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Old 25th April 2011, 08:17 PM   #1
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Default 807 p-p tube amp schematic

Well time has come for me to try and build an 807 amp. I have 2indentical power and OPT x-formers, and 4 identical chokes to work for this circuit. They should be a good match for this schematic.

807 Amps

Not entirely sure of the impedence of the opt's but should be 9k-10k, also has a CT'd tertiary winding.....which i'm not entirley sure what for, its seperate from the primary windings. Any thoughts or ideas.

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Jeremy
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Old 25th April 2011, 09:55 PM   #2
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Both the original Williamson amplifier and the The Musician's Amplifier (your schematic) used OPT's with a 10K primary impedance. The Williamson amplifier in America has a looong history. The original Heath WM-1 is a near direct copy of the Musician's design, and each succeeding amplifier just represented the latest version of the amplifier. Of greatest importance is the need to address stability -- both on the low end and high end. Williamson amplifiers demand very high quality OPTs, and will hardly work with just any transformer lying around. There were numerous stories of "breathing woofers" (low frequency instability), and burned out tweeters (high frequency instability) from poor copies of the design -- and they were all true. It took Heath to reissue their W-3M and W-4M amplifiers as the W-3AM and W-4AM amplifiers to finally produce decently stable amplifiers.

All that being said, the Williamson design has many virtues as well. Triode outputs, low output impedance, wide frequency response, class A operation, and low distortion. With proper attention to stability at both ends of the spectrum, they can be very nice amplifiers in deed. I have my own version of the design using triode EL34s and Heath W-4 output transformers. Both low and high frequency stability is excellent, and the sound is superb through my Cornwalls.

Dave

Last edited by dcgillespie; 25th April 2011 at 09:57 PM.
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Old 25th April 2011, 10:14 PM   #3
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Jeremy,

Either the tertiary winding is for the screen grids or (more likely) it's for cathode feedback. Determine the % of the primary the tertiary winding is and report back.

The linked schematic employs triode wired "finals", in Williamson style circuitry. By today's standards, PSU energy storage is very small. It's quite safe to increase the value of C10 and C11 to 15 μF. C12 should be 40 μF. for good bass performance. C8 and C9 are known as death caps. and they must be eliminated. Build with a proper, safety grounded, 3 wire power cord.
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Old 25th April 2011, 11:21 PM   #4
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I think shoog here used some 807s in an amp here - have a search and see.
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Old 25th April 2011, 11:41 PM   #5
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For these purposes, you can think of the 807 as a 6L6 with a top cap. The Musician's Amplifier was very popular in the day (807s were dirt cheap) and there were a lot of happy builders. Good, solid design.

Many of the Williamson stability issues were OPT related, many were from the RC coupling time constants being insufficiently staggered. The latter is an easy fix. The former just takes money.
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Old 26th April 2011, 05:49 AM   #6
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Quote:
Originally Posted by Eli Duttman View Post
Jeremy,

Either the tertiary winding is for the screen grids or (more likely) it's for cathode feedback. Determine the % of the primary the tertiary winding is and report back.

The linked schematic employs triode wired "finals", in Williamson style circuitry. By today's standards, PSU energy storage is very small. It's quite safe to increase the value of C10 and C11 to 15 μF. C12 should be 40 μF. for good bass performance. C8 and C9 are known as death caps. and they must be eliminated. Build with a proper, safety grounded, 3 wire power cord.
How do i figure ou the % of the primary the tertiary winding is? How does cathode feedback work? I'm thinking i'll build as is and eliminate the line caps, and use a line filter in thier place and a ground plug, and try using the tertiary windings later after i get all the bugs worked out.

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Jeremy
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Old 26th April 2011, 06:37 AM   #7
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given the legendary instability of the design, a bit of local feedback or use of the tertiary winding to go back to the finals could be a VERY good idea...

Certainly not a thing I would leave until later.
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Old 26th April 2011, 12:54 PM   #8
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Jeremy --

Determine your ultimate goal. If it is to build the original Williamson amplifier, then first determine if your OPT is capable of performing well with that design. You will need the specifications of your transformer to determine that. The requirements of a Williamson transformer are readily available on line. If your transformer is appropriate, the tertiary winding is not needed for that circuit.

If the transformer is not appropriate, and your goal is simply to build an 807 amplifier, then your transformers could still likely perform well, although using tertiary FB within an output stage using a triode connection provides diminished returns since the stage gain is so low. However, a triode design also allows transformers of unknown origin to perform to their best capabilities. Therefore, if your transformers are not appropriate for use in the Williamson design, then use a triode design that favors local feedback (of which tertiary is a form of) and much less global feedback. Stability will be enhanced, and performance will be very good.

To determine the % of of the primary winding that the tertiary winding represents, drive the full primary winding with 100.0 volts AC from a variac (using caution of course), and then measure the voltage of the full tertiary winding. If the tertiary winding produces 50.0 vac in this scenario, then the tertiary winding represents 50% of the turns ratio of the primary winding, meaning that for every volt developed across the primary, .50 vac will be developed at the tertiary winding. Perform this test with no load connected to the transformer, and preferably, out of circuit if the transformer is currently installed in some other application.

Dave

Last edited by dcgillespie; 26th April 2011 at 12:56 PM.
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Old 26th April 2011, 03:32 PM   #9
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It seems that Williamson is good choice, I had made few ones and the one with 807 works really fine Williamson amplifier with 807 tubes

Good luck!!!
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Old 27th April 2011, 03:42 AM   #10
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OK and the results are.... 125 volts on the primary. tertiary winding shows 25volts ct would show 12.5-0-12.5, and secondary shows 2.2. Show it would be 5%? Core dimensions 1 1/2H X3 3/16L X 3 3/4 W.

Ultimate goal is just to utilize some of this iron on my shelf. I'm not set on anything, just got a little time to spend... Idea was to use 807's, volt meter and current meter to make an amp that looked like a piece of broadcasting equipment. But i'm all for getting the most out of this iron. Power x-former was somehting like 450 VDC at 200 ma. BUt i'd have to go check that again, and that was using a 5U4 rect. choke look a little small, probly about 5-6 H @ 150-200 ma.


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