Journey of my Bassman Micro build

[joulupukki]

In terms of the perfect labels ... there's always next time. Indicator, input, etc. ...the intention was to copy a lot of what I've seen on Marshall amps. They may not be 100% accurate, but since there are a bazillion knobs on this thing (heh), I'll just twist them all 'til I get cool tones. Good points though and thank you all for explaining exactly what's going on in the circuit.

 

[joulupukki]

Before I go drilling into the chassis, I'm working on a to-scale layout for everything. I've got a few questions:

1. I rotated the preamp and power amp tube sockets 180° vs. what @robrob has in his layout diagram ... so that I can have the heater wires fly into pin 4 from the back of the amp through the gap between pins 1 and 9. I'm assuming this is okay to do? (wires not yet drawn)

2. There's a lot of space front to back. Should I mount the tube sockets closer to the board or centered between the board and the back of the amp? I've got them closer to the board because it seems like that may help to keep the wires from the board shorter.

3. The pilot light comes with wire leads on it already. I'm planning to use a terminal lug mounted to one of the PT's screws to solder the heater wires together. I assume this is good?

4. The power transformer, choke, and output transformers are drawn to scale and will be mounted to the top of the chassis (opposite the board). I assume the positioning here is generally okay to do? I'm not sure if the OT should be where I've shown it or maybe between the tubes and the back of the amp?

5. I'm planning to have the main preamp grounding bus wire go across the top turrets of the board. The alternate way would be to solder it across the back of all the potentiometers like I did on the Mojotone Princeton Reverb. Is one way better than the other?

6. Besides drawing all the connections, what general main things am I not thinking of? (panels are already etched, so those things can't be easily changed)

Also, thank you to anyone more experienced than me for your help!

 

[King Fan]

I like your planning. In an amp like this, with no set standard layout, I find it hard to over-plan. The littlest detail can cause trouble -- my favorite is the way board mounting nuts want to hide under the OT.

Speaking of OT, I'm gonna leave most of your questions for guys who do freestyle amp layout. Having mostly built Fender clones, I've never had to sort issues like OT location / transformer hum.

I assume you got the 6.3V indicator light? A tag strip to tie things to the heater secondaries is logical, but it doesn't have to go on a PT bolt; a #6 screw hole can go anywhere. I'm a bit confused about your transformer. I think you said which one you'd got, but I forget. Is it the laydown kind? Is that what you've drawn there? More generally, have we seen a pic of your chassis? Is it cut out and punched already? Your tube location question says no?

As for a Mojo/Marshall ground bus on the back of the pots, lots of folks still do that, but it has no advantage over a freestanding bus, and it has disadvantages. Merlin condemns its potential to create multiple small ground loops; practical drawbacks are it's a pain to solder to pots and it's even more of a pain to unsolder them if you need to swap out a pot.

 

[andrewRneumann]

my favorite is the way board mounting nuts want to hide under the OT.

Or the OT mounting nuts hide under the board! I have had to drill holes in my board just to get to the darn things.

 

[King Fan]

Or the OT mounting nuts hide under the board! I have had to drill holes in my board just to get to the darn things.

LOL, good one. I didn't mention that mostly cuz I've never had them *not* hide under the board.

 

[andrewRneumann]

Once you have the actual dimensions of the mounting situation for the magnetic components up top, lay those out accurately. @King Fan and I are lolling about the forehead smacking going once you see a screw that is impossible to install and/or tighten.

Just a few general opinions (not rules) about the layout. I would move the OT up to align its centerline with the centerline of the PT. I would also give a little more space for the choke in case you need to change its alignment for hum killing. Consider reducing the tube spacing a little by moving the power tubes a little to the right--it's nice if they stay in the vicinity of the OT. Leave the rectifier way off to the left like you have it.

I would keep the board close to the pots, but not so close that it becomes troublesome to work on. These pots, switches, and input jack(s) need some space and we don't want physical interference from the board. Some mains switches can get pretty large depending on the style. I would also keep the tubes close to the board, but again, give yourself some space to work. You should have easy access to all hardware and plenty of space to work in a soldering iron. You've got the space--if in doubt, space it out.

I like your orientation of the heater pins. I would probably not run the heater leads all the back to the back of the chassis for each tube. Just run them down an inch or so away from the valve socket, keep them pressed against the chassis, and then make the turn towards the next socket.

 

[Hyakuya]

I recently rebuilt my Champ micro and ended up with a loud hum. Turned out it was the OT / PT orientation. I used the headphone method to find a new quiet spot for the OT.

Of course that involves lots of unsoldering, removing the whole board and drilling new holes to mount it.

I also ended up with a board mount screw under the corner of the PT and another one under the OT..

I recommend using the headphone method to check the layout before you start drilling.

This was my Baseman layout... I think I centered the tubes between the board and the back of the chassis.

 

[2L man]

5. I'm planning to have the main preamp grounding bus wire go across the top turrets of the board. The alternate way would be to solder it across the back of all the potentiometers like I did on the Mojotone Princeton Reverb. Is one way better than the other?

Both are quite bad!

It is not good to use potentiometer cases to flow operative current soldering Common to them. When pots are mounted to metal Chassis they act as Shields with Chassis.

When you mention "pre amp grounding" and on layout the Common ground bus is not thru whole board length you make pre amp current "leak" to Chassis and that is bad idea as well. Although pre amp current is small it will pulsate and transfer electromagnetic noise to instrument cable shield and some of it transfer to instrument cable hot and then to amp input. Also when Chassis surrounds amp circuit and if it pulsate skme of it get to amp circuit. This is because Mains Safety Earth wire has some resistance and reactance.

Correct path for return current is shortest path to power supply. Thru single Common bus or few Star wires. On Star there is one wire for B+1 (Cathode current) and B+2 (Screen current) and then one for each following B+. Last B+ Common is connected also to Chassis using steel jack or ground lug when jack is insulated. There inevitably come some electromagnetic noise to Common but when Chassis is connected to Safety Earth it get "shorted" to Earth best possible way. Next you hope your band member gear do not use Safety Earth for return current.

For Negative FeedBack I use shielded cable which shield act as Common and its both ends are soldered, other to output jack and other to Common close where hot is soldered.

 

[joulupukki]

Thanks guys! One thing I forgot to say in case it wasn't obvious, but I have to keep things that protrude off the top of the amp in from each side about 1" ... because of the supports for affixing the front and back panels of the cab. Otherwise, I would put the PT farther to the left, etc.

I like your planning. In an amp like this, with no set standard layout, I find it hard to over-plan. The littlest detail can cause trouble -- my favorite is the way board mounting nuts want to hide under the OT.

Heh, well, I'm not sure where else to put them. But this is why I'm asking.

I assume you got the 6.3V indicator light?

Yup. I opted for the 6V Marshall-style indicator light as opposed to the 120V.

A tag strip to tie things to the heater secondaries is logical, but it doesn't have to go on a PT bolt; a #6 screw hole can go anywhere. I'm a bit confused about your transformer. I think you said which one you'd got, but I forget. Is it the laydown kind? Is that what you've drawn there? More generally, have we seen a pic of your chassis? Is it cut out and punched already? Your tube location question says no?

Chassis is undrilled. PT is the Hammond 270AX and mounts to the top of the chassis.

As for a Mojo/Marshall ground bus on the back of the pots, lots of folks still do that, but it has no advantage over a freestanding bus, and it has disadvantages. Merlin condemns its potential to create multiple small ground loops; practical drawbacks are it's a pain to solder to pots and it's even more of a pain to unsolder them if you need to swap out a pot.

Ok, that's what I was thinking too. I *was* a pain to solder them to the back of the pots on the PR build. Didn't wanna do that again. Hope I never have to change one out on that amp.

I recommend using the headphone method to check the layout before you start drilling.

Will do. I saw that mentioned elsewhere, but will make sure to do that before busting out the drill bits. Thank you.

This was my Bassman layout... I think I centered the tubes between the board and the back of the chassis.

View attachment 986610

@Hyakuya Is your OT here in-between your rectifier tube and power tube(s)?

I'll go back to the drawing board and try out a few more things but what y'all have said makes sense and points me in a good general direction.

 

[joulupukki]

Just a few general opinions (not rules) about the layout. I would move the OT up to align its centerline with the centerline of the PT. I would also give a little more space for the choke in case you need to change its alignment for hum killing. Consider reducing the tube spacing a little by moving the power tubes a little to the right--it's nice if they stay in the vicinity of the OT. Leave the rectifier way off to the left like you have it.

@andrewRneumann One of the reasons I put the OT lower and not centered with the PT is that according to the specs, the wires are maybe only about 5" long. Once I get it I'll double-check that. Otherwise the wires may not reach where they need to go.

I would keep the board close to the pots, but not so close that it becomes troublesome to work on. These pots, switches, and input jack(s) need some space and we don't want physical interference from the board. Some mains switches can get pretty large depending on the style. I would also keep the tubes close to the board, but again, give yourself some space to work. You should have easy access to all hardware and plenty of space to work in a soldering iron. You've got the space--if in doubt, space it out.

Good points. I've left about an inch of spacing from the front of the chassis before starting the board. I'll double-check that once I have all the parts.

I like your orientation of the heater pins. I would probably not run the heater leads all the back to the back of the chassis for each tube. Just run them down an inch or so away from the valve socket, keep them pressed against the chassis, and then make the turn towards the next socket.

That makes sense and will work well. Thx!

 

[Hyakuya]

@Hyakuya Is your OT here in-between your rectifier tube and power tube(s)?

Yup

 

[joulupukki]

Yup

Ok, good to know. Maybe I can squeeze the preamp tubes a little closer together and put my OT down there similar to what you've done. I'll check it with the headphone test first, but that may be a good option. Thanks!

 

[joulupukki]

Would the general location of the choke & OT in this layout be a good starting place? For reference, the major lines in the grid shown are 1" apart. The small lines are 1/8". This one has the three preamp tubes squeezed together a little more with 3/4" spacing between them. I guess that'd mean that the wires coming off the board would have to be a little longer to reach them instead of being right near the place they service (at least for V1 and V3).

 

[joulupukki]

A tag strip to tie things to the heater secondaries is logical, but it doesn't have to go on a PT bolt; a #6 screw hole can go anywhere.

Roger that. The only reason I put it there is because that screw will already be there.

 

[joulupukki]

Have been thinking...

I like the preamp tubes spacing on my earlier layout. What if I was to use JB Weld to affix the nuts that hold the transformers that would be underneath the board? That way they'd still be relatively easily serviceable ... and the preamp tubes could be spaced out better (to line up with their connections to the board and make them easier to get your fingers around them to remove/replace)?

 

[2L man]

I bought a "rivet nut tool" and think it has been very good ~$40 investment for transformer mounting to chassis and chassis mounting to cabinet.

 

[joulupukki]

I bought a "rivet nut tool" and think it has been very good ~$40 investment for transformer mounting to chassis and chassis mounting to cabinet.

Ah cool. I've never heard of those but sounds and looks interesting.

 

[joulupukki]

I haven't punched in all of the values for the caps/resistors (have that handy on Rob's layout diagram), but here are a couple of layout ideas I'm thinking.

In the first one, the PT is separated a little bit from the preamp tubes and the choke is mounted behind the board.

In the second one, the preamp and power tubes are all equally spaced and the choke is moved down. I guess I could probably move the choke down in both layouts since there's room. I'll use the headphone trick to see if this is a suitable position for the OT in relation to the PT.

I'm not sure which of the two options would look & perform best, but probably better to have at least one transformer (the choke) not have to have any screws hidden by the board. Is it generally better to keep more distance between preamp and power tubes or does that matter much?

 

[Len058]

I think it's better to rotate the tube sockets so the filament wires can stay somewhat out of the way of other wires. The pre-amp tubes are probably ok but the power tubes can have a nice path to the back of the chassis. You can run them between pin 1&9 but it's quite crowded over there.

Edit: Nevermind you probably don't need the artificial center tap resistors so there's room

 

[joulupukki]

I think the Hammond 270AX doesn’t have a 6v center tap, unless I’m reading it wrong?