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'Organ Donor' - Hammond AO-29 to 6V6 Plexi Conversion

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 3, 2017.

  1. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Great job Again! Congrats! Another awesome build. Love the videos.
     
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  2. kleydejong

    kleydejong Tele-Holic

    587
    Aug 30, 2010
    Orange City, IA
    I've had a few days to settle in with the amp. I feel good about where the voltages and bias have settled into. I'm still quite impressed with the clean tones on this amp. I always think Plexi = classic rock crunch. But the normal channel has a really warm, deep clean tone. I really want to experiment with a dual amp setup pairing the warm low end from this Plexi with the chimey top end from my AB763.

    But there are some tweaks I'm planning on making.

    Stage 1 Mods - Functionality

    - Move the input jack to the rear of the amp. I'm getting some noise and squealing at times. The normal channel won't let me go past 75% without squealing. By moving the input to the rear it can be closer to the tubes and the leads won't need to cross over the component board. I also believe getting it into an enclosure will help a lot.
    - Specifically that high voltage step down line on the bottom of the component board.
    - Check my input grid stopper resistor. I think I used 33k. I may bump it up to 68k. A little high end attenuation would be good I think. I was watching an Uncle Doug video about resistors and I believe 68k places a low pass filter at about 15k. Anything above 15k is not useful for my purposes.
    - Experiment with a switch to allow series-parallel options between the two input triodes. Not exactly sure how I'll accomplish this with one input jack. I did something like this previously with my 5E3, and believe it could be even more beneficial in this amp - ala JCM800.

    Stage 2 Mods - Voicing

    - Experiment with the tone slope resistor. I believe I currently have a 33k in place. Moving to 56k or 100k will give more maximum treble. In general I have been keeping the treble near max and the bass rather low.
    - Experiment with adding a vox style cut control in the power section. Shifting the EQ towards treble in the preamp will help with the voice of the distortion, but I am concerned about super high frequencies as I increase the master to saturate the power section. Also, I am just curious about this kind of tone control and want to try it out.
    - Big thanks to Rob's web page on how to voice an amp to inspire these ideas: https://robrobinette.com/Voicing_an_Amp.htm
     
    Last edited: Aug 16, 2017

  3. kleydejong

    kleydejong Tele-Holic

    587
    Aug 30, 2010
    Orange City, IA
    Updated tone clip. Strat - AO29 Plexi - 2x12 - mic'd with an SM57. Sorry if I got a little overboard with tone clips. I like to archive things.



    There is still something with the character of the distortion when I really turn the amp up that I'm not pleased with. In this clip the master is cranked and each channel is at about 60%. The touch sensitivity is good. The gain structure overall is about what I would expect. But there is a bit of a buzzy, scratchy, or crispy quality to the distortion when I really dig in. It could be a tube, so I will roll some different tubes in.

    But I also suspect that perhaps the OT is saturating. It isn't terribly large. It probably wasn't designed in anticipation of having guitar players gunning a bunch of overdrive through it. I'm going to start with the above described mods. But in the back of my mind I am considering a beefier OT. Rolling tubes or tweaking the EQ throughout the amp is cheaper and doesn't require any shipping time, so I'll start there. But this is a hunch I have.
     
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  4. kleydejong

    kleydejong Tele-Holic

    587
    Aug 30, 2010
    Orange City, IA
    Sorry to revive this old bugger, but I wanted to share a new tone clip and ask a few questions. First, here is the current tone I'm getting from the amp. Clip is at about 6:55.



    Master is at about 7. Normal and Bright volumes on about 5-6. Plenty of gain. In line with that classic rock, Marshall Plexi type tone. But I am a bit concerned about my voltages and the transformers. The original unmolested AO-29 ran vintage 6V6's. When I try EHX 6V6's my plate voltage is pretty high - like 425 vdc. If I put in a set of JJ 6L6GC's it comes down significantly to about 375 vdc. Safer, but still higher than what I believe the original amp would run at - maybe 300-ish vdc. Can anyone explain why this is happening? The remaining variables I can identify is that my amp runs significantly fewer tubes overall compared to the stock AO-29. Does having more tubes drawing current from the PT affect its voltage on the high tension output?

    Next, would you feel uncomfortable running the amp as it is set up? Thinking more about the Output Transformer. Again, the original AO-29 was meant to run 6V6's, perhaps at about 15 watts. If I'm running 6L6's at higher plate voltages then I imagine my output wattage is perhaps a fair bit higher than that, no? While I'm not 100% certain on this point, I suspect that I'm getting a lot of Output Transformer saturation going on. Most of my use so far has been pretty restrained. Cranked tone clips are limited to 5 minute runs or less. But I worry if I ran it pretty wide open at a gig that it would blow up.

    Assuming all the above to be correct, any input on corrective measures? Seems like a simple-ish solution would be to replace the OT or PT. Beef up the OT to handle a beefier 6L6GC's output section. Or slim down the PT towards the 18 watt Marshall style to match more closely with the Hoffman project. The downside of these approaches really just seems to be additional cost and the mental note that my initial goal was to use as much of the stock AO-29 as possible. But I feel that I've learned a lot and want to get the amp to an optimal place.
     

  5. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Voltages and current draw are intertwined.
    The heater current draw is an important consideration when running 6L6’s on a PT that was
    designed for 6V6’s. I don’t try to change an amp in that manner. I would run 6V6’s and come to understand
    The Biasing. IMHO, there is no reason to be overly concerned about 425vdc on 6V6’s. I like JJ6V6’s, and they will handle that easily, ime.....as will quality old U.S. 6V6’s from the ‘60’s and later.
     
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  6. kleydejong

    kleydejong Tele-Holic

    587
    Aug 30, 2010
    Orange City, IA
    I have an update. I had put the JJ 6V6's back in and biased them. It did not saturate the OT as harshly. But I had a (perhaps illogical) hankering to keep the big bottle 6L6GC's in there and keep this as my super loud 50 watt plexi. The deal was sealed when I scooped up an old mystery output transformer on eBay for $30.

    [​IMG]

    It is a bit non-descript so I had to do a lot of sleuthing to figure out what it is. My best guess is that it is an early version of the Classic Tone 40-18000. The marking on the OT of 40-16824 did not produce anything - but the second marking of 041754 does match with the description found here:

    http://www.classictone.net/40-18000.html

    The new OT is a behemoth. Easily 2x larger than the old one. Having a multi tap lets me use it with both of my speaker cabs. And it is rated for 50-60 watts, so I anticipate I won't saturate it nearly as easily. The thing was so large I had to do a lot of rearranging of the choke and reverb driver on the chassis to get it all to fit.

    I captured a before and after. Skip to 3:58 for the tone clip.



    To my ear the difference is significant. I find it to be an improvement as well. Quite a bit more open and uncompressed. There is quite a bit more range between clean to slightly dirty to medium dirty to full on drive. And at higher volume settings it seems to roar while previously it would get very overloaded. Some minor overloading can be great. See the 5E3 as an example. But this took it a few clicks further.

    Overall, the circuit seems to run much more optimally with a proper OT. I'm very happy with the change.

    A final note. The amp is basically a 1959 Plexi circuit with 6L6GC's. Channels permanently jumpered. Presence removed. Pre PI Master Volume. When I listen back to the clip, especially towards the end when I crank it up - it sounds like a Plexi to me. I haven't heard of a Plexi running 6L6's before. I haven't done a direct comparison between 6L6's and EL34's - but a thought I have right now is that perhaps the commonly held tonal differences of power tubes are more a result of the circuits they're used in. Not to say there are no meaningful differences in tone between types of output tubes. Just that the changes in circuit to accommodate those tubes are often just as likely to be a major cause for tonal change.

    Therefore - if I'm seeking the sound of a Plexi, I should look at the circuit design choices first and foremost. The way you bias and bypass the triodes. The tone stack. The location of the tone stack. the type of phase inverter. The power supply. The type of rectifier. The speakers. Power tubes are just one detail among many that determines the end result.
     
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  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The blueprint for all of the meaningful Marshall’s from the early years is an amp that ran 6L6/5881’s. That would be the Fender 5F6A. The first Marshall was...as Brother muchxs likes to note...the first Reissue of a Fender Tweed 5F6A Bassman.
    As you have found in your search, cIrcuits determine much more than does the choice of power tube, IMHO.
     
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