Well, the main concern, as it has been pointed out previously, is
whether he filament winding can handle an extra .5 amp per tube,
cause if the PT's filament winding is unable to do that, It can affect
the sound in a big way, like make it dissapear into a cloud of
somoke, well at least until you get a new PT anyway.And you will
need to modify your bias supply to produce aprox 8-10 LESS
negative volts, unless you like the sound of crossover distortion.
This can be done by increasing the value of the series resister
connected to either a bias tap on the power transformer, or one of
the the PT secondary wires. the 1.5k grid stoppers will need to to
have on end removed from pin 1 on each of the power tube
sockets. Pins 1 & 8 should be soldered together.I hope this helps a
little.

 Here is the
step-by-step mod on how to do this.
1) Open the amp and position it so that the control panel is facing
you and the circuitry side of the amp is up.
2) Remove the tubes. On the power tube sockets, locate the 470
ohm 2 watt resistors(Yellow, Violet, Brown) between pins 4 and 6.
Remove both resistors.
3) Remove the wire from pin 1 of each of the output tube
sockets(MAKE SURE TO LABEL THEM AS TO WHICH
TUBES THEY GO TO OR THE OUTPUT TUBES WILL BE
OUT OF PHASE). This wire is the bias voltage wire. Locate the
1.5K ohm resistors between pin 5 and pin 1 on the tube socket.
Remove these resistors and use a short piece of lead wire(you can
just take a new resistor and cut one of its leads off and use that)and
use it to connect pin 1 to pin 8. 
 
Caution: Most tubes do not use pin 1. This is because the
suppresor grid(or beam forming plates) is internally connected to
the cathode. However, EL34s DO use this pin for the suppresor
grid and it MUST BE GROUNDED. Since the cathode(pin 8) is
also grounded, PIN 1 MUST BE CONNECTED TO PIN 8.
 
4) Resolder the 1K5 ohm resistors to pin 5 of each output tube
socket. Then, take the bias wires from each output tube socket and
solder these to the free ends of each resistor.
5)Replace the 470 ohm 2 watt resistors you removed in step 2 with
1K ohm 5 watt resistors(preferably the white, wirewound ceramic
type since ceramic has a higher heat tolerance). Mount and solder
them to pins 4 and 6. If applicable, resolder the screen grid voltage
wire to pin 6.
6) Remove the 27K ohm resistor on the back side of the bias
adjust pot. Replace with a value between 10K and 15K ohms.
7) Turn the bias control fully counter-clockwise and put the EL34s
in. Rebias and you're done!
I didn't like the sound, but this is because I don't like EL34s. They
are too slow in response, resulting in a "saggier" and muddier
sound. But to each his own right?