Please help! How do I find the center tap of an o/p tranny?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tommyfobia, Oct 3, 2005.

  1. tommyfobia

    tommyfobia Member

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    Hi I have an old handwired 50watt class A/B head, which seems to be a very well built 'homebrew' amp. Inside the amp says that it was made on the 17/3/76.

    As with quite a few 70s amps I have come across, there is no standby switch. To increase the tube life of my amp I would like to install a standby switch, to ensure that the cathodes get chance to heat up before the tube is in operation.

    I have done a bit of research on the subject, and I found a Q&A session with Mike Soldano, which he suggests that you find the center tap of the primary winding of the output transformer, and put a switch along that wire to disconnect the center tap and power supply to the primary winding.

    So how do I find the centre tap of the primary winding of the OP?

    Thanks in advance!:)

    *edit*: From what I understand, the primary side of the transformer, is directly connected to some big caps next to the power transformer. Is that right?
     
  2. Wakarusa

    Wakarusa Member

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    The primary may have either three wires or five wires (the latter being an ultralinear setup). In both cases, all but one will make their way to the output tube sockets (two to the plates, two more to the screens if it is ultralinear).

    The one that doesn't go back to the output sockets is your center tap.
     
  3. loverocker

    loverocker Member

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    Hi Tommy - please don't kill yourself! This is an area where guesswork is a bad idea - especially on a homebrewed amp that might have an unusual wiring scheme.

    Do you have a multimeter, and do you know how to discharge the amp's filter caps (the "some big caps" that you mention)?

    Wakarusa is correct, but you must be cautious.

    Can you upload pix?
     
  4. tommyfobia

    tommyfobia Member

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    Thanks for the response guys!

    Yeah it is the wire I suspected. My tranny only has three wires on the primary side, so that suggests it is not ultra-linear.

    The amp has not been plugged in/switched on for a good six months now, due to the fact that I have quite a few amps to noodle around with, and I've only just got around to retubing it, so I assume the filter caps are fully discharged!

    The only way I am aware of, to drain the caps is to switch the amp off, but leave the standby on, :p which isn't exactly the best way to do it in this given situation:D. Any details on another way would be greatly appreciated!

    I am VERY careful inside amps, and never even attempt anything without adequate preparation, and talking it out with people who are more knowlegable than I. (Other wise I would have wrecked quite a few nice amps!)

    I do have a multimeter which I have left at my rehearsal space (doh!), which I will not be able to pick up for a few days. I understand that the resistance between the center tap and each of the other two wires should be pretty much the same, as it is directly in the middle of the coil, so I will check that when I pick up my meter.

    I am still learning, so please be patient with me, but I have another question:

    Between pins 4-6 of each power tube, I have a pretty mean looking resistor (which I assume will regulate the negative voltage of the grid). The amp doesn't have an adjustable bias pot, so I assume these resistors do the same job. Am I correct?

    If I am correct, and I were to retube the amp with E34Ls for a bit more headroom than EL34s, would I need to change the value of these resistor? If so, what would I replace it with, to keep the bias at a safe range?

    EDIT: I will post pics as soon as I can!
     
  5. Wakarusa

    Wakarusa Member

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    1,477
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    1) NEVER assume a capacitor is discharged unless you have discharged it yourself (think of them the same way you'd think about a firearm -- there's a bullet in the chamber until proven otherwise).

    2) The best way to discharge caps is with a simple shorting probe. In a pinch a straight wire with insulated probe/clips on either end will do, though it is STRONGLY preferred to have some series resistance in there. There's an old thread around here somewhere that goes into gory detail on the topic.


    3) Assuming that you've got octal sockets set up for 6L6/6V6/EL34 or similar then the ominous resistor between pins 4 and 6 is the screen resistor. Pin 6 is disconnected in the tube and is being used here for a tie point. (the tube's grid, fwiw, is on pin 5)
     
  6. Wakarusa

    Wakarusa Member

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    Oh yeah... screens and screen resistors have nothing to do with the bias (also sometimes called C-) supply in the amp.

    While I encourage folks to learn about working with tube amps (the more the merrier), I would highly recommend becoming familiar with the fundamentals before poking around inside -- certainly before picking up the soldering iron.

    Which pins do what (and how to find out for different tubes) would count as absolutely basic information.
    I'm starting to agree more with loverocker in wondering if you should be in there in the first place.
     
  7. Enzo

    Enzo Member

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    MAny many output transformers follow the color code:

    OT Primary - Blue and Brown wires to plates, Red wire is center tap. UL wires will also be blue or brown with a white stripe.

    So if your plate wires are blue and brown, the red one is the center tap. You can verify this with an ohm meter. The three leads when measured to either of the other leads will either show the resistance of half the primary or the whole primary. The two wires with the highest resistance between them will be the plate leads, while the one with half that resistance to each of the other two wiers will be the center tap.
     
  8. loverocker

    loverocker Member

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    +1 on never assuming a cap is discharged. Here's a page on discharging caps: http://www.ampmaker.com/se5aguide/discharge.asp

    Be aware that on a homebrewed amp, the maker could have done something 'unusual'. For example - put one cap in a position that doesn't get discharged at the same rate as the others. So be prepared to trace the B+ through the whole circuit. If it's a true PTP board - looks like a rat's nest - you've got to be doubly careful with your fingers.

    I think you may be OK with the E34Ls with the amp's bias circuit as-is. But first, you'll need to locate that (real) bias pot :)
     
  9. tommyfobia

    tommyfobia Member

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    Thanks for all the replies.

    I am still learning (and I have a LOT to learn), but as I said, I never touch an amp without talking it through with people who are much more knowlegable than myself. I do not have a gung-ho attitude to such things, so until I am ABSOLUTLEY sure of what I am doing, I won't crank up the ol' soldering iron.

    So now I know its the SCREEN resistors that look a bit burned. (Thanks for that.)

    There is no bias pot on the amp at all, but the wires from pin 5 of each tube dissappear underneath the board (which I will not be unscrewing until I have made sure those caps are drained). So I cannot tell you what happens to the wires from then on.

    Thank you all very much for all your help, so far and especially for your concern about my health. Least of everyone here, do I want me to get zapped!

    I'm sure I will post many other threads in the same vein as this one, but please be patient with me, I still have my 'L' plates on!

    This really IS the most helpful place on the net regarding amp electronics. Thanks again fellas.:)
     
  10. loverocker

    loverocker Member

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    :)

    If that's a typical 2xEL34 amp, it may use the well-proven layout that Marshall used. Is there a part of the board at the EL34 end that looks like this:
    [​IMG]

    This is for a fixed bias 50W Marshall, and in the lower part of the 'X' shape in the middle, there are a pair of 220k Rs that are joined together. Tracing this connection onwards (the green wire that goes up and to the right, leads to the bias circuit, part of which is just about visible on the edge of the photo.

    Typically, you'd be looking for a diode, pair of caps, couple of resistors and a board-mounted pot - like those you can see in the the full-board photo (the two bright blue caps on the left):
    [​IMG]

    BTW, are there any clues to the maker of the amp? Logo, etc? There were quite a lot of "me-too" EL34 amp makers inthe '70s. Be interesting to know more. As you're in Leeds, it might be an early "Music Ground" original. ;) :eek:
     
  11. Killa-B

    Killa-B Member

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    The way I drain the caps is to make a jumper wire from a piece of insulated wire with a "gator" clip on one end.

    I attach the gator clip to pin 1 of the first preamp tube (usually the one furthest from the output tubes), and simply short the other end of the wire to the chasis for about 1 minute. Keep one hand totally away from the amp until you are sure the caps are discharged.
     
  12. Wakarusa

    Wakarusa Member

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    Heh.. not a bad toy to have laying about for tech "how to" photos ;)

    I think we should scare him off with pictures more like this:
    (the bias adjust is the wee board left at the bottom...)

    [​IMG]
     
  13. tommyfobia

    tommyfobia Member

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    Yeah, there is a bit of the board that looks very similar to that! No board mounted pot though. :( The amp has a brilliant channel and a bass channel. There are two inputs per channel. (I assume hi and low impedence, but I haven't checked it with the multi). Each channel has volume, treble and bass controls.The amp is a head, but it is mounted in its box upside-down, like if it was a combo. The tolex is orange, and the only logo is near the power switch, which reads 'Multi-Purpose 50'. I believe it originally came from wales.
     
  14. tommyfobia

    tommyfobia Member

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    Erm.. Where???:p
     

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