ENGL screamer preamp mod question?

[Rod]

Hi everyone!

For me this amp has a problem that i would like to solve, when i increase the volume the sound becomes more thinner.
What should i mod in the attached preamp schematic to solve this problem?


Thanks.

[soundguruman]

change the 100K resistor on the grid of V1B to 270K
what do you mean by 'thinner?'

[nickb]

Does this happen on both channels? Which volume control makes the difference?

By 'thinner' I assume you mean loss of bass. Is this effect subtle or pronounced?

[Rod]

Soundgurumman, I have allready try changed that 100K resistor to 470K and 680K, to add more gain on channel 2 low gain, increasing that resistor value the signal becames stronger ,louder and adds more gain, right?

Nickb, this happens in the channel 2 only, the distortion channels, yes by thinner i mean loss of bass, well i notice this effect when i increase the volume from value 1, i think that this happens both in the lead and master volume.

[nickb]

What about Lo/Hi?

Rod - keep the resistor at 680k (or remove it all together) and change the 1nF cap that is in series with the 22k to 4.7nF and see how it sounds for you. I wouldn't go less than 10nf but you can experiment so see what you like best. It might make it sound rather 'farty' so dependent on the answer to the lo high question we can look at other ways.

[stormbringer]

First a question that is unlikely related to your problem - are there no grid stoppers on the Engl Screamer?! Not even on the input? If you are modding this amp, you might consider adding them.

For the tone change at different volumes, you might hear an improvement if you add a 100k resistor to the wiper of each volume control. This 'improvement' is detailed in KOC's TUT4 book. It is supposed to fix the resistance part of the RC filter made up by the (master) volume pot and the cap that come after it. Without the fixed resistance, the variable volume resistance acts to roll of high end as you dial down your volume.

Note: your sonic impression (of less bass at higher volumes) might be due to less high-end at low volume settings. So adding the fixed resistor on the volume-pot wiper might give you that 'thin' sound at lower volumes....

[yankeerob]

Note: your sonic impression (of less bass at higher volumes) might be due to less high-end at low volume settings. So adding the fixed resistor on the volume-pot wiper might give you that 'thin' sound at lower volumes....

Hi stormbringer,

Hope this thread's not too old for you to reply... when you say 'add' do you mean put a 100k in series?...

Many thanks,

Rob

[Jazz P Bass]

Yes, in series with the wiper.

[yankeerob]

Tell ya'll what I did... changed the .047uF coupling caps feeding the screen grids to 0.1uFs and hey presto... instantly better tone. I nearly changed the 330K bias feed resistors too but the .047uF/0.1uF swap was enough...

I suspect that doing the 100K mod would benefit as described but it would have been a ball ache to do whereas pulling the two WIMA .047s from the component side and soldering in two Roederstein 0.1uF/630Vs on the track side (so you could still change them very easily without having to un-nut the control PCB and pull the central PCB etc... - god I hate PCB amps!!!) has worked a treat...

Many thanks and hope this bit of info is useful...

All the best,

Rob

[Rod]

HI Stormbriger.

By add the resistor to the wiper do you mean something similar to the piture bellow, but how will i make this, if the pot is soldered to the pcb, and the tracks for the signal are printed in the pcb board?

First a question that is unlikely related to your problem - are there no grid stoppers on the Engl Screamer?! Not even on the input? If you are modding this amp, you might consider adding them.

For the tone change at different volumes, you might hear an improvement if you add a 100k resistor to the wiper of each volume control. This 'improvement' is detailed in KOC's TUT4 book. It is supposed to fix the resistance part of the RC filter made up by the (master) volume pot and the cap that come after it. Without the fixed resistance, the variable volume resistance acts to roll of high end as you dial down your volume.

Note: your sonic impression (of less bass at higher volumes) might be due to less high-end at low volume settings. So adding the fixed resistor on the volume-pot wiper might give you that 'thin' sound at lower volumes....

[stormbringer]

Apologies for not following the thread closely or seeing the PMs. Yes, that is what I meant by adding the 100k resistor after the wiper. Unfortunately I'm not sure how to add to a PCB amp with board-mounted pots.

which volume pot caused the amp to sound thinner: preamp (lead) gain, lead volume, or master volume?

[Rod]

the amp has one lead volume , and two selectable master volumes, and i notice that diference when increasing lead volume and in the two masters volumes.

The resistor has to be rightaway next to the wiper? i mean it has to be the first componente after the signal exits from the wiper?

[stormbringer]

I'm not sure if adding a 100k resistor after the lead volume will help because I don't know what comes after that pot. If it goes straight into an FX send gain stage via an existing grid-stop resistor, then adding the 100k R won't have much of an effect there.

My hunch is that the amp is probably just too bright for your tastes, at least when the volume is high enough to get into it's 'sweet spot'.

[Rod]

OK, the lead volume is the pot that makes the amp more thinner, but has you can see in the schematic bellow there is a resistor after the relay and before the signal enter the fx loop.
So i think that adding the resistor may not be the solution because the amp has one resistor allready.
What else can be the problema here? What is the pupose of the cap in the Picture CE6?

[Jazz P Bass]

CE6 (1uf electrolytic capacitor) is a 'coupling' cap.

It passes the AC signal while blocking the dc voltage.