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Webster-Chicago 66-1A resto-mod

Discussion in 'Shock Brother's DIY Amps' started by timfred, Dec 30, 2020.

  1. timfred

    timfred Tele-Meister

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    Greetings folks - my first amp build thread. The victim is a 1948ish Webster-Chicago 66-1A wire recorder amp. 2x6V6 in PP, one 6N7 phase inverter, one 5Y3 rectifier, 8" speaker.

    W-C had 2 amps for their wire recorders, this is the smaller and mostly unloved one. The larger 166 model has a 10 inch speaker and a much more interesting circuit - apparently harp players like that one. This one has no preamp to speak of, and really doesn't make sense from a design point of view. 2 6V6 but only 10W. 10W but only one 8" speaker with a tiny voice coil.

    But on the other hand we have nice iron, a cool chassis, 1 vol, 1 tone, and 4 octal sockets to play with. My plan is to fill them with three 6SL7 preamp tubes and one 6SN7 output tube in push-pull for ~1-2 watts.

    A 5F1 would also work here but I personally feel like the world doesn't need any more Champs. ;)

    [​IMG]

    [​IMG]
     
  2. timfred

    timfred Tele-Meister

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    Original tubes were Tung-sol for the 6V6 and 5Y3, Ken-Rad for the 6N7.

    [​IMG]

    Somebody had already modified the input cable with an RCA plug, but the 2-prog cord is unadulterated.

    [​IMG]
     
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  3. timfred

    timfred Tele-Meister

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    Mostly original inside. At some point one capacitor in the cap can failed and somebody put the big red electrolytic in. They cut the tab off the can and left a +360VDC chunk of solder floating around in the chassis with no insulation on it. Awesome.

    [​IMG]
     
  4. timfred

    timfred Tele-Meister

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    The rest of the capacitors are all wax-covered and Every Single One of Them is bad. That's OK because it's all coming out anyway.

    I took the power transformer off and the HT secondary leads' insulation crumbled off as I was handling it. So I took the bell covers off and cleaned up the wiring insulation with shrink wrap and new wire best I could all the way back to the winding. I won't be using the 5V winding in my SS rectifier design so I just tucked that out of the way in the bell cover.

    [​IMG]
     
  5. timfred

    timfred Tele-Meister

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    Clean slate. Seems like a good place to stop, for now.

    [​IMG]
     
  6. Nickfl

    Nickfl Friend of Leo's

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    Someone had to say it!

    Probably for the best that you're doing something more interesting than a 5e3 too...

    A couple of years ago, I turned a webcor reel to reel into a single ended amp with octal preamp tubes, feels like a similar direction here.
     
    Last edited: Dec 30, 2020
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  7. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Reverb o rocket octal front end with preamp bias wiggle tremolo. Into a single ended non NFB 6K6gt output. That could be cool.
     
  8. cometazzi

    cometazzi Tele-Holic Silver Supporter

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    I like it!

    I don't feel like I have the clout yet to say stuff like this, but I'm quietly agreeing with you here.

    (I do plan on building a 5E3 at some point though)
     
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  9. timfred

    timfred Tele-Meister

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    Funnily enough, I have an octal Reverberocket and that's what got me into octal preamps.

    Here's the circuit I'm planning. I'm sure a lot will change between now and when it's done as I tend to do a lot of tweaks as I go. Things I'm trying for the first time with this are a common first stage for two differently voiced channels, then mixing the two at a cathodyne PI. Also a garter bias circuit to try to balance the triodes as I've had problems in the past with difficulty in balancing 6SN7 triodes in PP.

    [​IMG]
     
    Last edited: Dec 31, 2020
  10. NTC

    NTC Tele-Holic

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    Sorry to comment on this, but on your schematic, I can't tell which connections go to ground and which don't. You should have dots where wires connect and ni dots where they don't.
     
  11. timfred

    timfred Tele-Meister

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    Thanks for the feedback. Updated!
     
  12. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    That is a cool circuit Timfred! One thing to consider would be the fact that each plate on a 6SN7 draws 10mA. That is 20mA per tube 80mA for four. And that is just the plates. I would think judging by the original tube complement your power transformer secondary may not be able to power your circuit. I could be wrong but it is something to look into.
     
  13. timfred

    timfred Tele-Meister

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    Only the power tube is a 6SN7, the other three are 6SL7. I figure I need something like 2x10mA + 6x3mA = 28mA. The original PT was powering 2 6V6 (20mA each) plus 6N7 (7mA) so I think I’m actually dropping the load on the PT by quite a bit. I can’t remember the details but I think I checked the heater draw as well.
     
  14. timfred

    timfred Tele-Meister

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    OK here's my low-tech circuit board manufacturing process for the rectifier and power filter bank. I use a milling machine to create pads in a copper clad board.

    [​IMG]

    Here it is populated and installed, alongside the OT and heater wiring. Octal preamps are nice because you can lay down the heater wiring at the pins instead of the acrobatics required for 9-pin tubes.

    [​IMG]
     
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  15. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    Looking good!
     
  16. timfred

    timfred Tele-Meister

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    OK, getting back to this. I didn't have any terminal strips so I made my own from PC boards and mounted them on standoffs.

    [​IMG]

    Then I wired up the output tube and the phase inverter, as I was excited to see how the garter bias circuit worked. Please excuse the wire nuts for AC power, that's temporary for testing.

    [​IMG]

    My DIY terminal strips worked really well for laying out the circuit in a small space, even if it is cramped and doesn't look exactly, um, elegant. Nobody will confuse this with a Hiwatt for sure.

    And the bias circuit? Wow. I tried 3 different 6SN7 tubes from 3 different manufacturers. Measuring the plate current through the OT I got differences of 0.4%, 0.8%, and 1.4%. Normally if I'm within 10% I'm happy. I don't know how it will sound - a little imbalance is sometimes good, right? - but I wanted to try this because the main advantage of a push-pull for a low-wattage amp is a lower noise floor and big imbalances in power tubes negate that. Also, I'm running the tube at its max combined plate dissipation and that doesn't leave much room for one triode to be way above another.

    What are the downsides of garter bias? Well, you lose 1 bias voltage's worth of B+. For a 6SN7 when bias voltage is only ~12V, that's not that big of a deal when you're starting with 390V. But if I were an amp manufacturer, losing 20V-35V of B+ from my 6V6 or 6L6 amp is going to cost power that I want to be able to sell. In addition, it costs 3 extra resistors and 1 more capacitor, all of which are parts that need labor to install and can fail. So I can see why it never caught on in mainstream production amps.
     
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  17. timfred

    timfred Tele-Meister

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    By the way, I'm updating the schematic as I go to reflect tweaks due to design and using whatever parts I have on hand.

    [​IMG]
     
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  18. timfred

    timfred Tele-Meister

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    OK - slowly tweaking this. I've got it to a very nice point, schematic-wise. Latest below.

    Both channels have a rather flat frequency response - the Yank channel is slightly bassier and the Wank channel has slightly more treble and a different overdrive character. I'm thinking about mods to create a little bit of a mid-scoop in the Yank channel (more blackface, less tweed).

    [​IMG]
     
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