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Old 12th Apr 2021, 10:20 am   #1
Bill
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Default Williamson stereo pair, a good idea or not?

I have 3 Williamson o/p transformers, one Partridge WWFB/.95 and two Gardners OT736. I have in mind a project to build a pair of amplifiers to original design but using 807s instead of KT66. I have a 400-0-400, 300mA mains tx , would this be enough to supply both amps, using 2x CV378/5U4 with separate chokes and filtering? Apart from considerations of efficiency and weight(!) would these sound any better than the Leak Stereo 30+ that I currently listen to? Possibly a mad project, but I do have most of the serious parts for this.Also, what is the power rating for these O/P txs? The specs say 20w but they look more than capable of 100w?The primaries are 10k a to a. Thanks.
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Old 12th Apr 2021, 10:45 am   #2
joebog1
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Default Re: Williamson stereo pair, a good idea or not?

Well, you are correct in surmising that they are 100 watts plus, BUT its Hi-Fi!! after all its basically Williamson that coined the phrase " Hi-Fi " SOO the traffos are at best 20 watts. Williamson amps are "rated" at 15 watts RMS, which translates today as 250billion PMPO watts.

Williamson amps are HIGHLY desirable !!! VERY VERY desirable.

Actual output power will be very close to 15 watts RMS, but with the amps very hot, and with a downhill slope you may catch em out at about 18 watts before clipping, at 400 Hz, the test frequency at that time. NOT 1kHz as is now used. Construction of "true" Williamson output transformers is VERY complex, NOT efficient!, but very complex indeed.
There were in the origional design TWO bobbins, wound identically. One bobbin was the spun around and placed on the core "backwards" to the other. There were TEN primary windings on each core half.
If you want "phat" distortion for your guitar, FORGET it, it just doesnt happen. BUT, listen to real music, and you will see what I mean. Especially female vocals, ( probably the hardest sound to reproduce).

Just my three bobs worth,

Joe

with great respect!
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Old 12th Apr 2021, 2:33 pm   #3
PJL
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Default Re: Williamson stereo pair, a good idea or not?

The Gardners transformers were wound for the Williamson design and have 8 separate secondary windings. !5W will be more than loud enough... and it will sound much better than your Leak Stereo 30 and any other early transistor amp that uses capacitive speaker coupling.

I would be inclined to use new EL34B's (made in china) as they are pretty cheap, reasonably reliable, and will use the same connections as the KT66.

The power transformer should be OK for HT current but you do not mention the LT windings?
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Old 12th Apr 2021, 2:48 pm   #4
TonyDuell
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Default Re: Williamson stereo pair, a good idea or not?

Quote:
Originally Posted by joebog1 View Post
Williamson amps are HIGHLY desirable !!! VERY VERY desirable.
Just out of curiousity, how 'desirable' would an original Radford Williamson amplifier, serial number 101, be?
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Old 12th Apr 2021, 3:30 pm   #5
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Default Re: Williamson stereo pair, a good idea or not?

Radford Williamson? Desirable enough that if you told some hifi fanatics you'd have to dump a bucket of iced water over them. They'll lose their interest once they get to thinking of the probability of finding another, and the cost if they did.

Transistor amps with output capacitors have a low frequency roll-off that can be avoided in DC output coupled designs. But then valve amps have a low frequency rolloff inherent in their output transformer. I'm not sure why one would be preferable to the other, though transformers have more imperfections than capacitors.

I wouldn't put the blame directly on the output capacitor per se, but transistor amp designers have learned a few things since the era of the Leak Stereo 30.... a few things which do actually matter.

If you subscribe to the "It's got to be valves" mindset then you reduce your choice. and the Williamson is a good valve amplifier. But if you have an open mind, there are also good transistor amplifiers.

Triodes, tetrodes, pentodes, beam tetrodes, bipolar transistors and field effect transistors ALL have limitations and foibles. A good designer can design a good audio amplifier around any of them. The art lies in doing designs which overcome or cover their problems. No one technology is necessarily superior to the others. But I'll take a good design over a bad one.

It's a doddle to look at an amplifier and say what technology of devices it uses. It's a lot harder to look and tell a good design from a bad one. So people put all the blame and plaudits on the type of device, not on the design it's used in.

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Old 12th Apr 2021, 7:57 pm   #6
Bill
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Default Re: Williamson stereo pair, a good idea or not?

Quote:
Originally Posted by PJL View Post
The power transformer should be OK for HT current but you do not mention the LT windings?
It has those as well! 5v 6a, 6.3 4.5a,6.3 10a. It's a big transformer but in any case I have two of them. I would be interested to know the make of them. Square oil filled can with rounded top corners, painted black with solder seal at the terminal end, glass insulated solder terminals, voltages etc printed in black, very heavy. I have no idea what equipment they came from , had them 50 years or so.
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Old 12th Apr 2021, 8:47 pm   #7
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Default Re: Williamson stereo pair, a good idea or not?

Hi Bill, probably Parmeko

Ed
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Old 12th Apr 2021, 9:21 pm   #8
joebog1
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Default Re: Williamson stereo pair, a good idea or not?

To Tony, Read what David has written, then double it.
To PJL, EL34/6CA7, are not plugin replacements for KT66. 6CA7's are pentodes not beam tetrodes and as such does not have beam forming plates, but does have a supressor grid that must be connected. Its pin 8 BTW. Also if 6CA7's are used in triode mode as are the KT66's in Williamsons design, the screen resistor must be upped to 470 ohms. One reason my amplifier is being made is that I am using 5B/254 AKA CV428 which are plentiful and cheap, made in England, and a very close sub for a KT66 or an 807 which is what Williamson used. My experience with "modern" output valves is sad to say the least. If the thought of top caps with 450 odd volts on them is a bit scary, you can use 5B/255 AKA CV391 which is the same valve but has the plate connected to a base pin and no top cap.


Just my take on it.
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Old 12th Apr 2021, 9:34 pm   #9
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Default Re: Williamson stereo pair, a good idea or not?

A modified Williamson circuit using 807's is described in the Radio Designers Handbook,807's strapped as triodes.Les
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Old 13th Apr 2021, 6:50 am   #10
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Default Re: Williamson stereo pair, a good idea or not?

I have in mind a project to build a pair of amplifiers to original design but using 807s instead of KT66. The only problem is 807's have a lower Vg2 max of 300v,but they can be run at higher voltage as long as Pdiss max is kept within limits. I agree, EL34's would be a good fit, Langrex do some good ones called Langrex Premium.

Your OPT's might drop into the Williamson circuit with no issues but you might need to do some testing and modifications.

Andy.
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Old 14th Apr 2021, 3:47 pm   #11
Wendymott
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Default Re: Williamson stereo pair, a good idea or not?

Many years ago, as an apprentice TV engineer, our local garage owner had a Mono Williamson amp..... no idea what the preamp was..... The Output transformer became faulty. Obviously there was no direct replacement..... well not known to us anyway... "no ebay" then.... so a Gardners transformer was ordered and fitted. The owner got it back and the very next day it was back with us...... apparently he was a Hi Fi "nerd" and he was most dischuffed. We took out the Gardners, he scrapped the amp... gave it to me as it happens..and bought some Fisher or Radford or Leak...and became an Audiophool.......The valves were PX25's
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