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Williamson preamp identification help wanted

2bz

Member
2010-02-11 8:45 pm
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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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.
 

2bz

Member
2010-02-11 8:45 pm
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.
 
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.
 
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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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.
 

2bz

Member
2010-02-11 8:45 pm
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.
 
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.
 

xraypro

Member
2013-05-04 7:40 am
Mine is very similar, but it's a G200. It doesn't have the stereo R+L, R-L mono - reverse switch. I added a grounding post and replaced the selenium rectifier with a silicon diode. I'm assuming the input selector switch does some equalization also. Mine works ok, but up until now I didn't know what curves the selector switch had. Maybe mine is close to yours. Thanks for your pictures. I'll post mine in a few days. Still looking for info too.
 
Given that there was no "Williamson" company (Theo only agreed to be associated with the British Radio Electronics/Acoustical Manufacturing Co. Hallmark model), and the name was generically used by all and sundry to identify the nominal circuitry used in the pre/amplifier, then any search is centred on Californian based companies making any such equipment at that time.

Are you able to limit the year range of the preamp? Eg. by any dating of components. Certainly the 'stereo' configuration and apparent SS diodes put it to the mid 1950's at the earliest.

Does the circuitry align with the original WW articles, or any other well known manufacturer of that type of equipment (eg. the Heathkit preamp).

Some of the Audio/electronics mags of that time in USA may also have run adverts for your preamp.
 

2bz

Member
2010-02-11 8:45 pm
Given that there was no "Williamson" company (Theo only agreed to be associated with the British Radio Electronics/Acoustical Manufacturing Co. Hallmark model), and the name was generically used by all and sundry to identify the nominal circuitry used in the pre/amplifier, then any search is centred on Californian based companies making any such equipment at that time.

Are you able to limit the year range of the preamp? Eg. by any dating of components. Certainly the 'stereo' configuration and apparent SS diodes put it to the mid 1950's at the earliest.

Does the circuitry align with the original WW articles, or any other well known manufacturer of that type of equipment (eg. the Heathkit preamp).

Some of the Audio/electronics mags of that time in USA may also have run adverts for your preamp.



The 12ax7s are dated mid-1958. The circuitry is somewhat similar to EICO. Thanks for the insight.
 

2bz

Member
2010-02-11 8:45 pm
Mine is very similar, but it's a G200. It doesn't have the stereo R+L, R-L mono - reverse switch. I added a grounding post and replaced the selenium rectifier with a silicon diode. I'm assuming the input selector switch does some equalization also. Mine works ok, but up until now I didn't know what curves the selector switch had. Maybe mine is close to yours. Thanks for your pictures. I'll post mine in a few days. Still looking for info too.

I too swapped out the selenium rectifier and added a ground. I would love to see some pictures, especially under the hood. Do you have a schematice, by chance?
 
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...(snip)

Ditto...Eli pointed out, quite correctly, that this circuit has a number of obvious deficiencies...nowhere does he suggest ripping it apart as trobbins implies, in fact, Eli cites a number of options that one is free to implement or ignore. And with this in mind, he specifically mentions the attributes of the 7025 which would be superior to the existing 12ax7s found in your unit. This would be a matter of simple substitution.

I am a hardcore minimalist myself and tend to first try my equipment in stock nick first and then apply a few modern/advanced remedies...in every case the direction provided by those more technically advanced is worth its price in saved time and wringing out the best performance out of dated designs for minimum kopecks.

It's one of the benefits of being a part of DIYaudio IMO...casting aspersions on the intentions of members with a long long track record of going above and beyond in assisting others here isn't helpful or respectful of their contributions:(