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Hi all,
I finally got the Sansui 1000a delivered today. It is in pristine condition! I am lucky in that I have a model with the 4 individual bias pots - one for each 7591. So, after I recap it I was wondering if anyone has the bias setting instructions. What do you measure at which pin to ground? Are we shooting for -22 volts? Pin 6? Or is there a test point where I measure current? I want to set the bias relatively cool as power output isn't really a consideration. Any detailed instructions would be appreciated. I know you need a load, with volume turned down and amp idling.
thanks in advance...
Don
Follow Ups:
Don
minus 22v or minus 21v is about the norm for those..if you want,stick a 10 ohm resistor on each cathode to ground and measure .35v and that will equate to about 35ma per tube..That would be close.
The original caps MUST go - a friend has a 1000A with ons side of a 7591 melted in to prove it!
The schematic I have doesn't show any provision for checking current - but then it only has a single bias pot too. Best way is to install a 10 Ohm current sample resistor in series from each cathode to ground (replace the wire from each pin 5 with a resistor. Maximum idle current would be about 40 mA (400 mV drop across 10 Ohms). 20-25 mA would be on the cold side.
I suspect that some value of current sensing resistor is already there - must be SOME way to adjust 'em...
Sorry Tom I see I said the same thing you said first without looking..Im getting old what I can I say? Anyway I have a single adjustment on mine but Im going to put separate bias pots in mine one day when I develop more ambition.
Thanks guys! I just got the Sansui 1000a delivered today. It is almost pristeen! Not even any dust inside! Pretty easy to work on I think. I see pin 5 is clearly marked on each socket and goes straight to a nice ground lug. I can easily solder a 10 ohm resister in that ground line. Now I just have to hunt up some good 1% tolerance resistors. It doesn't seem that a lower tolerance will do or you bias could potentially vary by 20% per tube. I was lucky and found a guy in Quebec who has Russian PIO's of every value I need. The 4 big coupling caps are 0.3 at 400v. He has PIO's that are 0.33 at 400v that are about the same size. Bingo. As per Mike's advice in the previous thread I will contact Jim McShane for all the big power supply caps and diodes. Might as well put new everything in. Man oh man, the iron on this thing (and all the sockets and everything) looks like it just left the factory. A few tubes are weak, but they are easy to get and I scored a quad of NOS GE 7591's with the amp. And the amp has separate bias pots. Yahoo.... thanks guys. Will send you a note in a week or two when I have it running.
DLS
Your going to have one kick ass receiver..Make sure you get k40s as much as you can and if he doesnt have those I have them in the sprague 118p which are a combo pio. I use them in my 1000a and they sound superb..Yes Jim will have those filter caps and the original caps in the doubler were 300v and I used the 800@350v Jjs but I think the 250v panasonics will work especially with the inrush limiter that you need to get from him also..Ill make you a parts list if you want.
Hi Mike,
Thanks for the info. Soldering a 10 ohm resistor pin 5 to ground will be easy. Am I correct in assuming that a 1% or 5% 1/4 watt would be plenty? Assuming a high current draw of 50 milliamps: 0.05 amps x 0.05 amps x 10 ohms=0.025 watts ? So a 1/4 watt resistor would be plenty.. Or is my ancient recollection of the power formula incorrect?
Yes, the pio's I ordered are k-40s. The large 0.33 ones for the 7591's are k-75's. Should be great.
If you have a parts list that would be great! I'm going to replace the little electrolytics in there, do I need to replace the MP-R's? Are those metalized polyprops? Shouldn't they still be good? Jim M. is on the road, when he returns I will get all the power supply stuff. Might as well do the diodes too for insurance... Too bad this receiver has those two resistors in it..otherwise it would be all tube. This thing is so clean you could eat off of it! The iron and all interior parts look like it just came out of the factory!
thanks,
Don
No
Use at least one watt and I would even go with 2 watt.You have to account for the fact that a tube can short and you dont want to be changing cathode resistors because you have a fuse that will open if you draw too much current.In theory you are correct that it would be enough but we always go higher wattage there.
OK, fair enough. Will have to hunt down some bigger ones. None available in town, but I know where to get some... thanks.
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