On The Bench

I’ll occasionally post here what’s been on the bench – what I found, what was needed, and some photos to round out the page. 

This month at MyTubeAudio,  an interesting amp turned up for some work – I’d call it the Fender FrankenAmp.  Basically it is a transition-era 1968 Bandmaster Reverb that’s been given a cosmetic facelift with the addition of a new brownface-era faceplate and knobs, and a new-re-issue type brown tolex Super-Reverb size cabinet (aftermarket). Just to add to the confusion, the unusual baffle supports a singe 12″ JBL driver and a single 10″ JBL driver, both with pincushion frames in characteristic JBL orange color.  Although the chassis once sported some non-original output transformer requiring 4 mounting holes, a quick check proved that all the iron present on the amp were in fact correct part numbers if not the original parts.

 Most of the tone caps were blue, and the tremolo LDR appeared to be a newer unit.  The HV caps were mostly a mixed bag of newer types, with one original part still present with original solder confirming the 1968 date.  One interesting note is that someone installed a useful Presence circuit into the feedback loop, and used one of the speaker output holes to do it. It’s detailed in the pictures. With an amp this modded, I guess that’s a nice touch, since the collector value on this amp is not a question.

 The amp itself was blowing primary fuses when it arrived, so the first order of business after opening it up and looking everything over was to check the 5AR4 rectifier and 6L6 tubes on the Hickock Gm tube tester.  Since the 5AR4 was built after the Hickock, I did not have a known setup for the tube, but it is the same pinout as many other American rectifier types.  Using a good, UK made NOS 5AR4 as a test pig, I set the Hickock up to measure something reasonable, and then subbed in the Fender rectifier. This test was inconclusive, but I did notice the Hickock fuse lamp glowing much more with the suspect tube. To make a long story short, the 6L6’s checked out OK but on the weak side, and I put a new 5AR4 into the amp, a new fuse and applied power.  No blown fuse.  With the original 5AR4 back in place, the primary fuse blew nearly instantly.  Time to order a new rectifier, and some new outputs as well.  After clearing everything with the owner first, I ordered the tubes, replaced the last original HV cap with a Sprague unit, and re-capped the bias section, which also had the original 40-year-old electrolytic in it.  That’s an area not worth risking in my opinion, period.  A leaky cap here can reduce the bias voltage over time and risk hurting the tubes or even the output transformer should it give up completely. 

 With all work completed and the tubes set up for about 35 mA bias current each, I burned the amp in for several hours using my CD of pre-recorded guitar signals and a dummy load, with the amp running near clip most of the time.  No issues. 

A live

play test with my favorite Stratocaster revealed that the choice of parts and speakers really worked well in this amp.

This is a very smooth-sounding Fender blues tone with lots of character and that hard-to-quantify slightly dirty sound  of 60’s Fenders that makes you feel like you can hear more than just one guitar playing, a density and complexity of tone probably brought on by a multitude of subtle distortion products that taken together are very musical.  I did not take time to put the amp on the THD meter or the IM distortion set, which I kind of regret now, but that’s a process I hope to investigate soon…. I’ve always wondered …. just how clean are the old Fenders, anyway?

 

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