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5F2A Weirdness...Troubleshooting

Discussion in 'Shock Brother's DIY Amps' started by keithb7, Nov 24, 2017.

  1. dan40

    dan40 Tele-Holic

    627
    Aug 19, 2015
    Richmond Va
    I recently had a similar issue in a Princeton build that turned out to be a loose solder joint at the screen grid connection on the tube socket. The joint looked good, but once I started pulling on the wires a bit I noticed the poor connection.
     

  2. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yeah the b+ does sound a bit high. Tubeswell discusses this a bit in your current build thread right around this post: New (and first) build: 5f2a Tweed Princeton
     

  3. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Sorry — what PT, Keith? What rectifier? Hey, I should know this — I studied your build back when I started mine.
     

  4. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Vintage 5Y3 rectifier tube. Heyboer HY022772 PT. Reading @theprofessor 's build, I like the zener diode voltage reduction package. Hmm.. Order 4 diodes from Dave Hoffman, send all the way up here to me, not far from the elves?
     
    Last edited: Nov 25, 2017

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, RLee. I went back and read @tubeswell's post you pointed me to, and it's right on the money! (should I be surprised? no.)

    Keith: Since you're not going to go back and change the PT, it seems the easiest way to go to use the zeners. I was really surprised at how easy the zener diode voltage reduction was. It really works! Perhaps I could have thought of a more elegant way to mount them, but they are doing their job so far. You could really shave some serious voltage off that B+ with several of these zeners in series. If I can do it, anyone can! And thanks again to @tubeswell for pointing all this out to me.
     

  6. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    It looks like with the 8k primary, the load line crosses just barely to the low side of the knee on the datasheet. That forces a rapid shift of current to the screen as the grid voltage approaches zero relative to the cathode, and flirts with overdissipation at that high a voltage. Looks also like adding some resistance to increase screen sag is helping.

    In plain English, that tube is just being asked to do too much at 400V in that circuit.
     
    dan40 likes this.

  7. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Thanks @clintj. I suspect some diodes in series to drop the b+ voltage is a good solution?
     
    clintj likes this.

  8. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    $20 US shipping for $4 in diodes from Mouser Arrghh... Not happening.
     

  9. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    That sucks. Usually when that happens I go find it on Amazon, and order it via amazon prime, but they don't always have the exact thing I need. But I've been surprised lately by being able to find a lot of specialty electronic components on amazon.
     

  10. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    trying to tie togethor arcing, increased loads, screen voltages, and even NFB. ( most already mentioned)


    Higher loading is a known cause of arcing, probably #1. Screens get mentioned too.


    The 5F2-A has twice the speaker load of the 5F1 but the power section is identical.



    More resistance at the OT primary, I'm not sure if the 5F2-A has it, would lower the plate voltage but not the screen voltage. This would have a bad effect on the plate - screen relationship.


    The NFB? The NFB must have a damping function which would help keep everything under control.
     

  11. Nickfl

    Nickfl Tele-Holic

    708
    May 24, 2016
    Florida

  12. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Not bad, and that's actually 25 pieces.
     

  13. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    which diodes do you need?
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Yeah, let us know what you need, Keith. We can figure out a way around $20 shipping, I'm sure!
     

  15. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Thanks for the offer folks. Mighty kind of you.

    Anything that crosses the border by way of any freight company, they get their palms well greased. Brokerage they call it. Or Import fees. Handling fees. Even Canada post and the Canadian Govt add their F-U fees. So to avoid this, I import 99% of what I buy from the USA myself. I have my parts shipped to a little border town. I literally walk across the border, sometimes with my mover's furniture dolly in tow. Pick up my packages and walk them back across the border into Canada. I do this about 10 times a year, as the border is 3 hour drive one way. However I go visit family in that direction and then I gather my parts. Works well, except when I am in a hurry for something, like these diodes.

    I found an electronics supplier in Canada. I loaded up on several parts to replenish my inventory. Caps, resistors, diodes etc. They are now on the way as I ordered them here today. Maybe I'll have them by Friday.
     
    Last edited: Nov 27, 2017
    King Fan, Axis29 and jsnwhite619 like this.

  16. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

    Jan 2, 2007
    Virginia, USA
    When I needed diodes for my Twin, to replace the selenium rectifier, I got them on eBay. I got like 10 of 'em for a few bucks. I was actually surprised at how cheap and how quickly they shipped. Might be something for the future?
     

  17. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    The diodes have arrived. I ordered 12V 5W 1N5349 and 15V 1N5352 5W options. 10 of each as I may get into this again on future builds.

    Question: Can I mix the 12V and the 15V diodes together to dial down just the right amount of voltage?
    12v X 6 qty = 72V reduction. The chassis is a tight space. Could I go with 4 x 15V plus 1 x 12V = 72V reduction however only using 5 diodes? Or say in another example I want to dial down 27V. Can hook up in series 1 12V and 1 15V diode?

    Last question for review. I am to lift the PT centre tap wire that currently goes to ground. It's red with a yellow stripe in my case. Insert diodes in series. Then wire from
    diodes to ground. Correct? I have a few little tag strips that outta do the job just fine I think. Thanks. -K
     

  18. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    You can mix and match the diodes in any combination you want. They go in series with the CT wire to ground. Leave some lead wire on the diodes to aid in heat dissipation.
     

  19. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Pre-diodes B+ was 428V today. After diode pack of 5 qty at 15V was installed, B+ is now 365V.
     

  20. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Sonic review, before and after? Any notable difference for better or worse?
     

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