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What was going on with the original PR heater CT?

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Mar 10, 2021.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Following along with a PR build here, the question came up of whether to use an artificial or actual CT (when one's available) for the 6.3V heaters. Aha, as always, let's ask Leo -- consult the original layout.

    upload_2021-3-10_12-19-2.png

    Hmm, wait. AFAIK, we're not supposed to use both!!!

    How about the original schematic? Here's the one I found online, at Prowess Amps, who have a large and (usually) high-res library of layouts and schematics.

    upload_2021-3-10_12-22-59.png

    So did Leo use both a real and artificial CT? No way.

    Look at the graphic quality in both cases. I suspect someone, at some time, hand-drew the 100ohm and ground symbols. They look hand-drawn in both cases, yes? And (always recalling the manifest mysteries of PR schematics) here's the version I have in my files.

    upload_2021-3-10_12-27-24.png

    Much mo better. No artificial CT, no squiggly drawing.

    So, speculation:

    See the 5U4 in the top schematic? Recall Fender changed PTs in ~1971. Did the original Schumacher 022772 (which would not supply enough 5V heater current for a 5U4) have an actual CT, and the later 010020, though with adequate 5V current for a 5U4, *not* have a 6.3V CT?

    That's my WAG and I'm sticking with it. Nice drawing skills, there, CBS. I could easily be wrong, tho.... :)
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    PRblackfacing.png
    This B1270 schematic only shows the artificial.

    PRblackfacing_layout.png
     
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  3. David Barnett

    David Barnett Doctor of Teleocity

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    The same transformer was used in Champs, and sometimes they'd just tape off the center tap. Champ heaters were wired up like the old tweed amps, with one side grounded and one green wire daisy chain.
     
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  4. James Knox

    James Knox Tele-Holic Silver Supporter

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    Interesting stuff. What got you thinking about this?
     
  5. theprofessor

    theprofessor Poster Extraordinaire

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    I love this kind of detective work, @King Fan ! And we're often warning rookie builders to avoid following the Mojotone layout and schematic to a "T"... Now we have to be on the watch for drawn-in redundancies on the Fender schematics!
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    So many pieces to the puzzle.

    Excellent point. Obviously these were later drawings, with the proud CBS graphic. I note also the expanded / updated list of transformers.

    upload_2021-3-10_19-48-25.png

    Excellent point also. If the older drawings had to have the artificial CT drawn in, does that mean the 125P1B / 022772 originally had an actual CT (that as you say they could tape off for single-strand Champ/Vibro Champ heaters) but then lost it? Did they decide it was simpler/cheaper to omit the actual CT and plunk for a couple 100Rs?

    As I say, it was another thread asking about PR actual vs. artificial CT that made me say: let's check the originals.

    LOL, yes. This is one of those many many areas where a single set of 'how to build an amp' instructions just won't work. How you go here depends on whether your PT has an actual CT. FWIW, of course, many modern PTs we might use in these amps still don't have a 6.3V CT.

    Of course, Fender is notorious for updating diagrams or tube charts or old parts only when they ran out of the old ones. The shop floor and the suppliers evolved the amps while the drawings department was only an occasional entity. So although I use Fender diagrams, I always study lots of vintage build pics and modern diagrams -- and pencil in my own squiggles and notes.
     
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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Aha. Doing some more searching, I now reaiize I'd read about this 'drawn in' artificial CT back when I was originally researching the PR. Nothing new under the sun. But my excuse is I got distracted back then by the *much more* confusing question of the notorious 'AA764 PR' tube chart yet the absence of any actual 'AA764 PR' schematic and the absence of any actual AA764 PRs -- the originals were always AA1164s inside despite the tube chart. Now there's a real detective story, but one for another day.
     
  8. pfarrell

    pfarrell TDPRI Member

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    My PR build has some noise—No experience to know if this is normal or not—certainly isn't a problem when actually playing. Things changed A LOT when I swapped the signal and vol and mid-mod pot to shielded cable—and carefully oriented the reverb tank. The floor goes up a tad when the reverb is on, might call it a little buzz. This is all in comparison to my 5E3 build—which you can't tell is on, when it's on. I'm considering redoing all the leads to the pots with twists, tremelo is silent—leads twisted per robrob's notes on his diagram. Also going to replace the input resistors w/ 1W to see what that does. This brings me to also possibly experimenting with undoing the "artificial" heater CT, and trying the heater CT on my transformer—to see/hear if there's a difference going that route. What do you guys think? Where's should the heater CT be connected—Power amp ground like 5E3?
     
    Last edited: Mar 15, 2021
  9. Jimy

    Jimy TDPRI Member

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    The Leo era Princetons used the 125P1B power transformer, which definitely had a center tap on the 6.3V winding. The virtual center taps came later, in the CBS era. Personally, I wouldn't look to CBS for guidance. Virtually every change they made to Leo's designs was aimed at cost cutting. I can't think of many that were for the better.

    Merlin Blencowe has some great information on heater circuits and fighting noise here:

    http://www.valvewizard.co.uk/heater.html
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good info, makes sense. IIRC Merlin doesn’t indicate that a real CT is preferable, but he seems to imply the approach I’ve taken out of simplicity—if you got real, why use artificial?

    For the noise issue. Reverb does make some natural noise, and OTOH there are treatable causes of excess noise in the tube, cables, connectors, tank. Heater noise is common, and the Merlin chapter can’t be beat. One thing he doesn’t mention is the simple-enough trick of disconnecting the heater wires to the tubes from the lamp and test powering them with a 6V battery. If hum disappears, you work on heater hum.

    If it’s 120Hz, you work on your filters and grounds.

    if it’s hiss, a different flowchart. Re the input resistors, for sure 1W will be quieter -- but before that, do you already have metal film there?

    Twisting sensitive signal wires to the pots is different from twisting the wires to the trem. I'd twist just the ones Rob shows twisted.

    You already know I'm not smart on whether a real heater CT is quieter than artificial. But if you have the time, I know a way to find out. :)

    If the noise persists after your fixes, of course, start an 'is this normal' thread and give us a sound sample and some gut shots.
     
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  11. pfarrell

    pfarrell TDPRI Member

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