SPECIAL PROJECTS

Up on the bench recently at MTA was a very nice and clean silverface Fender Champ which would not turn on.

This was one of those classic repairs where everything you find makes sense – you clearly see the damage, you find the root cause and the repair is limited to eliminating the root cause and reversing the damage. This Champ got a bit lucky – an oversize fuse (2A) had been soldered in place where the original fuse (1A) socket was, because the socket had suffered physical damage, and that pigtail replacement was someone’s “quick-fix”.

That shortcut could have cost the owner an output transformer replacement, or even the HV winding on the power transformer, but as things transpired, both transformers survived, while the oversize fuse and a number of other components finally burned up first.

Upon opening the amp up, I immediately noted that the cathode bias resistor had burned though (a 5W unit!) and melted the plastic-cased cathode bypass capacitor it was next to. I also saw that one of the high-voltage power supply resistors was also browned-out by overstress.

These are all points along the HV trail to the 6V6 output tube… so out he comes and hits the tester.

This is where things get a bit strange…. At first, it was clear that there were one or more internal shorts to the tube – three out of five of the shorts indicators on the Hickok 600A were dark.

Beyond that, the main fuse lamp (not ivisible in photo) of the tester was glowing – indicating too high of current flow for this type of tube. That settled, I turned back to the amp chassis to begin replacing the bad fuse holder and those burned-up components. A few minutes later, I looked again over to the tester, and the short-circuit indicator lamps were no longer indicating a short. After a few moments of warm-up, the tube no longer appeared bad. Strange indeed! I have seen many tubes that tested good at first and failed when hot — but not the other way around.

So now I understood the failure a little better… and why those parts kept taking too much stress for a limited time during each turn-on cycle. With the larger fuse in there, it probably only appeared that the amp took longer to warm up than it should, but left plugged in and switched on, after a while it would work. At least it did until the resistors and 2A fuse finally burned through from the repeated stress.

I replaced the fuse-holder, the fuse, the 1K HV power supply resistor, and the cathode bias components.

I know that a lot of people are pretty vehement about using original-type carbon composition resistors in older amps, but I don’t do that unless asked to. Yes, they look cool and old-school, and everyone likes to look into a beautiful old Fender chassis that looks untouched since new, but carbon comps almost always drift high with age, and are an inferior technology. Leo used what was on the market and reasonably priced. I don’t think physics supports the idea (or Leo would have ) that your ears can actually HEAR the way a resistors turns electrons to heat, or that they will change materially the way a tube clips or amplifies- but hey, lotsa folks believe in angels, too. I just prefer to put quality parts in that can’t burst into flames when they fail -and leave all the Mojo Magic stuff to the players who still think they can buy their secret way to tone heaven rather than working for it. Having done that, all that was left then was to order a new tube. Once that was installed, checked for idle current and everything play-tested for an hour or so, she was ready to go back home and rock and roll some more – still the Champ

Here’s another project from my past (I keep dredging these up as I run across the digital photos… our scanner is on the fritz and I can’t scan from the large body of analog project & repair photos for the moment….).

This was conceived as a rack-mount combination of an AB763 Fender preamp circuit and a Marshall JCM800 preamp (non-LED clipping diode version). The goal was to be able to combine them in one housing, and have silent switching between the two. To that end, I used Vactrols rather than relays for the switching. Other than some housekeeping functions for the switching circuit, it’s all tube. The front panel has your typical bass, mid, treble controls, a master volume for each channel and a balance pot for adjusting the final volume between the two. There are status LED’s on the front, as well as the footswitch input and the guitar input jack.

The construction was made with a neat system of push-in metal terminals inserted into a pre-punched board for which they are made. It’s a nice way of prototyping things, as you can change your layout or add components as needed. I used a special clear elastomeric adhesive that was available at the time to protect small leaded parts like capacitors from vibration. The iron is an NOS transformer that I had kicking around which had a 5V rectifier winding, which led me to the (probably pointless, but somehow cool) 5Y3 rectifier tube rather than using silicon diodes.

There is space for another tube, but I never took this any further than what you see. It’s fully functional, but I think it would be better if the output was transformer-buffered, as it tends to get some hum going when connected to other tube amps, as ground currents are no longer under control. Alternatively, I could/should install a ground switch. As it was a prototype project (this IS going back to the early 90’s) and I don’t actively use it, I will save that little detail project for another time. I have used this pre for a D.I. bass tracking preamp with good results – most folks don’t realize how well the Fender tone stack compliments electric bass, even though it is rarely used that way. For now, it’s a funtional musical conversation piece – I have the Fender and Marshall amps in their “real” forms, so when I need that sound, that’s what I reach for, mic it up in another room, and monitor it at a reasonable level with headphones. Like all the projects I have done, I did learn a lot during the process – metal cutting and bending, layout, schematic development and switching topologies – and that’s what it’s all about, isn’t it? (Yeah, I know, BESIDES being fun!)

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?