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Remember the Alamo?

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  • Remember the Alamo?

    This is kind of a general What do I do first? type post.

    I'm a relative newb to valve/tube amps, but have been doing electronic repair professionally for 17 years on military and medical test equipment, and as a hobby for years before that. I've worked on a few tube amps for friends, where it was just some simple stuff there's not a lot of tubes used out there in my field except for CRT's and X-Ray tubes, but I finally decided to get my feet wet, and get my own tube amp to work on.

    I found an old Alamo amp, I think it's a Paragon Special, but the interior label has been torn off, it's got 4 inputs, two channels a Normal and a Bass, a jack for the speaker cab, and another external speaker jack (neither of which have impedance marked on them). It has the following tube complement:
    • 2 - 12AX7
    • 1 - 7199
    • 2 - 5881
    • 1 - 5U4

    It also appears to have 3 transformers, the power transformer, the output transformer (from which the speaker outs are connected) and a third transformer, about size with the output transformer, which only has 2 wires running into it, so I have been told is probably just a big choke.

    The power cord had rotted away, and I replaced the 2 wire cord, with a 3 wire one, which I have been informed by our local amp guru is a bad idea, as the Alamo is a hot chassis type amp, or at least as I've noticed, grounds everything to the chassis. I had bypassed the Norm/Rev switch which connects either Live or neutral to ground via a cap, supposedley for noise purposes, and followed the original tranformer wiring by hooking live to the fuse and thence to the transformer and connecting the neutral to the power switch, which goes to the other side of the transformer, and then earth lug to one of the bolts on the power transformer to the chassis. The amp powers up, makes noise when played, needs a lot more cleaning as the pots are scratchy, and the connectors gunky and intermittent. So questions I have.
    • What do I do about earthing this beast?
    • Any other safety mods I should consider doing?
    • Whats a good way to figure out the speaker impedances I should be using for the output transformer?
    • Any good reference books for someone like me? I've been thinking of getting the Tom Mitchell book, I read it before and thought it seemed quite informative.
    • Anyone have any specific info on this amp?

    Thanks in advance, any and all suggestions and info gratefully received.

  • #2
    This isn't a "hot chassis" amp, 'Dub, though I'm sure Alamo made a few. The "hot chassis" type of design does away with the expense of a power transformer by powering the amp directly from the AC power line, much like an old "AC/DC" table radio from the fifties.

    "If you're the only person I irritate with my choice of words today I'll be surprised" Chuck H.


    • #3
      Originally posted by tboy
      This isn't a "hot chassis" amp, 'Dub, though I'm sure Alamo made a few. The "hot chassis" type of design does away with the expense of a power transformer by powering the amp directly from the AC power line, much like an old "AC/DC" table radio from the fifties.
      Ah, I thought I must have misunderstood the guy. He asked me about the tube complement, and then when I told him said, maybe not, and proceeded to tell me about the type of amp that would power the tubes straight from the AC, running the heaters in series. I thought this was just a side story he was telling me, doh.

      So it is safe to earth the panel then?


      • #4
        Yes, it is. BTW, I picked up a Paragon Bass amp last year. I haven't had a chance to do anything with it yet, it still has the old two wire cord attached. It's in need of new filter caps and freshening up. It's a little different from yours, it uses three 12AX7's in the pre amp, along with the two 5881's for power. Mine has a couple of new Ruby 6L6's I think, and a couple of the 12AX7's are old Bugle Boy's. Though it is in need of freshening up the tone is fantastic, I know that when I get around to it I'll end of with a fantastic amp.

        The cab is a large bass reflex style with a single C15N, Jensen, just great tone! Goodf luck with your amp, let us know if you need more help getting it going.


        • #5
          Well, I am thinking about drawing out a schematic for it, I thought it was going to take a bit of work. The Bass channel was working A-OK, but the Normal channel was making all kinds of weird sounds, and not only didn't amplify the input signal very much, but actually quieted the signal on the other channel too.

          In case it was just a bad preamp tube, I swapped the 2 12AX7's for each other, assuming the problem would maybe just jump channels (assuming they use one tube per channel and not half of each tube) but it didn't make any difference. The problem stayed with the Normal channel. However I left it on Standby out on my porch today , and it was quite a warm day, and suddenly when I was about to show a friend the problem with the Normal channel, it had gone, could be it's temprature dependant, or a bad solder joint somewhere, or maybe the amp just likes to warm up a lot.

          I looked at the schematic for the Alamo Montclair on Schematic Heaven, and it has a similar tube count, but has an extra tube in it for the reverb circuit, and uses the 5Y3 and not the 5U4 for a rectifier. Oddly it shows 3 triodes on the normal channel, and only a single on the bass channel, or maybe it's the other way about as it doesn't label the channels. I know the Bass channel is a lot meatier than the Normal channel, not just louder but has a fuller sound (I play bass btw).

          Mine looks to have been a closed box 15" unless your reflex port is on the back. The original Jensen driver has been replaced by a very interesting looking vintage driver with what looks like a field replaceable basket held in place by three large slot headed screws, it's obviously thicker than the original driver as the back panel has been cut to allow the magnet to fit in the cabinet, so my box is now also a 'reflex' box. It does sound good though, the Treble and Bass both do boost and cut, or so it seems, not sure how you do that with two tubes and two channels. I think possibly the tubes are the originals, as they are all Tung-Sol's with similar numbers on the base, and the tube type stamped on the glass (which idiot I am wiped off with Windex). My dilemma is whether to replace the original tubes or leave them the heck alone, I probably will recap it as a matter of course, but I haven't checked how bad the ripple is on the supplies as yet. So far all I've done is clean up the tube sockets (which seemed pretty clean), clean up the input jacks (which needed it) and the controls (which were scratchy), fit the 3 wire cord and bypass the Norm / Rev switch a cap to neutral switch. It sounds real nice to me, I should maybe find a guitarist and see what it sounds like on higher tones as well.

          Depending on my crappy digital camera, I may take some photos, or I may just not bother.

          Slight side issue, which may have been covered elsewhere on this forum, whats a good way to clean up tolex, and also the grille cloth? I have mud spatters on my grille which didn't sponge off.
          Last edited by LowBassDub; 06-26-2006, 02:49 AM.


          • #6
            Re. tubes & filter caps - It would make sense to replace them anyway, they don't cost a fortune. You could always A/B the new tubes against the originals to see if you could "get away" with them.

            Irrespective of the amount of ripple hum, replace the filter caps. Old caps just sound mushy & dull.

            Regarding speaker impedance, if your original speaker is still fitted, disconnect it and measure the DCR between terminals (power off of course, reconnect speaker before powering up). This will give you an idea of the impedance. E.G. an 8 ohm speaker may read from 5.5 to 7ohms (slightly less than rated impedance).

            If you don't have the original speaker fitted, then you need to apply a voltage to the OT secondaries and use 2 meters to establish impedance...see the "SF Deluxe Reverb" thread for more.


            • #7
              THis is what my Paragon Bass looks like:
              (looking at the pics I see the power tubes are not Rubys but Fender GT's I believe, the Rubys must be in another amp. Maybe the Super Reverb).


              • #8
                The layout of mine looks much the same as yours, in terms of the tubes, do you have what looks like 3 transformers but is actually power, output and a choke of some kind on yours?

                Does that seem like the original driver, what impedance is it?

                Mine is all very much of a mystery, the driver is almost surely non original, and is 8 ohms. I'd like to be lazy and not have to figure out the correct impedances the hard way, but that may come. Is there an impedance stamped on your driver? Any idea about the impedance for the Ext Speaker. jack? I have a part number on the output transformer of 549-6139 and the transformer type choke has two numbers C-1383 and ATC-142.

                Mine is a smaller cabinet, and is oddly designed where the head is meant to face back and the cabinet forward, as the speaker jack and the power cord come out what looks like the front of the head, and the back is just plain grille cloth to match the speaker. So you play it with the speaker facing away from you and the control panel facing you.


                • #9
                  I'll have to dig it out, it's kind of buried back there. I'll try to do that this afternoon.


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