Metropoulos Forum

I saw the Mojave Dirty Boy amp http://mojaveampworks.com/amplifiers/dirty-boy/ and was tempted to buy one.
However after contacting Victor he confirmed that it only runs of 120V, and being over in the UK it's not an option.

But now I'm only more determined to build a 50 Watt interpretation of this amp..
I'm going to include all the features including the built in variac and the feedback and bias adjustment and I think I've got a do idea for how to structure the power supply for preamp, poweramp and tube rectifirer. I'll update this post as I go along with details.

I assume most of the preamp is roughly based on the plexi type layout, but I'm a little stumped when it comes to how the "Boost Gain" and "Boost Vol" is structured. There is a photo that shows how the pots, switches and foot switch jack are hooked up - http://mojaveampworks.com/images/dirtyboy-08.jpg, I just can't figure out how it's implemented.

Any ideas about how you might add a gain boost with just a single SPDT switch?

It is more likely a gain cut when you switch that function off (shunting some signal to ground?)

I think you might be right.

On closer inspection of this photo http://mojaveampworks.com/images/dirtyboy-05.jpg, it looks as though based on the way the Treble pot is connected and looking at the way the "Guitar Volume" is hooked up in the very far right hand side of this pic http://mojaveampworks.com/images/dirtyboy-08.jpg, the "Guitar Volume" is in fact a pre phase inverter master volume. On this basis I would guess the amp is based on a 2203 type layout, but that still doesn't explain why there are 4 pre amp tubes?

I thought the switching arrangement might be something like the first switching stage of this layout from Thunder Tweaks - http://sonicdeli.com/ThunderTweakWeb/info/P-Matic2.pdf

Any more ideas?

As far as the 5 transformers needed for this project go this is the list I've complied so far:

1. Output transformer - Classic Tone 40-18025 -http://www.classictone.net/40-18025.pdf

2. Choke - Classic Tone 40-18058 http://www.classictone.net/40-18058.pdf

3. Variac - Staco, Variable Transformer 1020B, this will be used to control the voltage in the power amp section- http://www.stacoenergy.com/pdf/installa ... /1020B.pdf

4. Power Amp Transformer - Classic Tone 40-18023, this will run off the variac and used for the HT supply to the power amp tubes, the 6.3V taps won't be used - http://www.classictone.net/40-18023.pdf

5. Pre Amp & Heater Transformer - Classic Tone 40-18050 , this will supply the preamp and phase inverter as well as the heater supply for all the tubes including the tube rectifier - http://www.classictone.net/40-18050.pdf

2 x EL34 Power Amp Tubes
4 x 12AX7 Preamp Tubes
1 x GZ34 or 5U4 Rectifier Tube

I'm going to include the front panel user adjustable bias and feedback controls. I'm also tempted to include a PPIMV just for good measure.
If I'm going to use 4 pre amp tubes I'm tempted to have an amp that has the option to switch between Plexi, 2203 Mod 5 and 39 S.I.R layouts .

Those are great amps. Should it be able to run on whatever voltage you have with an appropriate stepdown transformer? I've hear they can change tone but I'm in doubt.

When I contacted Vic about buying one he made the following comment - "the 240 VAC were extremely difficult due to design and additional transformers. However, because the Dirty Boy was originally designed for 120 VAC and it turns out the performance was much better for it. At 240 VAC there were to many transformers bucking the AC using Dual Vacuum tube rectifiers brought the performance down." on that basis I'd be a little reluctant to run it with a step down transformer. Given the cost and weight of the amp in the first place the added expense and weight of having to use a step down transformer just didn't seem worth the risk.
I also now fancy the challenge of building my own interpretation of this amp to see what I can learn along the way.

To be quite honest, with some of the stuff I've seen him put in print in the past, I wouldn't read too much into Vic's opinions and theories. Some of his ideas are just ludicrous, and founded on nothing.

The only difference between 120V higher current and 240V lower current can be in the coupling of the windings. If you use a transformer with multi primaries you will inevitably get quite different coupling between primaries and secondaries in comparison to the original, but if you use a transformer with just the one set of primaries like the originals you should get pretty well approximated results. If you can't get what you want commercially, and know roughly what specs you need, have a word with ClassicTone.

It is a fairly well accepted phenomena that Marshalls sound better using our UK voltage, than lower voltages, but the majority of this difference will be down to the coupling within the transformer (different taps will have different proximity to the secondaries) rather than the difference between voltage/current/ and frequency, although it should also be noted the difference between the current on the primaries will also radiate differently.

Either way, I wouldn't expect the differences to be vast if using a purpose built PT, rather than an off the shelf multi-primary jobbie.