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Gillespie triode Williamson build

Discussion in 'Tube Audio' started by derekva, Nov 6, 2017.

  1. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,844
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    Woodinville, WA
    Back in April, I happened to be picking up some parts at my local electronics hut / surplus store, and I stumbled across a very nice condition Heathkit W4-AM monoblock amplifier at a very good price. I had never messed around with a 5881 amplifier or a Williamson circuit amplifier, so I decided to take a chance:

    [​IMG]

    I got it home, and decided to poke around AK for a bit to see if anyone had messed about with this particular beast, and, as luck had it, I came across mention of it by Dave Gillespie as a great amplifier when modified. I sent a message to Dave and began stripping down the chassis to rebuild it afresh.

    I decided to strip it down to the transformers and chassis and go with all new sockets while I was at it as the sockets were pretty cheap and some of the wiring was a bit crispy:
    [​IMG]

    Meanwhile, I awaited Dave's response.

    -D
     

     

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  2. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Dave responded a the following day with some details and a photo of his hand-drawn schematics with notes, which allowed me to create a BOM for Mouser:

    [​IMG]

    (NFB phase advance cap is cut off in the image above - should be 24pF for Chicago output transformer)

    The circuit is Dave's "take no prisoners" approach to the musical-but-unstable Williamson circuit - replacing the default W4-AM pentode 5881 outputs with a pair of triode-strapped EL34s, beefing up the power supply using a 5AR4 instead of the original 5V4G, and reworking the entire circuit including the feedback sections to make an amplifier that possessed "...a ruler flat frequency and power response from 20 Hz to 20 kHz at any power output level up to and including maximum power output, with excellent LF and HF stability, and extremely low distortion to boot." Given that I didn't need a ton of power with my speakers, that sounded like a perfect match for my room and speakers.

    When I gathered the appropriate parts and started to assemble, I had one troubling issue - no sound. With help from Dave, we figured out that the balance pot was shot and was not allowing voltage to flow through, so a new pot and new 1/4" jacks for bias test points later, and I finally had sound!

    [​IMG]

    There was still some optimization to be done, but it was definitely the right direction. Now I had to find another!

    -D
     
    Last edited: Nov 6, 2017
  3. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,844
    Location:
    Woodinville, WA
    Lucky for me, Scott Petersen had a spare, albeit a bit rough, W4-AM with Chicago transformers that he was willing to sell (without tubes) at a very good price.

    [​IMG]
    [​IMG]
    [​IMG]

    As with the first amplifier, I knew I was going to strip this one down as well (actually, I decided to strip this one down even more given the corrosion on the chassis and pots), so I went ahead and made another Mouser order and picked up some self-etching primer, some sanding disks, and some Hammerite paint.

    -D
     
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  4. nerdorama

    nerdorama AK member Subscriber

    Messages:
    2,351
    Location:
    Seattle area
    Fun project. Keep writing.
     
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  5. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,844
    Location:
    Woodinville, WA
    When I received the second amp, I stripped it to the ground (labeling all the transformer wires as I went along) and began sanding:
    [​IMG]

    [​IMG]

    Note the plastic insulators between the 1/4" jacks and the chassis - these were added when I discovered that if the jacks weren't floating off the chassis, you'd end up shorting the cathode directly to ground. Thankfully I didn't destroy anything in the process! All new hardware as well. McLendon Hardware has gotten used to me coming in and rummaging through the loose screws and random parts sections (including that hole cover) for amplifier builds. Also note that I replaced the standard auxiliary AC outlet with a sliding switch. Thankfully, the screw holes for a standard sliding switch are the same width as an auxiliary AC outlet, so it's a quick fix. I used my favorite resistors: KOA Speer SPR-series carbon film, plus an Ohmite brown devil in the bias section, along with a mix of Cornell Dubilier micas, Panasonic films, and K40Y-9 PIOs. Power supply capacitors were a CE Manufacturing 30/20/20/20 plus a pair of 22uF Illinois Capacitor axials. Fresh grommets for the wiring holes were for safety given that the leads on the transformers were pretty stiff and the wiring is somewhere around 65 years old!

    [​IMG]

    Note to self. Need to buy more phenolic sockets. With five per amplifier, it cleaned me right out of my stash.

    -D
     
  6. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Location:
    Woodinville, WA
    I finally finished the second amplifier and fired it up - not quite as good as the first as there was a little bit of hum / buzz / noise on the signal. Dave suggested I run both 6SN7s at the same B+ as well as modifying the power supply to elevate the heaters to reduce the overall noise. After a lot of back and forth, in which I came across as thicker as a whale omelette trying to figure out how the layout would look, I finally came up with the modified power supply:

    [​IMG]

    The 3k / 2W in the second section of the can capacitor (to the finals) and the 15k / 2W in the third section of the can cap (to the driver and phase splitter), got the voltages where they needed to be for optimum performance, while using the (now unused) fourth section of the can cap along with a 330k / 1W voltage drop and a 68k bypass to ground as a tie point for the center tap of the heater secondary (which is normally taped off). This elevates the heaters to remove any extraneous parasitic AC noise.

    This took me a while to implement (mostly because life has an inconvenient habit of getting in the way at the worst possible time), but once I had done so, it took the amps to a whole new level of performance.

    -D
     
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  7. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    And here the amps are in their natural habitat. They've managed to push themselves to the top of the heap quite definitively (against some pretty fierce competition - now I have to decide on what gets sold). I ended up doing a 1.5" x 1" black-walnut wrap on three sides that I finished in a few coats of tung oil. I still need to make bottom covers for these amplifiers, so the feet can be mounted a little more solidly than via foam tape. ;)

    [​IMG]

    I don't think these amplifiers will get a ton of playing time in August and September when the temperatures jump up into the triple-digits due to the amount of heat they pump out, but they definitely work quite well for wintertime. Who needs a fireplace!

    -D
     
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  8. kward

    kward AK Subscriber Subscriber

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    Excellent work!
     
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  9. s-petersen

    s-petersen Scott Subscriber

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    I am so happy to see that the amp worked out as well as it did. It came out great! Are both amps wired the same? It sounds that the amp from me was different in some way?
     
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  10. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Location:
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    The only difference that mattered in this case was that the amplifier I bought from you was gold instead of grey, but since it needed to be stripped down, no biggie. Both amplifiers use Chicago iron, and from the perspective of the new circuits are identical.

    Thanks again for selling the second amp to me at a very reasonable price, Scott. :)

    (actually, in hindsight, I should have stripped down the first amp as well since the hammerite looks so much better than the original grey...)

    -D
     
    Last edited: Nov 6, 2017
    s-petersen likes this.
  11. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Cool. They look very nice, especially with the wood trim.
     
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  12. NWGuy

    NWGuy New Member

    Messages:
    30
    Location:
    Seattle, WA
    This looks great. I love the concept. Thanks for presenting it.
    The paint job looks very clean.

    Please expand on your thought process during the painting process of when to paint over everything (including original Heathkit verbiage) and when to leave it and just touch up.

    I am working on a Heathkit W-7M pair and have left little boxes of the original words and since the new color selection was not close, I am not too happy with the result.
     
  13. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Location:
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    In my case (a) the outputs and rectifier were no longer the same as the original, (b) the circuit was significantly modified from the original, and (c) the chassis had a lot of corrosion on it, so it was sort of a no-brainer in this case. In the case of the first chassis, it was in pretty decent shape except for a few scratches, etc. The biggest issue is that there isn't a paint that matches the grey finish that was on the original that could be used for touch-up purposes, so there wasn't really a way to touch up the first amp. That being said, my painting skills are still not perfect, I managed to gouge a scratch into the finish. I need to figure out how to put a harder coat over the color, and I'm not sure how they did that back in the day.

    -D
     
  14. derekva

    derekva This ain't no picnic... Subscriber

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    Thanks!

    -D
     
  15. drtool

    drtool It might get loud In Houston Subscriber

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    Congrats D, they look great. Sounds like you have some wonderful mojo going on under the hood too.
     
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  16. NWGuy

    NWGuy New Member

    Messages:
    30
    Location:
    Seattle, WA
    I have had good luck putting the unit in the oven at the lowest "bake" rating (170F) for 1/2 hr or so between each coat. Then the same with a clear coats. Note: depending on the contents of the paint/clear-coat, you might want to let it air-dry first, especially if it has really hazardous contents.
     
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  17. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Good to know. The painting was done at ambient (e.g. 75 degrees & dry) over a period of 3-4 days as I laid coats down over the primer. Didn't think to bake in the oven, mostly because my wife would (rightfully) kill me. :D

    -D
     
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  18. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
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    Thank you.

    -D
     
  19. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
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    Location:
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    Those look great.
    My Mrs, is especially sensitive to paint smell
    I have a tough time painting anything inside (complaints)
    Last time I tried to paint outside I had wind and bug issues (now I'd be afraid of falling leaves.
    Nice work inside and out:thumbsup:
     
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  20. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Location:
    Southern NJ
    When I can, I like to paint things outside on the picnic table and let them sit in the sun to bake. A little extra heat really seems to help the durability of the finish. I have in the past painted stuff and put them in what amounts to an EZ-Bake oven made from a cardboard box and a light bulb to make things warm. It works when its a little too cool outside to really do a good painting job. Those banker's boxes work great for that kind of thing. Put the thing to be painted on the lid, paint it, then put the top with the light zip-tied inside on top of it.
     
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