Fixing a Mojotone TMB kit before I even start

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cmatte82
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Re: Fixing a Mojotone TMB kit before I even start

Post by cmatte82 »

I just realized I don’t have the cap across those pins like in the ceriatone layout. What purpose would that serve? Should I add that?
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Re: Fixing a Mojotone TMB kit before I even start

Post by Bieworm »

cmatte82 wrote:
Mon 03/20/23 8:30 pm
I just realized I don’t have the cap across those pins like in the ceriatone layout. What purpose would that serve? Should I add that?
I don't see a cap across any pins... what do you mean? Or do you mean the standby switch? I never put a cap there..
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Re: Fixing a Mojotone TMB kit before I even start

Post by JMPGuitars »

Bieworm wrote:
Tue 03/21/23 4:20 am
cmatte82 wrote:
Mon 03/20/23 8:30 pm
I just realized I don’t have the cap across those pins like in the ceriatone layout. What purpose would that serve? Should I add that?
I don't see a cap across any pins... what do you mean? Or do you mean the standby switch? I never put a cap there..
He's talking about V6 - the rectifier tube. I don't know what purpose it serves in that position.
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Re: Fixing a Mojotone TMB kit before I even start

Post by cmatte82 »

Bieworm wrote:
Tue 03/21/23 4:20 am
cmatte82 wrote:
Mon 03/20/23 8:30 pm
I just realized I don’t have the cap across those pins like in the ceriatone layout. What purpose would that serve? Should I add that?
I don't see a cap across any pins... what do you mean? Or do you mean the standby switch? I never put a cap there..
Yes I put the safety diodes on the rectifier tube like you suggested previously. I used the ceriatone 18w tmb layout as you also suggested. Unless I completely misunderstood and did the wrong thing. Which wouldn’t surprise me at all. Haha.
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Re: Fixing a Mojotone TMB kit before I even start

Post by chaliapin »

I thought I read somewhere the cap is too reduce noise from the diodes? Does adding the safety diodes affect the functioning of the amp at all?
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Re: Fixing a Mojotone TMB kit before I even start

Post by Bieworm »

Use ultra fast switching diodes… no noise there
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Re: Fixing a Mojotone TMB kit before I even start

Post by cmatte82 »

chaliapin wrote:
Tue 03/21/23 3:21 pm
I thought I read somewhere the cap is too reduce noise from the diodes? Does adding the safety diodes affect the functioning of the amp at all?
The safety diodes don’t seem to have changed anything. The tone is still the same. Volume is the same. I have noticed an occasional short burst of a buzz, but it’s very short and seemingly random. I’ve thought it was due to my phone being close by. Maybe not? In retrospect it seems to only happen in the first few minutes I’ve turned it on. Or maybe I t’s because I get to jamming and just can’t hear it? I’ll probably order a cap and see what happens. The one measurable change is the voltage at pins 1 and 7. I’m getting about half as much voltage as before. And on the other side of the diode I measure 0 volts. But the output is the same. I get about 360v on the output. I used a zener diode to lower the b+ down to the recommended 345v.

I finally got to look at the MV on the tmb channel. It shows to be a 1 Meg A pot. So it should be a log taper. I even measure it and it shows to be logarithmic. One leg is ground. Probably a silly question. But what would happen if I swapped other two legs? The guy at Mojotone mentioned a linear taper pot. But that doesn’t make sense to me.

Anyway I took some more pics once I put it back together. Maybe this will make the issue more understandable. I took a piece of sticky paper and put it behind the mv pot. I made a mark that lined up with it at “0”. Then the next mark is when the volume is “just starting to come up”. Then at loud enough to “jam with a drummer”. The last is where it seems to be at “full blast”. Honestly there is very little difference between the “jamming with a drummer” and the “full blast” levels. There is a lot of difference in level between “just starting to come up” and “jam with a drummer” settings. And that range is where I want more control. But is where there is the least amount of rotation in the volume knob.

Any help would be great appreciated.
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Re: Fixing a Mojotone TMB kit before I even start

Post by cmatte82 »

Bieworm wrote:
Tue 03/21/23 3:47 pm
Use ultra fast switching diodes… no noise there
I used uf4007 diodes. I believe those are supposed to be the ultra fast version of the 1n4007 diodes used in the ceriatone layout . I believe they’re vishay brand. I got them from mouser. So I figure they should be good ones.

And I have no idea why the picture is sideways. It’s not on my phone. Anyway it does show the “useful” range of the master volume.

Oh and fwiw I’m playing through a closed back 212 with Celestion g12m25s of that helps anything.
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Re: Fixing a Mojotone TMB kit before I even start

Post by Bieworm »

cmatte82 wrote:
Tue 03/21/23 4:09 pm
Bieworm wrote:
Tue 03/21/23 3:47 pm
Use ultra fast switching diodes… no noise there
I used uf4007 diodes. I believe those are supposed to be the ultra fast version of the 1n4007 diodes used in the ceriatone layout . I believe they’re vishay brand. I got them from mouser. So I figure they should be good ones.

And I have no idea why the picture is sideways. It’s not on my phone. Anyway it does show the “useful” range of the master volume.

Oh and fwiw I’m playing through a closed back 212 with Celestion g12m25s of that helps anything.
UF4007 are perfect.
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Re: Fixing a Mojotone TMB kit before I even start

Post by JMPGuitars »

Safety diodes wouldn't make an improvement in tone; the only alteration would be adding noise (if they were 1N4007 instead of UF4007 for example). The cap won't likely make any difference.

My guess is that you're seeing a difference in voltages at the rectifier because of the difference between reading AC and DC voltages. Check the AC voltage at the PT side of the diodes to see what your AC voltage is.

Try a 500K audio taper pot for the volume pot.

Thanks,
Josh
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Re: Fixing a Mojotone TMB kit before I even start

Post by cmatte82 »

JMPGuitars wrote:
Tue 03/21/23 7:43 pm
Safety diodes wouldn't make an improvement in tone; the only alteration would be adding noise (if they were 1N4007 instead of UF4007 for example). The cap won't likely make any difference.

My guess is that you're seeing a difference in voltages at the rectifier because of the difference between reading AC and DC voltages. Check the AC voltage at the PT side of the diodes to see what your AC voltage is.

Try a 500K audio taper pot for the volume pot.

Thanks,
Josh
The diodes are just for safety. It definitely wasn’t for tone. At least not directly. Once I get the circuit fairly well dialed in, I may do some tube rolling and see if I find a nice combo. And part of that will be comparing SS vs tube rectified. But I’d rather not fry the amp if the tube rectifier goes bad. So that’s why I put them in. And fwiw I ordered some caps too. I’ll add it and see if anything changes.

Not that anything needs to change. Because I was an idiot and measured for DC voltages. The AC voltages going into the diodes are around 309v like my previous measurements.

And for kicks I swapped in a used 500k audio taper pot from a guitar I had lying around. The range is probably a little better. But not much different. So I thought I would tie a 510k resistor across it to see what happens. Kind of turning it into a quasi 250k pot. At least in my mind. I didn’t notice and difference at all. I guess ideally I should mount it fully and mark it to see if I have any more range. Maybe tomorrow.

Guess I’ll order a proper 500k pot. Is there any thing specific I need to know about this pot? Like wattages or anything? Would a regular pot for a guitar work? I have some lying around. Maybe even one with a solid shaft. I just don’t want to use the wrong thing and it burn out or something. Thanks.
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Re: Fixing a Mojotone TMB kit before I even start

Post by JMPGuitars »

The pots are generally interchangeable with regular guitar pots. Shafts are the only concern, depending on the knobs you want.
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