Transformer Question

Discussion in 'Shock Brother's DIY Amps' started by SacDAve, Dec 9, 2014.

  1. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The resistor should not be in series with the OT secondary and the speaker connection. The resistor should connect from the 8 ohm tap to the cathode of V2-A. Alternatively, you can connect the resistor to a switch at the cathode of V2-A. With a single pole, three throw switch you could have negative feedback applied, no negative feedback with non by-passed cathode resistor, or no negative feedback with by-passed cathode resistor. Coming from the 8 ohm tap, the value of the resistor should be 56 K.
     
  2. KCStratMan

    KCStratMan Tele-Afflicted

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    Dave, you are talking about/showing a three position slide switch which takes the 4/8/16 ohm taps from the OT. Rob's example is showing a rotary style switch. Fred is also referring to switching the feedback features for negative feedback or not plus cathode resistor bypass. I like the slide switch for multiple impedence like on later style amps, but not sure about chassis space, maybe a mini you found? I need the same switch x2 for my stereo princeton, it has all three taps I only hooked up the 8ohms temporarily.
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The resistor taps into the 8 ohm wire at the switch but the other end of the resistor is connected to the feedback switch which then connects to V2A's cathode resistor.

    [​IMG]
     
  4. Fred Mertz

    Fred Mertz Tele-Afflicted

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    KC is correct regarding the bypass switch. But the resistor in series with the 8 ohm tap and speaker connection as drawn for the impedance selector switch is wrong. The resistor should not be in series with the speaker.
     
  5. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Robrob has the correct connection.
     
  6. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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    OK it's the feedback switch mod. I have the feedback switch mod in both my amps and will put it in the new one . So right now the 56 K resistor goes from the 3 way switch to the tip on speaker jack. So with a 4/8/16 ohm switch the 56K resistor only wires to the 8ohm leg from OP? If i wire the resistor to the speaker tip regardless of ohm setting the resistor is optional, so is there reason for not doing it that way?
     
  7. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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    .

    Not sure about what type switch I'll use . My thoughts on a sliding switch it would be harder to accidently change. What would be perfect is one that slide with a screwdriver blade.
     
  8. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The 56 K resistor is sized to work at the 8 ohm tap. If the feedback resistor is connected to the 8 ohm lead, rather than the tip on the speaker jack, the negative feedback level will be constant regardless of the output speaker ohms selected.
     
  9. KCStratMan

    KCStratMan Tele-Afflicted

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    Sorry, don't mean to confuse the issue or totally hijack Dave's thread, but I am also interested in slide style switch for multiple impedence plus the negative feedback switch for my stereo princeton. I bought a Carling dpdt toggle switch for NFB cut but did not install it (huge-barely fits), so have a hole near the output jacks already. Thanks, guys!
     
  10. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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    I e-mailed Patrick at MM (Fred) did ask about 325 volts to center tap under load. So I asked what they recommend for PT & OP 4/8/16 ohm taps. I also asked about the toneclone & Fatstack. I also found a smaller rotary switch at Mouser so I ordered one. So when I get a awnser from MM I'll post it.
    I also was thinking when I finish this Amp i can have two with the same Speaker Celestion Blue , same tube compliment and same value components so that should make for a good transformer comparison.
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Make sure both speakers are fully broken in before comparing.
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Dave, here's a layout showing a 3-way speaker impedance selector slide switch and switched feedback:

    [​IMG]
     
  13. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I look forward to your post with Patrick's response. Not to pick on MM, but it would be helpful if transformer manufacturers would provide specifications such as secondary DC resistance or HT voltages under expected load conditions. Providing supply voltages under a no-load condition is meaningless data. It does not provide any useful information.
     
  14. KCStratMan

    KCStratMan Tele-Afflicted

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    Rob, thanks that's exactly the kind of layout I had in mind, that will transplant almost identically to a 5F2A won't it?
    Dave, sounds like that will be a nice one, and yeah your first blue will have a head start on the maturation process. I'm curious to see the size of the small rotary switch, and what are the toneclone and fatstack?
     
  15. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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    Both blues are well broken in. Not my favorite speaker anymore now it's the Vintage P12Q & Clestion Green they sound just the same.
    Rob I understand the wiring now, your drawings are always well done and helpful, I know me and every else appreciate the time and effort you put into it.
     
  16. KCStratMan

    KCStratMan Tele-Afflicted

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    I forgot you scored a vintage P12Q, have you deciphered the date stamp if any mine both say 220 Jensen) 1 (1961) 26 (week 26). How would you describe the sound of the P12Q to the Celestion Blue?
    [​IMG]
     
  17. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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    So I contacted Mercury Magnetics talked to Pat his suggestion OT FTDO-M 4/8/16 ohm taps the PT FTDP 380-0-380. Then he said to buy it threw JMI music I'll get a better price. http://www.jmimusic.com So I called JMI asked more question asked about the Fat Stack PT. So he ended up calling Pat to ask about the FS size. So he called me back I ended buying the FTDO-M OT The PT I went with the FTDP-E-FS. The fat stack has tighter bottom end, more head room and runs cooler. I wont say what I paid it's not about the money it's about end results and when this amp is complete I will give an 'Honest" opinion.
    .
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 5F2A normally doesn't have a cathode bypass cap but you could do the mod as shown for the 5F3A:

    Switch 'OUT' would be normal 5F3A with feedback and no bypass cap
    The center position would be 'mild boost' with the feedback disconnected
    'IN' would be 'heavy boost' with a bypass capacitor and no feedback (5E3 mode)

    You should use your normal size feedback resistor connected to the normal tap (4 ohms I believe for the 5F2A) so you get the recommended amount of feedback. You can get more feedback by tapping the 8 ohm and even more by using the 16 ohm outputs so you can fine tune the feedback to meet your taste.
     
    Last edited: Dec 20, 2014
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Typically output transformers don't 'distort' when overdriven like tubes do. The OT's core will become magnetically saturated and no matter how much more current you run through the primary you won't get more out of the secondary so the signal gets compressed. The louder passages and heavy chords are capped at the saturation point so they aren't that much louder than the rest of the music. This compression is part of the amp's dynamics and character so some people don't like an oversize OT--at least in theory. Mercury Magnetics has sold a lot of upgrade transformers for small amps and people usually rave about the change in tone. If an amp is designed to saturate its OT then a larger core OT will usually add headroom and real dB SPL.
     
    Last edited: Dec 20, 2014
  20. onenotetom

    onenotetom Tele-Afflicted

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    Well put robrob. While my experience is limited I fall into the smaller OT group for tweed tone.
     
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