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Audio Guild amp schematics?

Discussion in 'Glowing Bottle Tube Amp Forum' started by mojo2001, Feb 7, 2015.

  1. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    Just picked up a two output Audio Guild converted to a head. As-received very interesting tone and cool vibrato. Love that clean creamy 7591 sound and, in this amp, all ceramic coupling caps lend a familiar period tone. In general, though, the unit seems quite well made.

    Only two problems with it: the reverb motorboats and the old US 7591s are kinda soft. I'm running low on old stock 7591s since I have several Ampegs. The sound deteriorates after it has been on for a while. Probably needs a recap and a fresh set of 7591s.

    Peering inside, it looks like the reverb send is tube but the recovery is 70s transistor.

    Does anybody have a schematic for this amp (or its mono sister)? I've been looking around for one for a while, since I have an extra bare chassis specimen of this amp, but never found anything.

    Still trying to figure out all of the switching. There are four inputs. low and high gain, on two channels plus a dual input jack with a switch to select between channels.

    I think the two outputs are high pass and low pass, each with its own set of 7591s and output trans. original cab had an 8" and a 12" in separate sealed compartments, if I'm not mistaken. And then there is a knob labeled "Ultraflex Balance" that appears to mix the bass and treble channels.

    Freaky setup indeed!

    Who else in TDPRI land is using one of these Audio Guild amps? Seems to have some serious potential!
     

    Attached Files:


  2. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc
    I checked and all I have is a hand written guild schem but I cant say how accurate it would be for your model of amp if all else fails Ill send it to you
     

  3. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    Thanks for the offer, but this is not a Guild...it is an Audio Guild.
     

  4. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc
    I checked under the A's it went from Ampeg to Bogen , sorry no Audio guild
     

  5. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc

  6. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc

  7. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    That magnatone site has a schematic and discussion of the wacky 2ch + stereo input circuit which is quite informative.

    Otherwise, the closest thing I found is a partial schematic of a Versatone 133 on the Yahoo Magnatone group file section...not really that close but it shows the bones of the design.

    There are a fair number of Audio Guild amps around with various branding. You would think there was a service network out there and hence documentation.

    Maybe I need to hang out more with old accordion amp repairmen?
     

  8. 24 track

    24 track Poster Extraordinaire Silver Supporter

    Nov 6, 2014
    kamloops bc
    I did read the wikipedia blurb on versatone audio guild amps it appears that the guy who built the amps his wife still has the rights to the amp , its along shot you may be able to contact her for the schematics , If you can find them send me a copy I collect amp schematics for hard to find amps. I have 2 old beltone amps and have the same problem trying to locate schematics.
     

  9. swellsound

    swellsound TDPRI Member

    11
    Apr 13, 2015
    Christiansburg, VA
    Hi there mojo2001, I recently acquired one of these guys and I desperately need some assistance. You wouldn't happen to have any pics of the guts, would you? Some of the wiring has been screwed with, and a capacitor has been swapped.

    Any help would be greatly appreciated!

     

  10. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    Do you have the two channel version? Is it currently working?


    I can get pics of the relevant section but much of the wiring connects underneath the boards. Let me know which section is of interest.


    Joe
     

  11. swellsound

    swellsound TDPRI Member

    11
    Apr 13, 2015
    Christiansburg, VA
    Hey Joe, thanks for the reply! Yes, it's the 2-channel version; looks identical to yours (except it hasn't been modded into a head)

    It works, but not consistently. The vibrato works at times, other times it doesn't. The output tubes (a mix of Sylvania and GE, and maybe one other brand haha) start arcing and misbehaving. Probably needs to be retubed, but I'm reluctant until I can figure out whether it has issues that might kill the new tubes.

    The speaker connector has four wires, but it doesn't appear that they're both connected directly to the two output trannys. I measure low resistance across two or three pins, but one of the pins has a very high resistance relative to ground. Maybe it passes through a cap? One of the two large caps on the board where the speaker wires emanate from has been replaced with a smaller -and possibly incorrect value- cap. The fragility of the wires and the "rat's nest" effect have kept me from tracing the wires and drawing out a partial schematic of this section. I've been holding out hoping to find a schematic (or some helpful pics of an unadulterated example! haha)

    Another weird observation: I removed the power tubes last night and connected my scope probes to pins 6 of the output tube sockets, and without output tubes I can see the signal being modulated (vibrato). But then I put the tubes back in and the signal appears to be "normal", as in not modulated. ???

    And the external speaker jack appears to have possibly been screwed with because there are two wires leading to it that have been soldered together with a pigtail lead going to the jack. It's a TRS jack with a switch on the tip(?).

    And the speaker wires leading to the 8" driver are the clear insulation type, and 18 AWG. Seems odd since the other speaker wires are 20 AWG, or whatever. A lot of little oddities like that make me question how "original" the wiring is in this amp. It's definitely not a cookie-cutter clone of anything else out there, so the lack of schematic an/or any other documentation (user manual?) has presented quite a challenge.

    Anyway, thanks a ton for offering to get some pics. If you can snap a few showing the areas I've mentioned it would be a tremendous help!

    Thanks,
    Sam

     

  12. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    Have a look at this "simplified" Versatone 133 amp schemo which may or may not provide insight into the Audio Guild output wiring and bias supply arrangements. This amp looks like our Audio Guilds from the control panel view.

    And then I think the stereo input wiring is along the lines of the Magnatone 480. Check out that schemo here:

    http://www.magnatoneamps.com/480.html

    Note center input is actual TRS stereo not mono jumpered to two channels. Plug in your Lucille in honor of BB.

    Like you, I have been hesitant to rip into this thing unless I am forced to, but I recently picked up a second bare chassis Audio Guild, so at least I'll have a reference.

    If the bias supply is ~ -22V and the coupling caps to the 7591s are not leaky, you might want to change the tubes. Mine were pretty flat on a tester when I got the amp. With a couple old Ampegs and two 2ch Audio Guilds, my vintage 7591 stash is highly endangered!

    Aside from the byzantine layout and impenetrable maze of jumpers, the amp seems very well built and uses good parts. You can probably get far with routine electrolytic swaps and general contact cleaning.

    My head came wired to a pigtail female stereo 1/4" jack going back to that old-fashioned Amphenol connector for the output connection
     

    Attached Files:


  13. swellsound

    swellsound TDPRI Member

    11
    Apr 13, 2015
    Christiansburg, VA
    Very helpful! Thanks for clarifying that the stereo input isn't essentially a jumpered input to both channels... I was confused as to why I didn't get the vibrato when plugged into that channel, now it makes sense.

    I haven't measured the bias current because I don't have a bias probe for 7591s, although I suppose I could throw one together easily enough. And if I can get to the bias supply without major deconstruction I think I'll add a bias pot because the tubes are seeing around -17, although the bias supply goes down to -22.

    The coupling caps may be a major source of my woes; if they are indeed leaky it might explain the arcing/redplating that sometimes occurs, as well as the fact that the modulation disappears when I plug in the output tubes! I'll test the caps today and probably replace them regardless of how they check out.

    Sounds like a good problem to have re: the Ampeg and Audio Guild amps that are threatening your supply of 7591s! haha I definitely want to change the tubes, if for no other reason than they are totally mismatched.

    Any chance you can send me a copy of the Versatone 133 schematic? I can't really make out the details from the thumbnail. And if it's not too much trouble could you possibly grab a few pics like the ones below? The speaker connector and the large nonpolarized cap that has been replaced (see yellow shrink-tubing on leads) are a couple of areas I'm having doubts about, as to their correctness.

    Thanks again!



     

    Attached Files:


  14. swellsound

    swellsound TDPRI Member

    11
    Apr 13, 2015
    Christiansburg, VA
    UPDATE: I replaced the 50uF/20VNP caps and the bias supply electrolytic and it seem to be working okay now, as far as the basic amp functionality goes. But the vibrato ain't vibin'. I pulled the board out that has the vibrato circuitry and replaced the large electrolytic. Also checked a few transistors and they were fine. Haven't fired it up yet to see if the vibrato works now. This seems to be the only example of the Magnatone/AG/Bonham family that doesn't use varistors, so I assume the vibrato is transistor based.

    BTW, I found a clearer copy of the Versatone 133 schematic on the Yahoo Magnatone group. And after tracing out the bias and output sections of my amp I see that those sections are pretty much identical to the 133.


     

  15. Johnny Cache

    Johnny Cache Former Member

    Mar 6, 2014
    Oregon
    Pretty rare amp! I've only seen a few, even though they were made close to where I was living back then. I only know of one today.
     

  16. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    Here's a pic of wiring at the jack. With this pic and the Versatone 133 schemo, things should begin to make sense.

    Wow...I think you are correct...no varistors! I never looked before. It would be real interesting to map this one out, because I must say that the vibrato is pretty cool.

    I don't see any perishable components on my vibrato board. And I am at a loss for further suggestions since I don't know the circuit. Did you check the pots? Footswitch...jack...contacts???

    Also check into the action of the Ultraflex Balance control? I think there is a vibrato/verb output channel and a normal output channel and, as I recall, this knob pans from one to the other. Both speakers working?

    I'm packing for a trip to Germany so I don't have time to verify at the moment but I'll try to dig into this amp more when I get back.
     

    Attached Files:


  17. swellsound

    swellsound TDPRI Member

    11
    Apr 13, 2015
    Christiansburg, VA
    Thanks for the pics, mojo2001! I've got the amp working solid now, with no weird behavior, tube malfunctions, redplating, etc. Sounds awesome! I also made a new baffle to replace the original particle board unit that was damaged in shipping.

    All in all it's working perfect, except for one minor detail: the vibrato is weak... extremely weak... almost impercepticble. Super subtle. OK, maybe that's not exactly a minor detail. And it takes a few minutes of warm up before it can be heard at all. This made me wonder if temperature might affect the performance of the vibrato ckt, so I pulled out my heat gun and heated up the vibrato PCB and its components. Voila', the intensity suddenly increased significantly! The temperature coefficient of Y5V ceramic caps is really crazy, and the capacitance drops like a rock as temperature rises. So maybe I have some caps that need to be replaced on the vibrato board. Another possible contributor is capacitance in/on the PCB from years of moisture absorption.

    Anyway, this gives me some ideas to play around with. Feeling encouraged!

    Enjoy your trip!
     

    Attached Files:


  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I don't know if the amp has bias tremolo but if it does then the bias level has an affect on tremolo intensity.
     

  19. mojo2001

    mojo2001 Tele-Holic

    687
    Feb 5, 2008
    DC
    I don't think there will be that much environmental temp swing on the vibrato board, but go ahead and replace the ceramic with an NP0 type if you want freedom from temperature coefficient effects. Modern monolythic ceramics are also quite a bit more stable, in general, and easier to find.

    Once in a while you get a real freaky ceramic disc cap. Happens.

    The board itself seems like a decent grade of resin phenolic board...not like a wax coated cardboard Fender eyelet board.

    Try also reflowing all the solder joints on that board while you have it out, then swab it down with a solvent to get flux and contaminants off of it. It is possible that there are micro separations in the traces or joints that close when it gets warmer. poking around with the old chopstick technique might also give you some hints.

    The vibrato on the one I am using can get strong, almost to the point of weird! At moderate levels it sure does add some funky swamp juice to the tone though.
     

  20. voodoo_idol

    voodoo_idol Friend of Leo's

    Mar 1, 2009
    Pasadena, CA
    Carol Kaye used Versatone amps. Here is a Facebook post in which she gives some background about the manufacturers. As I understand it, the Bonham guy mentioned is the one who worked with/for Magnatone and incorporated some of their circuit ideas into his Audio Guild amps. Apparently he then sold (or licensed) the plans or business to two other gentlemen.

    https://www.facebook.com/permalink.php?id=168440386566810&story_fbid=322123464520758
     

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