18w TMB with cascade and PPIMV mods (Tube Town 1st build) - Updated

[Lorenzo73]

Hi 18watters
(first post here)
After about 50 stompboxes I've finally come to a tube amp.
I've started from a Tube Town kit (TT kit Marshall style 18w TMB). They supply everything needed at very reasonable prices, with no import taxes for Europe (I'm in Italy).
Along with the kit I've taken also chassis, face and backplate, knobs, tolex, gold piping, rubber feet, handle.
At first I searched a SV20H or a SC20H second-hand, but both don't comply all the features that I need: mod friendly, vintage look, versatilty, plexi sounded.
So looking at the schematics I thought that the TMB 18w with some easy mods could be the live/practice tube head I was looking for.

So, this is my first take on a diy tube amp (my own schematic below)

Original Tube-town circuit with following mods:
1. Cascade mod (norm channel into TMB channel switchable with push pull potz)
2. Post phase inverter master volume (PPIMV type 2) - after tried the super-easy type 3 (works very well specially with linear potz) i've switched to type2 to see how it was, it's good and smooth regulation (double CST 1 meg lin potz in back plate)
3. Switchable Cathode bypass 0,68 uf cap in stage 2 of TMB (as tube-town suggest + 2,7k res) to get more gain with my strats (cap bypass) and to avoid muddy gain with my les paul (no cap bypass). Switch in backplate.

The head cabinet is made of poplar plywood taken at the local store, joint made with wood plugs and wood glue.
The hardest part was doing radius and bevels, but since I love to work with electronics and wood, this project satisfied me a lot.

The result is awesome to me, mostly tanks to the resources on 18watt.com and Rob Robinette pages.
The sound is great, way better a lots of Marshalls I've got or tried (JCM800 / 900). Silent, powerful, it goes from a almost clean norm channel to a fully overdriven jcm800 (maybe more).

The only issue, is the blocking distortion in cascade mode with all 2 volume knobs at 10. But since this isn't intended as an hi gain amp, I don't think I'm gonna modify it to use in this way. I'll mostly use it with the TMB channel eventually switching to norm channel with a deviator "Y" stomp box i'll build.

(more pics in the gallery - I didn't put much effort in a photogenic, superclean wiring)







UPDATE 20/10/2022 Schematic V2 with grid stoppers

18W TMB V2 - SCHEMATIC.pdf

[Iuri.moreira]

On my 18w TMB, I put a switch to choose between valve and diode rectification (for cascade mode). It was also essential for me to install two voltage dividers: the first on the normal/cascade switch itself (reducing the gain of the first stage by 50% when cascaded) and the second before the master volume (decreasing the gain by 80%). Now I have more range on the master volume in both TMB channel and cascaded. It turned out really good for my taste.

[JMPGuitars]

If you have issues with blocking distortion, you should look into the Paul Ruby mod.

Thanks,
Josh

[Lorenzo73]

If you have issues with blocking distortion, you should look into the Paul Ruby mod.

Thanks,
Josh

Thanks for the hint, I'm gonna try also this mod to get rid of that buzz, it's very easy to apply.

Others suggested me to try with grid stoppers before V3A (maybe 470K) and before the cathode follower V3B (up to 1M).

Actually many similar designs have such a grid stoppers before the 2nd stage of gain.

[Lorenzo73]

A little update, yesterday I've put two 470K grid stoppers: one before V3A (like all others "Richie" based TMBs), and another engaged only in cascade mode, before V1A. Maybe the updated schematic in the first message is clearer.

The result is very good. The standard TMB channel now is more controlled, and also with all volume and TMB controls at 10 there's no blocking distortion. Before this mod, with the volume past 7 there was a slight but clearly audible change of the hiss even with the guitar unplugged, like something gone wrong in the amplification chain. Now it's disappeared.

In cascade mode is engaged also the V1A grid stopper, and it's possibile to dial in the gain stages for a real ultra hi gain tone, without annoying blocking distortion, squeal and feedback. In this configuration there are 3 adjustable gain stages (NORM channel volume, TMB channel volume, Master Volume) and is better to set NORM channel not more than 5-6, and adjust the gain with TMB volume. Maybe also with the MV at full throttle is still manageable (even if is not the best for a tube amp run at full power, IMHO) but I don't have tried yet, I need to reassemble the Attenuator before.

This amp still confirm to have a great classic marshall sound, from Beano to Hendrix, ACDC and even EVH with cascade mode pushed hard.

It has a huge volume, it's really loud. I wouldn't have expected to have to build an attenuator for it.