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

Lorenz1973

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Hi tube geeks, I'd like to share here my experience: 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.


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  • 18W TMB V1.pdf
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Paul-T

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Looks great. Really impressed with your woodworking, especially the piping.

If you took any, post us photos of the guts - we don't see many tube-town kits!
 

Lorenz1973

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Looks great. Really impressed with your woodworking, especially the piping.

If you took any, post us photos of the guts - we don't see many tube-town kits!
Thanks Paul-T
The kit is really complete, with the chassis they supply also a large batch of all the nut-bolts, washer needed.

They supply the shematic and the useful layout, but there isn't a step to step manual. I thought it was useless since I've a good experience with electronics, but i was wrong, there are many important aspects that should not be underestimated such as wiring, assembly, welding sequence, for this I have referred to the guide of Trinity and RobRob.

Piping isn't so hard as it seems. Tube town has all the needed pipes, with flap (around the front panel) and simple "tubing" pipe. The one with flap is sapled to the side of front panel, the other pipe across front panel and above the top panel is pressed into a groove. To make the groove I've used the circular saw in photo. Much more relaxing work than tolexing.

Gut shot here, not the final wiring, a work in progress shot without potz wiring and mods.

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Lynxtrap

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Looks great!
Regarding distortion in cascade mode: Perhaps you could try a moderate gridstopper on V3A and a fairly large value stopper on V3B (the cathode follower). Blencowe recommends up to 1M on the CF!
It would be easy enough to try as you could solder both resistors directly to the tube socket, the stopper for the CF would be between pins 1 and 7.

That follower gets hit with some serious signal with both volumes on 10, by the looks of it.
 

Lorenz1973

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Looks great!
Regarding distortion in cascade mode: Perhaps you could try a moderate gridstopper on V3A and a fairly large value stopper on V3B (the cathode follower). Blencowe recommends up to 1M on the CF!
It would be easy enough to try as you could solder both resistors directly to the tube socket, the stopper for the CF would be between pins 1 and 7.

That follower gets hit with some serious signal with both volumes on 10, by the looks of it.
Thanks for the suggestion.
I'll play it for some weeks to see how it goes in different situations and with pedals, then I'll try to put the grid stoppers on V3A and V3B.

I'm asking if such a grid stopper won't change too much the TMB channel voicing... and even if is needed also a grid leak? Like the couple of 470k before V2A in jcm800.

I'll definitely get a copy of the Blencowe books, it's widely suggested...
 

Lynxtrap

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Thanks for the suggestion.
I'll play it for some weeks to see how it goes in different situations and with pedals, then I'll try to put the grid stoppers on V3A and V3B.

I'm asking if such a grid stopper won't change too much the TMB channel voicing... and even if is needed also a grid leak? Like the couple of 470k before V2A in jcm800.

I'll definitely get a copy of the Blencowe books, it's widely suggested...

A large grid stopper on the follower does not affect treble response, according to Blencowe. I have no experience with this myself.

The 470k combo in the JCM800 acts as a voltage divider, dumping 50% of the signal to ground before it hits V2A.

You have a volume pot in that position, which does the same thing.
 

JohnnyCrash

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I’ve built a few 18 watters… including a JCM800 TMB preamped version.

Grid stoppers will help and won’t affect the tone. I use grid stoppers on 50 watt Marshall/Fenders and/or 15 watt amps. It’s a good habit to get into.

It may also be how you setup cascade mode. JCM800s have a second stage that is a cold clipper. It’s got a 10k for its cathode. If your cascade mode simply uses the Normal channel (instead of a cold clipper), I’d definitely use grid stoppers.

The first Mesas were basically cascaded Fender preamps, with their fat 25uF cathodes and all. If you check a schematic you will note the use of grid stoppers throughout.
 

Lorenz1973

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Update: I've put two 470K grid stoppers: one before V3A and another engaged only in cascade mode, before V1A. (updated schematic below)

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.
 

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