Just finished the Mojotone TMB kit, PPIMV?

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Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

I might have gone a different route if I had read the forum first but overall I'm happy with the kit. The amp sounds nice and doesn't have a ton of background hum or anything. Has that raw JCM sound on the TMB channel and the V-T channel sounds great.

Right off the bat my first impression is its untenably loud. The master volume is essentially useless. It's very similar to the JCM800s I've owned so I guess I shouldn't be surprised. I'd like to add a PPIMV, would there be any issue incorporating the one in the downloads section to this amp? Any reason the keep the current MV as well?

I'm planning to revise the grounding to match the documentation I've seen posted on the forum. Not that it really hums or anything but I figure why not.

Also, the amp starts squealing around noon on the MV. It could stem from the fact that its really loud and my room is fairly small but my 50watt Fryette Pittbull doesn't squeal at similar volumes. Is there anything that can be done to reduce this? Is it a lead dress issue?

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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by JMPGuitars »

Start by fixing your ground scheme, that may cause or contribute to the the squeal. Then you'll need to chopstick if there's still a squeal.

Mojotone really needs their own support site.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

Yeah I know the more I look at this forum the most posts I see very similar to this one, sorry to add to the mess. I appreciate the feedback. Any thoughts on adding a PPIMV?
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by JMPGuitars »

ejendres wrote:
Sun 03/19/23 1:51 pm
Yeah I know the more I look at this forum the most posts I see very similar to this one, sorry to add to the mess. I appreciate the feedback. Any thoughts on adding a PPIMV?
No, and I wouldn't go that route. IMO PPIMV kills the sound. Either way, you need to sort out the issues before making mods.

I'd suggest starting with getting all your voltages with one of our charts in the downloads section. Voltages will test you a lot about why an amp behaves the way it does.

Once you have the issues sorted out, I would suggest an attenuator if you still can't get good sound at lower volume. We have one in the downloads section you can build yourself.

Thanks,
Josh
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

I just checked the tube voltages; on the whole my preamp voltages look low. I've got them listed below. I also noticed during some testing that the V-T channel picks up a radio station where the TMB does not. I'm guessing that's a grounding issue, hopefully that will be resolved when I rework the grounding scheme.

My B+ voltage is a bit high at 354V. Heaters are 6.5V. I tried to check the other values listed on the sheet for the filter caps but it seems that the Mojotone schematic differs from the schematic in the downloads section. I have a pair of 32uF can caps instead of just one.

V1:
1 - 125V
3 - 0.7V
6 - 132V
8 - 0.6V

V2:
1 - 195V
3 - 0.7V
6 - 180V
7 - 30V
8 - 65V

V3:
1 - 195V
2 - 33V
3 - 65V
6 - 180V
7 - 30V
8 - 65V

V4:
2 - 0.09V
3 - 11V
7 - 341V
9 - 312V

V5:
2 - 0.08V
3 - 11V
7 - 343V
9 - 312V
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by JMPGuitars »

Regarding the radio signal, your input grid resistors should be on the tube side, not the jack side. The closer to the tube, the more effective. Also, your photo has a red wire floating around, not connected to anything. What's that about?
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

JMPGuitars wrote:
Mon 03/20/23 8:55 am
Regarding the radio signal, your input grid resistors should be on the tube side, not the jack side. The closer to the tube, the more effective. Also, your photo has a red wire floating around, not connected to anything. What's that about?
Good to know. I'll switch that around. Any concerns with my voltages?

That was a pic right before I finished, if you look the fuse holder is also missing. It was just a spare piece of wire, it's been removed. I can post a high res pic of it now. I'll do that shortly.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by JMPGuitars »

Those photos are not of a finished amp.

Switch to shielded wire for the inputs. Solder the input tabs, and the pot tabs, etc. Highlighter verify every connection.

Watch the soldering videos in my signature a few times.

Thanks,
Josh
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by cmatte82 »

Hey man. I “finished” my kit a couple of weeks ago. I say that because I’m still tweaking mine too. Thanks to the helpful people here. :) The mv is my last thing to figure out. I contacted Mojotone about it. They seemed to imply I have a 1 Meg linear pot instead of a 1 Meg log/audio pot. I’m going to check it tomorrow and verify. They also said if a 1 Meg log/audio pot didn’t work I could try a 500k log/audio pot.

The main thing to check is the bias of the power tubes. Mine were over 100% when I first fired it up. I’m down to about 87% now.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by Bieworm »

About master volumes...
Lots of people have the wrong idea of what a master volume does. There are 2 common spots in the circuit where you can put them, and they both have their own properties:

1. the pre phase inverter master volume: this is a volume pot right before the input of the phase inverter. It attenuates the signal going into the phase inverter. This type of MV is best applied when
--> you have a preamp overdrive that needs to be tamed to prevent the phase inverter from going into overdrive too much (think JCM800 type preamp,...)
--> there is a cathodyne phase inverter after the preamp. Cathodyne PI's tend to deliver nasty distortion when overdriven. There are ways to handle that, but a pre PI MV will help.

2. the post phase inverter master volume: this is a gain control pot between the PI and the power tubes. The advantage is that you can get the PI to be overdriven and add some nice preamp overdrive to the signal (with a LTP PI!). This MV tames the signal going to the power tubes, resulting in a volume control before the power tubes.

So you can see these types act as an attenuator of signals going to the power tubes. They are only desirable when there is a nice form of preamp overdrive present. In no way these MV's will let you push the power tubes into overdrive. Many people like power tube overdrive and are mistakenly hoping they can achieve this with a master volume... but they just don't!
All a master volume does is attenuate the signal coming towards it and it always ends up with less overdrive and tone suck/affection.

I frequently use a post PI master volume type 2 (Lar Mar) and I'm very happy with those. But I use them to clean up the signal or to drive the preamp (when the circuit has a nice preamp overdrive) + I am aware I don't get to be picky about the tones I get.. like in a home playing situation...
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by cmatte82 »

Thanks for the info on the different types of master volumes. I’m definitely still learning. However being as he has the same kit as me, I’m betting he’s experiencing the same issue I am.

Somewhere around 1 you start to get some volume. Like most tube amps I’ve played it sounds pretty lousy at this point but is so quiet as to be all but useless. But in just the next couple of degrees of rotation like 1.1 to 1.2 (if there was a digital control) it suddenly at jamming with a drummer level. And around 1.4 to 1.5 it’s basically at full blast and not getting any louder.

Most any other amp I’ve played has a usable range of about 1-4 for useable master volume adjustment before it hits peak available volume.

But mine is way too touchy with very little adjustment range. Anyway I’m headed out the door to do some checking and see what I can find on mine.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

cmatte82 wrote:
Tue 03/21/23 1:19 pm
Thanks for the info on the different types of master volumes. I’m definitely still learning. However being as he has the same kit as me, I’m betting he’s experiencing the same issue I am.

Somewhere around 1 you start to get some volume. Like most tube amps I’ve played it sounds pretty lousy at this point but is so quiet as to be all but useless. But in just the next couple of degrees of rotation like 1.1 to 1.2 (if there was a digital control) it suddenly at jamming with a drummer level. And around 1.4 to 1.5 it’s basically at full blast and not getting any louder.

Most any other amp I’ve played has a usable range of about 1-4 for useable master volume adjustment before it hits peak available volume.

But mine is way too touchy with very little adjustment range. Anyway I’m headed out the door to do some checking and see what I can find on mine.
Yup. It's the worst master I've ever used. Wondering if it is a linear/audio taper issue
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by Bieworm »

That TMB master volume stinks big time! It’s useless. That’s one of the reasons Josh designed the Tremolo TMB initially… getting a useful 18 watt tremolo with treble mid and bass controls. If you don’t use trem there’s also the superlite TMB. It’s a bitcdifferent, but a really nice amp with useable volume and gain controls. I vouch for the Tremolo TMB… or my interpretation: the Ironside🤘
772323AB-CA8C-401E-9D18-C8C9A41FE671.jpeg
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by cmatte82 »

ejendres wrote:
Tue 03/21/23 4:24 pm
cmatte82 wrote:
Tue 03/21/23 1:19 pm
Thanks for the info on the different types of master volumes. I’m definitely still learning. However being as he has the same kit as me, I’m betting he’s experiencing the same issue I am.

Somewhere around 1 you start to get some volume. Like most tube amps I’ve played it sounds pretty lousy at this point but is so quiet as to be all but useless. But in just the next couple of degrees of rotation like 1.1 to 1.2 (if there was a digital control) it suddenly at jamming with a drummer level. And around 1.4 to 1.5 it’s basically at full blast and not getting any louder.

Most any other amp I’ve played has a usable range of about 1-4 for useable master volume adjustment before it hits peak available volume.

But mine is way too touchy with very little adjustment range. Anyway I’m headed out the door to do some checking and see what I can find on mine.
Yup. It's the worst master I've ever used. Wondering if it is a linear/audio taper issue
Fwiw I checked mine today. It was an audio taper pot. I had emailed the guys at Mojotone. He first said I should try an audio pot instead of a linear, or even a 500k pot. When I replied and pointed out the layout says it should be an audio taper, he said that was correct, and suggested I try a linear taper pot. Which just doesn’t make sense to me. To me it needs to be an “extra logarithmic” pot. Linear would make it worse in my mind. As has been suggested on my thread, I’m going to get a 500ka pot and see what happens.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

Hey gents. I've done some more work on the 18watt. I shielded the inputs, changed the grounding to be a star ground, added a 'zener clamp mod' to the he power amp to prevent bias excursion and added a grid stopper on v2b. I've attached the modified schematic (I used a 13V zener as that's what I could source).
Mods (1).png
I think the mods helped tighten up the low-end a bit but it seemed to make the low-end distortion more apparent, take a listen:
https://soundcloud.com/ejendres/loose-farts

I want to scope the amp and try and figure out what the distortion is. Is there a guide I could reference? I haven't used an oscilloscope since college nearly 15 years ago.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by Bieworm »

That zener thing is not the Paul Ruby Mod.
Look at how it’s done here: files/JMPGuitars_18_Watt_EF86_Modern_Cl ... ematic.pdf
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by chaliapin »

Have any of the knowledge members listened to the clip? Interested to know their opinions.
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Re: Just finished the Mojotone TMB kit, PPIMV?

Post by ejendres »

chaliapin wrote:
Tue 04/18/23 5:39 am
Have any of the knowledge members listened to the clip? Interested to know their opinions.
I was wondering the same thing.
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