My sentiments exactly. I'd even remove the switch and RCA jacks
HH Scott Owners Thread
- Thread starter RS Steve
- Start date
Well thanks for the refresher, I was a bit lost having not messed with any Scotts for some time. Tubes biased in and balanced nicely, amp is sounding stellar. Many thanks to all that helped, it was coming back to me as I went along, but always nice to bounce things off others to make sure.
Nice Job Steve!
Here’s my Stereomaster 312 with the Nuvistor front end. The previous owner had Joseph Chow work on it and also had a new cabinet made by a McIntosh cabinet maker. The sound is very nice.
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Scott-Model-312-Review.pdf
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Scott-Model-312-Review.pdf
NoTransistors
Dual Turntables Super-Restorer
What would have really complimented a nuvistor RF section?
A compactron IF section, of course.
A compactron IF section, of course.
Guys I thought I would share my newly restored LK48. On front it indicates a kit but actually this one was factory wired. And the 48B is the correct schematic. It came with the original wood case which I still have to clean up. I replaced all can caps from Hayseed Hamfest which I see other have also used. It must not have ran very much as the one balance control only measured 2k, so I ended up replacing both pots. Also the one coupling cap in the phono section was completely open.
For the bias, I removed the old wiring from the L/R switch and on the outer sections on the switch ran wires to indivigual 10 ohm cathode resisitors. The centre of the switch on either side goes to the left/right rca jack. This way flip it to left, now the left rca jack is connected to one tube on the left channel and the right rca jack is the other tube in the left channel. I made up some rca to pin jacks as seen in photos. Just balance out each channel for 0 volts and set bias for .22v or 22ma.
This unit came with 4 12ax7 tele's, 2 6U8 Bugle Boys, RCA 5AR4 and only 3 Telefunken 7189. For now I am running some RFT 7189 russian. I have just located another 7189 tele . The unit sounds amazing...only been running a few hours.
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NH-MAN
Addicted Member
Anyone have a LK-72 that when replacing the selenium rectifier with a silicone one ended up having too much negative voltage going to the output tubes not letting them bias up to .34V? This unit had a shorted selenium rectifier, and I replaced it. Brand new JJ 7591's will only get to .29V with bias turned all the way up. I have separate 10 ohm resistors going to each tube for sampling so stock bias for two tubes at a time is no longer relevant. It looks like a 22K resistor is used off the rectifier, most likely will need to increase this. Anyone with experience with this please chime in.
Just measure the voltage drop across the 22k resistor calculate the current going thru it, and then figure how much more resistance you need to lower the negative voltage by a couple of volts.
I think part of the problem that you're seeing is that the JJ 7591's need a different bias voltage than the American tubes the amp was designed around.
The schematic for the amp shows -43V going in to the bias pots, and also into the filament supply voltage for the series wiring of the 12AX7 tubes.
It would be a good idea to see where that is running when you are making the change to bias supply, and try to keep that within a volt or two of the -43V by modifying the 33ohm 5watt resistor (R209).
I think part of the problem that you're seeing is that the JJ 7591's need a different bias voltage than the American tubes the amp was designed around.
The schematic for the amp shows -43V going in to the bias pots, and also into the filament supply voltage for the series wiring of the 12AX7 tubes.
It would be a good idea to see where that is running when you are making the change to bias supply, and try to keep that within a volt or two of the -43V by modifying the 33ohm 5watt resistor (R209).
I dare say that in about every case of replacing an old selenium rectifier with a newer silicon device you'll find that some level of compensation is necessary. I think its generally considered to be true that a modern silicon rectifier will have less voltage drop than did it's selenium predecessors. As such, some change in either an increased supply side resistance or a reduction in any bleeder side is a common factor. Or so 'they' say.....
So the voltages are not that far off, coming off the rectifier I'm getting -59.6V instead of -56V as per schematic, -54.2V off the 33 ohm resistor instead of -51.5V, and -41.3 after the 22K resistor instead of -43V. Very possible the JJ tubes are the issue since I don't think those voltages are off enough to not allow the tubes to bias up high enough. I will try some different tubes, these tested good though. Thanks for the refresher course!
I haven't actually run JJ7591's in any of my projects, but I do recall reading a forum thread somewhere cautioning against running the JJ version of the tubes at the same idle current that the early USA tubes were run at.
Apparently there have been durability issues that show up with long term use of the tubes.
The Scott amps called for 350ma, and if you can get your JJ's to run with bias adjusted at 300ma, and want to use them, I'd try them at the lower setting.
Apparently there have been durability issues that show up with long term use of the tubes.
The Scott amps called for 350ma, and if you can get your JJ's to run with bias adjusted at 300ma, and want to use them, I'd try them at the lower setting.
This amp had all new JJ tubes in it except for the 6BL8's when it arrived, I like the 12ax7's, but like you have heard bad things about the 7591's. The amp is sounding pretty good, maybe a bit flabby in the bass department. I will let the amp have some break in time and see if that settles in a little better.
I think that's 35ma per tube, not 350ma. I've run plenty of JJ 7591 tubes, and honestly I think the issue with them is more just the luck of the draw as opposed to having to alter the operating points. I have had failures, but i've managed to find a suitable replacement to match up with the remaining set, and that usually solved the problem. And, I think they're getting better, as I've not had one of these failures in a good long time. Not saying its not a good plan to run the bias a bit more on the cooler side, and I do think that will help the overall life of the entire output quad. But i also think that if you've got a 'questionable' tube in there, then its ultimately going to shoot craps regardless of where your bias is set.
lol...... yea, tubes might be a little 'hot' at 350ma.......
I let the amp play for a good 4 hours yesterday with the Tung Sol's, and I did very much notice some break in after the total recap. The sound very much improved after a few hours with bass showing the most improvement. I give the nod to this Scott over my equivalent Fisher models for more open and bolder highs.