Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Pre-Build Bassman AA165 mods

Discussion in 'Glowing Bottle Tube Amp Forum' started by Peterquelle, Aug 9, 2017.

  1. Peterquelle

    Peterquelle Tele-Meister

    178
    Aug 1, 2012
    austria
    Hi,

    I am thinking about building an AA165 Bassman with a few mods. I settled on the AA165 because it seems simple enough to build it for me (I have experience in electrical Engineering, altough I never built a tube amp before) And I love the tone. I want to convert the bass channel to a drive channel by adding the unused half of a 12AX7 and changing the tone stack to Marshall specs. I also want to put a pre-PI master volume for the bass channel in.
    Are there other known mods for this circuit I should consider before starting to build ?

    Regards
     

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    If you are looking for a circuit with more gain possibilities, considering working with the AB165 scheme. Both channels have three gain stages. One can voice the two channels differently....and there is still an unused triodethat can be used for higher gain.
    If you are interested, I have a Guitar Player article from 1993 that outlines using that triode.p
     
    peteb likes this.

  3. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia

    That would be cool to see.


    I was thinking about the 2 vs 3 stages in the preamp and the effect it has on the gain.


    In the AA165 the normal channel has 2 gain stages in the preamp, while the bass channel has three.




    In the case of the AA165 bassman, the normal channel has more gain than the Bass Chanel.


    How is it known? The bass channel Needs to be set at a higher volume, by one number, than the normal channel, to reach equivalent volume. At least that's the way mine is. The bass Channel adds some width(fatness),but not the depth(volume).



    So three is not always more than two.
     

  4. Peterquelle

    Peterquelle Tele-Meister

    178
    Aug 1, 2012
    austria
    I thought about completely changing the bass channel to a jCM 800 2204 preamp with master volume. But I am gonna take a look at the AB165
     

  5. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Peter, I recently did a second AB165 type of circuit for a young pro player.....tele and pedal steel. On this second one...an AB165 BF...I revoiced the Bass channel for more mids and a bit more gain. Then, I looked at that Deep switch and all of the space that I have on the board after revoicing that channel...and I added a mid boost circuit using that switch.
    curiously.....this young fellow turned down an offer from a fellow who was playing a Sewell amp in the national act for which my young friend's band was fronting. That offer was more than twice the market value for a SF Bassman head. He heard something in that amp that he had not heard from one of these....and he wanted it...badly.
    There is a lot that one can do with that circuit. A Trainwreck express can be built there, too. Enjoy the project. And....the good Md. 2204 is the JTM series. The early JCM 2204's were identical to the JTM...but then there came some changes.
     

  6. Phrygian77

    Phrygian77 Tele-Holic

    588
    Apr 30, 2016
    Crawfordville, FL
    Considering how much I just spent to build my AA165 Trainwreck hybrid, if you're contemplating an AB165, you may just want to think about buying one.
     

  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    How much did you spend on the transformers???
     

  8. Phrygian77

    Phrygian77 Tele-Holic

    588
    Apr 30, 2016
    Crawfordville, FL
    TR-PW-CT-40-18005 ClassicTone Power Transformer # 40-18005 (1) @ $88.95 = $88.95
    TR-CK-CT-40-18003 ClassicTone Choke # 40-18003 (1) @ $21.95 = $21.95
    TR-OT-CT-40-18010 ClassicTone Output Transformer # 40-18010 (1) @ $71.95 = $71.95
    ============================================================ Subtotal: $182.85

    Shipping: Free Shipping on Qualified Orders $0.00

    ============================================================ Order Total: $182.85
     

  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I thought maybe you cut loose for some high dollar trannies the way you were talking. For instance, the Mercury Magnetics Trainwreck OT, which has dual primaries, is $292. The two PT's they offer for 'Trainwreck' are both a little over $300. That adds up, doesn't it?
     

  10. Phrygian77

    Phrygian77 Tele-Holic

    588
    Apr 30, 2016
    Crawfordville, FL
    No, but I did splurge on Jupiter caps. Add to that the cost of Belton sockets, CTS pots, Switchcraft jacks, and Tung Sol tubes (JAN 12AT7). The combo cabinet cost about $330 with shipping. Oh, and the repro faceplate was stupid expensive. Until yesterday, I had a $205 Weber Alnico paperweight in a box.
     

  11. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    If it's your first build - and assuming you understand all the safety, lead dress, ground scheme and other standards necessary to build a solid, working, tube amp - it would be advisable to work within an established schematic as a standard. In other words - keep it simple. When you start adding gain stages and/or altering the grain structure your lead dress, parts placement and possibly power supply needs change.

    This usually requires electrical design first, then layout - then usually redesign/re-layout. Building something like that is normally done in reverse stages - power supply/testing, power section/testing etc - working from back to front. When building a new/modified design building the whole thing and then trying to troubleshoot a completed amp can be extremely problematic.

    Part of the difficulty with a modified design is that you can't just place parts or run wires anywhere that's convenient. Problems with hum and parasitic oscillation are very common when mods are added without very specific design and layout considerations being taken into account. Those are things most first time builders...even those with electronics experience...don't have the practical knowledge to handle.

    "Amp assembly" and "amp design" are two very different processes.
     
    Wally likes this.

  12. peteb

    peteb Tele-Afflicted

    Apr 25, 2003
    Cascadia
    Thanks for the reply Wally.


    That's good to know.
     

  13. Peterquelle

    Peterquelle Tele-Meister

    178
    Aug 1, 2012
    austria
    Thank you for the answers :) I really dig that. I drawn a schematic with paint how I want to add the gain stage. Would that be correct ?
    Add_GAIN.jpg
     

  14. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    FWIW if you use Google there are at least a dozen different sites that discuss this type of mod. However, it's rarely done because of problems with added noise, hum, parasitic oscillation etc. - besides the simple fact increasing the gain will increase the noise floor *before* taking design problems into account.

    A schematic is the easy part. The problems start with parts placement, lead dress and grounding. With no design experience you'll be far better off working within an established, already-successful layout. The type of "mod" you are discussing is not generally done for the aforementioned reasons, and unless you already know how to prevent such problems it's simply not a good idea.
     

  15. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    41
    Mar 1, 2010
    Kent, OH
    Yeah, don't just go cascading gain stages, especially center biased, bypassed (or unbypassed) gain stages. It will sound like ass. At least mine did.

    Your original idea of JCM800 is a good one, it will sound like a Marshall, though, not a Bassman.

    That needn't be a deterrent. Some folks prefer to learn this by doing this. And that's part of the experience and dare I say it, fulfillment, of doing a project yourself.
     
    Last edited: Aug 10, 2017

  16. Phrygian77

    Phrygian77 Tele-Holic

    588
    Apr 30, 2016
    Crawfordville, FL
    Well doing the JCM800 preamp will require significant layout changes. That's why I went the Trainwreck route. Maybe I got lucky that it ended up working really well. The voltages are different from an Express, not to mention the whole power amp is completely different. The only change I made after the fact was changing the stock 500pf PI input cap to 1000pf.
     

  17. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    That's part of the several issues I was talking about. The type of project discussed isn't a bad one for someone with amp building/design experience, but the OP is starting from zero.
     

  18. Peterquelle

    Peterquelle Tele-Meister

    178
    Aug 1, 2012
    austria
    Thanks for all the inputs :) I looked the trainwreck schematic and the thread from Phrygian77. It seems the trainwreck gets its distiortion from the poweramp, right? The preamp gain-stages seem to have lower gain than a stock bassman bass channel because of the higher cathode resistors and the lower grid resitors on the last two stages ? Or is there something I am missing completely ?

    Edit: I guess I was on a wrong trail the whole time. I saw a video on youtube where I got the idea to add the gain-stage, but I missed that the guy modded the normal channel to a 3 gain-stage channel. So basically he converted the normal channel to a bass channel or something like that...
     
    Last edited: Aug 11, 2017

  19. Phrygian77

    Phrygian77 Tele-Holic

    588
    Apr 30, 2016
    Crawfordville, FL
    The cold clipper (that's the 3rd gain stage with the 10k cathode resistor) starts clipping at about the same time as the power amp (at least it did when I modeled it in LTspice), so some of the harmonics are also coming from that stage. I'm going to prove this with my scope as I soon as I get a chance. It's fairly crunchy by the time your up to 5 on the dial, and by 8 it's full on. The normal channel is mostly clean, and LOUD, on 5. Another thing to keep in mind is that the TW tone stack has very little mid scoop (almost none), so that extra amplitude for the midrange frequencies will add additional harmonics as the later gain stages start to clip.

    I don't understand the 'lower grid resistors' part of your question. I think you may be confusing something else for a grid resistor.
     

  20. Peterquelle

    Peterquelle Tele-Meister

    178
    Aug 1, 2012
    austria
    By grid resistor I meant resistor the goes from the grid of the tube to ground. Usually 1Meg I guess... But anyway, I looked around the net and gathered information. And I stumbled upon a used DIY Bassman pretty cheap, The whole is certainly cheaper than buing the parts seperatly, because the head is about what the 2 transformers and the choke would cost. So I thought about buying that and rebuilding it, or if it sounds nice enough just mod it or just try to learn something from it. But the seller did not build it himself and has no idea which bassman circuit it is. The odd thing about it, it has 9 tubes. And I cant find out which model it might be based on. Any idea ?

    Bassman_Poweramp.JPG Bassman_Preamp.JPG
     

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