18 Watt TMB Build

[sluckey]

In case you're not aware the Mojotone 18 Watt TMB channel is a high gain preamp, unlike the popular 18 Watt TMB developed over at 18watt.com. The mojo circuit has gain + gain + gain with plate driven tone stack. Layout is more critical.

[professormudd]

In case you're not aware the Mojotone 18 Watt TMB channel is a high gain preamp, unlike the popular 18 Watt TMB developed over at 18watt.com. The mojo circuit has gain + gain + gain with plate driven tone stack. Layout is more critical.

Thanks sluckey. I was aware it was high gain and I tried to be extra careful with my layout choices, but it appears I made a couple of bad decisions.

I re-routed the wiring to/from the gain pot and the board. I had originally ran the wire down from the pot, to the board, along the front of the board(slightly under the board) and up to their respective turrets. I re-routed to follow the length of the front of the chassis then 90 degrees to the board, and it seems to have resolved the oscillation issue (and I am willing to bet it might partially resolve the fizzy/sizzly overdrive I was hearing).

I still hear quite a bit of hiss proportional to the gain/vol on the high gain channel. The hiss is present whether or not an instrument is plugged in. I noticed that characteristics of the noise changed and the level would increase/decrease when I lightly chopstick the tip of the jack (I used switchcraft jacks rather than cliff jacks).

I wonder if the tip of the jack is acting as an antenna of sorts, I may try to rotate it when I go back out to the workbench. I am also considering running some shielded cable for the gain and/or the input.

[ViperDoc]

The first TMB I ever built was a M*jo, and it sounded like a bomb about to go off. I followed sluckey's advice and installed a cathode follower circuit and it sounded much better. Since then I've built the Valvestorm TMB and the 18watt.com TMB Superlite, both with great success. I like the 18watt version the best. It's more balanced and smoother, more useable. I used a GDS transformer set also, sounds great.

[professormudd]

I have done quite a bit of re-work and for the most part the amp sounds great except I still have noise issues. Namely there is hiss through the master volume even when no instrument is plugged in and the amp is at idle. The noise increases with increase in gain as well, sounding like it is ready to oscillate once I am dimed on both master and gain. On the clean channel I can dime it and it has very little noise at all.

So far I have installed shielded wire to the drive pot as well as the TMB input.
I moved the 470k grid resistor to connect directly to the tube, and shielded cable connecting to the resistor.
Swapped out grid and plate resistors on V2 to 1 watt metal film.

So far each change has removed various elements I was hearing in the noise. Now all that is left is the idle hiss.

The first TMB I ever built was a M*jo, and it sounded like a bomb about to go off. I followed sluckey's advice and installed a cathode follower circuit and it sounded much better. Since then I've built the Valvestorm TMB and the 18watt.com TMB Superlite, both with great success. I like the 18watt version the best. It's more balanced and smoother, more useable. I used a GDS transformer set also, sounds great.

I might try this as well.. do you have a layout or something you can share to illustrate that change on an 18 watt?

EDIT: Never mind the question about the layout. I believe I found the thread you are talking about. Thanks for chiming in. https://ampgarage.com/forum/viewtopic.php?t=32970

[professormudd]

I did the cathode-follower mod to this amp (which I shall now call slucky's mojo unfucky mod) and the high gain channel feels a lot less volatile. That 22nF cap coming off the plate of V2 was like a small high-sensitivity microphone.

I had obtained this board and decided to go with mojo's circuit because I was curious about the JCM800 aspect. Well my curiosity has been satisfied and it turns out I much prefer the cathode-follower. Thanks for the help.

[ViperDoc]

I felt the same way. Someone mentioned the plate-driven tone stack adds to the mayhem as well. I built a m0j0 JCM800 and had to have it fixed by the many talented builders here. I've stayed away since.

[GrayDigger]

professormud

Congrats on getting your TMB18 up and going! I’m new to this forum but have followed it for a while and learned a great deal. What an impressive group of amp guru-ness here!

My first build was actually a Mojotone TMB18 kit. I’ve learned a great deal over the years since, and have built three successively better TMB18s friends, incorporating some of the great stuff from the 18watt forums. My original build has been Frankenstein’d with various mods that I’ve tried, abandoned or kept, and it’s been an enjoyable amp I use with my cover band.

Regarding your TMB channel idle hiss, FWIW I think a certain amount is inevitable with the Volume and Gain cranked, given the high-gain nature of the preamp. Like you, I found that positioning the grid stopper resistor at the grid pin of V1 helped, as did using shielded wire from the jack. After reading Merlin Blencowe’s pre-amp book (absolutely brilliant), I re-worked my ground buss so that all the preamp grounds connected to the buss, which then connected to the chassis at a single point/lug near the normal channel input jacks. Using metal film for both the 1M high impedance input grid leak and grid stopper resistors (I ended up with a 33K) helped more. Another cheap part substitution was using a conductive plastic 1M pot for the TMB Master volume, helped again. On subsequent builds, I used Vishay 1 watt 1% 100Ks for plate load resistors (but kept carbon comps in the tone stack and phase inverter), which also helped (especially with cascaded gain mods). Finally, I played with shunting some of the input hiss to ground on my original amp using a 250pF cap to ground (between the TMB grid stopper and V1A grid), which was also helpful and didn’t seem to hurt the tone.

On my original TMB, I tried a number of speakers, but basically came back around to a Vintage 30. To try to tame some of the volume for smaller venues, I put in a PPI Master Volume, mounting the pot in the back. I would say it’s just OK (Trainwreck Lar-Mar Type 3), but playing out I find rarely use it. (The Frondelli PPI Master Volume I found on Rob Robinette’s site work much better in my JTM45s - they are loud) In my first TMB18, at moderate volumes, I had some harsh pick transients that were quite irritating. I eventually dealt with them by using a series resistor and shunt capacitor (after playing around a lot with other approaches - plate bypass caps, variable negative feedback, etc. all which took away some good juju) just after the grid stopper to V2A (I made a 50% voltage divider with a 470K resistor and ended up with a 150pF cap targeting 2.2KHz) which was just right.

Lastly, I found that rolling tubes made a big difference in the idle noise as well. Although I like the tone of several NOS Mullard ECC83s, CV4004s, and Amperexes etc. on the front end, some were so much noisier than others. Eventually, a buddy gave me an old Amperex that is living there very happily now.

I don’t know if any of this is remotely useful, I’m just excited for you.