Journey of my Bassman Micro build

robrob

Poster Extraordinaire
Ad Free Member
Joined
Dec 29, 2012
Posts
8,611
Location
United States
Those wires will work correctly. I have seen two or three jacks with three grounds that ground out part of the secondary.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,982
Location
Cincinnati, OH, USA
If 27K primary impedance would be “ok” then I’m definitely curious to see how using #4 as the common wiring (ground for both 4 & 8 ohm) would affect the behavior.

There was a thread about this recently. I believe the consensus was go for it, no big deal. Make sure you have wires 2 and 8 going to the top terminals of the switch. NFB from wire 2. Wire 4 can go to the bottom middle terminal in junction with the black wire--or you can leave the bottom terminals empty and just send wire 4 directly to the jack sleeves. With all this testing going on, remember to not switch impedance while anything is being amplified.

Oh and you will have to swap the OT primary leads because the phase of the NFB is now inverted with the swap of wires 2 and 4.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
There was a thread about this recently. I believe the consensus was go for it, no big deal. Make sure you have wires 2 and 8 going to the top terminals of the switch. NFB from wire 2. Wire 4 can go to the bottom middle terminal in junction with the black wire--or you can leave the bottom terminals empty and just send wire 4 directly to the jack sleeves. With all this testing going on, remember to not switch impedance while anything is being amplified.
Yeah, that thread may have been me. Sorry for all the noise. This OT is making more sense ... but only a little. I usually turn the amp off entirely before flicking the switch (and plugging or unplugging one speaker).
Oh and you will have to swap the OT primary leads because the phase of the NFB is now inverted with the swap of wires 2 and 4.
Yeah, I was wondering about that. I guess it's no biggie. I need to get better at not making the ends of the wires be a tiny bit melted. But I'll give it a go and see how it sounds.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
Bingo! We have a winning combination that works!

So, I could technically use a SPDT switch, but I’ve just left the DPDT switch in there and connected #4 (Yellow) to common/ground. The 4 ohm setting is now serviced by #5 (Violet) and the 8 ohm setting by #2 (Orange).

The presence knob works and so does the NFB switch in both 4 and 8 ohm. It’s a little more obvious in the 8 ohm mode, which is probably how I’ll be running this amp most the time anyway. I think I’m also going to leave the 2.6K resistor in for the JTM45 setting since there’s more of a drastic change between OFF and that position. Not too much change between OFF and 5F6A. But, at least it works.

I did have to switch the OT primary wires around so that the phase didn’t get reversed (apparent on the presence knob being backwards).

The #6 (White) wire gets a new permanent home zip-tied up outside the board area.

Once again, thank you for helping me through this. This really is a spectacular amp!

490ED0FE-FAE3-4856-9DEB-0FFB5962E10E.jpeg
24463237-BB73-4E62-AA6F-D7DC4EB034CB.jpeg
D3BAB2A9-3B85-4B01-A1C9-A2FFB5389C6C.jpeg
 

willholt92

Tele-Meister
Joined
Jun 13, 2019
Posts
307
Age
30
Location
Sheffield UK
Wow, this thread is putting mine to shame. Absolutely amazing work, and so fast haha.
Sounds killer from the sound bites and videos, it's been good to see you trouble shooting and the great advise from the very knowledgeable guys on shock bro's.

Top work, I'm making sure I am going to the workshop on my lunch today and making some progress on my cab haha.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
I’m curious, with the 12BH7 power tube, would I need to use a different cathode resistor to make it take full advantage of it’s power capabilities? When I put it in before just to test it, it didn’t sound any louder than the 12AU7. Or would that be more of the case of needing to use a different output transformer?
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,982
Location
Cincinnati, OH, USA
I’m curious, with the 12BH7 power tube, would I need to use a different cathode resistor to make it take full advantage of it’s power capabilities? When I put it in before just to test it, it didn’t sound any louder than the 12AU7. Or would that be more of the case of needing to use a different output transformer?

Brief me again on what your plate voltage is, what cathode resistor you have now, and what OT primary resistance you went with?

If I recall correctly, these micro amps dump a ton of signal at the power tube grid so they don't overdrive at volume 1. The BH7 probably could take a little more input signal at the grid, so those attenuating resistors would have to be adjusted.

Upshot--it's not just a matter of plugging in a different tube and *presto* louder amp. I guess you demonstrated that empirically didn't you.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
Yeah, just trying to understand & learn how it all works.

Plate-to-Cathode voltage: 302.3 V
Cathode Resistance: 816 Ohm

Actually, I think those ^^^ are the 12AU7. With the 12BH7 the values are…

Plate-to-Cathode voltage: 296 V
Cathode Resistance: 816 Ohm (obviously same)
 
Last edited:

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
I had left over tube socket retainer bases from my Princeton and 18w TMB build so I added them on here. Even though I don’t plan to put the covers on it does seem to make it more stable/sturdy when you insert them or take them out.
9C7E99DE-003F-4893-97DE-FB7BA6A25567.jpeg
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,982
Location
Cincinnati, OH, USA
Yeah, just trying to understand & learn how it all works.

Plate-to-Cathode voltage: 302.3 V
Cathode Resistance: 816 Ohm

Actually, I think those ^^^ are the 12AU7. With the 12BH7 the values are…

Plate-to-Cathode voltage: 296 V
Cathode Resistance: 816 Ohm (obviously same)

Looked at the load lines for the 12BH7. Similar input headroom as the 12AU7. Probably the only thing you need to do is drop the cathode resistor value to 550-ohms or thereabouts to bring up the bias current. Twice the power doesn't mean twice as loud, and the 12BH7 doesn't even have twice the power as the 12AU7. Still, you should hear a difference.

Maybe bump up the grid leaks to 100K on the power tubes as well.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
Looked at the load lines for the 12BH7. Similar input headroom as the 12AU7. Probably the only thing you need to do is drop the cathode resistor value to 550-ohms or thereabouts to bring up the bias current. Twice the power doesn't mean twice as loud, and the 12BH7 doesn't even have twice the power as the 12AU7. Still, you should hear a difference.

Maybe bump up the grid leaks to 100K on the power tubes as well.
Good to know there's that option. That's kind of what I was thinking (dropping the cathode resistor value). I think for now I will just play with it ... meaning ... play my guitar with it. If I need to get louder, I've now got the way-to-loud 18w TMB. I'm sure I will still experiment with mods here and there though and this might be a great candidate. ;)

One thing's for sure, I can't thank you guys on this forum enough for all the amazing help you've given me with this and my 18w build. I have found the entire process, learning about how it all works, etc., completely thrilling and very rewarding. Now I need to learn some more riffs to do these amps justice!

Oh, and I still need to figure out how to create a 3D file for printing my "Molinello" on all of these cabs. That should finish them all off pretty nicely.

IMG_1045.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
I thought it'd be cool to put a little label inside of the cab on the micro amp. Wow, it's tight in there! :) Should I just use something like 3M Super 77? Maybe just standard Elmer's glue? Just thinking of what I have on hand to mount it.

IMG_1061.jpeg
IMG_1062.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
Couple of things…

I’ve had a couple of house jams with the 18w and this micro. No drums but playing with a bass and two guitars. The micro is holding its weight. Of course it’s not as loud, but definitely loud enough for what we were doing. The jazz musician who was here preferred the tone of the micro. It definitely sounded good. Also, with both amps the Mojotone British Vintage BV-30H 30w was preferred over the Mojotone Greyhound 70w speaker. I thought that was interesting.

After building my 18w and seeing the different tube holder style (the low-profile 5x20mm holder) I ordered a Littelfuse 3455LS2 for a few bucks on eBay. The Fender-style one I had in there stuck out of the back farther than the edge of the amp head cab, which bothered me. Plus, after my fuse-blowing fiasco on the 18w, I have plenty of 5x20mm 1A slow blow fuses. Now the two amps use the same exact fuse which is nice.

C4AB7B6B-575A-45EA-8DA6-A493307C9603.jpeg
E2F86607-4EA5-497F-86EB-E5031B8768F1.jpeg
944DC51A-58B8-4EC9-986C-FC7319FAF230.jpeg


For whatever reason, the elephant-style tolex I ordered from Mojotone is a lot shinier than the stuff I had from a different supplier. I would have preferred it to match, but can’t complain with how it all turned out. :)
8DDD2215-E6A4-4336-AF7A-77DA3188F657.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
323
Location
Utah
We've been modifying our app (Volley) so that you don't have to have an account to view the conversations in a space that has been marked public. That means if you want to you can watch all the little play-by-play videos of me building the cab and the amp right here (works best right now from a desktop web browser). Click this link to get to it and here's a little screenshot of how it looks when you get there. If it doesn't start playing, just click the play button.

1655953017847.png
 
Last edited:




Top