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HELP

Discussion in 'Shock Brother's DIY Amps' started by originalmatthew, Feb 20, 2012.

  1. originalmatthew

    originalmatthew Tele-Holic

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    I got one of these recently from a Webcor extension speaker..now..am I wrong, or should the 12AU7 be a preamp section? seems I need line in power to the input to get any sound out of it.. It probably needs new tubes as well..output is low even for 9 watts...and it breaks up super quick. So far, the caps and resistors have tested within acceptable tolerances.
    Any ideas or suggestions would be helpful!
    Thanks!!
     

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  2. teleamp

    teleamp Poster Extraordinaire

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    That looks like a push pull power amp with phase inverter and no preamp. Try hitting it with an overdrive pedal. 12AB5 with 315 volts on the plate should get around 12 watts... probably a 15-20 watt amp.
     
  3. printer2

    printer2 Poster Extraordinaire

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    That is basically a phase inverter and a power amp section. You need at least a pentode on the input or a couple of triodes. You could do a quick swap putting a 12AX7 in place of the 12AU7. I can not read any of the values on the schematic so I can not help much more with it. The 12AB5's are a 12v 9 pin version of the 6V6, I have a few but have not tried them yet. The have the same voltage and power ratings as a 6V6 rather than the reduced limits that a 6AQ5 has. A 12AY7 on the input and a few parts will give you a Fender 5D3 Deluxe or if you can change the circuit and do a 5E3. I am jealous.
     
  4. teleamp

    teleamp Poster Extraordinaire

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    Great find, agreed.
     
  5. celeste

    celeste Friend of Leo's

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    Yep, just a power amp (I consider the PI to be an integral part of a PP output stage) Looks to be a paraphase inverter, which has more gain then any of the other common topologies. As printer says, drop a 12AX7 in in place of the AU7, and you would up the gain by at least a factor of 4. That might give you enough to drive it with a Pod one of the other effect processors
     
  6. originalmatthew

    originalmatthew Tele-Holic

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    Thanks for the input, (no pun intended). I had already considered the 12AX7 swap..now I am convinced. I think i can rig a line in easily enough. Any suggestions for a speaker? it came with a 6" woofer, 5" mid and a 2" horn all wired with caps inbetween...
    Thanks
     
  7. teleamp

    teleamp Poster Extraordinaire

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    A Bad Monkey and an instrument cable with a RCA plug on one end would work.
     
  8. teleamp

    teleamp Poster Extraordinaire

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    12" Alnico
     
  9. Platefire

    Platefire Friend of Leo's

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    I took a look at the schematic. Don't know how much space you have on your chassis, but I would dare say the circuit would handle another 12AX7 if you extended your heater wires from the 12AU7 and extended the power supply with some 100K plate resistors to a add in 12AX7.

    You could install the 9 pin socket and build a 5F2 tweed princton input, volume, tone stack just prior to the 12AU7 phase inverter and make a functioning amp out of it.

    It's presently just a power amp section needing a preamp section to drive it. Platefire
     
  10. printer2

    printer2 Poster Extraordinaire

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    Something that I gave a thought to but was unsure of before I just stumbled upon.

    http://archive.anthemav.com/OldSitev1/pdf/taste.pdf

    Page 16 has a brief description of a cathode feedback scheme used in a Quad, It looks like you may have the same sort of feedback going on. It is supose to be good at cleaning up the distortion in pentodes. Just what we are not looking for.
     
  11. originalmatthew

    originalmatthew Tele-Holic

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    I am back at this thing... Any thoughts about a ART MP tube preamp driving this thing?
    I would prefer to build my own preamp tho. Can anyone point me to ( or draw if possible) a simple preamp circuit using a single input, a 12AX7, with a volume and tone control? my heater voltage is 12.6 coming from the OT... thanks in advance for any help!
     
  12. Keyser Soze

    Keyser Soze Tele-Holic

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    There are any number of ways you could do a 12ax7 preamp. The basic choice would be to cascade one triode into another, with volume and tone in between.

    Similar to this, but where your first triode takes the place of one of the 12AY7 stages, and then your second triode is the subsequent 12ax7 triode

    http://www.webphix.com/schematic heaven/www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf

    When wiring a 12ax7 for 12.6 volts you attach one wire to 4, the other to 5 and nothing to pin 9.
     
  13. originalmatthew

    originalmatthew Tele-Holic

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    Thanks Keyser... I knew about the heater wiring and the potential drawback of having the heaters wired in series but I gotta do what I gotta do... as for cascading the triodes..thats where I need help...are we talking connecting the coupling cap that connects to pin 1 and jumping it to pin 6?
    Thanks
    Matthew
     
  14. kleuck

    kleuck Tele-Holic

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    Yep : ART Tube **** are just plain crap (owned two, burned two, noisy as hell, forget about these)
    For the pre, you can't go simpler than a 5E3, for more gain take a look at the VHT Special 6 (just don't consider the fixed tone stack)
     
  15. Keyser Soze

    Keyser Soze Tele-Holic

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    If you want volume/gain control you'd attach that coupling cap to the wiper of an audio pot, bottom to ground and top to a grid stopper resistor (33k-100k, not shown in the linked shematic) that would then attach to the grid of the other triode (pin 7.)

    Tone control would consist of a cap attached to the junction of the coupling cap/wiper mentioned above, connected to an audio pot, wiper to the grid stopper, and bottom to another cap attached to ground.

    See the schematic for suitable values.

    Edit: wiring a 12ax7 for 12.6v operation is not necessarily series wiring - if your transformer has a 12 volt tap they still get wired in parallel. Otherwise you will need a heater tap of higher voltage (series heaters sum voltages.)
     
    Last edited: Mar 9, 2012
  16. originalmatthew

    originalmatthew Tele-Holic

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    The VHT circuit (minus the tone stack) looks like it might fit the bill..thanks!
     
  17. originalmatthew

    originalmatthew Tele-Holic

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    Well, I got hum.

    So.. I am using this schematic to build a preamp for the little webcor power amp mentioned above..
    I have the heaters (pins 4 and 5) jumpered from 4 and 5 on the PI tube, B+ at 250V jumpered from the transformer, and all grounds jumpered to the chassis of the webcor. All cap values are exact to the schematic, resistors too except I use 1/4W instead of 1/2W. The output on the schematic is the input to the webcor. I know the problem is somewhere in my wiring, I had the webcor up and running with a line in source....

    I get hum. The volume knob works, the gain knob works, the tone pot works.. you can hear them increase and decrease and change the tone of the hum, but I seem to get no signal from the guitar. Voltages seem to be where they should be when I check at the pins on the preamp tube.
    I am thinking something is grounding or shorting.. the hum sounds like when you grab the end of a plugged in guitar cable.
    Any Ideas? and sorry, I don't have pics of my incredibly messy layout and wiring. I didn't want you guys to laugh and then throw up. Solder joints are all good as far as I can tell.....
    Thanks
    Matthew
     

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  18. celeste

    celeste Friend of Leo's

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    voltage readings and gut shots, otherwise it is just a stab in the dark.

    Well, blowing up the schematic enough to see the typos might be a clue as well. Tell my you did not really use a 15k for the cathode resistor of the input stage. If you did, you got 6.6v on that cathode? Your input stage biased into cutoff?

    That resistor should be 1.5k 1500 ohms, not 15000 ohms.
    If that is the issue, you are in good company, and are far from the first to have done it
     
  19. originalmatthew

    originalmatthew Tele-Holic

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    Blew up the schematic to the size of a house. It says 15K...hmm..will start by replacing that with the 1.5K. I am a total noob...thats why I don't catch stuff like that.
    Thanks for the help..will try it!!
     
  20. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Hey Celeste,

    Check out that output transformer wiring!

    BTW, Webcor used to be Webster Company out of Chicago back in the day. They were into wire recorders in the 30's. At the end of the 1940's they got into the tape machine racket and upgraded their name to "Webcor" (short for "Webster Tape Recorder")

    My very first guitar amplifiers was...A Webcor tape machine from the 50's. That's how I ended up becoming a recording engineer!

    Mine was an SE design using one 12AB5.

    Cheers,

    CBG
     
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