AB668 Pro Reverb Questions

Discussion in 'Amp Tech Center' started by CroMagnonDan, Jun 10, 2020.

  1. CroMagnonDan

    CroMagnonDan TDPRI Member

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    I have a Pro Reverb combo whose chassis code dates it to 1981. I use it primarily as a head to power an external 4x12. I use a Wampler Tumnus Deluxe for my drive, and a TC Electronic Nova System for all other effects.
    In the future, I want to clone, then modify this circuit into a head and have an effects loop, removal of the pull out overdrive, eventually maybe switchable impedances.

    My first question is: Is there a way I can improvise an effects loop, so I can have my Nova running after the preamp, but before the power amp, without frying my Nova?
     
  2. CroMagnonDan

    CroMagnonDan TDPRI Member

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    15917626553506015818262222213818.jpg 15917627212054423672871743549035.jpg 15917627980056514201694818281335.jpg
     
  3. Wally

    Wally Telefied Ad Free Member

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    Welcome, CroMagnonDan. FYI, here is the schematic for your Pro Reverb, which can not possibly be an AB668 circuit since the AB668 was drawn in June, 1968 as the first CBS circuit for the Pro Reverb.
    https://schematicheaven.net/fenderamps/cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

    as for an effects loop perhaps this bump will bring on the higher pay grades to share their wisdom on that subject. One can do a search and find much info. Some hold that this circuit would best be served with a cathode follower driven loop, which makes the deal a bit more complicated.
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Welcome CMD!

    I agree with Wally. You can incorporate a effects loop in your Pro Reverb. Basically you have two options.

    One is a *quick and dirty* loop which is a simple circuit. The problem with the simple circuit is not all pedals *play well* due to the higher impedance.

    The second option is to lower the impedance for the effects loop. Cathode follower or transistor options are available. Will you be able to add a tube? Do you have the space available? Do you have a Power Transformer that will handle the extra current?
    If you choose the transistor option, you need to come up with voltages to serve that circuit.

    Rather than design a new low impedance circuit, I would borrow one from a Marshall or any of the several other manufacturers.
     
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  5. CroMagnonDan

    CroMagnonDan TDPRI Member

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    Thank you so much Wally. I will keep that schematic handy. The one i referenced previously must be inaccurate.
     
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  6. CroMagnonDan

    CroMagnonDan TDPRI Member

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    proreverbab668.gif
     
  7. Wally

    Wally Telefied Ad Free Member

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    Yes, as you noted and the serial number shows, your amp is from 1981....the last year of production for the handwired Pro Reverbs. The Pro Reverb amps were introduced in January, 1965....AA165. The AB668 was the second version. There was a version from late 1969, another in 1970, and the MV amp came out after that. Your amp is the 70 watt Ultralinear version which was introduced in 1977. this...https://schematicheaven.net/fender.html...is one place to find schematics. In the BF/SF era, the digits reveal the month and year of the schematic. The two letters indicate whether the circuit is the first of its type..AA, or whether it is a revision...AB,AC...with B being the first revision and C being the second revision.
    I can read the speaker manufacturer’s EIA,code...1098..which indicates Pyle. The last three digits are either not readable or the speakers do not date to the rest of the amp. Am I correctly reading that whole code as 1098701? If so, the ‘701’ indicates the first week of either 1977 or 1987. Usually, those date codes would be more closely in line with the codes on the rest of the amp.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Cro, in the upper left hand corner of that schematic info box the lower right of the schematic you will see a 1968 date.
     
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  9. Ten Over

    Ten Over Tele-Holic

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    If you don't like the Fender schematic style at the time, here is an alternate:

    Pro and Bandmaster 70W.png
     
  10. CroMagnonDan

    CroMagnonDan TDPRI Member

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    My plan is to remove all of the reverb and vibrato circuitry, but leave the "vibrato channel" and "normal channel" circuits otherwise intact, add an effects loop (hopefully just in place of the reverb in & out), and cram it all into a head about the size of a standard 100 watt head. Like a Marshall DSL 100 or equivalent. And it will be built from the ground up. Will be my first foray into amp building and be guided by my tech, but i want to come to him as prepared as possible so he doesnt end up with a stack of homework any bigger than necessary.
    To answer more directly, i want to keep as much of the original circuit as possible, and i would like to run a solid state effects loop, buuuuuut whatever i have to do to make an amp with this tone and modern flexibility and features, i will. I am also just a cooooomplete greenhorn, so much of this is confusing for me, but as i learn, it will all be helpful.
     
  11. CroMagnonDan

    CroMagnonDan TDPRI Member

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    Speaker code is 1098201
     
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  12. CroMagnonDan

    CroMagnonDan TDPRI Member

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  13. Wally

    Wally Telefied Ad Free Member

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    1098201.....first week of 1981.
     
  14. Wally

    Wally Telefied Ad Free Member

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    CroMagnonDan, it is you world there; but there is no way that I would tear a Fender amp that clean and original apart. I say sell it as is and use the proceeds to fund a new build. Sit with your tech...pay him if need be....and design what it is you want from the ground up. It would be easier, and a pristine Fender Pro Reverb will be maintained. Ymmv.....
    I have a BF Bandmaster chassis that was butchered to build a SOLID STATE amp. I have had the chassis repaired to carry the original 4 x 12A-7 uses. There are four octal holes. I have the PT and OT to make a killer amp as you describe. I have a Bandmaster head cab for the amp. I haven’t concluded what I want to do with it.....it changes from a TW Express to a 5F8A!!!!! If we were closer, I would want to work up a trade with you to save that Pro Reverb...even if it is a UL amp. I have turned UL amps into snarling animals before....but all of those mods were easily reversible.
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You may want something different than what you are asking.
    I would want to at least be able to jumper the channels but this is your amp.

    Solid State:
    Rob Robinette suggests a *Metroamp Zero Loss FX Loop*. This had good reviews on amazon". Metroamp" may have been replaced with "Metropoulos".

    This could be placed between the MV and the PI.

    Cathode Follower:
    By removing the reverb and vibrato from the circuit, you will eliminate two of the 12a_7 tubes. It would be easy to retain one of the 12a_7 tubes and use it for the FX loop.

    This would replace the master volume in the Pro and Bandmstr circuit. You may want to retain the 1M pot and use it for the "send level". (The "send level" in this circuit is just a "master vol".)
    You can find it here:
    https://el34world.com/Forum/index.p...ion=dlattach;topic=10208.0;attach=19466;image
     
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  16. Ten Over

    Ten Over Tele-Holic

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    Now there is a lame effects loop. It uses the cathode follower for the send, but it doesn't take advantage of the very low output impedance at the cathode. Instead, the loop is capable of an output impedance of 63K which will limit you to a 4'10" cable if you use a cable with 33pF/ft and you want a cutoff frequency of 16KHz.

    A lot of people want an effects loop that is transparent which means, among other things, a flat frequency response. This loop doesn't have a flat frequency response. And 22M in the local feedback loop? Gimme a break. Even if you had a reasonable feedback loop, you wouldn't put a frequency altering network within it.
     
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  17. Ten Over

    Ten Over Tele-Holic

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    The way I understand it, you aren't going to tear-up your existing Pro Reverb. You are going to build another amplifier that is a clone of the Pro Reverb with some mods, right?

    Here is an idea for what I think you mean. It doesn't have a SS loop, but it could easily be adapted to one.

    70W Pro for Cro MagnonDan.png
     
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  18. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    If you were going to rip that much apart, I'd just build a new amp from the ground up with what you want, or sell the Pro and get something else closer to what you want. Anything you do with that plan will only make the Pro take a big value hit.
     
  19. Wally

    Wally Telefied Ad Free Member

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    Apologies...I misunderstood the process.....this will be a new build not a tear down.
     
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  20. Wally

    Wally Telefied Ad Free Member

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    Ten Over, what do you think of the effects loop in the Fender Pro Sonic?

    44C372DE-3D2C-4E75-B03F-5FEB60B88094.png
     
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