Fixing a Mojotone TMB kit before I even start

[cmatte82]

Will do. I didn't play it or mess with the cathode resistor yet, as I wanted to make sure the voltages were okay first. Basically I didn't want to chase my tail if changing the voltages would affect the bias. And yes I had forgot to mention I had calculated the dissipation and knew it was even further out. I do have a nice 180 ohm resistor. I'll get it put in tomorrow and report back. I also have a 150 and 160 ohm resistor also. Well I have 2 of each of all 3 values. Building up a selection for future use.

[cmatte82]

Okay so I have installed a new resistor. This got the plate dissipation down to “91.7% per tube” with the output being “11.6w and 96.7% output”. I only remeasured the necessary voltages for these calculations. I didn’t follow the voltages through the caps. Or the voltages at the preamp tubes.

Played on it briefly. I think it sounds punchier and better. But are the tubes still running too hot? Just don’t want to fry anything.

Lastly the master volume is still very touchy. It goes from off, to I can kind of hear it, to jamming with a drummer by about 2. It gets a little louder after that, but by 3-4 it’s maxed and just gets more saturated. Is this tweakable? And I don’t mind how loud it gets. In fact I kind of like that’s it’s not quite killing my ears when maxed. But I wouldn’t mind being able to dial in a lower volume jam kind of volume. Not bedroom/in the house quiet. But it really goes from a whisper to a scream without any real in between. And if it’s not tweakable I’ll get an attenuator. Just curious. And maybe the tubes are still running too hot?

Again many thanks for your guidance and help so far. I think we’re almost there. And I couldn’t have got to where I am without y’all.

p.s. I also got the tubes and parts in for the next amp. And I’ve signed up for Bruce Egnater’s amp building class in May. I think I may have a problem. Haha.

[JMPGuitars]

91% is fine, that's not that far off from our target of 85%. You should take all your voltages though. If your PI plate voltages are still low, you can reduce the 100K plate resistors. Try 82K and see if that gets you to the target. This is optional.

Roll off the guitar volume a little and see how that sounds to you when messing around. What's the volume pot resistance value?

[cmatte82]

Okay. So I swapped the 100k PI plate resistors for 82k as suggested. Then I took another round of voltages. The things I'm seeing, which may not be an issue, are

The v1a plate voltage is 194. The sheet shows 150.

V1b is a little high too at 163. The cathode voltage is only .69 and the sheet shows 1.1

V2a looks close, but v2b is just not close to the sheet at all except for plate voltage. This may be okay??

V3 now looks really close to me. It's still a little low on plate voltage. Would it be worth swapping for something around 70k? or is that close enough?

The EL84's seem to be very close too. The plates are a little higher than the sheet, but that may be fine?

I'm just not sure what the margin of error is.

And the amp still sounds quite good. It may seem a little "grainier" to me, but still not fizzy or anything. I can dial in a nice thick rock tone that I really really like.

If the voltages need some tweaking I'll gladly do that, but the main thing right now is the volume knob. I took some not so great pics. Sorry about that. The chassis was upside down and I kept unplugging so the cable wouldn't be in the way. Anyway the first two pics are the difference between that fizzy barely audible just starting to make noise volume and a decent enjoyable volume. And it takes a pretty steady hand to get there. Then the last pic is around the point of no more volume. Just more saturation. I don't mind it hitting peak volume in the 5-7 range on the dial. In fact that is probably preferable so I can dial in some power tube saturation if needed, but I wouldn't mind a little more usable range along the bottom of the dial. If possible. The pot is a 1meg audio taper. I'm really not sure what to do to change it. But I'll gladly take any advice. Thanks again

And I flipped the first two pics, but for some reason when I upload them they're still upside down?

3-14-23 Voltages.jpg

Barely On.JPEG

Decent Volume.JPEG

Max Volume.JPEG

[Bieworm]

Let your ears be your guide. When you like it, it’s ok. My tremolo TMB V1 voltages are way higher than soec’d too. That’s part of why I like it that much

[JMPGuitars]

I think typically these amps get pretty dirty by 3 or 4 on a 10 digit dial, then maybe a little more volume, then more dirt.

The question is, how do you like what hear through that sweep? If it generally sounds good, cool. If not, determine why.

Sometimes tubes themselves will cause a voltage discrepancy. If swapping V1 and V2 with each other doesn't make a big difference, you can look at the solder quality, and resistor values. Small variations (even up to 20%) aren't that big of a deal. Hitting on the 30 to 50 % variation line might change how the amp feels.

Before I considered any of that though, I would look at dropping about 15VDC from your B+ line going to "A" on the chart. If you're using a 2.2K resistor between A and B, I'd change it to a 1.5K resistor after dropping your B+, and then test your voltages again.

Thanks,
Josh

[cmatte82]

Thanks. I’ll check and see what resistor is there and look at dropping the voltage. I thought it was kind of high, but then the others seem kind of low. But swapping that resistor after lowering the initial voltage makes sense.

And as far the volume goes. Looking at the pic it’s more like on 1.5 when it hits max volume. And from when it just starts to make sound to a comfortable volume is like 2 degrees out of 360 degrees of rotation. Basically you have to either be very careful and slow, or twist it up and down multiple times to get it to a comfortable volume. It really wants to go from literally barely on to nearly full blast within maybe 5 degrees of the sweep. Fwiw I’m not trying to play it inside a house . I’m in a 30’x40’ foot shop out in the country. Too loud only bothers me. But imo it gets too loud too fast imo. I’ve never had another amp that goes from off to blasting so fast. Any guidance here would be greatly appreciated.

Thanks

[cmatte82]

Okay so here's where I'm at. I got another package from mouser today. So I did as mentioned in the dropping b+ voltages post and inserted a 15v zener after the first cap but before it went to any other part of the circuit. My resistor across to the other side of the cap was actually a 2.7k. So I ordered several sizes between 2.7k and 1.5k. But I just went with it and installed the 1.5k to start with. While I was there I also went ahead and installed the uf4007 diodes going to the rectifier as mentioned earlier in the thread. To help prevent damage if the tube rectifier goes. Right now I do still have the Weber SS rectifier replacement in there, but at some point I will try it with a better tube rectifier and see how it goes. I just neglected to order one with my last tube order. At leas the amp will be ready to use it. I think. I say that because the voltages at the rectifier pins 1 and 7 where I installed the diodes are now about half of what they were before? I guess this may make sense if they're rectifying before the actual rectifier? Which makes sense to me, I just wanted to double check that this was expected? Anyway the rest of the voltages are getting really close now. I did a quick jam on it and it still works. I actually grabbed a strat with lower output pickups. And it was a little lacking in gain, but I think that's the pickups. I should have taken one of my regular humbucker guitars. Anyway here's the voltages.

3-20-23 Voltages.jpg

If those all seem alright, then I guess my last issue is the master volume. I contacted mojotone about this too. They said to switch it out for a 1 meg audio taper pot instead of a linear. The layout and schematic call for an audio taper pot. But maybe the did send a linear one. That would make sense. I need to check that. I was too worried about the voltages and forgot to check it. Anyway they did mention I could try a 500k pot too. And it would warm up the sound some. Not sure I need that, but we'll see what I find on the pot they sent.

Thanks again.

[JMPGuitars]

Assuming you're using a 180R cathode resistor, your voltages are much better now. With a 180R, you're at 87.5% dissipation, which is great. If it's lower than 180 ohms, then they're biased too hot still.

A linear pot makes sense based on your description. It should definitely be audio taper for the volume pots. This graphic compares them pretty well:

pot-taper.jpg

I have no idea what you're doing with your rectifier, but the voltages after it are good now.

Thanks,
Josh

[cmatte82]

Thanks. Yes it’s a 180 ohm resistor. And I had checked the dissipation and thought it was good. I just forgot to mention it.

Yeah it being a linear pot would explain it. The one thing I didn’t measure before installation was the pots. And I’m pretty sure I would have noticed if it was a linear pot and it called for an audio taper. But I very well have missed it, or just assumed they knew what they were doing. But I’ll take it out tomorrow and see what it says. And maybe even measure it.

And what I did at the rectifier was install two uf4007 diodes. One each on the red wires feeding the rectifier tube from the PT. Basically like the first two diodes in your layout for the “18 watt tremolo tmb”. But they’re attached to pins 1 and 7. I even bought a stand-off like in the ceriatone layout. I mean if I got them backwards I wouldn’t have any voltage? Right?

313E0A21-62C5-44ED-9151-BAA419011907.jpeg

[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?

[Bieworm]

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..

[JMPGuitars]

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.

[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?

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.

[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?

[Bieworm]

Use ultra fast switching diodes… no noise there

[cmatte82]

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.

8B39A75E-A562-4EE4-A519-AFBA1CE2E083.jpeg

[cmatte82]

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.

[Bieworm]

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.

[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