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Author Topic: Randall RG100 hybrid  (Read 32846 times)

ENGR BUGO aka DimebuGG

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Randall RG100 hybrid
« on: October 12, 2011, 12:37:45 PM »
This is my take on the Randall RG-100ES amp minus the original poweramp and reverb section - not to make it as complicated as hell for some beginners. For simplicity, poweramp is a TDA7293 based from Rod Elliot's application. Reverb section based from BELTON reverb brick.

Few notes:

1. In the reverb buffer section(starting from the 82n capacitor), I just directly input values from the Rebote 2.5 delay as this was originaly my plan to put a delay instead of a reverb. Feel free to experiment on the values of each resistors and capacitors or better yet use the values recommended according to the datasheet of Belton Reverb Brick.

2. Transformer is of course, as seen on the attached file, 25-0-25 / 3A to 4A.

3. 50K trimmers(0.5W) are used instead of fix 33K resistor(can be also used either) to adjust the correct Drain voltages of each FET according to the schematic or for your own tuning.

4. I didn't provide pads for the channel switching( just refer to schematic, it's very simple).

Comments, suggestions, and (violent) reactions are welcome.  :)
Enjoy!. ;)

tra

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Re: Randall RG100 hybrid
« Reply #1 on: October 12, 2011, 12:50:10 PM »
Thank you so much! Do you have it up and running? How does it sound?

ENGR BUGO aka DimebuGG

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Re: Randall RG100 hybrid
« Reply #2 on: October 12, 2011, 02:32:06 PM »
Thank you so much! Do you have it up and running? How does it sound?
Sounds like a Mesa..:)..just kidding. You'll never now if you won't try, buddy. Haven't build it yet but I'm pretty sure that thing will work.

J M Fahey

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Re: Randall RG100 hybrid
« Reply #3 on: October 13, 2011, 04:34:54 AM »
Looks good  :tu:
Thanks for sharing  :D

ENGR BUGO aka DimebuGG

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Re: Randall RG100 hybrid
« Reply #4 on: October 13, 2011, 05:35:26 AM »
"100n C" = 100n Multilayer Ceramic Capacitor

furrball

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Re: Randall RG100 hybrid
« Reply #5 on: October 21, 2011, 12:40:08 PM »
(50K trimmers are used instead of fixed 33K resistors to adjust the correct Drain voltages of each FET or for your own tuning.)

What affect does this due to the circuit? and how does it change the sound of the pre-amp? more gain?

I have an old rg100es and looking to do a few simple mods with it.I am wondering if changing the fixed resistors to trim pots will help me get different tones/gain.

thank you!

J M Fahey

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Re: Randall RG100 hybrid
« Reply #6 on: November 06, 2011, 04:30:22 PM »
Your RG100 is "the real thing", properly biased and powered with, say, 24V or higher.
The trimmers are a kludge to make poorly designed preamps work, even if fed absolutely inapproppriate voltages such as 9V.
You turn the little wheel or screw until you get 4 or 5V at their Drains, no matter how poorly biased they are.
Leave your excellent Randall alone, they have practically extracted the maximum from what they used.
Add some clean boost pedal between your guitar and it to slam it into heavier distortion, that´s all it takes.

allan_belt

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Re: Randall RG100 hybrid
« Reply #7 on: April 04, 2012, 12:15:14 AM »
Hello from Peru and congrats for the Randall Hybrid .. recently finished this project only have two issues:
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???
2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

on my experience with power amps the transistor ones are most pleasing sounding for guitar, and adds a little distortion to the signal when driving hard.. anybody know a 100 watts transistor power amp project with pcb and layout?? please let me know...
thanks for the project and please post some advances or maybe an audio demo..  :tu:
Allan Belt.


Loudthud

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Re: Randall RG100 hybrid
« Reply #8 on: April 04, 2012, 08:27:01 PM »
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???

Those FETs are pretty close. The transconductance of the 2N5457 is spec'd at 1000 to 5000 uMho. The 2N5484 is spec'd at 3000 to 6000 uMho so on average, it's a little higher but it depends on the exact part you are using.

2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

I would look for a bad component or wiring error. Try feeding another preamp into the power amp. Or just feed your guitar straight into the power amp.

allan_belt

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Re: Randall RG100 hybrid
« Reply #9 on: April 19, 2012, 04:56:11 PM »
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???

Those FETs are pretty close. The transconductance of the 2N5457 is spec'd at 1000 to 5000 uMho. The 2N5484 is spec'd at 3000 to 6000 uMho so on average, it's a little higher but it depends on the exact part you are using.

Thanx a lot for your support. i recently found the 2n5484 Jfets and the sound is improved now very tight and gainier than 2n5457.
Just one more question...the power suply needs 25-0-25 volts 3 or 4 A ... my question is       :is necesary to use 1/2w watt resistors at least? or is ti ok with 1/4 watt? i mean in the preamp section.. Thanks in advance..

allan_belt

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Re: Randall RG100 hybrid
« Reply #10 on: April 19, 2012, 05:02:28 PM »

2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

I would look for a bad component or wiring error. Try feeding another preamp into the power amp. Or just feed your guitar straight into the power amp.

Yes you are right i had a mismatched component  :-[ now the power amp sounds great and powerful ... thanks for your help.. i `ll post the amp when i put it in a Dynacord Chasis i have empty.. and maybe some demos too...  ;)

Davelectro

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Re: Randall RG100 hybrid
« Reply #11 on: April 22, 2012, 11:44:58 AM »
While this TDA7293-based power amp may sound very good, you won't get the original amp's response with it. The RG100 uses mixed-mode feedback to raise its output impedance.

allan_belt

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Re: Randall RG100 hybrid
« Reply #12 on: April 24, 2012, 09:40:36 PM »
Hello again .. 8|
Does anybody know any project to make a power amp close to the Randall RG100 with PCB and Layout?
i mean that uses Transistors intead of IC... ;)

J M Fahey

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Re: Randall RG100 hybrid
« Reply #13 on: April 24, 2012, 11:34:26 PM »
Not that I know of.
And I have searched, that and many other things.
I laugh when people say "you can find *anything* on the Net"
Yea, sure !!!  Keep searching !!!
You'll find (maybe) *some* of what anybody posted ... and what never was, never was.
You'll find lots of stuff on classic 40 years old tube amps, little on modern tube ones, almost nothing on SS ones.
Closest solution would be getting some classic discrete 100W into 4 ohms power amp kit and mod it slightly to add the mixed mode feedback.
You'd save 90 or 95% of the work.
Good luck.

phatt

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Re: Randall RG100 hybrid
« Reply #14 on: April 25, 2012, 09:35:20 AM »
Hello from Peru and congrats for the Randall Hybrid .. recently finished this project only have two issues:
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???
2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

on my experience with power amps the transistor ones are most pleasing sounding for guitar, and adds a little distortion to the signal when driving hard.. anybody know a 100 watts transistor power amp project with pcb and layout?? please let me know...
thanks for the project and please post some advances or maybe an audio demo..  :tu:
Allan Belt.

Re 2/ above?
What do you mean exactly?
If you plugged into the front (Where you normally insert a guitar ) then yes a big big difference would be heard as you have two preamps in series.

If you like it use it that way and save a power amp build. :tu:

Re poweramps.
If 100watts or more is desired then yes plenty of circuits abound if you take time to search.
Rod Elliot site ESP has some good big amp circuits.

But at 50 watts or less just use a chip ,,I have built and use an LM3886 chip which is a serious 40~50 ish watt output.
Phil.