Tone and tremolo tweaks for 'British" 18 watt

[MacGwyn]

Just finished my first "British" style amp - an 18 watt 2-channel with tremolo.  It's the Mojotone Version.
Being more familiar with Fender style amps... I was wondering if anyone had good ideas for 'tweaking' the Brit type SINGLE tone controls, i.e. swapping caps or resistors to different values.  I'd like to get a little more top-end 'sizzle' on the normal channel, for one thing.  It's a really a nice sounding amp as-is.
Also, can anyone identify which component(s) control tremolo speed?  Mine seems a little slow at max.  Thanks in advance for any input!

[sluckey]

I circled the trem caps. Remove the one in red to speed up.

[MacGwyn]

Trem speed is perfect with that mod.  Now, can anyone help with reducing tremolo depth?  When the trem intensity control is turned all the way down, a bit of tremolo effect is still present... unless you turn it completely by shorting it out with the foot switch. 

[HotBluePlates]

Just finished my first "British" style amp - an 18 watt 2-channel with tremolo.  ... I'd like to get a little more top-end 'sizzle' on the normal channel

Turning up the Tone control doesn't get you there?

In general, it's hard to usefully boost just highs. Mostly, the approach taken is to cut bass, which gives the impression that highs are boosted by comparison.

Look at the cathode bypass cap for the 2 Normal channel input stages; it's a 50uF cap. Try making that thing 1-5uF. That will shave bass, but maybe only enough to make it less thick without leaving the amp sounding gutless.

Or maybe you don't like the effect of the Normal channel tone circuit. Look hard at the schematic. It uses a 0.01uF cap and a 0.0047uF cap, each of which pass some amount of highs to ground when the Tone control is turned to their side of the Tone pot. You could try making the 0.0047uF cap much smaller, like 250pF or less. That would shunt less highs to ground.

However, the Normal channel tone control is wired differently than the Trem channel (which is basically like any small tweed Fender). When you turn this tone to max-Treble, it doesn't just use the 0.0047uF cap (high-treble side of the Tone pot) to pass highs around the Volume control (which is how the Trem channel Tone control works). The tone circuit places a 100kΩ resistor from the 0.0047uF cap to ground. So this still cuts high (but moves the roll-off point higher than the bass-side of the Tone control), but it also places a heavier load on the input stages (100kΩ instead of 1MΩ). That would reduce the gain of the input stages some, compared to the bass setting.

So now you have a bunch of options to try. I highly recommend trying 1 at a time, then listening. It might also be a good idea to put the amp back to stock before trying out a different option (meaning if a smaller cathode bypass cap didn't trip your trigger, maybe put back the 50uF before changing the Tone control circuit or cap values).

When the trem intensity control is turned all the way down, a bit of tremolo effect is still present... unless you turn it completely by shorting it out with the foot switch. 

That is probably unavoidable due to how the trem is applied.


Fender used essentially the same circuit in the VibroChamp. Over the years, they had at least 4 other variants of a tremolo/vibrato circuit to eliminate pulsing background noise. In the old days, I think the store salesman would try to upsell you to a better amp (and trem circuit) to sidestep that issue. "Well, that's just our student model. Now over here in our Professional line, we have ..."

[MacGwyn]

Here's a question for you experienced tube amp guys:   If lowering the value of the 50 uf cathode bypass cap would cut bass response, would raising that value increase bass?   Would it hurt to try say, a 100 uf cap at that location?  Specifically, I'm talking about cath bypass for V3, the preamp for the tremolo channel on the British 18 watt amp.

[labb]

Do a google search for 18 watt marshall tone tweaking: find "Learneconometrics.com/pdf/Trinity_18.pdf" This is the Trinity Amp's build manual for the 18 watt. Go to page 49 for recommended tone tweaking. Tried to post a link but couldn't get it to work.

[MacGwyn]

Thanks - that looks very helpful!

[PRR]

> If lowering the value of the 50 uf cathode bypass cap would cut bass response, would raising that value increase bass? 

50uFd in a small-triode cathode is good (no loss) down to maybe 4Hz.

Far-far below any "bass audio".

In fact, in this case, it had to be picked to support the trem oscillator (which is on the same cathode network). The osc needs that cap (without it, it would almost-work), and big enough for the lowest trem rate (a few Hz).

So I don't think that value, in this circuit, can be tampered much.

[MacGwyn]

Here's a real rookie question... but I'm trying to learn as I go:

When it comes to capacitor values, I read various places about using "higher" and "lower" values for comparison.

For instance, if I'm instructed on a .0047 cap to try a HIGHER value... wouldn't that be a .01, as an example?  Or would HIGHER value than .0047 be the other way-round... .0001?   

[sluckey]

higher value = larger number

[HotBluePlates]

For instance, if I'm instructed on a .0047 cap to try a HIGHER value... wouldn't that be a .01, as an example?  Or would HIGHER value than .0047 be the other way-round... .0001?   

You need to include the units (some fraction of a Farad). 2000 is bigger than 0.01, but 2000pF is smaller than 0.01uF.

[dude]

This can get confusing, which cap will be brighter than the other according to the values.

Example:  A .01 cap will be brighter than a .02 cap. (because it's smaller) and a .047 cap will be less bright than a .02 and so forth. So the lowest number is brighter than the higher number.

al

[MacGwyn]

Example:  A .01 cap will be brighter than a .02 cap. (because it's smaller) and a .047 cap will be less bright than a .02 and so forth. So the lowest number is brighter than the higher number.

Thanks Al... so in the pico realm... a 250 pf would be brighter than a 500 pf, right?

[dude]

Example:  A .01 cap will be brighter than a .02 cap. (because it's smaller) and a .047 cap will be less bright than a .02 and so forth. So the lowest number is brighter than the higher number.

Thanks Al... so in the pico realm... a 250 pf would be brighter than a 500 pf, right?

Can someone chime in.  I think the 500pf would bleed more highs to ground than the 250PF

al

[HotBluePlates]

Example:  A .01 cap will be brighter than a .02 cap. (because it's smaller) and a .047 cap will be less bright than a .02 and so forth. So the lowest number is brighter than the higher number.

Thanks Al... so in the pico realm... a 250 pf would be brighter than a 500 pf, right?

Can someone chime in.  I think the 500pf would bleed more highs to ground than the 250PF

You need to specify where/how the cap is being used in the circuit (see a theme with "you need to specify ..."?).

Bottom-line: A cap passes some very high frequency with almost no opposition, but looks like an open-circuit to d.c. (the lowest frequency you can have, 0Hz). As you make a cap larger, the frequency where it goes from being "no opposition" to "significant opposition" moves lower.

So in a coupling cap position in a circuit, you might say the cap shaves bass; but if the cap is made a higher value, it shaves bass at a lower frequency, which sounds like bass has increased somewhat.

If the cap is connected from signal to ground, then a very small cap shaves only the highest highs (because it presents a big opposition to most of the signal). If that cap is made bigger, it passes highs and now also high mids, etc. The circuit now sounds as though more highs are rolled off, so the sound is more bassy by comparison.

Example: let's pretend a 250pF cap to ground has a roll-off due to circuit resistance at 2kHz. The circuit doesn't sound treble, but has bass, midrange and some low treble. Now you swap in a 500pF cap. The circuit now has a roll-off at 1kHz, and sounds less "present" or "biting". Sub in a 1000pF cap, and the roll-off is shifted downward to 500Hz, and so the circuit sounds muddy or dark; all treble and upper mids are gone.

The absolute value of the cap isn't the only thing that determines frequency roll-off, but also the other resistance or reactance in the circuit. Just because you like a 0.022uF cap as a coupling cap in one part of the preamp doesn't mean the 0.001uF cap leading into the long-tail phase inverter is stripping out all your bass.

[MacGwyn]

Found a very helpful youtube video on tone stacks.  Conclusion:
A HIGHER VALUE cap or resistor will LOWER the cutoff frequency of a tone filter...
a LOWER VALUE cap or resistor will RAISE the cutoff frequency.
Example... in a high pass (treble) filter, a 250 pf cap would move the filter's cutoff to a higher frequency, compared to a
500 pf cap (higher value) which would move the cut LOWER (toward the midrange).
Thanks to "Uncle Doug" for the great tone stack videos!   
Here's the url:    https://www.youtube.com/watch?v=uZHM5BwPLRw&list=UUuR4hQTXkG_KxozLxwPzEjQ