Randall RH50 tube amp mod!

   Here is a new modification for the Randall RH50 or RG50tc tube combo/head. This 50W Randall tube amp is an affordable model made for "metal" music but there are a few drawbacks. First of all, it has almost no clean channel headroom. The clean channel distorts when its level is set above 12:00 o clock, especially when the clean boost is on. Then, the overdrive (1 or 2) channel just doesn't have all the gain and lead to support "metal" playing styles and it gets quite bassy. Note definition disappears when the gain is set to max.
   To address those issues, I looked at several other tube amp/preamp designs and found an interesting story about the Mesa Boogie V-twin pedal. The V-twin has a small power amp module in it that's based on a LM-386 opamp chip. The LM-386 chip is also famous for being the heart of all "Smokey:" amps and many distortion pedals(especially d.i.y. ones). The LM-386 sounds great when driven by another dist./od/metal pedal and it's fairly easy to work with. That's how I decided to incorporate an LM-386 chip module inside the RG50 (OD channel) preamp. This lm386 module can be used with various tube or ss amplifiers to boost the guitar signal or as  a nice od channel/preamp.
  As for the clean channel, I made it less bassy and gave it less gain/distortion by changing a few resistors and capacitors. Then, the eh12AT7 driver tube was swapped with a more open sounding sovtek 12AX7a one.

   The original and modified schematics of the RG50T amp are shown below. The LM-386 module is drawn as a yellow box on the RG50 schematic but you can see its details on the LM-386 schematic. The lm386 module power Voltage is +15V (6-16V are O.K.) and can be found inside the pcb at pin number8 of the jrc4558 IC chips (it's marked as "+15V" on the pcb, nearby the power relays).  The caps and resistors I changed are drawn in small red boxes on the rg50 schematic and beside them is their new value. All resistors are 1/4W unless otherwise specified. Be careful with the polarity (+/-) of some large(a few uFarads) electrolytic caps .

 I also changed the appearance of my Rh50 amp as I like to see the tubes when running hot! It's easy: I just flipped the amp's metal chassis and put its faceplate at the back side of the wooden box (see pics).

Randall RG50, RH50, RG50tc tube amp design:
http://www.echostar.pl/~zelman/RG50TCSchemat.pdf

My modified RG50 design based on the above schematic: 
(right click on it and "open in new tab" to enlarge).

The lm-386 post-od preamp module :

LM-386 datasheet(pins etc)

http://www.ti.com/lit/ds/symlink/lm386.pdf

http://www.national.com/mpf/LM/LM386.html#Overview

The jrc4558 pins:

http://www.diystompboxes.com/pedals/ampins.GIF

The mesa V-twin dist pedal:

The new Randall RH50t head box:

the old Randall rh50 look:

Here is something very important:
*** SAFETY WARNING ***
READ THIS FIRST!!!!!

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Working inside a tube amplifier can be dangerous if you don't know the basic safety practices for this kind of work. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure yourself or get yourself killed. This section is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one.

BASIC REQUIREMENTS

• UNPLUG IT FIRST Pretty self explanatory. Do not, ever, ever, leave the equipment plugged in and start work on it. Leaving it plugged in guarantees that you will have hazardous voltages inside the chassis where you are about to work. This is like setting a trap for yourself.
• LET IT DRAIN If the amp has been turned on recently, the caps will still have some high voltage left in them after the switch is turned off. Let it sit for five minutes after you turn it off.
• SUCK IT DRY When you open up an amp, you need to find a way to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground. This jumper will drain any high voltage to ground through the 50k to 100K plate resistor on the tube. To do this successfully, you will need to know which pins are the plate pins. Look it up for the amp you're going to be working on. You'll need to know this for the work anyway. Leave the jumper in place while you do your work ( high voltage electrolytic caps can "regrow" voltage like a battery sometimes. Really. ) Remember to remove it when you finish your work.
• TEST IT Take your multimeter and ground the (-) lead. Probe the high voltage caps and be sure the voltage across them is down, preferably to less than 10V.
• BUTTON IT BACK UP FIRST Take the shorting jumper out. Put the chassis back in the cabinet, making sure all of your tools, stray bits of solder, wire, etc. are out of it. You don't have to actually put all the screws and so forth back in if you believe more work might be needed, but make sure that the chassis is sitting stably in the cabinet and won't fall out. At the end of a listening test, either continue buttoning up if you're done, or go back to UNPLUG IT FIRST.

                                 Source: http://www.geofex.com/tubeampfaq/tube_amp.htm