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Thread: Flea market find

  1. #1
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    Flea market find

    I went to a hamfest/flea market yesterday and stumbled across a stereo amp that looks like it came out of a console. It's a Columbia "stereophonic" but it is a circuit I've never seen before. I'm sure some of you have so maybe you can enlighten me. It has two 6BQ5s, a 12AX7 and a 6CA4. Close examination tells me the OT is a dual transformer in one, as each side has three wires like the attached schematic has. I can't find the right schematic but this one is very close to the layout of the one I have.

    It looks to me like the separate sides run in a sort of push-pull configuration but in separate output circuits. I say "pseudo" PP, because it only uses the one transformer, but electronically it can be separate signal paths, right?


    chassis.jpgot1.jpgot2.jpgot3.jpg
    Attached Thumbnails Attached Thumbnails stereo-amp2.png  
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    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  2. #2
    Supporting Member Chuck H's Avatar
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    I've read that it's possible to create a stereo signal in a single OT with the CT secondary arrangement. I never gave it much thought because, well, how often would we use this for a guitar amp? But the schematic is pretty clear. Has to be strictly class A with that arrangement I expect. And strictly a low enough power tube grid input that the bias doesn't shift at any point. Pretty useless for a guitar amp as it's wired I think. The PI uses grid leak bias!?! You could probably test the ratios and run the formulas for figuring impedance on the different secondary taps and figure out how to wire the OT for mono use in push/pull. It'll probably handle it. The tube compliment is good. Since you have a 6ca4 (probably on the same filament supply?) you can change to a diode rectifier and add another 12ax7 to facilitate a more typical "guitar amp" topology. The voltage is probably somewhat low now because of the careful, strictly clean class A arrangement so the diode rectifier may not cause a high voltage problem. If not you can always drop the voltage with zeners.

    Neat design to see and probably a good find for a conversion
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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  3. #3
    Supporting Member Jazz P Bass's Avatar
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    I think the 'trick' is the secondary coil going to the CT of the output transformer secondary.
    That and the NP caps.

    I would sell the amp long before I gutted it.

    It is probably a really nice sounding low power amp.

  4. #4
    Supporting Member eschertron's Avatar
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    Back in the days of vinyl recordings, the stereo encoding made it difficult to get a phono needle to track phase-variant low frequencies without the needle flying out of the groove
    So if the phase-variant (stereo) information is limited to high frequencies (like most live space reverb is) then I'd wager the electrical limitations of the OT are inconsequential when conservatively-mastered vinyl recordings are played back. All bets are off in the digital age!
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  5. #5
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    I have no intentions of using this as a guitar amp, but I have been wanting to build a tube stereo amp for a while and this might just fit the bill. I don't listen to music up loud anymore, playing in a live band for years has cured me of that.
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    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  6. #6
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    I'm drawing out the schematic (in between tv shows) and this actually has one leg of the heater wires going to ground as do two other leads from the PT. There is no fuse, it's definitely not safe as is. There are also a few capacitors that have five leads and I haven't run into these before. Time for a search.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  7. #7
    Senior Member Enzo's Avatar
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    Probably the mains fuse was elsewhere in the unit this assembly lived in.

    Five-legged things are not caps, strictly speaking, they are caps and resistors in a small circuit. AMpeg used them in a number of models, had the tone stack parts in one thing, just wire it to the pots - like your photo shows - for a complete tone stack. Look at Ampeg V4 or VT40 for examples. Look up Ampeg PEC. I think that stands for "printed electronic circuit".

    I don't know your TV schedule, but I watch the Untouchables at 4PM, and the Man From Uncle at 4AM. I have a lot of time between shows...
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    Education is what you're left with after you have forgotten what you have learned.

  8. #8
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    Quote Originally Posted by eschertron View Post
    Back in the days of vinyl recordings, the stereo encoding made it difficult to get a phono needle to track phase-variant low frequencies without the needle flying out of the groove
    So if the phase-variant (stereo) information is limited to high frequencies (like most live space reverb is) then I'd wager the electrical limitations of the OT are inconsequential when conservatively-mastered vinyl recordings are played back. All bets are off in the digital age!
    The RIAA curve de-emphasizes bass on recording but a preamp specifically for vinyl (pretty much every phono stage has this) re-emphasizes the bass using an inversion of the same curve. So on playback the bass is present as per the original source. You still need good transformers to reproduce the bass properly.

  9. #9
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    Here's what I have done so far, I just need to verify the preamp and connections to the power amp so they're not connected.
    Attached Thumbnails Attached Thumbnails stereo-amp2.png  
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  10. #10
    Supporting Member Jazz P Bass's Avatar
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    I would redo the drawing for the B+ to the Output Transformer.
    The OT should feed directly off of the rectifier.

    Not too sure what is up with the 270/ 5 watt resistor.

  11. #11
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    Redo it why? That's how it's wired. I looked this thing over and over and over again because it's not what I'm used to seeing. I even put it down for a couple of days and went back just because I wanted to make sure. I followed the wires forward and backward.

    Look at the first picture I posted of the underside. B+ comes off the rectifier through the orange wire to the first cap in the can. Then it goes through the cement resistor (270/5w) to the second cap and no other wire comes off that first cap. Unusual? Yep. Why? I have no idea but if you follow the second cap it goes to the first transformer that then goes to the OT.
    Last edited by gui_tarzan; 05-13-2017 at 08:27 PM.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  12. #12
    Supporting Member Jazz P Bass's Avatar
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    You got me.

    The original posted schematic is correct.

    Why would you want to limit the B+ to the output tubes?

  13. #13
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    I don't know, it seems very odd to me. But I looked it over carefully and everything is original and hasn't been worked on so your guess is as good as mine. Maybe Enzo has some insight?

    That schematic is not the one for this amp, it uses different power tubes but it was the closest I could find. This seems to be a very uncommon stereo amp.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  14. #14
    Senior Member Enzo's Avatar
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    I'm not the circuit guru around here really. I have no issue with the thing as drawn. It simply has an extra stage of filtration before it starts using it. Less ripple hits things. Might be gilding the lily, since push pull stages tend to cancel B+ ripple. But it costs a few volts to get smoother DC.

    Look at Champ 5C1 and 5E1 as examples.
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  15. #15
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Enzo View Post
    II have no issue with the thing as drawn. It simply has an extra stage of filtration before it starts using it. Less ripple hits things. Might be gilding the lily, since push pull stages tend to cancel B+ ripple.
    Just thinking aloud:
    "push pull stages tend to cancel B+ ripple" : true, they work in a differential way, so whateverīs present on both plates at the same time, such as ripple, gets cancelled out.
    But in this funny amp, I *think* it works as a push pull mono amp combining "signals which are the same left and right", typically the bass guitar, often the bass drum, some vocals, (hey, they are already "mono" by definition) , and/or "low frequency signals were mixed to mono to make needle tracking groove easier"
    Notice full power reaches a woofer boc, with an added low pass filter.
    I wonder why you crossed it out because itīs *essential* for this system.
    Mid/high frequencies, which are easier to handle and provide stereo information are differentiated and sent to satellite speakers , notice 100uF NP caps in series with each, that would give as a 200Hz highpass if running 8 ohms speaker.

    That said, they are getting differential signals to recover right/left information.
    ***Maybe*** that re-differentiation kills the hum-cancelling in "normal" push pull and they had annoying hum present in satellites, so they needed super clean +B
    Losing a few Volts would not be a big deal.

    In any case, the amp looks more like a "proof of concept" job than anuthing else.
    Maybe it was designed by an Engineer experienced in disc cutting Electronics and he applied his specialized knowledge here.

    I faintly remember some 60īs designs which did that (decoding stereo/mono information hidden in commercial vinyl records) but in the preamp, using some complex decoding system, never saw somebody doing it in such a direct way.
    Juan Manuel Fahey

  16. #16
    Supporting Member Chuck H's Avatar
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    Interesting. Also just thinking out loud now, but wouldn't 60Hz be a low enough frequency to be included in the mono portion of the amplification with almost none getting past the 200Hz filter? More lily gilding?
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  17. #17
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    Quote Originally Posted by J M Fahey View Post
    Notice full power reaches a woofer boc, with an added low pass filter. I wonder why you crossed it out because itīs *essential* for this system.
    As I said, the top schematic is not this amp, but it was the closest schematic I could find after searching for three hours. That's why I crossed it off, this amp does not have those components.

    Sorry for the confusion, that's why I am drawing my own based on the actual amp.

    In any case, the amp looks more like a "proof of concept" job than anuthing else.
    Maybe it was designed by an Engineer experienced in disc cutting Electronics and he applied his specialized knowledge here..
    Could be, however the only connectors on this were normal power in, power out through a harness, audio in and audio out. The best I can come up with is it's a CBS/Columbia stereo unit, possibly out of a phonograph console. The tubes have CBS printed on them along with "made in Great Britain" on the rectifier and the 12AX7/ECC83, and "made in Germany" on the one 6BQ5/EL84. I wish the other 6BQ5 was there but it was gone when I got it.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  18. #18
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    Quote Originally Posted by Enzo View Post
    I'm not the circuit guru around here really. I have no issue with the thing as drawn. It simply has an extra stage of filtration before it starts using it. Less ripple hits things. Might be gilding the lily, since push pull stages tend to cancel B+ ripple. But it costs a few volts to get smoother DC.
    The more I think about this, the more I'm thinking you're probably right about extra filtration. That's the only thing I can come up with since there are no other parts hooked up to it.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  19. #19
    Senior Member Enzo's Avatar
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    I still believe that, same reason those Champ models added the stage. But Juan appears to be right as well, I was ignoring the preamp, so this is essentially a pair of single ended amps sharing a transformer. On the other hand, the two sides don't know their grid signals come from different sources, so they still ought to cancel B+ ripple in the power tube stage.
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  20. #20
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    Here we go. I will double check this again after I get some sleep but I am pretty confident it's exactly the way the amp is set up. I just need to identify the combo resistor/cap parts and the pot values. I have to believe the plates on the 12AX7 are getting B+ through that combo part since lead four goes to B+.

    I haven't checked voltages since all I've done is map out the circuit.
    Attached Thumbnails Attached Thumbnails stereo-amp2.png  
    Last edited by gui_tarzan; 05-20-2017 at 02:03 AM.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  21. #21
    Senior Member Enzo's Avatar
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    So get out the ohm meter.

    B+ goes into D and I expect a resistor (100k?) from D to A and F.

    From A and F I expect a coupling cap to B and E.

    B and E ought to also have a resistor each to C. (220k-470k?)

    What's that, six parts? Likesiwe the other one is tone stack parts.
    Education is what you're left with after you have forgotten what you have learned.

  22. #22
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    Oh yeah, that's the next step.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

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    Hey Enzo,
    How can I start a new thread ?

  24. #24
    Supporting Member Jazz P Bass's Avatar
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    http://music-electronics-forum.com/n...newthread&f=48

    Up top: click on 'Forum'
    Then pick which forum you want.

  25. #25
    Senior Member Enzo's Avatar
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    New thread? Look at the main menu, pick a topic (music electronics, or schematic request, or whatever). Open the topic. Upper left or the topic menu, is a start new thread button. CLick it.
    Education is what you're left with after you have forgotten what you have learned.

  26. #26
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    Thanks Mr. Jazz P Bass,
    My first guitar was a Schecter C4 Custom Bass, Love it, But never really understood how to play it as a solo instrument
    Thanks for the help regarding the forum

    Joe

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    Mr.Enzo,
    That's great information and I thank you for acknowledging my question. I really like this type of forum because I actually went through an Aviation Communication and Navigation system electronic course 10 years ago. The bench tests and basic principles of resistors, diodes, transformers never really sunk in. Especially how to test using an oscilliscope and reading the sine wave. Now I have a greater appreciation for the Art of repairing micro circuits and semi conductors.

    Thanks for the assistance
    Joe

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    Wow, I am very impressed with the depth of knowledge you have Mr. Fahey,

    I've also traveled extensively through the Country and have even lived their temporarily. Always considered relocating there.

    Joe
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    Thanks, I'll try participating

    Joe

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