12au7 Bassman Micro build

[King Fan]

Yes. I asked Rob about this, and he noted the 250uF was intended (from the original *Bass*man) but not important; that 25uF will sound the same — maybe only theoretical differences.

 

[Kev-wilson]

A little further on, I’ve the circuit board to cut to length next and PT to position and drill for, then I want to stretch a diylc file to the board size to mark the turret holes from then adjusting to put a couple of new turrets in too (and a bus Lane )

 

[Kev-wilson]

Ok, I think I see the question. Yeah, a 100K dual-gang pot with an appropriately high resistance safety resistor (470K) seems correct. You still want the 470K "grid stop" resistors.

2off 470k safety resistors added in parallel to a 100k pot gives me the number 70kohms, which is very near the original 68k grid leaks, if I'm following what's going on and the safety resistors are in parallel as I think they are?

If I've got that right I'll be pleased

 

[andrewRneumann]

2off 470k safety resistors added in parallel to a 100k pot gives me the number 70kohms, which is very near the original 68k grid leaks, if I'm following what's going on and the safety resistors are in parallel as I think they are?

If I've got that right I'll be pleased

Looks like you’ve got it my friend.

 

[Kev-wilson]

This 55 year old was doing childish fist pumps in the kitchen after reading the above this morning whilst the coffee brewed

And this amplification malarky is sorcery at work so getting an insight into what's happening is a great feeling

 

[Lynxtrap]

For those wondering about cathode bypass caps. This is from ValveWizard, it shows the effect of different cap values for a 12AX7 with a 1.5K cathode resistor and 100K plate resistor.

 

[Kev-wilson]

More questions if I may

My first valve rectifier & choke and 3 more valves than the Micro Deluxes I made so I'm unsure of placement as the chassis is homemade, I can see the different layout styles on Robs site but wondered if there are 'rules' involved to minimise hum?

The output & power tranny's core's are at 90' to each other and I dig the reasoning why, but is there a rule for the choke as I can see them pointing both way (and the choke's not polarised?), and how far from the PT would the rectifier like to be?

I suppose that breaks down to 'what are the minimum spacings for these parts?

I've done some reading on the choke, and think I've learned it's used instead of a resistor to preserve the B+ voltage as there's more voltage drop in a resistor/capacitor filter than a choke/capacitor one so is preferable when rectification is done by a valve due to the voltage drop (& valve rectification limits the capacitor size) and isn't necessary for solid state rectification's small voltage drop and the filter/reservoir capacitors can be much bigger as they don't overload the ss diodes.

 

[Lynxtrap]

IMO a choke is not necessary for a micro amp with a 12AU7 output stage that draws relatively little current. Others might have a different opinion.

Chokes often sit right between the PT and the OT, and at least in many Fenders they are mounted in the same direction as the OT (90 degrees to the PT). I don't think it matters much really.

Place it as close as possible to the circuit point where it is connected and you should be fine.

 

[Kev-wilson]

View attachment 1052567
That’s where I’m at with the faceplate, the unmarked circle to the right is for a bright switch, I also want a tone stack on/off switch, if I go under the bright switch (above when wiring as the bottom is open) my wires are longer than if the switch was near the mid pot, but I want that space for graphics!

There will be a 1/4” border around the outside from a wooden panel so my spacing allows for that, the return jack needs to come up but what are the ‘rules’ or best practice for this wiring?

Usual edit, my 3rd amp and so far I've ordered the wrong control knobs each time, the 16mm ones (5/8") look a little small, last time I found 5 'Dakaware Davies' 1510's on ebay, but I'd forgotten the name, remembering them to be American I've just searched long and hard for a set of 'Delaware Baker' knobs until the penny dropped.

 

[Kev-wilson]

Is there any reason that layout won’t work? The 2 small circles bottom left would be an nfb switch and tone stack cut and I get room for nice graphics, the v1 bypass cap switch would go to the rear.

I’m using 16mm Alpha pots.

Edit, it's very tempting to see how much it'd cost to have a faceplate in stainless or aluminium engraved, I have an old work mate in another town who I think got into laser stuff so may chase him up, if not then it'll be done 'Uncle Doug' stylee

2nd edit, the number dial centre is off but that's on another sheet as I'll use the drill template one to create the faceplate graphic from to keep the hole centres together, it's just one so I could check it'll fit.

 

[King Fan]

View attachment 1052826
Is there any reason that layout won’t work? The 2 small circles bottom left would be an nfb switch and tone stack cut and I get room for nice graphics, the v1 bypass cap switch would go to the rear.

I’m using 16mm Alpha pots.

Heh, it looks nice. I'm not aware of what would keep it from working -- but my awareness may be limited. Mine was made Fender-style, 'upside down' in a Marshall chassis, and so I'm not able to rotate all the wiring paths, for example. And my brain is too stubborn to let Rob's Marshall-ized layout (flipped north-south) help.

Folks who've built in a right-side-up Marshall chassis may have better insight; our friend @joulupukki is good at wiring details, I know.

On a much less important note, how are you actually labeling your three volume knobs? I suspect you saw my goofy thread about their sonic functions, and how Rob's 'bright' volume pot was just a carryover from the original two-channel Bassman which also had a normal channel. Folks suggested lots of good (and many quasi-humorous) options there. As someone said, it doesn't matter; you'll know what they do anyway. I'm just curious.

 

[Kev-wilson]

Heh, it looks nice. I'm not aware of what would keep it from working -- but my awareness may be limited. Mine was made Fender-style, 'upside down' in a Marshall chassis, and so I'm not able to rotate all the wiring paths, for example. And my brain is too stubborn to let Rob's Marshall-ized layout (flipped north-south) help.

View attachment 1052876

Folks who've built in a right-side-up Marshall chassis may have better insight; our friend @joulupukki is good at wiring details, I know.

On a much less important note, how are you actually labeling your three volume knobs? I suspect you saw my goofy thread about their sonic functions, and how Rob's 'bright' volume pot was just a carryover from the original two-channel Bassman which also had a normal channel. Folks suggested lots of good (and many quasi-humorous) options there. As someone said, it doesn't matter; you'll know what they do anyway. I'm just curious.

Cheers I used it as a template on a Weetabix box so with all the parts in place I'll get the iron in ok if I work in order and as for labelling, mmm, I'm not a Fender purist by any means so I'll wait to see what size font/how many letters I've room for before I give them names although the micro Deluxes I built have the pre amp volumes named as gain as I couldn't fit 'master volume on 2 lines so called the latter 'volume'. I'll aim to label them as per Rob.

I've only ever played one amp with a presence control, a ss Marshal and sounded poor wherever/however it was set

I emailed my old workmate and he confirms he owns a co2 laser but hasn't replied to my faceplate proposal yet, tho I have some persistence in my tool bag so will work on that.

 

[King Fan]

Yeah, from that thread, I actually ended up with Rob's suggestion: Volume, Preamp, and Master or MV...

 

[joulupukki]

Is there any reason that layout won’t work? The 2 small circles bottom left would be an nfb switch and tone stack cut and I get room for nice graphics, the v1 bypass cap switch would go to the rear.

Yeah, seems like it should work fine. It might get tight wiring the pots and the only thing really to consider there is serviceability in the future. At the bottom of this post there’s a picture of the front of my amp.

 

[Kev-wilson]

Yeah, seems like it should work fine. It might get tight wiring the pots and the only thing really to consider there is serviceability in the future. At the bottom of this post there’s a picture of the front of my amp.

Thanks so much that photo there helps too

I'm a 'meddler', but have hole drilling experience, usual holes I drill are to drawings, so I'm not used to choosing where they go, and if I did work to a layout I'd drawn, it'd be for something I'd designed so I'd have some intuitive waving of warning flags before putting them in the wrong place, here I'm finding out things like 1 guy has his rectifier 'here' and another guy's is 'there' (yours is around where I'd like to fit mine ) so I'm blundering around trying to identify benefits and pitfalls of component placement...

Bless Rob for the 12" x 7" x 3" chassis suggestion as there's room to work in there!

 

[joulupukki]

I'm glad the photos helped! 3" deep would be nice. I used the Hammond 1444-1372 (13" x 7" x 2"). It definitely got cramped trying to fit the Carling power switch in there without it getting too close to the edge. Which reminds me, I should probably add some heat shrink around those terminals just to be on the safer side.

 

[Kev-wilson]

I emailed my old workmate and he confirms he owns a co2 laser but hasn't replied to my faceplate proposal yet, tho I have some persistence in my tool bag so will work on that.

A funny old world but my old mate reckons to have done some custom faceplates for 'Matamp' and mentions "used a laminate sheet to engrave the top layer off giving white letters on a black background or other colors".

Out of interest (and not electronics) can anyone elaborate on what a 'laminate sheet' is? Or is it likely to be less than 1.5mm thick (1/16") and would let me get a black faced white lettered plate?

My mate was always brighter than me and I hate to appear ignorant/don't want said ignorance to lead to mistakes!

I think the logo will move to the left and the nfb switch around and above the bass knob, and instead of a tone stack cut the mid can get a 100k audio taper pot.

I stretched the layout to suit the board before remembering I need extra turrets in

 

[Kev-wilson]

That'll be the front, my jigsaw wandered roughing the wooden trim but the palm router should save it (hope so as the oak's now 10mm thick from 20mm and I hope to avoid having to thin another plank :/ )

I've emailed my mate the faceplate drawing I've drilled to, red circles for cuts and black for engraving, most likely in black laminate with white writing as he doesn't sound keen on stainless steel :/ failing that a friend of a friend has a diode laser at home he doesn't know how to use so I've been invited to try and set it up for him, if I took a fire extinguisher with me it can't be that hard can it?

NFB, sounds good so when I read up on it, it seems there's a bit of uncertainty on the resistor's values, reading suggests a value between 2.6k to 6k for the 'Marshall' NFB resistor and the 15k one for the Bassman so I'll have to find a 2.6k one, and at the back there'll be an on/on switch for the bypass cap, 220uf and 2.2uf, I'm going to use a 100k pot for the mid as there's no room for a tone stack cut switch

I'm enjoying this

 

[Kev-wilson]

I'm glad the photos helped! 3" deep would be nice. I used the Hammond 1444-1372 (13" x 7" x 2"). It definitely got cramped trying to fit the Carling power switch in there without it getting too close to the edge. Which reminds me, I should probably add some heat shrink around those terminals just to be on the safer side.

I've re-read your thread I remember it starting but at the time I was looking at building my first amp and the Bassman was too complicated for me so I admired your woodwork photos whilst scratching my head at terms such as 'ltp & nfb' etc, I mean, a long tailed pair when there's the one valve, and 'negative feedback' had me imaging a screech going from high to low.

Anyway, I was starting to stress about nfb resistor values but you've given me some values to work with for the jtm feedback and your wiring saves me two turrets if I copy it!

I love this site

 

[ping-ping-clicka]

I'm preparing to start an attempt at the RobRob Bassman Micro, I've ordered the parts from Tube-Town in Germany so will have the correct OT/choke and the PT is the Hammond 370AX which has a CT on the 6.3v secondary, the rectifier valve is made by 'Zaerix' but it was that or a JJ one which was 10 euro cheaper so thought I'd play it safe.

The valve bases will be the ones mounted from the top unfortunately (Belton VT9 ST2) I'd have preferred the internal mounts but it'd have meant another order elsewhere.

I've oak left to make a cab with that matches the cab one of my Deluxe Micro's uses, and have ordered a 1mm sheet of aluminium for the chassis (1.5mmm is a pain to bend without a proper bender )

I'd like to use an NFB switch and make the pre amp bypass cap switchable and possibly have a switch on the input jack for lo/hi inputs but will wait till I've a chassis in hand to see what room I've got

I'm looking forward to this

Usual edit, I'd like the version 2 'lar mar' volume arrangement so have a 250k dual pot on its way, but it's linear not a log one as they had no log ones.

I have no idea what your talking about building an amp from scratch and being discriminating about the parts you use indicative of caring and I say, GOOD ,Great I am proud of you guys that build amps and old fashion reverbs for yourselves. I am happy for you.
You know every time some guitar players post about building gear from scratch it helps end the myth that all guitarists are dumber than drummers.