bluesmkc

Volume eon troi Rcmove the wiring froin the Normal inpuis, if you haven^ht already. These inputs will no longer function,

10. Shicld the wiring from the preamp out-puts to the Balance control. Shicld wiring from the Volumc control to the Bright switch. hastly, shield the wire from the center terminal of the Treble control to the 470k resistor. I Remember to heat-shrink both ends and I ground only one end of all shielded cables.

if the amp is old enough, it may usc a center tap on the 6.3 VAC heater suppiy. Remove This center tap wire from the groundj cm the bare portion off, heat-shrink the end and tie-wrap it safely to the other tran sfor mer wires. Install a 10G£2 1/2-watt resistor from cach leg of the 6.3 winding to ground. Some amps may already have this done.

Initlol power-up. Remove all the tubes. Double-check all conncctions for proper sol-der connections and location assignment against the schematic. Using a Variac_h slowly raise the AC voJtage. Monitor the B+ and bias voltages* lf everything looks good, inerease tlie AC voltage; monitor carefully until you've applied 117 VAC. Remove the power and take a deep breath. You can nnw install your new tubes, For best results use only ąuality matched tubes, such as Groove Tubes or Ruby flibes,

Betore testing the Bluesmaker, set the bias, Bring the amp up to proper linę voltage and let it warm up for a few minutes. Take tt off standby; with a DVM, measure across one of the 1£2 resistors, Adjust the bias control until you see .030 volts DC Check the voltage on the other resistor and make surę its read-ing is close to .030, If it differs by morę than .004 volts₅ then your tubes aren't matched or your output transformeris primary winding isn’t perfectly balanced.

Finding The Parts

Włth tfl6 exception of the lOk push-putl pots_v whkh Musiłedi stocks, dl parte shoyldbe reódily availdbfe ot focd ełec-frank whołesalers or amp repoir shops.

You can obtain a complete Bluesmaker parte kit—inducflng matched tubes—łrom Musitech, If you want MusHech to perform lhe conversion, cali (403) 243*8952,9:30-5:30 M5T Monday thraugh Sahjrday, or senda stomped, setf-addressed envelope to Musitech Electronics Lfd., Boy 110-4040 BlockfoałTr./Cdgary, Alberta, Canada T2G4E6.

Testing. Connect the amp to a 4tt dum my load and hook up an oscilloscope to monitor the output waveform, Set all Controls counter-clockwise. While applylng a -Z0dBV (Q.100V) 1kHz sine wave, slowly bring up the volume control and observe the waveform. Look for even clipping and linie crossover distortion. Re duce the volume again and test all the other functions. To check tonę Controls, I use a 400Hz square wave. If you notice any prob-iems, use normal troubleshootlng procedures to isolate and resolve them*

Now the fun begins! Connect the amp to a suitable speaker cabinet and pług in your fa-vorite guitar. Tum all the Controls down. Bring up The Volume knob to 4 or 5. Rotate the Balance control back and forth, listening to the timbre and gain changes. Hit the Gain-Boost (Deep) switch and repeat the process. Replace any preamp tubes that are worn or micro-phonic. Check all the tonę Controls and puli switchcs. The Mid-Boost switch adds extra bite for morę sustain and extra harmonie feedback. Using a Strat and a Les Paul Cus-tum, I played the Bluesmaker through a 4x12 Marshall cab and found the distortion to be thick and rich, with very little buzz, The sound cleans up nicely as you back off your guitar volume (of course, this depends on bow you set your amp). The overall sound is noticeably unprocessed and very natural. I hope that anyone who builds this amp enjoys some killer tonę!    *1

John Mclntyre kas been designing_t modifying, and repairing amps for over 23 years. He is the president of Musitech Electronics, whose product linę includes Mclntyre Gustom Tubę Amplifiers, Mid/Boosters, Tubę Coolers, and Bridgelockers.

114 GUITAR PLAYER Fehruory 1993