BRIAN MAY’S RUNNING METHODS FOR THE VOX AC30 TBX AMPLIFIER

Information supplied by Greg Fryer
for www.brianmayworld.com
 
 

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 EL84’s 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 AC30’s 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.
 

VALVES:

Choice of power valves (EL84’s) is critical to tone and failure-free running.
Derek Rocco from Watford Valves (UK) gave us invaluable assistance in the lead-up to Brian’s 1998 World Tour, and built special test rigs in order to test and match EL84’s run at full stage BM volume!
More details can be obtained, including Brian’s choice of valves, on Derek’s web site: www.watfordvalves.com

SPEAKERS:

Brian May prefers the sound of the current re-issue Blue speakers.
 

For AC30’s built in the 1960’s (JMI) or 70’s (CBS-Arbiter), or 1980’s (Rose-Morris Era) the following preamp tube configuration applies almost all of them:

Normal Channel needs No.4 and 5
Brilliant Channel needs No’s. 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 May’s 1998 World Tour, we stuck with the standard issue Vox  (Chinese) 12AX7’s which sounded fine. Since then I have had the chance to test more of Derek Rocco’s stock and can recommend highly these 12AX7’s:

1. Sylvania (USA).
2. Philips JAN (USA).
3. GE (USA).
 

GZ34 RECTIFIER TUBE:

Dave Peterson, who advised us closely during the amp preparation of Brian’s ’98 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 “Brian May full volume” 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!)
 

MAINS FUSE:

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.
Suggested types:
Radio Spares Catalogue Numbers:

1. No. 415-610 or
2. 416-528

(5 X 20mm size Anti-Surge type)
 

Then: Get ready to Rock!
 
 

BRIAN MAY’S USE OF VOX AC30 TBX (Blue Speaker Model):