tightening up the trainwreck lowend

[Curranproducer]

I built myself a cute little trainwreck express with a ton of silly mods. Marshall style MV(NOT a PPIMV) slope resistor/potentiometer to go from 50K-150K so every where from a marshall slope to a dumble. vox style clarity control (250kA-4uF7 cap). MV solo boost (1M pot on a grounded jack for power section boost) dual rectifier switching between a 5U4GB recto tube and diode rectification, and also a 300V-260V switch for the power transformer(classic tone TW) to be able to switch between the two voltage taps coming off of the secondary for the amp. this amp is very very fun. BUT I am a high gain metal player type of guy as most of you know. i have modified the 2204 platform to a high gain fire breather of an amp and now I want to blaspheme all over the TW express I have built. I would like to remove SOME of the flubby loose low-end from the amplifier where would I start? I automatically look at the (2).1uF caps in the PI section of the amp. BUT with the 2203 I built I had way more lowend and gain success in the 1st-3rd stages of the preamp itself. what would be the smart move?

[Littlewyan]

If the amp is built to spec then the low end is definitely not loose.

Is everything else stock in this amp?

[Curranproducer]

oh come on, the low end on these are very traditional in the lowend. they are not high gain tight they have some flab and tub in there. I only want to tighten it up a bit

[Littlewyan]

I certainly don't find that on mine? How have you got the controls set?

And rather importantly. Is the amp stock according to the schematic on here? Apart from the mods you've listed of course. Can't help you unless I know what I'm working with

[martin manning]

How about posting a marked-up schematic showing all of your mods?

[geetarpicker]

What is your net B+ at the power tubes? A stock Express (KF build) is in the 410-420v range.

[Curranproducer]

I will go through and do up a schematic. I also added an adjustable NFB control to the amp, using a 50k Linear pot and 50k resistor. The amp is not farty by any means, BUT it is not Soldano low-end tight. I have some high-gain madness I have built and love, I also REALLY enjoy the help out of the touch sensitivity of the trainwreck. I just wanted to see if i cna stiffen up a few things in the lowend so it is not so saggy overall. I will get that schematic up and also a voltage chart this week!!!

[geetarpicker]

I usually run the bass at the most at 1pm on my Express, but usually on 1/2 or even 11am if I'm diming the amp or close to it. At that point it's fairly tight, perhaps as tight as a lead spec 1987 circuit Marshall JMP 50. Then using a 4x12 helps to fill it back out in the lows. Run the amp circuit lean in the bass, then make up for it with a big 4x12 works for me. Or a Bogner Cube (ported 1x12) works to fatten it up with a smaller cab.

That said, what is your B+? 417v give or take is typical on an original Express.

Quite a few mods may get you what you want, or perhaps not and then you are on your own to figure it out for the most part!

Hope you can figure it out but post more info and we will try here...

[Littlewyan]

Yeah a stock Express with everything at 12 O Clock and the bright switch on the 500pF setting is pretty tight.

What pots are you using? I use Alpha pots which are 15% at 12 O Clock, so I set my log pots to 10:30 so they're the same as a real Express with everything set to halfway (10% rotation).

[jelle]

You could tighten the power amp up with a choke instead of that 1K resistor. That will take it away from Trainwreck land into something new and tighter-land.

[Curranproducer]

I do have a nice tri-100 choke in there. I always like to put chokes in an amp even if the schematic does not call for it. good idea though, I need to get my schematic/mod list up here for everyone to look at

[M Fowler]

A resonance pot might do the trick.

Mark

[pdf64]

I do have a nice tri-100 choke in there. I always like to put chokes in an amp even if the schematic does not call for it.

Have you considered this can be a big change to an amp's operating conditions under load?

Changing the 1k resistor supply the screen grid node to a choke (assume its resistance is <500 ohms, likely <200 ohms) is significant.
It is likely to change the character of the amp's overdrive response, most particularly its 'clean to mean' transition.

[Littlewyan]

Also the Cut Control, Slope Resistor Control, Master Volume and Rectifier controls will all affect bass.

Cut will remove treble frequencies which may make the bottom end seem a bit mushy.

Slope resistor will affect the bass A LOT, especially if you have it set to a Marshall value (33K vs 100K TW).

The MV isn't going to drive the power valves very hard which means the presence control isn't going to be very active, again loss of high end.

If you use a valve rectifier that can cause sag which will make it a bit mushy.

Also the lower voltage could give you a squishy bass.

[Curranproducer]

here is an MS paint schematic for the "shipwreck" I have built. I really like this amp right out of the gate it is super dynamic and crunchy. plenty of pleasing gain and smooth saturation. I would like for the low end to be a little tighter over all. it is not much or flabby BUT it is lacking in the tight SLO-100/Splawn lowend tightness I enjoy.