Randall RG100ES Emu

[Maik]

@meffcio
...file not found...

[Xaar]

I'm confused now. What was used in the original? 1n914 or 1n4148? 2n5458, 2n5484 or any other?
And btw - hitting the sustain switch will actually cause a big volume drop, yes? And a volume pot even before the tonestack is kinda strange for me..

The original uses 1n914's for clipping diodes, and 2n5484's for jfets. There is indeed a big volume drop when the clipping diodes are switched in (the sustain switch), which was addressed in later Randall designs with iirc a makeup gain stage. I like the sound of it without the makeup gain stage, and besides, I've never had an issue with the RG's not being loud enough! As for stompboxes, that's another story. If it were me, if I were making a pedal out of this, I'd move the volume control to right before the output. I also agree with teemuk about the scaling for 9v operation. That's where I had left off with my version. Well, that and what to do with the tonestack. I got a fairly close response curve to what I like about original tonestack with a big muff style tonestack in Duncan's tonestack calculator.

Out of curiosity, has anyone had a look at what's in MXR's Dime Distortion circuit-wise?

I'm sorry but there's some confusion here, I have the official Randall Warhead X2 schem and the transistors are 2N5484, exactly as in the RG100.
The clipping diodes in the real X2 Warhead are 4148, same as in the X2fet pedal (but in the RG100 it's a pair of 4V zeners and a pair of 914) .

the "original" you are referring to is not the X2 Warhead but the RG100 and it's not the schem that was used to make the X2FET pedal.
X2FET was based on the official schem of the Warhead X2, only it was simplified  (I posted the main differences here : http://www.diystompboxes.com/smfforum/index.php?topic=70489.msg760186#msg760186 )

Truth is, that Randall used the fet preamp of the RG100, revised it (still keeping it at a 24V supply),removed the twin zener stage, added an input buffer and a big EQ +presence section just after .

In one word, X2FET is based on Randall's X2 Warhead schematic with extremely minor modifications. And there has never been any J201 neither in the RG100 or the X2.

Concerning the "mistake " in the famous "preamp from hell" (RG100 Red channel for pedal use) that was available on the net some time ago (still?) , it's the exact same dist channel as in the RG100 with no modification at all ( besides the reverb section removed ) .
Plus those who were in charge of the preamp from hell project advised the DIYers to use a 9v to 25V converter circuit and to reduce 25V to 24V with a resistor (citing exactly).

[space_ryerson]

the "original" you are referring to is not the X2 Warhead but the RG100 and it's not the schem that was used to make the X2FET pedal.
X2FET was based on the official schem of the Warhead X2, only it was simplified  (I posted the main differences here : http://www.diystompboxes.com/smfforum/index.php?topic=70489.msg760186#msg760186 )

Correct, I was referring to the RG100.

Concerning the "mistake " in the famous "preamp from hell" (RG100 Red channel for pedal use) that was available on the net some time ago (still?) , it's the exact same dist channel as in the RG100 with no modification at all ( besides the reverb section removed ) .
Plus those who were in charge of the preamp from hell project advised the DIYers to use a 9v to 25V converter circuit and to reduce 25V to 24V with a resistor (citing exactly).

It's funny you mention that, because the guy who said that there was a mistake (Munky), is the one who put together the 'preamp from hell' 

[Xaar]

Concerning the "mistake " in the famous "preamp from hell" (RG100 Red channel for pedal use) that was available on the net some time ago (still?) , it's the exact same dist channel as in the RG100 with no modification at all ( besides the reverb section removed ) .
Plus those who were in charge of the preamp from hell project advised the DIYers to use a 9v to 25V converter circuit and to reduce 25V to 24V with a resistor (citing exactly).

It's funny you mention that, because the guy who said that there was a mistake (Munky), is the one who put together the 'preamp from hell' 
[/quote]

Strange not many were interested with the preamp from hell (the "mistake" was effectively corrected a long time ago, but I didn't know it was forum member Munky who was responsible of the project) and now all kinds of people are raving about having a Dime in a box (or even a Lynch in  box, since G.L. used a RG1OO with a screamer in front apparently).

RG100 had a status in the 80's and 90's similar to the ampeg vh140c in the metal community and still both are esteemed by trash/tech death fanatics. With reason!

Knowing that some people are using some of Tech21's character series pedals (the lowest gain ones ) as clean channel right before the poweramp and just after an overdrive/distortion pedal, it would be interesting to see the DIY community put together more high quality stompboxes that could fill in this "solidstate preamp clean channel" duty, wouldn't it?

[bmwfreq]

With regards to the RG100ES and the biasing resistors for Q-1 and Q-2, which resistors will I want/need to change in order to acheive the 4.5 volts biasing?  (i.e.: R6, R7...)  I would like to get the optimal gain from the Red channel, and decrease the amount of gain on the green channel.

Also, where would I measure this 4.5 voltage drop?  Accross the biasing resistors - or at one of the legs of the transistors being modified?

Any help would be most appreciated.

Thanks in advance-

BMW FREQ

[space_ryerson]

I'm sure which version you are referring to, but if you were to use the schematic in the first post of this thread, it would be R4 and R7. I would measure it by either drain leg of the transistor and the ground, or the end of the resistor/trimpot that is connected to the drain leg of the transistor and the ground.

You want less gain on the green channel? That is a very clean channel! When I was corresponding with Gary Sunda (who designed the amp), he mentioned that he modded his green channel to have more gain! Even if I wanted the cleanest signal possible, I would still bias the JFETS at half of the supply voltage, and just keep the gain knob low.

[bmwfreq]

Okay - so when you say 'drain leg', are you referring to the transistor Emitter, Collector or Base leg?  If I want to bias the transistors to half the supply voltage, and the supply voltage is 15v, they will be biased to 7.5v, not 4.5v - correct?  Or, do I want to bias the transistor to 4.5v regrdless of the supply voltage?

Thanks in advance - I really appreciate the reply.

BMW FREQ

[caspercody]

Go to runoffgrove.com and look at the Fetzer valve article. This has some good info and diagrams about Jfet amp emu and in the second diagram you will see the schematic look at the Jfet and how it is labeled. You want the voltage to be half of your power supply to start with. I use trim pots at these locations (in this case R4 and R7). I adjust the voltage at the drain to be half the power supply voltage. Then using the trim pots you can adjust them to sound.

[bmwfreq]

Alright, so I think I misunderstood at some point.  You said the transistors should be biased to 4.5 volts, but that was because the circuit in question had a supply voltage of 9 volts.  If the transistors 'I' am trying to bias have a supply voltage of 25.6 volts, then I will want to bias them to 12.80 volts...correct?  Also, is this a good rule of thumb when biasing transistors?

Thanks in advance-

[caspercody]

Yes

[Maik]

Hi,

can´t get the 4V Zener. Are 3.6V or 4.7V OK?

[space_ryerson]

If you are using the 1n914 clipping diodes (sustain boost), then if I understand this correctly, the 1n914's clipping supersedes the zeners, since the 1n914's clip at a lower threshold. If the sustain boost is on, then I don't think the zeners will have any effect on the sound--you may not need them. Here is a good link showing what I mean.

[Maik]

Thanks 

[therecordingart]

A PCB layout has been created based on Mensur's schem that have the "Voicing" circuit of the Randall Century.

http://www.diystompboxes.com/smfforum/index.php?topic=91777.0

[Marcos - Munky]

what I did in the "preamp from hell" was to take the red channel from the RG100 amp, and add the tone stack. the "mistake" I once said was in the layout I did, not in the schematic. I've been out of effects building for some time, college is taking me too much time, so I don't remember where's exactly was the error. I remember someone emailed me, and we figured out the error, and he got his build to work. I think it's somewhere near the last transistor, before the tone stack, but I'm not sure.

[allan_belt]

what I did in the "preamp from hell" was to take the red channel from the RG100 amp, and add the tone stack. the "mistake" I once said was in the layout I did, not in the schematic. I've been out of effects building for some time, college is taking me too much time, so I don't remember where's exactly was the error. I remember someone emailed me, and we figured out the error, and he got his build to work. I think it's somewhere near the last transistor, before the tone stack, but I'm not sure.

I just finished the one posted in Plexilandia modded some values according to the original schematics added a 33k resistor at the input.. and worked but too low sound.. found amistake on the Layout:
C16 must be connected to source of the las Fet not to drain ..
corrected the loss of volume issue...
I only have one question i tested with 24v but with different tansformers with 3amp sounds a lot better to may ears .. anybody tested with different amperes?

[caspercody]

I do not believe you can put more current into a circuit. A circuit will draw what it needs for current, and that is it. You can apply more voltage..

[kcarring]

I took a stab at building this one dead bug style and actually it sounded nice. I surprised that the gain quality was good (no trim pots for biasing - different JFETs), I used:


(4) J201 JFETs
(2) TOSHIBA 1S1588 Silicon Epitaxial Planar Diodes for clipping


I just have one knob: Gain

Instead of a Master volume, a I have a voltage divider with 9.2k ohms one way, and 680 ohms the other, simulating the volume knob turned way down. Mids scooped out on the amp. I added a capacitor for smoothing on the power rails.
No tone stack at all. No end resistor either. No zener diodes.

Cacmcorder recorded audio *behind* the amp which is a partially openback. Volume on as low as the amp will go, making it a volume that can be talked over (you'll notice at the end I am actually talking over it - it's not loud)
That's it.