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Old 17th February 2013, 01:20 PM   #1
2bz is offline 2bz  United States
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Default Williamson preamp identification help wanted

I could use some help identifying an old tube premp that I recently purchased. It is marked “Williamson Model G???, L.A., 16 CALIF.” Google comes up with nothing of use. Any information about this unit/company would be greatly appreciated. A link to a schematic gets you a friend for life.

Thanks. John
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Old 17th February 2013, 04:34 PM   #2
DUG is offline DUG  Canada
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Did you try the links at bottom of this page:

Consumer Audio Information

not much help...you are right...not much information on this pre-amp
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Old 17th February 2013, 04:37 PM   #3
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2bz
i was on your "dogbone cap" thread about this one.
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Old 17th February 2013, 06:29 PM   #4
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2bz,

The 2 clearly labeled ECC83/12AX7s account for the low level, mag. source, preamp. What is the 3rd bottle, rectifier or twin triode? Even if the B+ is SS rectified, the tube complement appears inadequate for the features provided. A single triode in each channel supporting both the tone controls and driving the line level O/Ps is (IMO) OVERWORKED.

The hum balance adjustment leads me to believe that the low mag. level section employs (YUCK!!) AC heating. My gut reaction is that very, very, much work will be needed to get that "beast" functioning at currently acceptable performance levels.

A point I'm constantly making about units of this "vintage" is the need to replace the source selection circuitry. Tape head I/Ps are passe and switches in lines carrying mV. level signals is bad. One set of I/Ps hard wired to a RIAA preamp and all other I/Ps "line" level is (IMO/IME) what's needed today. Fortunately, that can be done at modest expense by installing a 2 pole non-shorting switch by Lorlin purchased from Mouser.
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Old 17th February 2013, 06:40 PM   #5
2bz is offline 2bz  United States
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Quote:
Originally Posted by Eli Duttman View Post
2bz,

The 2 clearly labeled ECC83/12AX7s account for the low level, mag. source, preamp. What is the 3rd bottle, rectifier or twin triode? Even if the B+ is SS rectified, the tube complement appears inadequate for the features provided. A single triode in each channel supporting both the tone controls and driving the line level O/Ps is (IMO) OVERWORKED.

The hum balance adjustment leads me to believe that the low mag. level section employs (YUCK!!) AC heating. My gut reaction is that very, very, much work will be needed to get that "beast" functioning at currently acceptable performance levels.

A point I'm constantly making about units of this "vintage" is the need to replace the source selection circuitry. Tape head I/Ps are passe and switches in lines carrying mV. level signals is bad. One set of I/Ps hard wired to a RIAA preamp and all other I/Ps "line" level is (IMO/IME) what's needed today. Fortunately, that can be done at modest expense by installing a 2 pole non-shorting switch by Lorlin purchased from Mouser.
All three tubes are 12ax7s. Thanks for the info.
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Old 17th February 2013, 07:45 PM   #6
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Quote:
Originally Posted by Eli Duttman View Post
2bz,



The hum balance adjustment leads me to believe that the low mag. level section employs (YUCK!!) AC heating. My gut reaction is that very, very, much work will be needed to get that "beast" functioning at currently acceptable performance levels.

A point I'm constantly making about units of this "vintage" is the need to replace the source selection circuitry. Tape head I/Ps are passe and switches in lines carrying mV. level signals is bad. One set of I/Ps hard wired to a RIAA preamp and all other I/Ps "line" level is (IMO/IME) what's needed today. Fortunately, that can be done at modest expense by installing a 2 pole non-shorting switch by Lorlin purchased from Mouser.
ECC83 valves were designed to be used with A/C heaters.
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Old 17th February 2013, 08:03 PM   #7
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It is EXTREMELY difficult to get acceptable hum levels in low level, mag. source, preamps, while employing AC heating. Those older AC heated designs with reasonably low hum levels employ either 6EU7s or 7025s, which contain spiral wound, hum bucking, heaters. The triodes inside the bottle are "identical" to those in 12AX7/ECC83s, but the hum bucking heater changes things considerably. In their seminal passive EQ design, RCA specified the 7025. That specification was anything but accidental.

BTW, the quite decent, current production, Sovtek 12AX7LPS is a genuine 7025 equivalent, with the hum bucking, spiral wound, heater.
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Old 18th February 2013, 07:11 AM   #8
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2bz I think its great that you're initially trying to get to know and understand the preamp first. Some people just instantly want to rip apart a circuit and put in a modern approach.

Horses for courses - the amp is circa 1960 - when pickup improvement was frenetic and stereo was just blossoming. The 50's were dominated by higher output cartridges and hum was not such an in-your-face issue.

I would suggest initially trying to get the amp working as is, and going through the learning process about restoration and what subtle changes can be initially made to improve performance, and how to test and compare such equipment. External workarounds such as a higher output pickup, or step-up transformers may bypass any perceived phono issue, and add some more insight in to technical performance.
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Old 18th February 2013, 11:59 AM   #9
2bz is offline 2bz  United States
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Quote:
Originally Posted by trobbins View Post
2bz I think its great that you're initially trying to get to know and understand the preamp first. Some people just instantly want to rip apart a circuit and put in a modern approach.

Horses for courses - the amp is circa 1960 - when pickup improvement was frenetic and stereo was just blossoming. The 50's were dominated by higher output cartridges and hum was not such an in-your-face issue.

I would suggest initially trying to get the amp working as is, and going through the learning process about restoration and what subtle changes can be initially made to improve performance, and how to test and compare such equipment. External workarounds such as a higher output pickup, or step-up transformers may bypass any perceived phono issue, and add some more insight in to technical performance.
Thanks for the response. Greatly appreciated. I am simply looking for a little company info and a schematic to make the restoration go a little easier.

After reading Eli's reply though, I went back to my original post to verify that I hadn't accidentilly asked for "an opinion on circuit topology from a sactimonious blowhard". I didn't.
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Old 18th February 2013, 05:09 PM   #10
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I think the information about the humbucking 7025 tube and its Russian equivalent is quite useful. When you start a thread you are not the only reader. Dynaco took over the preamp market with their DC heated 12AX7 circuit; it is interesting to know what the competition was doing. I drooled over Allied catalogs about 1962-68; I was lucky when I finally had some money to spend I stumbled on a tired 1961 Dynakit PAS2.
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