TUBES
Just like all other parts in the Convertible amps, the tubes were selected for
providing the most musical tones. Tubes can be temperamental, but there are
things that can be done to avoid problems.
Module Preamp Tubes
Convertible amps are designed with EL-34 power tubes. These tubes will give
you the sweetest tone and are generally more "musical".
The EL-34 tubes installed in your amp are from Groove Tubes. They are
specially matched for cathode current to reduce hum and noise - especially
noticeable when playing quiet passages and in the recording studio. Clipping
characteristics are also matched to give the most sustain and harmonic balance.
All notes will decay together. When you replace your output tubes, make sure to
ask for matched sets to keep your amp sounding like new. A bias balance control
under the amp lets you adjust for minimum hum when you replace power tubes.
Bias Balance Adjustment:
1. Turn both volume and overdrive controls off.
2. Remove guitar cord from input jack.
3. Liste n for hum level.
4. Turn bias balance control clockwise for lowest hum. If hum increases while
turning clockwise, turn the control counter-clockwise until the hum reaches its
lowest audible level.
If you re transporting your amp often and subjecting it to rough handling, there
are other power tubes that will better withstand the abuse.
The 6CA7 is an industrial version of the EL-34. While the tone is not as sweet as
EL-34's, it is better suited to handle bumps and bangs. You can plug 6CA7 tubes
directly i nto the power tube sockets because the amp bias will not have to be
changed. If you try 6CA7 tubes, just make sure you adjust the bias balance
described above.
The 6L6 and 6550 tubes will also stand up to road knocks. Like the 6CA7 tubes,
they are not as musical as EL-34's. You'll notice a harder sound. Although the pin
matchups allow you to directly plug 6L6 and 0 tubes into the power tube sockets,
you'll need to have a service technician replace a bias resistor one that's value
matches the 6L6's or 6550's. See your authorized service center for cost.
