Where is the preamp ground in this Mojotone 18w TMB layout?

joulupukki

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My 18w TMB kit should arrive tomorrow. I’m not planning to follow what Mojotone’s layout diagram shows, which is putting the preamp ground buss across the back of the pots. I did that on the PR kit and I now know it’s just a bad idea. Instead, I’ll be mounting a ground buss in between the board and the pots using some terminal posts with a ground lug close to the input jack.

But, my question is, in their layout diagram, where is the actual ground point of the preamp buss to the chassis? What am I not seeing?

 

BigDaddy23

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The ground wire from the preamp is the black one running to the buss between the tone and vol pots. It grounds to the chassis via the pot body when their nuts are done up tight.

Theses TMB’s are great amps btw. Good luck with the build!!
 

joulupukki

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The ground wire from the preamp is the black one running to the buss between the tone and vol pots. It grounds to the chassis via the pot body when their nuts are done up tight.

Theses TMB’s are great amps btw. Good luck with the build!!
Oh duh, of course! Thanks for clearing up my late night brain freeze. So it basically a *lot* of different grounds. I feel dumb for asking the question.

In any case I‘m excited to hear it. It seems like it’s gonna be pretty versatile. Unfortunately they still don’t have power transformers in stock with no end in sight of when they might get some in so I’ll have it all finished soon except for that. I’m excited to get it fitted into the amp head I made … and I hope it fits! :)

AB48A2F5-EECF-4A2B-89D3-9629C3FFEF85.jpeg
 

joulupukki

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If you don't want to wait for their PT, there are alternatives available.
I'm guessing I'd need to convince MT to refund a bit of $ if I went that route. Speaking of, what'd be a good alternative? Doesn't MT use Heyboer Transformers?
 

BenTobith

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I'm guessing I'd need to convince MT to refund a bit of $ if I went that route. Speaking of, what'd be a good alternative? Doesn't MT use Heyboer Transformers?
I think they use Heyboer, which are difficult to come by right now (supply chain issues, I'm sure).

I've used Hammond 290PAZ in 18-watt Marshalls, and they work great. The OT is more important, I think, and I do recommend a Heyboer there from having tried both Hammond and Heyboer OTs.
 

2L man

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Antekinc 100VA toroid PT are $42. Two separate HV output coils and two separate 6,3 filament coils. That reguire tweaking when 5V rectifier tube is used but nothing for EZ81. Mount flat above the hole using wax-thread or tie wraps or make an aluminium plate and use a screws. I hope their mounting kit is neater now. Those on pictures are ugly :(
 

6stringcowboy

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I jut bit the bullet on some Mercury Magnetics iron for an upcoming build. Nothing but positive reviews if you can stand the price. Also Pacific Audio Magnetics. Went with MM as PAM'S output xformer for the 6G3 is 350-0-350! While MM offers a reduced B+ (315-0-315) version.
 

joulupukki

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Antekinc 100VA toroid PT are $42. Two separate HV output coils and two separate 6,3 filament coils. That reguire tweaking when 5V rectifier tube is used but nothing for EZ81. Mount flat above the hole using wax-thread or tie wraps or make an aluminium plate and use a screws. I hope their mounting kit is neater now. Those on pictures are ugly :(
Which model of the Antek PT would you use as a replacement? One of these?


or


Mojotone's TMB schematic shows an EZ81 but their wiring diagram shows an EL84 for the rectifier. Odd.
 

2L man

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Which model of the Antek PT would you use as a replacement? One of these?


or


Mojotone's TMB schematic shows an EZ81 but their wiring diagram shows an EL84 for the rectifier. Odd.
I have began using lower power tube plate voltages than most classic schematics have. Because most amps are way too loud and because I like tube distortion and using lower voltage it is one way to reduce clean headroom. For EL84 amps I use russian 6P14P which are said to be more prone to high plate voltage.

So I would use 275VAC Antek. Antek usually list few voltages for few output currents and I try to think what would idle plate voltage come for this and 300V PT.
 

2L man

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Which model of the Antek PT would you use as a replacement? One of these?

I have used only once a EZ81 rectifier and if I did much measuring I did not find them. Looking the datasheet its voltage loss increase quite a lot when current flow increase.

Now I think this 300V Antek transformer would be safer choice so that power tube Anode voltage does not become too low. It easy to drop High Voltage but difficult to increase. When transformer output is 300-0-300 the DC might be 350VDC when amp idles at about 60mA.

I am not sure is this following transformer output resistance calculation done right here but Antek datasheet list voltages and currents both 120V primarys parallel and input is 120VAC/60Hz. Also both 300V output coils are parallel. Zero current output is 310VAC . When current is 0.31A the output voltage drop to 291VAC. Now direct resistance calculation is (310V-291V) / 0.31A = 61 ohms. So one coil resistance is double, 121 ohms when CT is used.

EZ81 datasheet list minimum resistance for each Anode circuits 190 ohms for RC filtering, when filter capacitor is max 50uF. So at least 47 ohm current limiter resistors should be installed to rectifier Anodes but bit higher value is always safer. Smallest value current limiter I have ever installed has been two 100 ohms and I recall highest was two 270 ohms for 5Y3. Yes they increase Sag-effect but Sag is the only reason I use tube rectifier :)
 




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