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Brown Box/VVA/bucking transformer meets 5E3: Report

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Nov 21, 2020 at 3:04 PM.

  1. King Fan

    King Fan Poster Extraordinaire

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    So this summer I built a version of R.G. Keen's "vintage voltage adapter" based on Rob's good ideas (his "bucking transformer"). Links to those sources and build details here. I liked it a lot and use it all the time. It lets you choose between standard wall voltage or two reduced voltages, ~6% or 12% lower.

    VVA - 2.jpeg

    Almost all my amps sound better down 6%. Between some luck and some tedious PT picking, I've been fortunate enough to get B+ and heater voltages only (about) that much higher than vintage. This has meant I don't have to use the -12% setting much -- though it'd be handy for folks with hotter PTs or higher B+ issues.

    When I rolled it out, I re-biased my fixed bias amps to suit (in a couple cases they didn't need re-bias) but I left my cathode bias amps alone. Today I had a chance to drop the back panel on my 5E3 and did some measurements on full and –6% wall voltage.

    upload_2020-11-21_12-55-28.png

    How's it sound? It sounds good at both voltages, but to me it sounds *better* at the reduced voltage. Fuller, rounder, richer, more 'tube.' It's still a 5E3, but it sounds a bit more balanced and relaxed, a bit less annoyed or anxious.

    Whew, that's about as far as I'm gonna go trying to describe tone with words. :D But I think the optimized heater voltage is really useful too; I hope my 1947 Ken-Rads like it as much as I do.
     
    Last edited: Nov 21, 2020 at 5:28 PM
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  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Nicely done! I know some techs and players are really attached to high voltages, having gotten used to how real 1950's amps behave when plugged into modern wall voltages. Maybe their tones really do depend on it, I dunno.

    But in my opinion, low-voltage operation doesn't have anything to apologize for! My Deluxe-ish build ran at 286 volts (not a typo!) on the low-voltage secondary, and it sounded beautiful in that mode. "Smoother" and "relaxed" might sound like BS terms to anyone else, but it's true! And it's not like I was hurting for clean headroom at that lower wattage in my basement shop.

    (I use the past tense because that PT fried and the ClassicTone I got to replace it didn't have a low-voltage secondary. Dang it, I should have spent the extra $30 to get a 40-18028...)
     
    Last edited: Nov 21, 2020 at 4:36 PM
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  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I love seeing real data & numbers here. It's crazy how much it actually can affect tone & response. A Harvard I built for someone last year tripped me up when I was getting it dialed in. I think I'd set the bias at maybe 55% range in my shop. I brought it inside to play it some in a room that didn't have a concrete floor. It sounded way different when I started playing. Too thin, clean, just anemic sounding. I checked the bias again and it was somewhere near 45%-48% I think! I checked the wall voltage in my shop and in the living room and it was 5-6v difference between them. You just never know how many things you have to account for with this stuff!
     
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  4. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    Yep, I need one. My ‘66 AB165 with the original trannies is pumping out well over 7 volts for the heaters and is very fidley when trying to dial in the bias voltage. I’m sure my clones could benefit too. I remember a lot of head scratching while trying to tame the B+. Old vintage rectifier tubes helped a great deal though.
     
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  5. King Fan

    King Fan Poster Extraordinaire

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    Good stuff, all. I said here recently that this kind of thing really rewards the effort to make bias and voltage measurement easy. Like external test points and bias-adjust. Hmm, now I gotta think of a way to wire a TP for heater voltage.:)
     
  6. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Unless you really wanted to mentally double whatever you measure, I'd probably just bring both sides of the winding out to their own TP. :)
     
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