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FA: Sherwood s 8000 Tube Receiver - Awesome Condition
On Aug 30, 1:05*am, John Byrns wrote:
In article , *Patrick Turner wrote: That is an impressive tubed stereo receiver. If it is a 1960 model then does it not predate the addoption of the Zenith-GE multiplexing for stereo info using a 38kHz sub-carrier? I searched for a free online available schematic for the S-8000 which this receiever seems to be. Probably it is after 1961 and with the addopted standard MPX stereo decoding circuits of the time. Anyone bidding for this would be able to check this out because you have a picture of the front cover of the owner manual. I could not find a free schematic although someone was able to sell a service manual for just about all Sherwoods from those ancient times. The top of chassis view clearly shows the presence of a post June 1961 Pilot Tone Stereo decoder. These old sets usually develop a number of faults over the years. It happens also with Scotts and Fishers et all. But they do respond to having all dud tubes and R&C parts replaced along with some upgrade to electrolytic cap values, and some possible revisions to output tube biasing methods. What revisions do you recommend to the output tube biasing methods? * Most of these old receivers originally used a "fixed bias" scheme. I cannot see the full schematic of the receiver in question. But many such things were seriously compromised by very poor bean counter inspired biasing of OP tubes. It was common practice to use the heater filaments of the phono stages for the common cathode resistance for all 4 output tubes so that you could have Ek at every OP tube at +25V at a total Ia = 300mA, if there were 4 x 12AX7. Trouble occurs when OP tubes drift and don't all bias the same with Ia variations of +/- 70% as I have seen in bad cases. Almost none of the well known brands used separate individual RC cathode biasing for each OP tube which was the most user friendly method. None I know of ever had separate trim-pots to set the fixed bias which most owners got wrong causing smoke and burned out parts. As I have so often said, much of the design methods used in 1960 receivers was bloody crook ****e, and they always used a minimum cost design method which meant poor lowest common denominator performance. Patrick Turner. -- Regards, John Byrns Surf my web pages at, *http://fmamradios.com/ |
#2
Posted to rec.audio.tubes
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FA: Sherwood s 8000 Tube Receiver - Awesome Condition
"Patrick Turner" wrote in message ... On Aug 30, 1:05 am, John Byrns wrote: In article , Patrick Turner wrote: That is an impressive tubed stereo receiver. If it is a 1960 model then does it not predate the addoption of the Zenith-GE multiplexing for stereo info using a 38kHz sub-carrier? I searched for a free online available schematic for the S-8000 which this receiever seems to be. Probably it is after 1961 and with the addopted standard MPX stereo decoding circuits of the time. Anyone bidding for this would be able to check this out because you have a picture of the front cover of the owner manual. I could not find a free schematic although someone was able to sell a service manual for just about all Sherwoods from those ancient times. The top of chassis view clearly shows the presence of a post June 1961 Pilot Tone Stereo decoder. These old sets usually develop a number of faults over the years. It happens also with Scotts and Fishers et all. But they do respond to having all dud tubes and R&C parts replaced along with some upgrade to electrolytic cap values, and some possible revisions to output tube biasing methods. What revisions do you recommend to the output tube biasing methods? Most of these old receivers originally used a "fixed bias" scheme. I cannot see the full schematic of the receiver in question. But many such things were seriously compromised by very poor bean counter inspired biasing of OP tubes. It was common practice to use the heater filaments of the phono stages for the common cathode resistance for all 4 output tubes so that you could have Ek at every OP tube at +25V at a total Ia = 300mA, if there were 4 x 12AX7. Trouble occurs when OP tubes drift and don't all bias the same with Ia variations of +/- 70% as I have seen in bad cases. Almost none of the well known brands used separate individual RC cathode biasing for each OP tube which was the most user friendly method. None I know of ever had separate trim-pots to set the fixed bias which most owners got wrong causing smoke and burned out parts. As I have so often said, much of the design methods used in 1960 receivers was bloody crook ****e, and they always used a minimum cost design method which meant poor lowest common denominator performance. Patrick Turner. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ One integrated amp, the Fisher KX 200, had a nice arrangement for setting the fixed bias. A bias and balance pot for each pair of output tubes. It also had a little meter and a rotary switch so that each pair could be biased and balanced fairly easily. The KX 200 was a kit, the kit instruction manual has some tough in cheek humor to it which is a little entertaining. Pretty cool. My LK72B Scott also has the bias and balance pots but no meter. Probably the exceptions rather than the rule I would imagine. MarkS |
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