My 18w TMB Kit Build

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
Now that I finished the JTM45/Bassman Micro build the 18w TMB kit I ordered months ago arrived. It’s a Mojotone kit and so far everything is going well. One thing I don’t have is a power transformer. They are still backordered with no ETA on when they’ll get one. But, they were kind enough to send me everything else so I could at least start on the project. Unlike my other builds, I’m not planning to make any special mods to the circuit along the way other than a couple of wiring differences.

Fortunately the head cabinet I built fits correctly. Since I already had a bunch of threaded rivet nuts I decided I’d use those for mounting the chassis into the cabinet. Those holes are now drilled and the test fit looks like it’ll work.

This board layout is pretty tight. I’m not super happy with the one capacitor I had to put on but at least everything is in place and well-soldered.

@andrewRneumann, you’ll be happy to know there are no jumpers on the back side of the board. I’m also happy about that. :)

@2L man I’m thinking I’ll order this Antek power transformer to use until the Mojotone/Heyboer PT comes. I know you mentioned the EZ81 may need something like 47ohm current limiter resistors, but I still don’t fully understand how sizing of the PT works. Before ordering an Antek though I should probably make sure I understand how all this would work. I’ve tried to study up a bit on transformer power requirements but still it’s not all making sense. https://www.antekinc.com/as-1t300-100va-300v-transformer/

Here’s how things are looking so far…

C611334D-B2DF-470F-96B1-504BCB00B937.jpeg
ECD7CE4B-7CAA-4301-9D81-D4192D937366.jpeg
6280DCC5-1CB1-4ED5-B196-D63546178368.jpeg
B8230972-EAE8-4497-A1FD-808DF25D5C2C.jpeg
AC3E09CF-D2C4-4709-8B15-86A80987F838.jpeg
3F45DE0B-23EA-4EA8-AC00-7AAC74CD49AE.jpeg
D00278F0-BFE8-4FB0-A98A-558E0978C4CB.jpeg
C71FC415-873E-4763-B530-D289CB4507A1.jpeg
8C0E9E9A-44A0-4139-9BA3-C0FCFE2E31A0.jpeg
82BB74A6-9082-49AD-BCB8-11E6952D579F.jpeg
 

BenTobith

Tele-Meister
Joined
Jan 3, 2020
Posts
236
Location
Austin, Texas
I don’t know why so many cap makers insist on making their 68nF caps comically large. The voltage rating is way overkill, which doesn’t help.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,556
Age
62
Location
Finland
@2L man I’m thinking I’ll order this Antek power transformer to use until the Mojotone/Heyboer PT comes. I know you mentioned the EZ81 may need something like 47ohm current limiter resistors, but I still don’t fully understand how sizing of the PT works. Before ordering an Antek though I should probably make sure I understand how all this would work. I’ve tried to study up a bit on transformer power requirements but still it’s not all making sense. https://www.antekinc.com/as-1t300-100va-300v-transformer/
Antek is great company because they list few voltage outputs for different currents. Obviously HiFi listening builders are more demanding than instrument playing builders ;)

I think Antek "load" the AC output using just resistor so it open some speculation because on rectified and filtered power supply PT output current produce current pulses which width change. For low power the current is small and only the tips of rectified sine wave output are used to charge filter capacitors. When amplifier produce more power it use more current and the DC tends to drop and then current flow longer. Sine wave width comes wider when the voltage get lower.

I recall the voltage drop from zero current was 19V when current was 0.31A and then output voltage was 291VAC. This make RMS power 90VA. The VA is almost like power W but phase shifting between voltage and current has a role but 18W amp won't use 90W from Mains where also filament is included. So I think that antek output stay about 300VAC when amp is dimed. Perhaps during power chords PT output drop below 300VAC?

Then I just searched EZ81 datasheet and assumed power tube bias about 2x28mA and few mA for pre amp and followed 300VAC curve to 60mA and there output is 350VDC.

I think it is good practice to install current limiter resistors to rectifier tube anodes. Reason is modern transformers which wire insulation coating is thinner than lacquer is, Mylar insulation is thinner than oiled paper. Iron core is purer and plates are smoother and stack more dense than old transformer cores were. And when coil is machine wound it comes tighter. All this produce higher efficiency transformer which does not Sag as much as transformers did when tube technology was developed almost 100 years ago.

I think biggest difference are Eletrolyt Capacitors which internal resistance has become much smaller. This mean they can deliver hign peak current but also take more current when they charge and increase current peaks!

It is often said new production rectifier tubes are not as reliable than NOS, but NOS are expensive so I think it is good to limit their peak current. Current limiter resistors increase Sag but for me the Sag-effect is only reason why I use tube rectifier in the first place :)
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,556
Age
62
Location
Finland
Antek toroids are quite a lot bigger and heavier what same VA european toroids are. 60Hz require about 15% smaller core that 50Hz transformers so they could be smaller and when core is smaller less wire is needed. Perhaps this is also because of HiFi use where weight is not much of an issue? Anyway now after I have followed you I think you won't have any problem to find use it later when "correct" PT arrive ;)

I have participated to few other builders theads where is interest to use toroid PTs as Output Transformers and if you are going to order that 300V PT and postage won't double consider buying also 50VA low voltage output toroid which are only $12. They have two separate 115V primarys so technically they become a push pull OT. They have few outputs options but for example 6VAC winding ratio produce about 10k impedance for 8 ohm speaker. Idle output is usually bit higher than the rating but if it is 6,5V, then 230VAC is used winding ratio calculation 230/6,5=35 and 35x35x8=10000. If their datasheets have zero current output voltages they should be used to calculate very accurate winding ratios. 230V should be used as primary calculation because two primary coils are used in series for push pull.
 
Last edited:

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
Antek toroids are quite a lot bigger and heavier what same VA european toroids are. 60Hz require about 15% smaller core that 50Hz transformers so they could be smaller and when core is smaller less wire is needed. Perhaps this is also because of HiFi use where weight is not much of an issue? Anyway now after I have followed you I think you won't have any problem to find use it later when "correct" PT arrive ;)
Haha. I swear this is my last build, promise. ;) …at least it’s the last one that I have planned.

I think it is good practice to install current limiter resistors to rectifier tube anodes.
The rectifier I’ve got is an ElectroHarmonix 6CA4. I think I could use, like I did with the Bassman Micro project, the 1N4007 diodes from the PT’s secondary output before they connect to the plates of the rectifier tube … as current limiters, right? I’ve got a couple extra of those.
 
Last edited:

BenTobith

Tele-Meister
Joined
Jan 3, 2020
Posts
236
Location
Austin, Texas
The rectifier I’ve got is an ElectroHarmonix 6CA4. I think I could use, like I did with the Bassman Micro project, the 1N4007 diodes from the PT’s secondary output before they connect to the plates of the rectifier tube … as current limiters, right? I’ve got a couple extra of those.
On a 6CA4/EZ81 you can use the "NC" pins to attach the protection diodes. You want the stripe of the diode connecting to the plate end (pin 1 and pin 7).

On mine, I've used pin 2 to go to pin 1 and pin 8 to go to pin 7. In that case, the PT HT leads would connect to pins 2 and 8 instead of the plate/anode pins (1/7).

I used 1N4007 on mine. I found that beefier diodes have leads too thick for noval socket pins, but you could use them for an octal rectifier socket's pins.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
I wonder if I should have just ordered a spool of Kester solder in the beginning. Haha. I’m about out so I’ll put it on hold for a couple days, but I did get quite a bit done this weekend on it.

The front panel is 95% wired (missing a PT connection), the heater wires between power and preamp tube sockets are done, and V1 is all connected.

I didn’t want to run a buss ground wire across the back of the pots so instead I have a ground lug right near the input jacks on the far right and then a terminal strip that suspends the ground buss wire between the pots and the board. I think it worked out pretty well and I tried to keep all the wiring there nice and clean.

1654495163008.jpeg


Here’s maybe a better picture of the buss wire before I wired everything up. It’s hard to see the lug that connects it to the chassis, but it’s there. I connected the bus wire to one of the ground terminals of the input jacks so it’d be suspended at the right level.

1654495396397.jpeg

1654495494252.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
I have participated to few other builders theads where is interest to use toroid PTs as Output Transformers and if you are going to order that 300V PT and postage won't double consider buying also 50VA low voltage output toroid which are only $12.

That’s an interesting idea. I looked through their stuff and tried to make sense of which one I could order and couldn’t make sense of it — I assume for a future build or some sort. They do sell output transformers as well but they aren’t as low as $12.
 

mountainhick

Tele-Meister
Joined
May 2, 2021
Posts
426
Age
64
Location
Rocky Mountains
That’s an interesting idea. I looked through their stuff and tried to make sense of which one I could order and couldn’t make sense of it — I assume for a future build or some sort. They do sell output transformers as well but they aren’t as low as $12.
He is talking about their power transformers. Start on post #6 here: https://www.tdpri.com/threads/small...-16-test-data-attached.1099367/#post-11399331

I ordered a couple to play with. might be some time before I get to it, but will post my thoughts once i do.
 
Last edited:

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,556
Age
62
Location
Finland
That’s an interesting idea. I looked through their stuff and tried to make sense of which one I could order and couldn’t make sense of it — I assume for a future build or some sort. They do sell output transformers as well but they aren’t as low as $12.
There is a phrase "flea hifi" where HiFi builders try to find solutions to save cost because HiFi iron is much more expensive than guitar iron. If they have succesfully used an OT for hifi we can use same transformer at least double or even quadruple power for guitar. Hifi 20Hz vs guitar 80Hz.

Antekink 10W HiFi toroid OT should be good for at least 40W guitar amp but I think even for higher power. It is not cheap but against 40W guitar OTs it is cheaper. And looks like they might have them in stock? Obviously because of toroid OT frequency range go higher there comes interesting airy chime to guitar sound.

This is just one of the threads there came when I searched "flea hifi toroid"

 

Cyberi4n

Tele-Afflicted
Joined
Oct 28, 2020
Posts
1,007
Age
49
Location
Chester, Uk
On your turret board - I notice that you've wrapped the cap ends around the turrets rather than insert them into the turrets themselves then fill with solder, as I've seen others do.

Any particular pros/cons to doing it either way?
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
On your turret board - I notice that you've wrapped the cap ends around the turrets rather than insert them into the turrets themselves then fill with solder, as I've seen others do.

Any particular pros/cons to doing it either way?
I am no expert and this is only the second time I’ve built a board with these type of turrets. In my extremely limited experience it seems like it uses less solder and is easier to solder with the wrap around the side of the turret. Also, for aesthetics I wanted to only use the top hole for connections from off of the board.

From a couple of the things I’ve read about soldering these turrets you should try to use the bottom section first and also keep multiple wires in separate sections, if possible. I tried to do that where possible and only doubled-up if there were more than two things connecting to a single turret.

The other thing I like with the wrap around is that it’s a better mechanical connection. The components held themselves in place before I added solder.

One other thing I’m not positive on but it may help with strain relief to do the wrap around the sides. Someone with more knowledge and experience should comment on that one.

Here’s a video I found about soldering turrets from one of the 18watt.com’s posts on soldering technique:

 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,455
Location
Salt Lake City
On your turret board - I notice that you've wrapped the cap ends around the turrets rather than insert them into the turrets themselves then fill with solder, as I've seen others do.

Any particular pros/cons to doing it either way?
You’re right, ‘caps in the hole’ are sometimes used to make them easier to change out, either due to age (e-caps) or, more often, to mod (coupling caps). A downside of using the top hole at all is its tendency to drink solder. Hard to argue with the nice results here, in any case.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
Haha, I realized looking back that I had taken pictures of the board oriented the wrong way 180°. Here are a few shots of where I'm at right now. More Kester solder arrives today. I've got an AnTek power transformer on order that should arrive later this week.

IMG_0945.jpeg
IMG_0946.jpeg
IMG_0947.jpeg
IMG_0948.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
So here's a couple of close-up shots of the turret joints. They aren't perfect, but should get the job done. In the second picture here I'm not super happy about how I connected the 22nF cap, but there wasn't room between the turret posts. I guess I could have had it be higher off of the board and had the leads come from the inside instead of flying outside. I dunno. It should "work" at least.

IMG_0917.jpeg
IMG_0916.jpeg
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
Another thing I’m not sure how well it’ll work but on V1 I decided to follow the advice that the grid resistors from the input jack should be close to the tube socket instead of mounted on the input jack. So I added a couple of terminal strips near V1 and ran the input wires as shielded wires. The terminal strips gave a nice grounding point for the shielded wire (grounded on just that side to the chassis). But, when I was wiring V1B the shielded wire I cut was a bit too short so the distance between where it ends and pin 2 is a bit long. That also made the plate connection wire on pin 1 run a little closer to V2. I dunno if it’ll be a problem, but I may clean that up later if it seems it’s causing issues. Also you can see some PVC melt on the wires on pin 1 & 3. Bad form. ;) But it probably won’t affect the sound.

1654613456729.jpeg
 
Last edited:

BenTobith

Tele-Meister
Joined
Jan 3, 2020
Posts
236
Location
Austin, Texas
I’m a little concerned about the wires on pins 7 and 8 of V2. They look like they are almost touching? You’ll want to trim things up a bit and try to make sure none of the pins or their wires have too loose of ends or are touching (except pins 4-5 on V1-3 obviously).
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
I’m a little concerned about the wires on pins 7 and 8 of V2. They look like they are almost touching? You’ll want to trim things up a bit and try to make sure none of the pins or their wires have too loose of ends or are touching (except pins 4-5 on V1-3 obviously).
Good call. They weren't actually touching, but I've tidied them up:

IMG_0950.jpeg
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,455
Location
Salt Lake City
Your photos are as detailed, and tidy, as your build -- strong work. And nice job getting the grid resistors right on the pins -- widely recommended, but we don't see it done much.
 

joulupukki

Tele-Meister
Joined
Nov 26, 2020
Posts
354
Location
Utah
All that’s left is the power transformer.
E9B3099B-A10B-4C90-A389-EB644AEFEA98.jpeg


Here’s a close-up of the speaker output jacks & impedance switch wiring:
A9F75065-BAE0-48FF-9FB8-C7FDD9D28294.jpeg


The 1N4007 diodes (current limiters) are ready to go on the rectifier tube socket. I’ll connect the power transformer’s HV to pins 2 and 6:
72D4DB4F-99A3-487A-BD37-B9E5A33994F3.jpeg


Knobs are all on the front panel:
1032C6D4-EAB7-4132-9C9C-1B9AAC96C9CD.jpeg
 




Top