'65 Twin Reverb Amp with 25W Output Power
Oct 29, 2013
The '65 Twin Reverb amplifier is a high-powered amplifier which is often
considerably more than needed in small venues where amps with 25-30W are adequate. For better control, it is desirable to lower the audio output power of the twin.
The configuration below shows one way to properly lower the ouput power of a
'65 Twin Reverb amplifier from the normal 85W to 25W while keeping the original twin speaker connection.
Removing two tubes, (one from each side of the push pull) and using the same (4 ohm) speaker load will lower the output power to ~ 30W but the remaining
2 6L6GC power tubes will, under maximum power output, exceed the plate dissipation specification which can shorten the
lifetime of the power tubes. This is because the load line is changed and the tubes are "under loaded". The correct and safe load for 2 tubes in the '65 Twin Reverb amp
is 8 ohms. A simple solution is to simply disconnect one of the two 8 ohm speakers. This will provide about 45W output power into the correct single-speaker 8ohm load.
It is possible to keep the original 4 ohm (twin speaker) load and simply add a 4 ohm series power resistor of sufficient power rating to achieve the correct total output load.
This will of course waste power in the series resistor but will limit the maximum output power from the twin speakers to ~ 23 W,
similar to that of the Deluxe Reverb amp except that 2 speakers are powered.
In this case, the speakers will operate well below their maximum power ratings.
The configuration below is:
The total load impedance "seen" by the amplifier is 7.3 ohm (sufficiently close to the proper 8 ohm load). In this case the maximum speaker output
power is ~ 25W and the maximum power dissipated across the series resistor load is ~ 20W, within the 30W power dissipation limit of the 3 resistors.
The three power resistors can be rigidly housed in a small box with phone jacks and a short phone-plug patch chord (with 18AWG speaker cable) as shown:
- two power tubes with standard 4 ohm (double speaker) load
- 3 x 10 ohm/10W paralleled power resistors (for a net resistance of 3.33 ohm) in series with the 4 ohm speaker output