before. There are different levels of repairing to achieve the same result. It is just a matter of personal preference. Each way has its own reward. The most basic is to get the amp up and running and therefore, what goes on under the chassis is less of an issue than sound quality. To some, the underneath appearance of a stock looking unit is just as important as the sound quality and the time and effort required to remove the can cap, very carefully so you don't break off the mounting tabs, cut it open, clean out the chemical components clean it then install the new caps, etc. is worth the effort. Or you can buy a replacement cap for 25 bucks which seems to be a good thing if they are still available. Or you can get 4 Elna Silmic from Handmade Electronics (no affiliation but i have been getting high quality parts from this outfit for years) for 10.00 and a suitable 6 place solder lug to accommodate the caps. It is all a matter of what you feel comfortable doing and not get overwhelmed by the project so that you lose interest or run into a situation you can not cope with.
 
Nimbley said:
I just rebuilt my 299C and it uses the 75uf positive com can. Mine is the first version with the big output transformers. I know the later ones followed some of the 233 schematics. When in doubt use what's installed. I got the cap from New Old Sounds for around $25.
Click to expand...
That's a good site! Which multi cap did you order please. There are so many listed in their pages.
 
Nimbley said:
I just rebuilt my 299C and it uses the 75uf positive com can. Mine is the first version with the big output transformers. I know the later ones followed some of the 233 schematics. When in doubt use what's installed. I got the cap from New Old Sounds for around $25.
Click to expand...
That's good to know; I didn't know there was anyone out there making these multi-value electrolytics with positive ground. But it's a great idea! I was probably going to run all of the lower voltage caps under the chassis and replace a few cans. Thank you for your help everyone.
 
Thanks for the replies. I will probably go with 100s. I'll stuff them under the chassis along with the other low voltage lytics in the stereo. Thanks for all the responses. I will probably be back with more questions once I go to set the bias. Finding correct bias instructions for mine has been tough. I have the earlier version 299C with large output transformers, running the 6Ug phase splitters.
Thanks!
 
Diverted said:
Thanks for the replies. I will probably go with 100s. I'll stuff them under the chassis along with the other low voltage lytics in the stereo. Thanks for all the responses. I will probably be back with more questions once I go to set the bias. Finding correct bias instructions for mine has been tough. I have the earlier version 299C with large output transformers, running the 6Ug phase splitters.
Thanks!
Click to expand...
What almost everyone does for determining the bias voltage is to add a matched set of 10 ohm or 1 ohm resistors to the cathode of each of the 7591 to ground. Now, when the amp is warmed up you can measure the cathode voltage on the tube and adjust the pot to achieve the desired idle current that you want to use for each power tube. To make this more user friendly add a test jack for each 7591 in a convenient spot and run a wire from the cathode to the jack. Now you can measure the cathode voltage from the top without removing the bottom plate.
 
triode17 said:
That's a good site! Which multi cap did you order please. There are so many listed in their pages.
Click to expand...

I had to go back and look for the invoice. I was wrong. I got it from Antique Electronic Supply. Its a CE Mfg and fits in place of the factory Mallory.
 
I was able to rebuild the entire unit without restuffing cans or adding extra caps under the chassis. I used two of the JJ's dual 32uF epoxied to the wafer in place of the power supply cap (pos jumped together to get close to the 60uF on the schematic) and one replacing the dual 30uF cap. I got the other from Hayseed. I wasn't to concerned with keeping the factory look because its in a walnut box, but they all had to be short enough to slide in the box. It sounds incredible and and can get uncomfortably loud driving a pair of Heresy's.
 
Nimbley said:
I just rebuilt my 299C and it uses the 75uf positive com can. Mine is the first version with the big output transformers. I know the later ones followed some of the 233 schematics. When in doubt use what's installed. I got the cap from New Old Sounds for around $25.
Click to expand...
Was the cap. custom made? I don't see it on the New Old Sounds site. What brand is it?
 
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