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Thread: Alamo Paragon amp O.T. wiring

  1. #1
    Senior Member Mars Amp Repair's Avatar
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    Alamo Paragon amp O.T. wiring

    Well,
    Look at this wiring diagram drawn from an originally wired Paragon amp. It kinda makes sense & then again...BUT it does work.

    If you connect your load the 'rear jack', you'll need to measure output with a dual trace scope in the add mode, owing to the fact that both + and - speaker leads have signal on them.

    The 6L6 cath resistor connected to the black (hot) side of the output transformer is rather perplexing. It is also the negative feedback tap, too. the cath resist is not bypassed & I guess as this is set up, it really couldn't be as it would shunt the output transformer secondary. I suppose if someone were to do a network analysis on this it might make more sense.

    this information just FYI for anyone who might encounter one of these amps, as there doesn't seem to be a schematic to be had for this amp anywhere.
    the rest of the amp is pretty much just a tube amp you can find your way through like any other.
    the issue with this one was leaky coupling caps to the outputs as well as a few in the preamp that as the amp got hotter, would leak more & cause distortion.

    Pardon my rough drawing. I have to use Excel & convert it to a PDF.
    glen
    Attached Thumbnails Attached Thumbnails alamo-paragon-2x12-cab.jpg   alamo-paragon-front-panel.jpg   alamo-paragon-underside.jpg  
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  2. #2
    Senior Member Mars Amp Repair's Avatar
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    Oh yeah,
    It appears that the 'Extension' jack on the front panel 'shorts' out that winding...& I guess it does, but the amp delivers full output with either the "int' jack or 'ext' jack or both, which puts them in series for 16ohms. Weird, huh...g

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    Senior Member Enzo's Avatar
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    Might look more familiar drawn right side up.

    I am not sure looking at the drawing, but does the rear jack just stick the speaker plugged into it in series with the other? That would explain both sides "hot."

    Plug the amp into an isolation transformer, then you can connect the scope ground to that jack if you want.

    Do I read right, that 400 ohm is the cathode resistor for the output stage, but it goes through the OT secondary to get to ground? That would be another form of NFB. Some amps had a special winding just for that purpose instead of using the speaker winding.

    Hey Glen, you got a scanner? Draw on a sheet of paper and scan it, if using excel isn't getting you where you want to be.

    If you bypassed the cathodes to ground it might shunt the output, but what if you bypassed across the 400 ohm ONLY? Seems that wouldn't shunt the output. But I am just guessing.
    Education is what you're left with after you have forgotten what you have learned.

  4. #4
    Senior Member Mars Amp Repair's Avatar
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    Yeah,
    Excel works pretty well...it's just timely as you have to hand draw much of the componentry. since the new version allows you to save it in a PDF form, I can upload it to the forum.

    Hand draw? With my drawing 'abilities', that would basically mean illegible ;-]

    I see where the cathode resistor (400ohms) connected to the speaker output would essentially act as a type of negative feedback, but there also is a neg feedback path to the PI off that same 16ohm tap...I just didn't bother to trace that out...so go figure.

    It is weird that they can short the 8ohm winding of the xformer with no ill effects, but it delivers full power into an 8ohm load from the 'rear' jack'. If you use the rear jack w/8ohm load & the 'extension' jack (front panel) with 8ohm load, yes you get the 2 speakers in series across the 16ohm tap hence the floating speaker neg.

    I'm think you might have meant isolating the 'ground' on the amp (or scope), but still I'm not certain that isolating either the amp or the scope from ground would provide the results you mentioned. That would mean that the scope 'ground' would have to be varying with transformer output. would make me nervous.

    It's an easy affair to just use both scope channels & 'add' the waveforms for a sum of both like we do on the IC driven power stages where the common of the speaker is 'hot' as well as the positive.

    I'll see about drawing the neg FB loop for grins. This schemo just informational for whomever needs it. as I mentioned, there doesn't seem to be a schemo anywhere for this model....thanx, glen

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    Old Timer Gtr_tech's Avatar
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    Heh....noticed the little Dan-o amp there....Nifty Fifty? I have one of those I use for a bench test amp.
    The farmer takes a wife, the barber takes a pole....

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    Senior Member Mars Amp Repair's Avatar
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    yep,
    we use all the amps that have either been exchanged (ie:Fender G15's & such) as small speaker monitors when you need the sound right up in your face.

    Also when running an amp up on a load, we put a 470ohm resistor in line with the small bench speakers so we can monitor what's going on at a tolerable volume level...g

  7. #7
    Senior Member Mars Amp Repair's Avatar
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    GEEZE!
    I'm definitely working too much & too hard. the extension jack does not short the 8ohm winding. the jack switch in that jack merely 'forwards' the 8ohm tap to the 'internal rear' jack when only the rear jack is used. Then if only the 'extension' jack is used, that gets the 8ohm tap, too.

    If both are used as mentioned, you get both speakers in series to 16ohm tap.

    Vaca coming up in July...haven't taken a vaca for 5yrs while buiding the biz...man do I need it now! glen

  8. #8
    rf7
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    I worked on one of those a while back. If I remember correctly, it had Ultralinear taps on the OT. Is that one like that too? I actually drew up a schematic at the time. I'll see if I can find it again. Is that cab 2 12 inch speakers or 2 15's?

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    Wow. It's my old amp, or its twin brother.

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    Senior Member Enzo's Avatar
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    Glen. No, I meant isolating the amp. Running on iso, NOTHING in the amp is referenced to the mains ground, so any point in the circuit can be connected to the earth. Imagine a portable transistor radio or walkman. it sits there running with no earth connection. You could connect a wire to earth to any ONE point in it without affecting it. Same deal with scope. That is how you can scope out some little low volt supply in a switcher that is refernced to the -170v rail.

    What is happening in some cosmic sense when signal flows is this: Your scope ground connected at a hot speaker terminal references the whole amp to that point - as far as the rest of the world is concerned. The "ground" WITHIN the amp would then be bouncing up and down to the signal, but only with respect to the scope. The amp itself would not notice any difference.


    I use that to explain those power amps with "grounded output" and the speaker connected in the filter cap return. They are really no different from conventional amps, just the grou d is connected to a different point than usual. I suggest to them that they look at it as if there were no ground at all, and it becomes apparent the same loop of power supply, drive circuit, speaker exists in both systems. Grounds are wherever you make them.
    Education is what you're left with after you have forgotten what you have learned.

  11. #11
    Senior Member Mars Amp Repair's Avatar
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    I hear ya on that. It's just easier to use both probes & the 'add' function.

    Here's the Isolation station I built for my techs when working on switchmode stuff. I just got lucky & had this toroidal xformer I saved from some monster SS amp that had secondaries that were about 117V.

    thanx, g
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    Senior Member Mars Amp Repair's Avatar
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    yep, screen taps on OT.

    I kinda drew that in the attached diagram. that would be cool for all to have a schemo on this amp. thanx, g

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    Senior Member hasserl's Avatar
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    Great sounding amps BTW. Also uses grid leak biasing on the input stages.

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    Senior Member Mars Amp Repair's Avatar
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    Grid leak bias?
    I always understood that the sole purpose of the 'grid leak' resistors was to merely bleed off the excess electrons that accumulate on the control grid and hence tend to build up a negative charge on the grid as a matter of just being in the path of the electron flow.

    They install them in high impedance input circuits where there is no other path to ground that might also serve to do the bleeding off, hence the many circuits that do not have a specific 'grid leak' resistor. They really don't 'bias' the stage per-se, just keep the stage from buiding up a negative voltage on the control grin there by cutting the tube off.

    The 'biasing' of the input stage is performed by the cathode resistor & to some extent the plate load resistor value.
    glen

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    Senior Member hasserl's Avatar
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    Grid leak bias inputs use no cathode resistor, the tube cathode is connected directly to ground. The bias voltage is supplied by grid conduction during the positive swing of the input signal and from input cap discharge during the negative swing. Basic Electronics - Google Books

    Fender used this type of input on some early tweed amps (see 5C5). Other companies used it as well, like this '58 National: http://www.valcoamp.com/files/Nation...2_6X11_amp.jpg

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    Old Timer Gtr_tech's Avatar
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    Silvertone and Valco used grid leak input stages on some models too. I can't stand it myself....but thats just me. Breaks up too early with a hot input and sounds kinda nasty when it does. You'll see this type of input stage used alot on old tube PA amps for the mic inputs.
    The farmer takes a wife, the barber takes a pole....

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    Senior Member Mars Amp Repair's Avatar
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    Ahhh cool...and I have the bloody RCA Receiving tube manual to boot!

    Grid leak bias must be used exclusively when the cathode goes to ground...only reason I can think they'd do this is to save on components or just a cheap way to design a hi gain low signal-in input stage. There also would have to be a coupling cap the way they describe it to provide a charge to build up the bias voltage from the input AC signal.

    The true 'grid-leak' (not bias) resistor as I described it will be seen in a ckt with cathode bias & a high impedance input. You can also see the 1meg resistor to ground on the control grid of interstages that as I recall helps to keep the plate voltage from the previous stage from charging up the coupling cap & causing unwanted bias on the control grid.

    I'm just recalling from my tube training back at RCA Institute of Electronics in NYC from about 1973!! Thanx for the info....g

  18. #18
    Senior Member hasserl's Avatar
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    Reduction in parts is not the only reason, tone is also. Gtr tech may not like it, but plenty of other people do. It is a subjective value and individual preference is the only thing that matters.

    Re the Paragon Bass, it's a great sounding amp IMO. I recall one evening here in my garage I had a fellow guitar player buddy over and we were just playing thru a bunch of amps, making a lot of noise and generally having a really great time at it. At one point we connected my Super Reverb and my Paragon Bass amp up thru an AB pedal and we sat and spent a lot of time just switching between the two amps, and we both ended up preferring the Paragon Bass. Everyone knows how good a Super Reverb is, great amps for sure. But in that situation, at that time and place, the Paragon Bass was the better amp. Better cleans, better overdrive, more interesting tones, more easily controlled with the guitar volume, just better overall. And that was with an old amp in need of service and new caps, 3 prong cord, etc.

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    Quote Originally Posted by rf7 View Post
    I worked on one of those a while back. If I remember correctly, it had Ultralinear taps on the OT. Is that one like that too? I actually drew up a schematic at the time. I'll see if I can find it again. Is that cab 2 12 inch speakers or 2 15's?
    I wouldn't say no to you posting a schematic. I've got one of these, or something very similar, it's an Alamo, no model name, similar guts and tube complement. It's suffering from the dreaded red plating, on just one of the power tubes. I'm about to recap it, as the caps look original.

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    Does anyone have a copy of this schematic? It doesn't appear to be accessible anymore.

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