06-15-2005, 12:39 PM
I own a restored Scott 340B receiver. I replaced all coupling caps in the amp and preamp stage as well as all under chassi electrolytics. The filter caps are good and have had their capacitence increased slightly. All voltages are to spec. The unit is still operating with the selenium rectifier in place. I want to replace it. I know I may have to compensate for the higher voltage from a silicon bridge but I do not know what value resister to use or which leg of the new rectifier put it on. Can anyone advise. This is a beautifull receiver that sounds fantastic, I don't want it to suffer a melt down.
06-15-2005, 01:16 PM
I think you have to figure out empirically how much resistance you'll need to pad down the new DC supplied by the silicon bridge :-( Gary Kaufman suggested using a variable resistor of suitable power capacity (pot or rheostat) to set the voltage, then just measure the resistance. In a bias supply, you don't need much in terms of power rating (is that what the Scott's using the selenium rectifier for?).
As to where to put it: on the output side of the bridge, upstream of whatever other P/S components (caps, dropping resistor, etc.) are now. Guess it could be either + or - depending upon what the supply is for. :-)
06-15-2005, 04:05 PM
The selenium rectifier is for the grid supply for the output tubes. The bias on the 340B is adjustable with the bias pots, no modification should be necessary. Just re-adjust the bias for the output tubes, adjust the balance for symmetrical clipping, and you're done. If the bias pot desn't give enough adjustment range you can change the resistor in series with the bias pot and ground (47K) to a lower value, or raise the value of the resistor between the output of the rectifier and the bias pot (10K).
06-15-2005, 06:53 PM
Don't want to step on your feet here but your way off on this one. The Selenium rectifier in all Scott amps supply filament voltage to heat the 12AX7 preamp tubes. If left as is when you replace it you will slowly smoke the 12AX7 heaters.
The new SS Bridge rectifier is marked the same as the original Selenium so replacement is simple in that respect they both have a +, - and 2 ~ to denote AC. As far as the resistor value required you replace the very first resistor off the bridge it should now be a 10 Ohm 2 watt try something like 33 Ohms the exact value is a trial and error thing so you may want to buy a few values above and below 33 Ohms just tack them in until you find the proper value. To determine when you have it correct test the negative voltage entering the first 12AX7 V1 pin 5 this should be around -46 to -48. Then bias the amp and your all set.
Good Luck Craig
06-15-2005, 10:37 PM
Oops, I didn't notice that supply feeds the filaments as well. I stand corrected.
06-16-2005, 06:54 AM
Thanks for the response. I figured it was fairly straight foward but choosing a resistor value would have been a shot in the dark for me. I shall purchase some resistors and experiment, and have peace of mind while listening.
06-16-2005, 08:30 AM
or you can use a pot/rheostat, and back substitute a fixed value resistor after you (literally) dial in the right resistance. Need a pretty good power-handler (rheostat, probably) if there are many filaments in that circuit.
You still may not need any resistor. I just checked the voltage on the 12AX7's in my LK-72B and they are running at 12.4v with no resistor. That's actually a little on the low side, so they must have really been starved with the original selenium. I wouldn't even consider adding a resistor until you've confirmed that the heater voltage in above 12.6v (or 6.3 v if they are in parallel).
06-16-2005, 05:05 PM
Scott ran the heaters at 10.5 to 11.5 volts each on all these amps and since I have yet to get a bad original 12AX7 out of hundreds in these amps I think I would leave it there why mess with success?