BRIAN MAYíS RUNNING METHODS FOR THE VOX AC30 TBX AMPLIFIER
Information supplied by Greg Fryer
All preamp valves except the two 12AX7/ECC83 stress that the normal
channel use are taken out.
This provides cooler running for the amplifier, which is extremely stressed in full volume Brian May mode, and also frees up a little extra power for the power amp section. This latter situation means that the EL84 power valves will be pushed even harder than normal, giving extra sustain and distortion, but making it essential that the EL84ís used are robust and extremely good quality, as well as being good sounding valves.
12AX7 / ECC83 Preamp tubes to be removed:
*Normal Channel uses 4 and 6
Brilliant Channel uses 4, 5 and 6
This is the valve arrangement for current AC30ís 1992 onwards. Older
models are different.
VOLUME KNOB SETTINGS:
Vib-Trem and Brilliant Channel volume knobs are turned off (anti-clockwise).
Normal Channel volume control is set on FULL volume (clockwise).
TONE KNOB SETTINGS:
Treble, Bass and Cut controls are all wound fully off (anti-clockwise).
Only the Cut control directly effects the Normal Channel, however the Treble and Bass (which function with the Brilliant Channel) still have
some bleed-through tonal coloration.
Choice of power valves (EL84ís) is critical to tone and failure-free
Derek Rocco from Watford Valves (UK) gave us invaluable assistance in the lead-up to Brianís 1998 World Tour, and built special test rigs in order to test and match EL84ís run at full stage BM volume!
More details can be obtained, including Brianís choice of valves, on Derekís web site: www.watfordvalves.com
Brian May prefers the sound of the current re-issue Blue speakers.
For AC30ís built in the 1960ís (JMI) or 70ís (CBS-Arbiter), or 1980ís (Rose-Morris Era) the following preamp tube configuration applies almost all of them:
Normal Channel needs No.4 and 5
Brilliant Channel needs Noís. 4,5 and 6
PREAMP TUBE SELECTION:
The AC30 Normal channel has a very clean low distortion preamp section
and requires a 12AX7/ECC83 that is not a high gain type, but above all
has low microphonic characteristics.
For Brian Mayís 1998 World Tour, we stuck with the standard issue Vox (Chinese) 12AX7ís which sounded fine. Since then I have had the chance to test more of Derek Roccoís stock and can recommend highly these 12AX7ís:
1. Sylvania (USA).
2. Philips JAN (USA).
3. GE (USA).
GZ34 RECTIFIER TUBE:
Dave Peterson, who advised us closely during the amp preparation of
Brianís í98 Tour strongly recommended that we replace the standard Vox
issue Chinese GZ34 rectifier with more robust models, preferably the hard
to obtain legendary Mullard GZ34.
Dave felt that because of the great stresses placed on the GZ34 during ďBrian May full volumeĒ operation that the Chinese tube was working only marginally within the limits of failure. The Mullards proved to be bullet proof and delivered a superior sound. (Enquire at Watford Valves. GZ34 Mullards are all gone now. See Derek for alternatives. Brian May bought the last of these!)
Dave Peterson also recommended that we replace the standard 2-amp mains
fuse with a 3.15 amp higher rating fuse. He has found (as I did back in
Sydney) that when the amp is first turned on, the amount of current being
pulled is sometimes sufficient to trip the 2-amp fuse.
Radio Spares Catalogue Numbers:
1. No. 415-610 or
(5 X 20mm size Anti-Surge type)
Then: Get ready to Rock!
BRIAN MAYíS USE OF VOX AC30 TBX (Blue Speaker Model):