Help needed with 5f1 PT specification for SS rectifier

Discussion in 'Shock Brother's DIY Amps' started by Lemon, Mar 9, 2018.

  1. Lemon

    Lemon TDPRI Member

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    Hi everyone,
    After abandoning (for now) the mystery PT and OT I found in the trash, despite great help from forum members, I'm now starting to source parts for a diode rectified 5f1 build. I already have a Hammond 1760C OT to go along with an 8Ω 12" speaker, and the next step is ordering a bespoke PT from a local shop.

    I know that SS rectifiers upset the vacuum tube gods, but it really simplifies the logistics and dramatically lowers the costs in my part of the world. What I'm not sure about is the specification for the PT for this specific build and any additional variables that should be considered for this type of rectification. Here are some problem points -

    What B+ voltage should I be aiming for? I see this specified in different schematics and layouts ranging from 340V to 370V. Why is that?

    How do I determine the PT's secondary AC voltage for that B+ through a bridge rectifier? Being a total layman to this world, I'm a bit confused by the different calculations for the different types of voltages - RMS, average, peak, etc.

    What should be the PT rating? How many amps for each winding? What other rating should be taken into account?

    What diodes should be used? What rating? Should they be UF?

    What about the filtering caps and resistors? How are they affected by this rectifier? I also see different capacitance values between schematics. Why is that?

    Should I insist on ordering a PT with a 6.3V winding center tapped, or can I get the same performance with an artificial CT?

    Should I use a standby switch when implementing a SS rectifier to compensate for the lack of gradual startup?

    What changes should be made in order to use the 8Ω winding on the OT?

    I'm sure that there are a lot more questions that I don't even know that I should be asking about this, so any information, suggestion or comment will be greatly appreciated. Even a friendly RTFM.
     
  2. clintj

    clintj Friend of Leo's

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    Figure your unloaded B+ (no tubes in) to be 1.414 times the HT voltage of your PT. For example, for a 300-0-300, you'd get about 420-ish volts. So, if you want 350V to the tubes look for something in the neighborhood of 250-0-250V. The actual plate voltage will droop a little bit from various resistances in the power supply, but it'll be reasonably close to your 350V goal.

    A pair of 1N4007 diodes will do quite nicely for rectification.

    Look up a typical Champ PT from Hammond or whoever to see what current capacity you need for HT and 6.3V. Those numbers will still be perfectly good.

    You have some freedom to go larger in your filter caps, but part of the Champ sound is that set of values. I build to the original Fender values, staying within the 20% tolerance listed. Upsizing the first cap may get you a little less hum.

    The 8r speaker tap will give you slightly more negative feedback. I've built both ways, and the slight extra NFB is not a bad thing for some players. Slightly cleaner further up the dial, and a more pronounced break into overdrive, are the main effects I've noticed.

    I don't use or install standby switches unless someone is absolutely insistent on having one. Your tubes will be fine, and as long as your filter caps are high enough voltage rated they'll be fine too. 450V is a common rating, and very adequate for this amp. 350V caps would be really flirting with overvoltage at power up, though.
     
  3. Inglese

    Inglese Tele-Meister

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    I'd suggest to leave the center tap secondary needed in the time only rectifier avaiable was a tube and go with a bridge rectifier; PT will be smaller and lighter with the same performances.
    Really don't undestand why so many amps still have that old configuration.
     
  4. Lemon

    Lemon TDPRI Member

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    Thanks for responding. Since this PT will be made to order, I will use a four diode bridge. The HT winding will be untapped, but I wasn't sure about the 6.3V winding. I know that there is a way to create an "artificial" CT, but if the option exists, should I still insist on a CT for the 6.3V?

    Isn't there a risk of damaging the tubes when they're hit with high voltage before heating up?
     
  5. clintj

    clintj Friend of Leo's

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    Last edited: Mar 10, 2018
    the fatch likes this.
  6. Lemon

    Lemon TDPRI Member

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    That was indeed my follow up question! Thanks for the link. Very educational.
     
  7. jonrpick

    jonrpick Friend of Leo's

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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Hammond 290CAX has a 275v secondary option. It's 115mA rating. I used to on mine and with a 5y3 it hit the vintage layout voltages spot on at 340v B+. The power section ended up right on vintage levels and the preamp section was right on Mojo's layout specs. I didn't work the math for a SS, but maybe those numbers will help.
     
  9. Lemon

    Lemon TDPRI Member

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    Thanks everyone. What do you think about these specs?

    [​IMG]

    I guess the 6.3V winding can have a lower amperage rating because it's only feeding one 12ax7 and one 6v6GT (about 750mA combined).
     
  10. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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  11. Nickfl

    Nickfl Tele-Afflicted

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    That looks like it would put you in the right ballpark. Don't worry about having a center tap on the 6.3v winding, an artificial center tap works just fine, if you are worried about hum you can elevate it on the 6v6 cathode resistor to reduce hum potential.

    My only thought is that 115ma is a bit beefy for a 5f1. Between that and the solid state rectifier, you are going eliminate any potential for sag that the original amp may have had. This may or may not be desirable, personally I'd be just fine with it. That being said, something like a 75ma secondary would still be more than sufficient and possibly a bit cheaper.
     
  12. Lemon

    Lemon TDPRI Member

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    VintageSG - Thanks for the link. Since the WF-55 is essentially the same amp that I'm planning to build, the instructions will be very handy.
    The transformer in that kit is designed to cover a lot more options than I need.

    Nickfl - You're right about the expected current draw (shouldn't be more than ~55mA, right?), but now I remember reading about a change in current draw when using a full wave bridge. I think it also depends on the type of filtering and it might call for a 10% to 80% increase in amperage rating. The classic Hammond drop-in PT for the Champ is 81mA, but it wasn't designed for a full wave bridge rectifier. This is way beyond my very limited knowledge level, so for anyone reading this in the future - everything written above may very well be total BS.
     
  13. gusfinley

    gusfinley Tele-Holic

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    A lot of Fender Standalone Reverb transforms work great for a SS rectified 5F1 and are some of the cheapest transformers out there. Just watch ratings on the secondary, some are rated as high a 100mA and some as low as 30ma!

    I've build 4 or so of these using hammonds, webers and mojos and like the mojo (heyboers) the best.

    Also, I like using 1% resistors for an artificial center tap, I've used 5% resistors that still had lots of hum. If one 5% is high and one is low there could be as much as a 10% imbalance between them.
     
  14. tubeswell

    tubeswell Friend of Leo's

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    With a FW bridge rectifier, it should be good enough for a Push pull amp with 2 x 6V6s at 340-350V with a couple of 12AX7 pre-amp tubes.
     
  15. OneHenry

    OneHenry Tele-Holic

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    Actually I much prefer tubes for amplification stages, but I really like solid state power supply rectifiers. The four lead full wave rectifiers are Good Things.
     
  16. Lemon

    Lemon TDPRI Member

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    I still haven't ordered a PT, I'm sorry to say. Things just got in the way...
    Upon returning to this dilemma, I found a forum thread with two differing opinions - one says that the PT should be rated in excess of 60-80% of the expected current draw, and one says that 11% is enough. Here's the link to that thread - https://forum.allaboutcircuits.com/threads/current-rating-and-full-wave-rectification.32697/

    Is it safe to say that the 5F1 tubes are expected to draw about 55mA? If so, 11% increase is 61mA and 80% increase is 99mA. Other than the higher cost for added copper, is there a downside for playing it safe with a 100mA rating? (I'm not trying to achieve any sag by overworking the circuit).
     
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