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Author Topic: RG100ES Biasing the POWER transistors???  (Read 2736 times)

BMWFREQ

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RG100ES Biasing the POWER transistors???
« on: March 23, 2011, 08:35:46 PM »

I just replaced all four of the power transistors in my RG100ES, and would like to make sure they are biased to the correct voltage.  I know there is a trimmer pot on the PC board that is used for this purpose, but I am not sure where the voltage is taken.  I believe they should be biased to -.6volts, just not sure where to probe the board when setting this voltage.

If anyone has done this and can help me out, it would be greatly appreciated.

BMW FREQ
« Last Edit: March 24, 2011, 04:38:52 AM by BMWFREQ »
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #1 on: March 24, 2011, 12:18:33 AM »

It's not exactly as on tube amps, and transistors are 1000000 times more consistent than tubes, probably it won't even be needed, but to take that idea out of your mind, measure voltage across cathode resistors ... he he, I mean those 0.47 ohm emitter resistors (or were they 0.33 ohm?).
Anyway, any voltage between 5 mV and 20mV across them is fine.
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BMWFREQ

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Re: RG100ES Biasing the POWER transistors???
« Reply #2 on: March 24, 2011, 04:15:01 AM »

Thanks for the reply.  You sound like you know your way around an RG100ES.  Would you know what would happen if the value of these resistors were to be changed?

There are two .6 ohm and two .27 ohm 5 watt resistors on the board.  Are two of these for the clean channel and the other two for the gain channel?  The reason I ask is, I have been looking for a way to add more gain to the red channel, and clean-up the green channel.  I know I know...you're gonna say "The red channel already has monsterous gain, and the clean channel is already crisp and clean - why would you want to change them?".  And to answer that question before you ask it...it's just my nature; I like to tweak.

If you would happen to know how to occomplish this, your knowledge would be greatly appreciated.

Thanks in advance-

Jay
« Last Edit: March 24, 2011, 04:39:35 AM by BMWFREQ »
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #3 on: March 24, 2011, 05:06:54 AM »

Quote
There are two .6 ohm and two .72 ohm 5 watt resistors on the board
Read those ".72" the right way and they will be the correct  .27  ;).
That's the power amp side, which you must leave well alone.
Randalls are finely tuned, excellent design, they really squeezed all juice possible, no untapped gain there; I suggest you build a clean boost ahead for a scary sound.
The MXR MicroAmp is very good and layout is available; if you don't find it, it's just a Distortion+ without the clipping diodes and the output volume control; the board is the same with those parts omitted.
Or you can use a Tube Screamer, more as a gain booster than for its (light) distortion which will *not* be noticed after being brutalized by Randall's own..
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JHow

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Re: RG100ES Biasing the POWER transistors???
« Reply #4 on: March 29, 2011, 09:20:34 PM »

It's all personal preference... but I like my JRC4558-based BYOC "British Blues overdrive" in the effect send-receive of my rg80 - I'm not going for hardcore metal, but it leaves a lot of room for rude sounds in the red channel.  Think of Zep live Killing Floor.  If you want extra crazy, you can turn on the diode-clipping.  The green channel I like with the reverb up high and no boost.  Think Earl Hooker  on dust my broom. 

The modification I am most interested in for the randall is the trick to set it up to switch the channels via relays.  I have read that people have done this to cut some of the noise introduced by the long footswitch cable.
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BMWFREQ

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Re: RG100ES Biasing the POWER transistors???
« Reply #5 on: April 01, 2011, 08:13:54 PM »

So, does changing the Op Amp really change the sound of the RG100ES?  I thought the Op Amp chip was used to amplify the reverb and effects loop?  If it DOES change the sound of the amp, how does it do that? If I look at the schematic, I do not see how it has an effect on the sound of the amp...
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JHow

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Re: RG100ES Biasing the POWER transistors???
« Reply #6 on: April 01, 2011, 08:54:41 PM »

The op amp on the pc board is for the reverb send and return circuit.  It happens to be 4558 on the schematic, but in my post I was referring to a pedal that I particularly like, which concidentally also uses the same chip.  Regarding, changing the op amp, I have done that.  When I got my amp it had a socket for the chip already installed.  I don't know if Randall did that or someone else, but a previous owner or experimenter had mis-installed the op amp upside down.  When I got the amp the owner (he said it was his brother's amp) told me the reverb didn't work, and really it was just the chip inserted wrong, I flipped it over and it worked.  Anyhow, the chip in my amp (the upside down one) when I got it was a 5332.  I also tried a 4558, which worked fine.  It may be the 5332 was a little quiter, less noisy, but I wondered if it wouldn't pull a little more current, so I left the 4558 in there.  I didn't notice a big difference in the reverb, to be honest.
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vidmachine

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Re: RG100ES Biasing the POWER transistors???
« Reply #7 on: March 17, 2017, 03:36:30 AM »

what are the symptoms of a bad transistor?
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #8 on: March 17, 2017, 10:46:39 PM »

what are the symptoms of a bad transistor?
It doesn´t work that way.
You tell us what is the actual problem with your amp or describe its symptoms, then we proceed.
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