Harvard/AA964 Hybrid

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, May 17, 2021.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've been looking a lot lately about fixed value tone stacks that would work with my Harvard/5F2 type builds. With some guidance from a ChasingTone article and some Blackface studying, I added a Blackface tone stack to my College Dropout (Harvard based with parallel inputs). It's temporary because I actually had drilled and set eyelets on a small square of board that's sitting on top of the guts-up chassis on my bench, and wired it in with longer leads just to try things out. Right now, the bass & mid are set values, and the treble is adjustable. I really thought I'd come up with something unique until I got to comparing schematics and layouts and realized I built a 2-knob AA964 Princeton... :rolleyes:

    The idea was a basic mid-level Blackface tone hardwired into it with some level of adjustment to fine tune it with just a volume/tone setup. It does work. It also kills a lot of gain, and I've already got TWO 25uf bypass stages in this, and 2x 12ax7 tubes in it. I've never played a non-reverb Princeton before, so I don't know what to compare it to in output, but it definitely takes a hit. The parallel input is really the only reason I'm still messing with it, however, I'm not happy with the pots in it right now. I have grown to prefer linear pots on my 2-knob amps, and this is from when I still had CTS audio pots. The only real output is when the knob is at 9 or above, and when I measured it, 80% of the adjustment is covered by the last 25% of the turn. I think a linear pot would help get things up to a working level sooner and then fine tune the last half of the dial. Anyway, biased at 65% right now, my Tele stays clean & fat all the up to 12 on the dial. I haven't tried any pedals yet, but I imagine it would love them.

    The tone stack calculator download has been great in seeing what I have and where to go. I didn't have exact values I wanted for a copy + paste Blackface tone stack so it's taken some playing around. I haven't played with the NFB at all to dirty it up, and it definitely likes a hotter bias now. I only have a 5y3 rectifier on hand at the moment, but I want to try out a 5u4. A gz34 will just take it hotter than I want to have it I think. The mid value is 2x or more from standard Fender, but I was experimenting trying to get the output up to a more reasonable level. There's still some more tweaking to do after getting the calculator app to visualize everything, but it's all a step in the right direction I was wanting to go. I may disconnect the NFB next time just to see what happens.

    Here is my Nashville Tele through a Legend 1058 in a small Blues Jr sized pine cab. The "reverb" is the echo from my concrete shop floor.

     
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  2. Cali Dude

    Cali Dude Tele-Afflicted

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    Nice job. You've got some cool Tele twang going on. I've got a tweed Harvard, and your tone reminds me of the sound of the 2nd input.
     
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  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've thought about trying the treble & bass as set values, then make the tone knob adjustable mids to dial in from straight scooped 60's to full tweed. Will probably try it at some point.
     
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I fell in love with building these things since the first one a few years ago. It's so simple inside with room to work. I don't try to make any of them a swiss army knife that can do everything, but you can definitely tune them for great cleans up the dial or an a total barn burner rock & roller. Stock setup is the nice "meet in the middle" point for both extremes.
     
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  5. NTC

    NTC Tele-Holic

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    You hiy my hot button with this thread. My "5F11" has voltages more appropriate for an AA964. The speaker in it needs a higher treble setting than normal to sound good and I have been contemplating the BF tone stack with fixed values for bass and mids, but the gain hit is going to be huge - as you have found.
     
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  6. jsnwhite619

    jsnwhite619 Friend of Leo's

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    You nailed it! That clip was done with 2x 12ax7 tubes installed and the amp on 11. This clip was done with the same amp using the stock Volume/Tone but with a 12ay7 in V1 to keep the gain down!





    It's been too long since I've dived into 5e3 world, but I have a similar shared cathode & bypass on this with parallel inputs. How does the math work out when two sides share a bypass cap? I know the resistor value doubles for a shared cathode resistor, but what does the capacitor value work out to? Half for each? If that's the case, then I would still have room to work on my V1 to maybe bring the gain up a bit.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime it would be beneficial to have the treble adjustable as well. The treble spectrum is changed the most when dealing with different rooms. Live rooms. Dead rooms. Large and small rooms. I suppose you could set it bright and control the treble from the guitar?
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, the treble/brightness probably would be a must-have for versatility.

    However, I had a few minutes to try my Blues Driver with it this morning. I finally see the allure of clean amps and overdrive pedals. The amp is super clean and low output on it's own, but that means you've got miles to boost it on the front end. With the amp vol. on 4 and the normal input, my acoustic guitar is probably louder than my Tele was putting out. Turned both dials on my Blues Driver to 2:00 or so and BOOM! An amazing, thick overdrive tone with lots of clarity still, and between the pedal and the amp, definitely a volume you could gig with.

    So, as of now, for anything below 8 on the dial, I essentially have a fixed bias 2x 6v6 power amp with a low powered Blackface preamp that loves an overdrive pedal.
     
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  9. NTC

    NTC Tele-Holic

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    That is what I envisioned. My Bandmaster Reverb with an SD-1 in front has a delicious distortion tone and I keep thinking I could approach that with the little amp as well.

    Note that my amp can also easily run 6L6's and an extension speaker. Its 1 louder...
     
    Last edited: May 19, 2021
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  10. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    That “reverb” is nice!

    For a second, without my morning caffeine fix, I thought it was a slapback delay.

    I’ve been dying to build a Harvard. Maybe a brown Princeton with a 5751 would suffice.
     
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  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    That thought definitely crossed my mind as well. I wouldn't do it with these transformers, but for so many people wanting a super clean amp that let's the pedals do all the work, this setup with 6L6's would be a clean machine without a doubt. Even this with a clean boost would go a long way. While it's pretty anemic right now on it's own, it's due to loss in the preamp, not lack of the power section. A clean boost up front vs a gain stage down stream, you can still make up for a lot of volume loss with a boost in front.


    If you are after the basic tone & sound, I'd go for whatever is easiest to work with in the chassis selection and run with it. The more of this stuff I do, there always turns out to be more similarities than differences between them. I started my modded Harvard a year or two ago and added the Princeton style NFB that adds the second gain stage. Now, when I said, "What if I added a Blackface tone stack to it? But no more knobs, so I guess it will have to be fixed values." It didn't cross my mind until I saw it on the screen that I 99% modded it into another existing Fender without specifically trying to do so.
     
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  12. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, first of all, it NEEDS the linear volume pot. Probably would be better on the Tone/treble as well, but I don't have the correct value - got one tacked across a 1M to get closer to a 250k, and yes, that made a huge difference as well. Only want to change it once, so I'll wait on the proper pot. But, the perceived working volume comes in around 4-5 out of 12 now instead of only from 9-12. Like I said, 50% of the change occurred between 8.5-12 before that. So, it is definitely louder than I previously thought. I know the final output shouldn't change, but there's so much more room to work with now. Which is why I've been using linear volume pots for a while now. After playing with the tone stack calculator, I got it closer to the Fender EQ curve, and it still doesn't really breakup even with a Les Paul in the parallel channel and on 12. But...

    It LOVES a Blues Driver! It's SO clean and bare that none of the usual fuzzy, swirling, phase inverter, etc. trouble is happening at all. Just pick whatever volume level you want and hit it with the OD. In my research I found a post - probably an old one here - where someone said his non-reverb Princeton was his secret weapon because it took pedals so well. I can see why now.

    I also swapped the leads and dropped down to the 275v tap and rebiased for a GZ34. I tried it before but it was in the 420v+ range and I didn't want that. I think I'm at 365v B+ now with the GZ34.

    Here is my Les Paul with the Blues Driver. I click it on & off a couple times for the comparison. My DAW kept crashing my computer for some reason today every time I turned it on, so I'm stuck with a cell clip until I get that figured out.

     
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  13. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’m looking for as straight a shot towards a Harvard as possible. I’ve built so many amps over the years, but that old Stax sound is calling.

    Which leads me down another rabbit hole—finding ways to cop that Steve Cropper sound from that era.

    His earliest stuff was done on an Esquire, and he mentioned he NEVER uses the bridge pickup on his Teles by itself... but I can’t find of his “it had to be an early ‘50s Esquire” was one of the transitional models with two pickups.

    I realized I’ve customized myself out of traditional Tele neck pickups (my #1 has a Charlie Christian in the neck).

    He also transitioned to a Super Reverb for some later classic tracks — but I’ve got my 1x10” SR in a Princeton chassis/cab for that.

    I think I’m looking at a tweed Vibrolux kit in my future (building my own from scratch isn’t feasible in my old age


    [QUOTE="jsnwhite619]The more of this stuff I do, there always turns out to be more similarities than differences between them. I started my modded Harvard a year or two ago and added the Princeton style NFB that adds the second gain stage. Now, when I said, "What if I added a Blackface tone stack to it? But no more knobs, so I guess it will have to be fixed values." It didn't cross my mind until I saw it on the screen that I 99% modded it into another existing Fender without specifically trying to do so.[/QUOTE]


    That is pretty funny!
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I'll suggest the Hammond 290AX PT like this one. The 275v tap can nail the vintage Harvard schematic levels across the board. The 325v tap takes it up to 375-380v range of you want some more punch out of it.
     
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  15. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Thanks for the tip, Jason!
     
  16. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I was able to get my recording issues sorted out today and did a couple demos with it. Here is my Tele & Blues Driver, swapping back & forth as I go and kinda repetitive, but I wanted to do a clear A/B clean/OD as I went. Legend 1058 speaker in a small pine cab.

    I have to get some 15k resistors for the mid setting. 6.8k stock just KILLS the output, this is a 22k that was night & day difference.

     
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  17. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Well, I woke up thinking about this thing, and I realized where I'm losing so much gain, and at first glance in comparison I didn't realize it. Same tube lineup, nearly the same inside now - minus tremolo - but this thing is just too low output. Not a headphone amp or anything, but just lower than it should be. I have the inputs paralleled, and the parallel option sounds great & full, and both side share a cathode and bypass resistor. BUT, I'm missing a gain stage now. The AA964 has the first half of V1 bypassed, then the second half is bypassed after the tone stack. So, AA964 is bypassed before the tone stack, afterward, and in the tremolo circuit. I have the same type V2 bypass/NFB circuit, but my two halves of V1 are working before the tone stack only. A Harvard design left one half open, so I paralleled it and both halves are up front now.

    With only a tone knob before it was a screamin' demon with that setup. Drop -20dB of mids out of the signal and not so much screaming anymore... Working backwards from my dB meter reading, I'm guessing around 6-8W of output with the guitar straight into the amp and the current preamp setup.

    But, it's still a fixed bias 6v6 power section with a GZ34, so obviously the power section is capable of more than that if fed a proper signal from the preamp - roughly twice the current output is available from the power section. So, where does that leave things? I guess the next thing I try will be using my Blues Driver or maybe a tube preamp I've got as a clean boost and see what I can find out. How much gain can you make up on the front end with pedals and clean boost? I guess as of now for output level, it's kinda like a hot rodded Champ that you can boost the hell out of and turn into a Princeton?
     
  18. jsnwhite619

    jsnwhite619 Friend of Leo's

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  19. TwoBear

    TwoBear Tele-Afflicted

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    I think it sounds great I don’t know if you’ve gone back to the one knob tone, but I would vote for that. I think all the small output amplifiers and even 30 or maybe 40 Watt sounds great with just the tone control. I played lead in a band with a song writer who just played his telecaster straight into a white knob black face Princeton, While I played my Strat into my silverfaced Princeton. I had always got compliments about my tone but I really thought his sounded better. It sounded so good I think that was actually the beginning or at least what solidified my thoughts on single volume single tone control amplifiers, and that they sound so darn good. When my other buddy got a 58 or so Pro from the Salvation Army (bone stock with original Jensen 15 for $200) That just hammered it home. I even did a tone Stack lift on one of my super reverb’s first channel. The tone stacks just dump a bunch of signal, And I would usually set them to the same place all the time anyway. I don’t mean on the same model but different amplifier that I would set them alike, but rather on two different super reverb’s If I set the bass on 4, the middle 9, and treble 7, I’d probably set that one up ‘about’ the same every time. And on the other one I would set the controls differently for that amp, but they would probably be about the same each time for that amp. Seems I should’ve been able to word that more succinctly... I vote volume and tone?
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, that was still the volume/tone in the demo, but the fixed resistor tone stack. I think the most sensible thing at this point is to keep this one as it is - or revert back to what it was, I just wired in a section of board to try this out - and start from the ground up on either Rob's Princeton design with no trem/reverb, or Blackvibe. It was interesting in this to see how much that midrange setting affects things. I tried from 6.8k to 22k and with it lifted out of the circuit completely. 6.8k was a complete volume killer; 22k low powered but sounded good; mid-cut lifted didn't really sound Blackface, but different and punchier.
     
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