Guidance on 18w TMB Layout adjustments.

[MartinL]

Hello, first time poster here but I've been lurking for several months. A couple years back I had purchased two Mojotone 18w TMB kits as a fun project build with my father, but life got in the way and before we ever got a chance to work on them he had to move out of state. I decided to build the two kits myself, and thanks to the wealth of information and guidance on this forum I was able to get my first kit up and running with a few adjustments (mainly just a rework of the grounding scheme).

Now I'm onto the second build, and with a little more understanding of the circuit and the layout I'd like to address some of the issues I have with the Mojotone Kit. Apart from the terrible grounding scheme that I fixed with the first build, the two big issues I have are the messy layout of the turret board and the floating filter cap hanging leads over the rest of the circuit, so my plan was to build my own turret board and replace that cap can with a board mounted filter cap. While comparing layouts and schematics I had a couple questions I was hoping to get some clarity on. This 1st kit was my 1st amp build, so I'm still very new to all this. I'm doing my best to learn as I go, so forgive me for any stupid questions.

*For these questions I'm cross referencing Mojotone's TMB schematic and Trinity's 18w TMB Schematic, which seems very similar to Richie's TMB in the downloads. I'll attach both schematics for clarity, and I've marked and annotated the mojotone schem to help clarify some of my questions.

1. In the other 18w TMB schematics I'm looking at, the two preamp stages share a single 16uf filter cap located on the board, with a separate board mounted 16uf filter cap for the phase inverter. Is there any specific reason Mojotone decided to add in another 32uf/32uf cap can for v2 and v3 instead of the seemingly common way to do it? If I were to scrap the cap can for a second 16uf filter cap on the board, would it be important I also adjust the value of the resistors between them for proper functionality? Seems like there are differences in values for these resistors in every schematic. Assuming I should just use the Trinity values here.

2. On the Mojotone Schematic I've included, I've highlighted a section of a few caps/resistors between the plate of v2a and grid of v2b, something that the the other schematics don't have. What's the purpose of this part of the mojotone circuit? It's sort of crammed into the mojotone board layout as well, which makes me think they thought it was important for this to be there.

3. The Mojo Schematic has the TMB tone stack connected to the Plate of V2B, and in the Trinity, along with a few other Schematics I see, the tone stack is connected to the cathode. Does this have significant impact on the function of that tone stack?

4. Is there any benefit to the 47pf cap between the 100k resistors connecting the plates of V3 and the filter cap in the Mojotone Schematic? This is another thing missing from the other schematics.

5. What's the difference in using higher value resistors coming from the control grids of the output tubes?

6. Trinity's schematic shows a single 100 ohm screen grid resistor instead of mojotone's 2 100hm r for the output tubes. Would using 1 be more sensible here?

Another poster here had made similar adjustments with the Mojotone Kit using the Trinity amp layout, so I think I'll follow in their footsteps. Current plan is to salvage what I can from the Mojo parts and move forward referencing the Trinity Documents for component layout and values. Making my own turretboard seems easy enough, so I'll pick up some G10 fiberboard and turrets along with the components I'll need for value differences between the two schematics.

Sorry for the long post, but I just want to make sure I'm going about this the right way, and aim to understand the changes from a technical standpoint as best I can.

[Bieworm]

Hello, first time poster here but I've been lurking for several months. A couple years back I had purchased two Mojotone 18w TMB kits as a fun project build with my father, but life got in the way and before we ever got a chance to work on them he had to move out of state. I decided to build the two kits myself, and thanks to the wealth of information and guidance on this forum I was able to get my first kit up and running with a few adjustments (mainly just a rework of the grounding scheme).

Now I'm onto the second build, and with a little more understanding of the circuit and the layout I'd like to address some of the issues I have with the Mojotone Kit. Apart from the terrible grounding scheme that I fixed with the first build, the two big issues I have are the messy layout of the turret board and the floating filter cap hanging leads over the rest of the circuit, so my plan was to build my own turret board and replace that cap can with a board mounted filter cap. While comparing layouts and schematics I had a couple questions I was hoping to get some clarity on. This 1st kit was my 1st amp build, so I'm still very new to all this. I'm doing my best to learn as I go, so forgive me for any stupid questions.

*For these questions I'm cross referencing Mojotone's TMB schematic and Trinity's 18w TMB Schematic, which seems very similar to Richie's TMB in the downloads. I'll attach both schematics for clarity, and I've marked and annotated the mojotone schem to help clarify some of my questions.

1. In the other 18w TMB schematics I'm looking at, the two preamp stages share a single 16uf filter cap located on the board, with a separate board mounted 16uf filter cap for the phase inverter. Is there any specific reason Mojotone decided to add in another 32uf/32uf cap can for v2 and v3 instead of the seemingly common way to do it? If I were to scrap the cap can for a second 16uf filter cap on the board, would it be important I also adjust the value of the resistors between them for proper functionality? Seems like there are differences in values for these resistors in every schematic. Assuming I should just use the Trinity values here.

—> in an amp with few preamp tubes like this one there are generally 4 filter caps. One for the EL84 plates, followed by one for the screens ( this would be the 32+32uf cap can). Then one for the PI and lastly a shared one for the preamps. For dropping resistor values refer to the classic 18 watt schematic and you’ll be fine.

2. On the Mojotone Schematic I've included, I've highlighted a section of a few caps/resistors between the plate of v2a and grid of v2b, something that the the other schematics don't have. What's the purpose of this part of the mojotone circuit? It's sort of crammed into the mojotone board layout as well, which makes me think they thought it was important for this to be there.

—> the .022uf is the coupling cap between two stages, and necessary. Then the 470k in series with a 470k to ground is a voltage divider. This is to drop excess signal to ground in order to not push the next stage too hard. Sometimes this voltage divider is replaced by a 1M pot. ( ref Tremolo TMB in the downloads). The 500pf cap is there to avoid loss of treble by the 470k resistor.

3. The Mojo Schematic has the TMB tone stack connected to the Plate of V2B, and in the Trinity, along with a few other Schematics I see, the tone stack is connected to the cathode. Does this have significant impact on the function of that tone stack?

—> it’s an impedance thing. Pretty technical. Both are good and work fine. The difference is that the plate driven TS will have signal loss while the cathode driven TS does not. It’s a designers choice. You can read up on the topic here: https://www.aikenamps.com/index.php/ton ... ol-scaling

4. Is there any benefit to the 47pf cap between the 100k resistors connecting the plates of V3 and the filter cap in the Mojotone Schematic? This is another thing missing from the other schematics.

—> this is a snubber cap. It would eliminate parasitic oscillation if there would be. Search for snubber cap on google

5. What's the difference in using higher value resistors coming from the control grids of the output tubes?

—> the grid stoppers we generally use are 8.2k. Fender uses 1.5k. In case of crossover distortion there are some that up this value to 100k or more. The higher the value, the more affect on tone in a bad way (muffled tone)

6. Trinity's schematic shows a single 100 ohm screen grid resistor instead of mojotone's 2 100hm r for the output tubes. Would using 1 be more sensible here?

—> both are fine. We generally add 2x 1k/3W

Another poster here had made similar adjustments with the Mojotone Kit using the Trinity amp layout, so I think I'll follow in their footsteps. Current plan is to salvage what I can from the Mojo parts and move forward referencing the Trinity Documents for component layout and values. Making my own turretboard seems easy enough, so I'll pick up some G10 fiberboard and turrets along with the components I'll need for value differences between the two schematics.

Sorry for the long post, but I just want to make sure I'm going about this the right way, and aim to understand the changes from a technical standpoint as best I can.

See my answers in bold, between your questions