Bassman Micro features and functions -- my experience

[King Fan]

I am so deep in these NFB maths right now…

I’ve got the original 5F6A and JTM45 sorted out now, but for the life of me I can’t rationalize why @robrob selected a 15k NFB resistor for the micro. 1+2=4 and it doesn’t seem to provide the proper amount of feedback at the moment, and it’s making me nuts.

As soon as I can grok what’s going on in the Micro NFB and EF80 math, I’ll post the proper resistors for a switched mod.

(Ditto for the switched Micro “lead”/Plexi mod.)

Good question, one I'm even less able to tackle. I looked into this a bit, decided just to try 15K, and found I liked it -- in the '5F6a' 12AY7 setup. Again, I think @joulupukki gets this a lot more than I do -- his comments above are relevant.

What little I know is what Rob says in his 5E3 NFB discussion: "The 56k feedback resistor is correct for a speaker output of 8 ohm and will supply a light level of feedback. You can increase the feedback by reducing the feedback resistor value. A 39k resistor will increase the feedback by 41% and a 20k resistor will give you a Marshall JTM45 2.8 times more feedback. The Fender tweed Princeton amp used 22k feedback resistors and the tweed Harvardused a 56k (both with 8 ohm speakers) so feel free to try anything between 20k to 100k with an 8 ohm speaker tap (10k will give more feedback, civility and clean headroom)." So it sounds like you could tack in a resistor you choose and see how it sounds, then adjust. Or some folks have suggested a pot?

My take on Rob's page is he suggests the 15K because he found it sounded best. I know Rob is very supportive of us micro amp builders. @robrob -- any NFB thoughts?

 

[mountainhick]

King Fan,

Thanks for your threads about your Bassman Micro build. I am in the early stages of building a Bassman Micro EF80 Lar-Mar. You've already answered a couple of questions for me. Your threads have been incredibly helpful. One more quick question, I am planning to use the same Marshall 18 watt chassis you used. However, I will also use a Marshall 18 watt style cabinet which means the chassis mounts on the bottom upside down compared to yours. Rob's website has a Marshall layout for the Micro with north and south reversed. This has caused me to question my spatial awareness because, as far as I can tell, I can use the normal layout as you did and, when flipped over, everything will be in the right place (though the dials and plugs will be aligned in the opposite direction compared to yours). Is this correct? Thanks!

You need to be mindful of the tube pin configurations. The actual tubes will not mirror by flipping a layout diagram.

 

[SerpentRuss]

I've thought about including the FX loop in my next build but I wonder if most pedals can "take the heat". You can easily generate 14 volts peak to peak in that location if you bone-headedly dime your MV. Will that fry the typical reverb pedal?

 

[joulupukki]

Heh, you're asking the wrong guy. I almost melted by brain on the upside-down and backwards thing when I was planning. I *think* you're right. I think I asked this same question and someone posted a pic of a Marshall-built combo with front-mounted controls "upside down". But I also bet @joulupukki can tell us more clearly and confidently -- he built his Marshall style.

I did build mine with the Marshall style in mind and have it set up as a JTM 45 Micro. At the end of this particular post is a picture of the front of my micro: https://www.tdpri.com/threads/journey-of-my-bassman-micro-build.1100749/page-13#post-11431290

Jump to page 1 to have way too much reading. Haha.

When I was drilling the holes in the chassis I realized (fortunately in time) that I needed to print off my layout completely reversed to tape it to the top side of the chassis. I'm glad I caught that before I started drilling holes. It definitely took plenty of brainpower to quadruple check that I was doing it right.

I used a Hammond 1444-1372 (13" wide, 7" deep, and 2" tall) for my build and not an 18 watt size chassis/cabinet. You'd have plenty of room with an 18w chassis for a micro build. Incidentally, right after finishing this JTM45 Micro I built a Mojotone 18w TMB kit (build thread here) in a head cabinet I built (so it'd match the size of my 1x12 speaker cab). Both use the same orientation of input jacks on the right-hand side and power switches on the left (when you're looking at the front).

If I could rebuild my Bassman/JTM45 Micro again one thing I would do for the layout is to put the MID control in between the Bass and Treble (B M T) instead of how Rob has it in his layout (M B T). Not a huge deal but would definitely be a little more intuitive.

 

[joulupukki]

Good question, one I'm even less able to tackle. I looked into this a bit, decided just to try 15K, and found I liked it -- in the '5F6a' 12AY7 setup. Again, I think @joulupukki gets this a lot more than I do -- his comments above are relevant.

Well, don't try to get me to understand the maths of it all ... I'll fall flat on my face. Haha. I can't remember where I read Rob talking about what value of resistor to use for the NFB resistor for JTM 45 option but if I remember right, it was something like 6.6K or 6.8K. In my experimenting once I had it built, I really couldn't tell the difference in sound between the 15K and 6.6K so I lowered the value even more for the JTM45 setting to 2.6K (I had to go look up my post about it to remember). That basically is letting a lot more feedback come back into the circuit and, as @King Fan explained to me in person one day, helps tame the overdrive a bit ... at least if I'm remembering right.

In any case, I usually leave it on the 2.6K setting on my three-way switch (2.6 — OFF — 15K) anyway. But every now and then it's fun to flip the switch around just to see what other kinds of sounds I can get (more breakup when set to 15K and even more raw when it's OFF).

It's a fun amp to build and definitely rewarding ... and the amp I most often turn on when I'm here at home just noodling. If you hit any snags or want me to check whatever reading/value on my build, I'm happy to do so, just let me know!

 

[Lancer X]

In my experimenting once I had it built, I really couldn't tell the difference in sound between the 15K and 6.6K so I lowered the value even more for the JTM45 setting to 2.6K (I had to go look up my post about it to remember).

2.6k would definitely be the JTM45-correct NF resistor in the Micro, assuming that 15k is correct for the 5F6A Micro. (I'm not second-guessing @robrob here - just want to understand the math and having a terrible time showing on paper why he chose a 15k for the Micro Bassman.) Instead of hijacking @kingfan 's thread any further, I'll continue in my own thread here in this subforum.