Looking for 36w mods

Discussion in 'Shock Brother's DIY Amps' started by soggybag, Jul 18, 2019.

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  1. soggybag

    soggybag Tele-Meister

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    I built this 36w TMB combo a couple years ago from a kit. It works and sounds okay. I’ve identified a few things I’d like to change.

    1) it gets really loud after about 1/8th of the range on the volume pot. This applies to both channels. The TMB channel is especially touchy. What can I do about this? I’m pretty sure the volume pot is a A500K. I think I’ll check this first in case it’s B500K by mistake.

    Q: How can I make the volume pot work across a usable range? I have a Bassman that shows a nice smooth gradual range.

    2) I’d like to get more bass out of the amp as a whole. I’m not sure where to start.

    Q: would C2 and 3 be a good place to start, or C6 and 13?

    3) the TMB channel tone stack lacks range. It would more useful if the controls had range.

    4) the amp is really loud.

    Q: is it possible to easily implement a half power switch that took two of the EL84 offline?

    [​IMG]
     
  2. JD0x0

    JD0x0 Poster Extraordinaire

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    1. Make sure you're using audio and not linear taper, but this sounds relatively normal. Amps are loud and IME, the volume can be touchy, without a master volume taming things down. Non-master is essentially like having a master volume amp with the volume on '10' and using the 'preamp' or 'gain' control to adjust volume. Sometimes you can compensate for this with special pot tapers to smooth things out in the lower portion.

    Are you using R37?


    2. You wont gain any perceivable bass with those caps from what I can tell. If they're 50uF they're fully bypassed already, and more bass isn't obtainable. C1/C14 looks like the only area where you'd pick up much percievable bass. C2 and C3 look a bit on the small side, but I don't think you'd pick up much bass with an increase, according to my calculations, and you risk blocking distortion/flub if you go too high. What cab are you using? An open back combo is naturally going to have a weaker bass response than a sealed cab would.


    3. Check your wiring. With the cathode follower that tonestack should have a fairly decent range. (You'll get more response from the treble and bass controls if the mids are set lower) A malfunctioning tonestack also may explain the lack of bass.


    4. Yeah it's easy, but if you ask me, it's not going to help that much. Halving power reduces output by about 3dB which is just on the threshold of being perceivable, for most people. You might be better off with some sort of master volume.
     
  3. soggybag

    soggybag Tele-Meister

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    Hey thanks for the reply.

    I think I’m using R37, and if I recall I changed it to 1M. It’s been a while since I built this. I seem to remember thinking it was really loud after building it I changed this resistor to try and tame the volume. It did make much difference.

    It’s an open back cab with a single Weber 12” blue.

    The math is a little over my head but I think I get it. Going from 10w to 100w would create a perceived double in volume so going from 36w down to 18 is not much change at all.

    How would you go about adding a master volume?
     
  4. Wally

    Wally Telefied Ad Free Member

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    You could try a 5751 or a 12AY in V1. This will change the sweep of the volume pots...while decreasing the maximum amount of gain. If that improves the way the volumes work for you, then you have options. For the TMB channel, which looks a lot like a 5F6A Bassman circuit, you could increase the size of the bass tone cap....C11...to add more low end. You could apply a bypass capacitor across R38 on the cathode of V1B to bring more lows in earlier.
    Do some research on master volumes. Our brother Robrob...google Rob Robinette amps....has some good tutorials. The Valve Wizard will have some good insight. I vote for an attenuator because I prefer power tube involvement, but MV’s can be useful.
     
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  5. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Swapping to a lower sensitivity speaker will help with the volume. I have been using the Celestion EVH model in my 18 watter and it does help to tame the volume slightly. The EVH is the same model as their Heritage Greenback but at a slightly lower price.

    If your volume pot has a bright cap installed across it, removing the cap will help to smooth the taper slightly. This PEC pot has the smoothest, most even volume taper of all the pots that I tested in my amp. A little pricey but worth it...https://www.tubesandmore.com/products/potentiometer-precision-electronics-audio-slotted-shaft

    Many TMB channels will include a pre phase inverter master volume which will also help with your volume control issues. Check out Robs sight for suggestions on how to install one. You can also opt for a post PI MV that will allow for better volume control over both channels. You will need to drill another hole in the chassis to install either type of MV though...https://robrobinette.com/Generic_Tube_Amp_Mods.htm
     
  6. soggybag

    soggybag Tele-Meister

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    Thanks Wally, that’s great advice. I think you’re right an attenuator might be the best choice and doesn’t require any mods.

    I’d like to see what’s involved with installing a master volume. This might be the best choice for taking the volume and would give more tonal possibilities.
     
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  7. soggybag

    soggybag Tele-Meister

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    Hey dan40, that is exactly what I was looking for, and like magic I asked the question as you posted the answer. This forum is prescient!

    It looks like I could probably implement the type-3 master volume on my amp without too much trouble.

    I’d need to find a location for the extra pot. I’ve seen people use on of the inputs for this. Might be better than drilling another hole.
     
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  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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    I prefer to use a home built attenuator with my 18 watt. This way I can get the power section working a little bit and not be blown out of the room. I built the "B" version of this attenuator circuit that was designed by JohnH over at the "MarshallForum". It was fairly cheap to build and really retains the amp's natural sound at 4 different levels of attenuation....http://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/page-4
     
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  9. soggybag

    soggybag Tele-Meister

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    I bought a cheap attenuator, I think it’s just a rehostat in a box. I think I’ll plug that between the speaker and the amp and see what it sounds like.
     
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