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

Thoughts on a Bassman Micro to 5e8-a micro idea

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Oct 29, 2017.

  1. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    I built @robrob 's bassman micro circuit last year, its a great little amp and I am happy with it. But recently I've been going back to all of my old builds and tweaking things and this one may be next on my list...

    Today, for whatever reason, I started reading about tweed twins, both high and low power and thinking about the differences between the various cathode follower tweed amps. The bassman and tweed twins are all closely related, with differences in the number of power tubes and some preamp differences in the 5e8a. So I started thinking about taking a couple of ideas from the 5e8a and trying them in the micro bassman.

    Basically I am thinking about adding a switchable triode gain stage with NFB between the master volume and the phase inverter. I would remove the bright switch on my build (I don't really use it much) and replace it with a dpdt switch which I would use to switch the signal from the master volume wiper from its normal path to an extra triode gain stage modeled after the 5e8a gain stage right before its phase inverter. I built my bassman micro in a princeton reverb chassis, so I have a spare noval socket hole to use and it would be reasonably easy to find room for the extra components for the simple gain stage. I would run the nfb to this new gain stage and I would then be able to switch between a non NFB bassman preamp and something somewhat like the 5e8 preamp (though with the 5f6a tone stack).

    One thing that has struck me about the bassman micro is the use of a cathodyne PI instead of a LTP as in the original 5f6a. Obviously this was done because it works well with the 12bh7 power section and honestly I can't argue with the results, but that and the cathode bias are the two places where the micro diverges from the big amp. Related to this, I notice that Rob removed the negative feedback in the most recent version of his design, because not having a LTP PI meant that there wasn't a good place to put the NFB. Adding a post tone stack gain stage in front of the PI as in the 5e8a, gives you a perfect landing spot for the NFB and the extra gain stage may be a good match the cathodyne PI since that is how it was done in the low power twin.

    Does this make any sense, or am I just up too late obsessing about amps again?
     
    Last edited: Oct 29, 2017

  2. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    Here's a drawing of what I'm thinking:

    IMG_20171029_023522.jpg
     

  3. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    Hmm, I thought this would generate a little more interest...
     

  4. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I like all of your ideas, give it a try. Be prepared to play the NFB level to get the tone right.

    (I thought I posted this yesterday but I must have forgotten to hit "post reply")
     
    Nickfl likes this.

  5. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    I did some calculations and (I think) a 33K NFB resistor should be a good starting place to get me something equivalent to the NFB in the 5e8a. This is taking into account that I am using the 16ohm tap on my OT.

    I also addressed the issue of the unused second triode on the tube I will be adding. Following the philosophy of "too much is not enough" I have added a "fat" switch to allow me to use that extra triode in parallel with the other one. I have put a smallish bypass cap on the parallel triode to add a little extra variety to the fat switch option. I have left the negative feedback off of the parallel triode cathode, but I don't know if that a good idea or not, so I'd be interested to hear opinions on that.

    Here's a drawing:

    IMG_20171031_012802.jpg
     

  6. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    I bought some parts and went ahead and tried it out. I ended up making a couple of changes from the drawing I posted originally.

    Wired according to my original schematic, everything worked fine with the switch set to stock bassman micro, but I was getting severe blocking distortion with the 5e8a mod active and even worse with the parallel triode switched on. Adding a dedicated grid leak resistor in front of that section eliminated the blocking distortion, but I was still getting a significant unintended volume boost. I ended up adding a 270K grid stopper before a 68K grid leak resistor to cut the signal to the grid down to 1/5, which gives me a similar level on the 5e8 mod as on the stock bassman micro, but still a significant boost when the parallel triode is switched on.

    I also ended up moving the signal tap to a point before the master volume rather than after as I had it arranged in my earlier schematic. This should make the master volume more usable with the extra gain stages engaged and allow it to behave more like it does in the stock design.

    Here is my updated schematic:

    IMG_20171110_005500.jpg

    And a picture of the wiring:

    IMG_20171110_004724.jpg

    Not my most attractive layout, but it works!

    I haven't had a chance to play it other than for a couple of minutes while testing, I'll report back once I have a chance to really use it and decide if it was worth the effort.
     
    awasson and robrob like this.

  7. awasson

    awasson Friend of Leo's Gold Supporter

    Age:
    53
    Nov 18, 2010
    Vancouver
    Cool ideas! The micro Bassman is next on my list of amp projects. I’ve been looking at it for a while and I was just looking at it again on Rob’s website yesterday assembling a parts list. Looking forward to hearing more of your opinions about the mods you made.
     
    robrob likes this.

  8. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    The micro bassman is a great little amp and you will be happy with it with or without these mods. After playing it at full volume for a while I do think it is a worthwhile, but not game changing mod (this is referencing the 5e8a style gain stage, not the parallel triode addition, which we will get to later). Honestly, the amp as Rob designed it is so good that there isn't a lot of room for improvement and I think the main advantage of what I have done here is that it gives you a better way to implement the NFB and the opportunity to add a presence control. Other than that the change to the actual sound is minor. It makes the amp sound a little hotter and emphasizes the high end a little, but fundamentally its the same tone... which isn't surprising since the real bassman and LP twin are pretty closely related amps. If I were doing it again, I would probably use a trim pot in the input to the 5e8 gain stage grid since I ended up needing a voltage divider to reduce the signal there and adding a trimmer would make dialing that in easier.

    As for the parallel triode switch, I am not as into that and might even consider changing it somehow. It provides a noticeable volume boost, but little else and just really doesn't strike me as all that useful. I think it might be better used as a cascaded rather than parallel stage and I might try that out if I feel motivated to do the significant rewiring that would require. If I did that I would probably use a smaller coupling cap and maybe some signal filtering that would be switched out with that gain stage. If I get around to actually trying it, I'll definitely post it.

    I will try to record some clips of the amp in the next couple of days and post them here for reference to give you and idea of what they actually do for the amp.
     
    robrob likes this.

  9. Nickfl

    Nickfl Tele-Holic

    684
    May 24, 2016
    Florida
    I actually went ahead and switched the parallel/boost option to an additional cascaded gain stage after the "5e8" triode. It wasn't all that much work to rewire it, but it took me all afternoon yesterday to get it to work properly. This modification adds a huge amount of gain to the circuit and made it squeal badly. I ended up having to use a trim pot setup before the grid, a 100pf snubber between the signal path and ground and used a shielded cable for the leads to and from the "5e8" switch, which is located up by the input jack. Even with all of this I had to switch the tube for these two stages from a 12ax7 to a 12ay7 to fully tame the squeal. I imagine that if I were doing this from scratch rather than cramming it all into an existing build I could avoid a lot of this trouble with better lead dress, but it ended up taking a lot to compensate for the rats nest it has become in there.

    Anyway, I recorded a clip of the current (I think finished) version of the amp. The whole thing is with the bridge pickup of a LP, the speaker is a 1x12 greenback clone in a closed back cab. No effects or processing, just the guitar, the amp, and a sm57 mic.

    It starts with the gain at noon on with the regular micro bassman circuit, then:

    0:50 - 5e8 mode, gain at noon
    1:10 - 5e8 plus extra gain stage, gain at noon
    2:20 - Stock Micro Bassman, gain at 3 o'clock
    3:35 - 5e8 mode, gain at 3 o'clock
    4:00 - 5e8 plus boost, gain at 3 o'clock
    6:00 - Stock micro bassman, gain max
    7:00 - 5e8 mode, gain at max
    8:45 - 5e8 plus boost, gain at max



    The 5e8 mode is not hugely different from the stock micro, but I do think I have a slight preference for the mod. The boost gain stage makes more of a difference, almost creating a fuzz effect with the gain maxed. Overall, its not a game changer, but I do think it was worth doing considering it only cost me about $15 worth of parts and a tube I already had lying around.
     
    6stringcowboy and robrob like this.

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Wow, you've got a great sounding amp there with a lot of different tones and overdrive available. Very nice, congrats.
     
    Nickfl likes this.

  11. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    Or simply not to end up with an unused triode?

    Strangely enough, there are amps with unused triodes, check out Trainwreck.
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The phase inverter is from the earlier 5E6A Bassman and the extra drive from an LTP just isn't needed with the tiny bottle power amp.
     

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