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Before I finally put my TMB/MV in the fine purple box which Stephen is sending me, I'd like to reduce the bass in the TMB channel. In my builder's guide there are two methods described. If I remember correctly one alters the cathode resistor/capacitor setup on the TMB 1/2 of the phase inverter. The other involves changing values in the tone stack.

What are the relative merits of the different approaches?

Or could I simply swap out the bass pot?

thanks

I'd prefer to modify the front end. Here's some

Back End - To change the Low Frequency Response of the Output Stage

Output tubes, cathode bypass filter: The original Marshall 18Ws used 500uF, and the Watkins Dominators they derived from used 50uF, there is no right value.

Larger values will give more bass response. What is used depends on the type of speakers, cab, OT and even guitar. The -3db low frequency cutoff point is approximately 1/(2 x pi x R x C), so for example, 125 ohms and 50uF has a lower cutoff of 25 Hz.

Try either a 50 or 100uF. Some people find that lower values keep the low end from getting muddy or boomy.

This is not like a conventional high pass filter in that there is still some gain/band pass at Low Frequencies. Instead of a continuous cutoff there is a step in the gain response, lower gain below the crossover point, and higher above.


Front End - Bass Roll-Off

On the 2 cathodes of V1, the R-C filter is set to 1uF X 820 ohms. This will roll the bass off slightly. Increase the capacitance up to 25uF to increase bass response of the front-end pre amplifier. Try the following combinations for the pair for the TMB channel connected to V1a.

2.7K/.68uF; 1.5K/25uf; 2.2K/2.2uf;

The lower the capacitor value, the less bass response.

If you want to modify the Effect that the Bass Control has, change the value of the potentiometer to 500K or 250K.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

My normal channel seems fine. If I mod the output tube stage it would also affect the normal channel , right? I only want to reduce the bass of the TMB channel.

If the initial setup on the cathodes of V1 is 1micro farad and 820 ohms, how do I lower the value of this capacitor? All the pairs listed in your post have a capacitor that's greater in value than 1.0 micro farad.

Find a 0.68 uf cap and try that. It wont be electrolytic.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I found a .56 micro farad in the bin at work and made that substitution, It seems to have tamed the bass nicely and there's more adjustment range available with the bass pot. I'll have to play it some more but I think that might have done the trick. The wife and kids prevent further testing at this time. Before the change it was too bassy even with my son's strat. The strat sounds good now, much tighter bass, not so boomy and more like the normal channel's bass response. I'll check my '76 SG with it tomorrow.

Thanks !

Hi. I ran into the same problem building a TMB off the same schematic

I love channel one however want to modify channel two to reduce bass. With the volume cranked and the master at 2-4 and the mids 4/4/ treble 3/4 and bass at 1/4 it's super boomy

Replace the1 uF electrolytic to the channel two side on V1 across the 820 resistor with .68 regular axial caps?

Thanks for the great post

What if i was to leave the normal channel 1 x 820 and 1 uF

then

the TMB channel 2.7K x 0.68 uF? Every 2203 schematic uses the 2.7k in conjunction with a 0.68 uF cap.

or should I go 820 x 0.68 uF?

Thanks,

Dan

Dan , keep in mind that I built this amp almost 9 years ago.

I was using the TMB schematic that was posted back then - I only changed the cap, not the resistor, and ony on the gain channel. I would say that reducing the value of the cap reduces the bass so try.68 instead of 1.0 and see of you like it. Only change one thing at a time, ifyou do experiment with other resistir values. You can go lower than .68 if still too much bass. As far as cap formulation , I don't know if that has an effect. Keep trying till you like it, that's what's nice about building your own.

Bob

We are using the same schematic, I got a 0.68 uF cap (630 volt) today at the electronics supplier with a 2.7k resistor.

When I look at the 2203 tone stack (what this one is derived from) I see a 2.7k resistor with a .68 uF cap.

I am going to try it both ways with the .68 uF with the 820 and with the 2.7k ohm resistor.

I was wondering, on the revisions there is a 68k screen resistor on V1 Our builds go straight to V1 without a 68k resistor.

What would be the advantage to putting in a 68k on the high feed for the TMB channel if any?

Thanks for the opinions man

Much appreciated!

Regards,

Dan

If the math is right u would probably need 2.2uf with the 820R. Did try it with the normal channel sounds great totally cranked but when you dial the gain back to clean things it starts to sound a little thin. In my case ended up wiring a fat switch to kick in a big cap on the 820R so you can get the best of both worlds.

The are various circuit description of input jacks you can google. This is the one for the standard marshall.

Mines single high input gain channel. It's the old schematic

That being said do you think I should proceed with a 68k resistor to pin 2?

With out a grid resistor you have maximum gain. The only draw back is possible para oscillation.

If you want to match the standard marshall High gain input you put to 68K in parallel which matches the circuit from above (or a 33k resistor)

Or I Have seen some with just a 22k resistor.

There is no right or wrong answer. Go ahead and stick it in if you don't like it you can always pull it back out. The only one that can tell you whats right for you is you

So in theroy because the resistor isn't there would that particular channel seem louder compared to the other?

I put in a 2.7k resistor and 0.68 uF cap on the tone stack on the TMB channel and that tamed a lot of bass.

Thanks!

Adding a resistor to the the input of v1 probably wont make a hill of beans of difference. from what im reading on amp garage. I always thought using a lower value would me more gain but looks like i'm wrong on that

Might want to look at this thread as well

http://www.trinityamps.com/phpbb/viewtopic.php?f=1&t=273

As for comparing Volume the TMB will have more just because there's two tube of gain stages vs one tube in the normal channel

If all else fails, Try a .01 coupling cap between the TMB channel and the PI.
This may unbalance the output of the PI slightly but probably not enough to get excited about.

Cheers

Or even less such as .047.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps