"odd" cap can?

Discussion in 'Amp Tech Center' started by marshman, Nov 3, 2014.

  1. marshman

    marshman Friend of Leo's

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    Meter sez that the black wires are ground. Glad that is settled.

    I went back and re-read the thread I started when I bought this little gem and started wondering again in muchxs still has that cab. I dunno where in NE muchxs is, but I have to drive missus marshman to Parsippany , NJ tomorrow night and could try and arrange a meeting for Thursday if it's not too far.
     
  2. marshman

    marshman Friend of Leo's

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    Well, it's alive...still very hissy, a few pops thrown in for good measure, but it's sitting on my workbench under a flourescent light with my cheapest guitar plugged in and the tubes I would miss most if they blew up. I'll try to get a new, 3-prong power cord installed and put some decent tubes in it and see what shakes out.

    Any thing I oughtta check with my meter before I get too busy pattin' myself on the back?

    Thanks for the help.
     
  3. Wally

    Wally Telefied Ad Free Member

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    MAarshman wrote: "Meter sez that the black wires are ground. Glad that is settled."

    You thik you are glad?? I mentioned/asked about that 3 or 4 times, at least.

    That meter is a trip, right? Now you have learned where the center tap for the B+ secondary..probably--is. The least you have learned is where the ground for the at capacitor is, right? So...now you know how to build that parallel 50/50 into a single 100mfd cap, right?
    And....you have learned that without a doubt the B+ on that amp is gonig to be that 600V+ number...I don't know why they show two different voltages there---660 adn 620, IIRC----....some variation on the theme, I guess.
    That 330V figure you have been looking at are the voltage supplies for the screens.....way low compared to the plates. That preamp votlage is way low on that schematic....outsdie my realm of expereince. My '52 Pro has maybe 60V to the plates of the preamp tubes, and that was shocking to me.
    Is this a bass amp, by any chance?
     
  4. marshman

    marshman Friend of Leo's

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    It was built as a bass amp, and came loaded with 12AU7s through the entire preamp. Headed out of town for a few days, will have to get back to it when I return. Thanks for all the guidance.
     
  5. marshman

    marshman Friend of Leo's

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    So, we're back on the job. Busted out the multimeter and took some measurements. Tubes are randomly selected from my stash of lightly used but removed for being cheap-a$$ junkers-matching unlikely. The amp is a bass amp, but I played guitar through it for a few minutes before beginning the measurements...aside from a bit of scratchiness, it sounds ok. All the pots are set to the middle of their range. One hand is in my pocket at all times.

    AC at the socket:119V
    AC in the chassis: it's odd...I get 60VAC at the PT, and 11VAC where the other line meets the switch.

    Power tubes:
    pin4: 290VDC
    pin5: 8.3VDC and 5.3VDC (6L6GC reading 5.3 had very slight redplating)
    pin 8: 3VDC
    I expected to get readings at pin 3, but didn't.

    OT shows 385VDC coming into the center of the primary.

    The 10/50 cap in the amp was replaced with a Sprague 25/50, and reads -41VDC. Schematic suggests it should read -32.

    The 22/350 in the amp was replaced with an F&T 30/500, and reads 250VDC. Schematic shows 240VDC.

    The 50/150 was replaced with a JJ 47/500 and shows 389VDC on the + end. For some reason, it shows -284 at the other end, which doesn't sound right to me.

    The 100/350 was replaced with a JJ can, 50/50/500 and reads 275VDC.

    My heaters read 2.8VAC, which I presume is @ 5.6--seems a bit low.

    Any thoughts?
     
  6. Wally

    Wally Telefied Ad Free Member

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    Marrsh, on that schematic, there are these 'nodes' for votlage supply...B1, b@, B3, and B4.
    B1 is you B+ voltage to the OT...and should be 660V.
    B2 is the screen voltage suply...330V....pins 4 on the power tubes. Your voltage is a bit low there. That is half of that B+, and you can see in the schematic that it derives its votlage at the connection between those two 50mfd/350V caps that are in series.
    B3 is the voltage supply for the PI and tthe next filter cap...a 50mfd/350V cap..and thereby to the 12AX7(V2). B4 follows that 10K power resistor and feeds the 10mfd/350v filter cap that feeds the preamp 12AU7.
    I am going to make this observation. Either 1) that schematic is not for your amp or 2) someone has been working on that amp befroe you got it, and it doesn't 'mirror' the schematic any longer. I vote for #2. The telltale sign is that low B+ on the center tap of the OT.
    I am thinking that the 100mfd/350v cap was incorrect----by error of previous work.
    Do this....check the voltage at the connection point between those first two filter caps you have installed...the 50mfd/350v caps that are---or should be--- in series. IN fact, check that you do have them in series...and that you do have them oriented correctly....which I think you do...one of them at least. The first cap has a negative pole to ground and the positive pole tied to the negative of the second 50/350 cap. This point is where the B2 voltage should read something like 330V....and this voltage feeds the screens--pins 4---on the power tubes....330VDc. You are reading 290VDC there, right? The voltage at the other-- positive-- end of that series arrangement should read something like 660VDC...this is the B1 or B+ feed to the OT's center tap. YOu are reading 385VDC here...adn that isn't correct...according to the schematic. This arrangement is why that single cap in that 'old' can...the 100mfd/350V that you showed us on page 1....cannot be correct for the schematic that you show us....and I think that you have the first two caps..tfhat series arrangement in the schematic...wired incorretly for the B1 and B2...you have the B2 but you dont' have the B1. IF you have it wired correctly then.......teh OT is a quesiton...read on..

    As for why you dont' have any plate votlage even though you have votlage--incorrect though it is...at the OT's center tap???? You may have a bad OT...due to the bad work that was done before. IT is hard to evaluate this from afar...but I think that my crude analysis here may be correct.
    to check that OT...disconect the primaries at the pins 3 of the power tubes and check for resistances....center tap to each end and end to end. The two measurements from center tap to each end will be fairly close but not exactly equal to each other. The end to end---primary to primary...should equal the total of those two measuremetns from the center tap to each end. IF not...bad OT.....adn you cna think whoever left that low votlage on the pins of the power tubes. Low plate voltage equals excessively high current draw...which can equal component failures.

    that 660V B+ got the attention of many of use when we first saw it...but we have to believe the schematic, right? And....that schematic shows two separate points of contact for the B1(B+) and the B2 voltages. One cap cannot handle those two jobs as the schematic shows us.....so that old 100mfd cap can was not 'correct'....or something else is not 'correct;'...like the schematic...for this amp.
     
  7. marshman

    marshman Friend of Leo's

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    As to the things I can address regarding the schematic...the amp chassis is stamped for 3 12AU7s and a pair of 6L6GCs, but the schematic has been tampered with at least a bit, as the 12AX7 shown is in a different font, as if it'd been white-out-ed and typed over.

    2 of the installed caps were out of spec with the schematic, either the voltage rating had been changed or the capacitance. As to the 'improper' cap values, I have tried to replace all caps with slightly higher value caps than listed in the schematic, which, as noted, is our baseline, right?

    I do get continuity between the 2 tabs I've connected on the 50/50 to the neg lead of the 47/500 next to it on the board. While the amp was cool, I switched the power tubes around to see if the redplate followed the tube (it did, though less effect) and now I do get 330VDC on pin 4 and the neg end of the single 47/500V cap. The positive end now reads 442VDC (which sounds much better, though it doesn't match the scematic)

    I wondered if the "660V(620V)" and similar markings were to reflect wether or not the 100V or 115V PT connections had been used. The 115V is the one that's attached to the power cord.

    The OT halves measure 116 and 106 ohms on both halves and 222 all the way across. I didn't disconnect anything, as the tabs are accesible on top of the chassis.

    To add insult to injury, the switch is now acting up, 'jiggling' itself in and out of connection!

    I have a few photos of the state it was in when I purchased it, if anyone wants to see 'em.
     

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  8. Wally

    Wally Telefied Ad Free Member

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    I erased a long post. There are situations here that would be rather quickly checked if an experienced tech had this amp on the bench. IMho, this may be beyond a learner's
    ability. Imho, this amp was not correct or original when you opened it up. That 100mfd/350V cap ius the proof.....or I am not understanding anything at all about this amp. IF I am correct in this thinking, then you have the B1 miswired at that 50x50/350 cap.
    You also have problems with the bias voltage. I don't understand why you dont' have a votlage reading on the paltes....adn I don't understand how you can have a redplated
    6L6 is you don't have some kind of votlage on the plate.
     
  9. marshman

    marshman Friend of Leo's

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    I'm inclined to agree at this point, Wally. The last list of thoughts was just beyond my grasp, and I managed to limp through it without understanding what I was looking at or for. I did play it for abobut 10 minutes and it sounds ok with guitar through it (bass was shaky, but I don't have a bass cab handy) except for the shaky power switch...at least I think it's the switch, as it comes back on if I wiggle the switch.

    Anyway, I think at this point I'm just going to rustle up a new power switch (up-on/middle-off/down-standby) and then see what it does. Fortunately, I have a pretty good tech within 10 miles, if i can't sort it, he certainly can.

    Thanks for all the tips, hints and pointers, folks. And I definitely owe Wally a beer or a coffee, your choice.
     
  10. Wally

    Wally Telefied Ad Free Member

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    Well....I may be correcton this one...or I might be misunderstanding thigns. IF I had the amp, I would know for sure which of those two things I am on this one!! Let us know what goes on and what makes the amp work.

    And...if you are getting sound out of that amp, then there is voltage on the power tubes' plates...pins 3.
     
  11. marshman

    marshman Friend of Leo's

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    I know that there's voltage there, I Just can't get a measurement...it's as if the decades of airborne mung has created a layer of non-conductive film. I really had to wiggle the probe around on the OT tabs to get resistance readings there, as well.

    I also noticed I have no specs for the secondary impedance on the OT...I had a 16ohm cab hooked up at first, and it sounded a little dull. When I hooked up an 8ohm cab, that changed a bit, though there were other things that changed as well.
     
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