Tubes
Three Electro Harmonix 12AX7's in the main chassis and a matched pair of Ruby 6550A-STR power tubes giving around 80W of output power (the Ruby's are a new remake of the GE 6550, with tough straight bottle and polished metal base).
The Randall RM80 is basically a RM50 with 6550 power tubes. If you install EL34's or 6L6's and re-bias then you've got an RM50 (or at least about 50W output power).
Output transformer
Output transformer has "OT-75" designation. Not sure if this is the same as the RM50. "OT-75" may mean 75W?
Speakers
Two Celestion G12T-75 speakers arranged diagonally in the cabinet.
Presence/Density board
Here is a comparison between the stock components on my version of the RM80C and the more recent production values identified by Jaded Faith here:
| Last production values | My RM80C | |
|---|---|---|
| Presence Pot | B10K | B25K |
| Density Pot | A1M | A1M |
| Density Cap | 4n7F 472 (250V) | 4n7F 472 (250V) |
| Presence Cap | 0.1uF 104 (250V) | 0.1uF 104 (250V) |
| Electrolytic Cap | 100uF (100V) | 100uF (100V) |
| Resistor | 56K (1/2W) | 56K (1/2W) |
Presence pot replacement
I replaced the B25K Ohm presence pot with the later B10K Ohm value pot. Process is quite straightforward:
- remove amp chassis from head/combo;
- remove volume, effects, density and presence knobs from the front;
- undo retaining nuts from all 4 pots;
- remove 2 screws that hold presence/density board in place;
- remove board from the case enough that you can get access to the legs of the pot. There's no need to remove the various wires whose connectors are glued;
- unsolder the pot and replace with new pot. Take your time unsoldering the pot and don't overheat the board or damage the traces. If you work carefully, it should look like it came that way from the factory.
Master volume boost
The RM80's (and RM50's) are unique in that they have a 1/4 rear jack for a "Master Volume Boost". Plugging a volume pedal in will allow you to boost the volume using the pedal. Plugging a footswitch in seems to boost the volume to the max. It is possible to use a 250k Ohm pot in parallel with the switch to give you a foot pedal with adjustable volume boost.
See this thread for details on how to build a footswitch with volume boost pot.
Problems biasing EL34's
It seems some of the RM50 (and RM80 as a consequence) heads have difficulty getting the right bias voltages when running EL34 output tubes. If you run into this problem, then change the value of R35 from 1.8K to 3.3K Ohm.
Fuses
Internal
There are three fuses inside the unit. Two on the power board and one on the main board.
External
There are also two fuses for the power tubes accessible via some ports at the back of the amp near the bias adjustment area.
Mains power
There is also a mains power fuse located near the IEC power inlet. This is replaceable from the rear by removing the 240/220/120/100 voltage selector. The fuse for Australia is a 1.6A 250V fuse, dimensions 5mm x 20mm.
Reverb tank
The reverb tank is a horizontally mounted made in the USA Accutronics unit - model number 9AB2C1A. Model number decodes as follows:
- 9 = Long tank (16.75") with 3 transmission springs
- A = 10 Ohm input impedance
- B = 2,575 Ohm output impedance
- 2 = medium decay time (1.75 to 3.0s)
- C = input insulated, output grounded
- 1 = no locking mechanism
- A = open side up
Known issues/fixes
- Carbonizing around plate voltage connectors - mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
- Arcing on the main board - mtsforum.grailtone.com/viewtopic.php?t=20534&highlight=