FENDER VIBROLUX REVERB 1970 AA270 CHASSIS

ISSUE: CUTS IN AN OUT

CAUSE OF PROBLEM: SHORTED POWER TUBE

CURE: REPLACED AND BIASED POWER TUBES (PAIR) @34mA

UPON PULLING THE TUBES IT WAS APPARENT THAT THE TUBES WHERE GROSSLY UNDER BIASED CAUSING EXCESSIVE HEAT AND EVENTUAL TUBE FAILURE. UPON PULLING THE CHASSIS, IT WAS EVIDENT THAT THE EXCESSIVE HEAT WAS PASSED ON TO THE TURRET BOARD (CIRCUIT BOARD) PARTICULLY RIGHT BELOW THE OUTPUT TRANNY, SO MUCH THAT THE “WAX” INSULATINNG TREATMENT THAT IS APPLIED TO THE BOARD WAS LEECHING OUT. FORTUNATELY NO DAMAGE WAS DONE TO ANY COMPONET OTHER THAN THE TUBES.

AFTER EXAIMING THE BIAS CIRCUIT AND BY THE WAY THE TUBE CHART LISTING THIS AS A AA864 CIRCUIT IS WRONG, IT IS A AA270 WHICH HAS BALANCE POT NOT A TRUE ADJUSTABLE BIAS POT. THAT BEING SAID AT SOME POINT IN THIS AMPS LIFE, AN ATTEMPT WAS MADE TO MAKE IT AN ADJUSTABLE BIAS POT LIKE THE EARILIER BLACK FACE MODELS..BUT IT WAS NOT DONE PROPERLY NO DOUBT LEADING TO THE SMOKED AND SHORTED TUBE..CAN’T IMAGINE THAT IT SOUNDED AS IT SHOULD EITHER!

REMEDY WAS TO RETURN IT BACK STOCK BALANCE CIRCUIT AND ADD AN ADDITIONAL TRIM POT TO FACILITATE ACTUAL BIAS ADJUSTMENT OF POWER TUBES TO MAXIMIZE TONE AND TUBE LIFE.

NOW THE REST, THIS AMP IS 45YRS OLD WHICH IS A GOOD THING, BUT NOT SO GOOD FOR THE ELECTROLYTIC CAPACITORS, PARTICULLY THE HIGH VOLTAGE ONES THAT HAVE BEEN ON BORROWED TIME. TO ME ITS BAD BUSINESS JUST TO FIX WHAT IS BROKE TODAY AND NOT FIX WHAT IS SURELY GOING TO BREAK IN THE NEAR FUTURE WITH NOT SO GOOD AND COSTLY RESULTS, TUBE FAILURE, POWER TRANSFORMER, OUTPUT TRANSFORMER ETC.. THAT BEING SAID, THIS IS WHAT WAS REPLACED:

ALL PARTS MEET OR EXCEED ORIGINAL SPECIFICATIONS

ALL H.V. CAPS (5) UNDER THE “DOG HOUSE” INCLUDING VOLTAGE DIVIDING RESISTORS.

BIAS SUPPLY COMPLETELY REBUILT 2 CAPS3 RESISTORS1 DIODE1 TRIM POT

2 SOVTEK (RUSSIAN) 5881 POWER TUBE REPLACED WITH PHILIPS 6L6WGB (USA) NEW OLD STOCK COURTESY OF THE USA MILITARY..NICE CLOSELY MATCH PAIR.

ALL SOVTEK 12AX7 TUBES WHERE QUESTIONABLE AT BEST, 2 OF THE STRONGEST ONES WHERE RETAINED FOR THE REVERB CIRCUIT AND ½ OF THE VIRBRATO CIRCUIT, THESE ARE IN THE V3 AND V4 POSITION AND SHOULD BE REPLACED IN THE NEAR FUTURE..THE ARE JUST PLUG AND PLAY LIKE ALL TUBES IN THE FRONT SECTION OF ANY AMP.

THE 2 12AT7s (GROOVE TUBES) V3 AND V6 POSITION ARE FINE.

REPLACED THE PRE-AMP 12AX7s IN THE DRY (V1) AND VIBRATO (V2) WITH (NOS) SLYVANIA 12AX7 LONG PLATE AND RAYTHEON 12AX7 SHORT PLATE. I MUST SAY THAT I AM NOT A BIG FAN OF TUBES MANUFACTURED IN COMMUNIST COUNTRIES, LIKE RUSSIA AND CHINA, THERE ARE SOME DECENT ONES OUT THERE, BUT QUALITY CONTROL IS STILL A PROBLEM FROM MY EXPERIECE.

NOT NECESSARY BUT I ALSO REPLACED THE GROVE TUBE 5U4 RUSSIAN (SOVTEK) RECTIFER TUBE, WITH A WESTINGHOUSE 5U4 (USA). THE 5U4 WAS ONE OF THE MOST COMMON TUBES BACK IN THE DAY AND KNOWN GOOD QUALITY USA MANUFACTURED ONES CAN BE HAD VERY INEXPENSIVELY SO ITS ON THE HOUSE (NO CHARGE) AND I WILL SLEEP BETTER AT NIGHT KNOWING I DIDN’T LET AN AMP OUT THE DOOR WITH A FORGEIN BOTTLE ROCKET IN IT..AND PLUS YOU GET A SPARE IN A PINCH. THE AC CORD WAS ALSO REPLACED, IT WAS PRETTY MUCH ATE UP..THE NEW CORD IS GROUNDED AND WIRED CORRECTLY AND THAT’S NOT A GIVEN, I HAVE SEEN QUIT A FEW THAT WHERE DONE BY SO-CALLED PROFESSIONLS NOT DONE CORRECTLY..NOT A GOOD PLACE TO TAKE SHORT-CUTS.

ODDS AND ENDS:CABINET AND FACE CLEANED AND LOOSE TOLEX TACKED BACK DOWNSPEAKERS AND BAFFLED BOARD RE-TORQUEDPILOT LAMP REPLACED WITH STOCK BULB, IF YOU LIKE IT DIMMER JUST PUT THE OLD ONE BACK IN, I BELIVE IT IS A 12 VOLT ONE, STOCK IS A #47 6.3VOLT. THE POTS WHERE ALL IN GOOD SHAPE, NO SCRATCH OR NOISE THAT I COULD DETECT.

SO TO SUM IT UP THE AMP HAS BEEN GONE OVER TO PROVIDE HOPEFULLY MANY MORE DECADES OF ENJOYMENT…OTHER THAN REPLACING POWER TUBES WHEN THEY WEAR DOWN AND EVEN LESS OFTEN THE PREAMP TUBES… THE WILDCARD BEING IF THEY WERE PROPERLY BIASED (POWER TUBES) AND OF GOOD QUALITY TO BEGIN WITH.

ANY QUESTIONS PLEASE FEEL TO GIVE ME A CALL. ALL WORK IS DOCUMENTATED WITH PICTURES ALSO FOR MY FUTURE REFERENCE. IF YOU LIKE, GET AN EMAIL ADDRESS TO ME AND I WOULD MORE THAN HAPPY TO SEND THEM TO YOU AS RECORD OF SERVICE FOR VINTAGE AMP.

THANK YOU FOR SELECTING GT AUDIO,GEORGE TOGNARINIgeo_tog@yahoo.com