SubZero T20R - (FBJ Clone) -
I am no longer modifying these Amps - The Following is for Information Only.
If you Own a Stock T20R (NOT the RAT Modified Amp) I suggest you STOP using the Amp and get it Safety Checked ASAP. Some of the Amps I have seen are LETHAL and there is a serious threat to the user of receiving an electric Shock. The two worst problems relate to the Rear Panel Mounting Screws being too long, one of them can touch the Mains (240vac) Power Fuse on the lower rear panel of the Amp, the other drives itself into the Mains (240vac) On/Off Power Switch on the Top Control Panel of the Amp. NOT Good. The Stock Amp has NO Internal (HT and Heater) Fuses, this is a recipe for disaster.
If you don't have one of these Amps - DON'T BUY ONE!
This Amp is a Copy of a Fender Blues Junior (FBJ), The Cabinet Looks a Bit Different but other than that, it's an FBJ. If you need a Circuit Diagram for the T20R, you can get it on the Fender Website, The Circuit is Identical and 99% of the Component Ident numbers on the PCB are the same. The Amp is Sadly let down by VERY Poor Attention to detail during Assembly. Every Nut, Bolt, Screw and Fastener in the Amp is usually Loose or has already fallen out. The First thing I did when I got one of these Amps was to collect all of the 'Spare' parts from the Speaker Magnet. I think if you purchased a Stock Amp and took it directly to a Gig, it would be 50/50 that the Amp would Survive the duration of the Gig....Not so Good. However, once all of the 'Loose' Issues were corrected (and the RAT Modifications done), it is a Good Sounding Amp (No real Surprise as it's a Copy of the FBJ). There are a stack of 'Minor' Issues that need to be corrected on the Amp in order to assure it's Reliability, nothing Serious, just the usual Mass Production short cuts (Poor Quality Capacitors, Underrated Resistors, Cheap Switches, No Fuses). Most of the Important Components (Transformers, Valve Bases, Valves, Speaker, Pots and Sockets) are usually decent Quality.
- Additional RAT Features and Modifications -
1. - 'OPR' - Output Power Reduction (Power Scaling).
2. - The Amp is completely Stripped and then rebuilt, making sure that all fasteners are Tight or Thread-Locked as required.
3. - Twin Rectifier System - The Mains Power Switch is replaced with a '3 Position' Switch offering the Standard 'Silicon Rectifier' -or- 'EZ80 Valve Rectifier' Operation. Center Position of the Switch is 'Power-Off'. The Valve Rectifier gives the Amp some Great 'SAG' - The Silicon Rectifier gives the Amp a Nice 'Tight' Sound. There is No 'Standby' Switch, this is not required Providing the Amp is First Switched 'On' to the Valve Rectifier Position.
4. - The HT Voltage (B+) is Spot-On but the Heater Voltage is WAY Too High - I have corrected that problem and the Heaters now run at 6.3vac, as they Should.
5. - I have done Several Modifications to the Power Supply - Increased the Smoothing Capacity for 'Tighter Bass' - Fitted a 'Safety' Resistor so that the Amp can Not hold a Lethal Charge when switched off - All of the Main Power Supply Resistors have been replaced with 'Wire Wound' or 'Metal Film' Components.
6. - All of the 'Tone' Capacitors have been replaced with 'Orange Drop' Parts. This gives the Amp much better definition. The Tone Controls Sweep from 'Zero-to-Max' on each Frequency.
7. - Full Lead Dress - All Transformer and Heater wires in a Valve Amp must be tightly twisted or they will induce 'Noise' into the Amp Circuit. Twisting all of the wires and placing them in the correct location does make this Amp quieter (No Background Noise - Buzz+Hum).
8. - The Standard Input socket is a Problem. It is PCB Mounted and made from 'Hard' plastic. The first time you 'trip-up' over your Guitar lead, you will destroy the input socket. It is then a Major job to replace the socket as the Amplifiers PCB must be removed. The Socket has been replaced with a Quality part that can be easily changed in about 5 minutes (if you ever need to change it).
9. - The Stock Amp is not Fitted with HT and Heater Fuses - It is Now.
10. - Preamp and PI Heaters Converted to 'DC' - The 2x Power Valve Heaters are 'AC'. This Stops any Hum and Buzz and also allows any Make of Valves to be used in the Amp.
11. - The Amp has been converted to 'Auto-Bias' - Class A/B. You will NEVER need to Re-Bias the Valves. Output is 15 Watts RMS - 22.5 Watts Peak Power.
12. - Boost Footswitch in Metal Housing with 3m Lead. A 'RED' LED Illuminates when the 'Boost' (Fender 'FAT') Circuit is Active.
13. -Full RAT User Manual detailing all of the Modifications.
14. - All Amps are 'PAT' Approved.
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