Pro Audio, Amps and Marketing

Fender 5F1 Champ DIY Project
I had a bit of time between work this week, so I gutted an old Marshall practice amp and started to build a Fender 5F1 Champ circuit in the chassis. The Marshall sounded like you’d expect it to – pure junk, so there were no hard feelings about ripping out all of the $10 in electronics inside. Once it was clean, I reused some of the front panel holes and enlarged a few others. I wanted a couple of tone options too, so I used those two switches you see in the front. It ended up looking pretty awesome in the end and cost about $75, not including the donor amp. With the small Marshall speaker in there, it sounds decent enough, yet not too loud. However, the amp is amazing when plugged into a 2×10 cabinet or a 2×12. It’s portable, and easily loud enough to deal with a drummer if you don’t mind bringing an extra cabinet along. Not bad for 5 watts!

The Build
Everything went smoothly enough. I cut off the turret board from a larger one and had no trouble finding the right pieces from the parts collection that I’ve made over the years. It really helps to just buy every resistor and capacitor you’d ever need up front. It will save a lot of time later when you’re right in the middle of something. It also helps to have them for last minute amp repairs in the studio if a band’s gear quits suddenly, which happens more often than you might think.

Once the turret board was filled, I tried to be smart and attach all of the connecting wires at the outset, yet I screwed one or two of them up and had to change a few connections later on. It looks a lot better and the layout is more like Fender did it back in the 50′s and 60′s – most connections ran to the tubes and front panel from under the board, not from the top of it. This being a quick build, I generally gave up on trying to make thing very neat. I did pay attention to some things: for one, you have to be careful about your lead dress, especially when dealing with the filament wires.

I had to wait for the power and output transformers to arrive from Antique Electronic supply. They’re generally cheaper than the Mojotone ones, but while I like Antique Electronic’s storefront for a lot of things, I tend to like the transformers from MojoTone better. However, both stores are great and my ordering preference has more to do with what they have in stock than any serious need for a certain product.

Once I received the transformers, everything went together well (nothing some metal snips couldn’t handle!) and the last joint was finished. I deviated from the original Fender schematic only in the wiring of the power transformer and the tube filaments. Instead of grounding one side of the filament wires, I wired each lead to its own side of the lamp as the Weber layouts suggest. I also ran two 100 ohm filament resistors to ground – I see this more in later 60′s amps than the 50′s circuits, but it tends to stabilize the amp better, so most of my amps have this configuration.

I always have a fire extinguisher ready when I fire up a home brewed amp. I haven’t torched anything yet, but I am sure that the day will come. This one didn’t blow up, but instead, it squealed like a pig. Had I built this 2 years ago, I would have known that there was a simple solution to this problem. However, I forgot what that solution was this morning. I checked voltages and while they were high in the power section, I chased that problem around for an hour with no change. I did change out the 10K ohm resistor that drops the voltage down the line. While the 5K and 7K resistors were generally usable, the voltages were most happy with the 10K, yet this still didn’t solve my pig problem.

Perplexed, I looked online as I always do and found that I was not the only one with a squeal in their Fender 5F1 Champ. Some blame the period incorrect Hammond output transformers, but the best piece of advice smacked me in the head. The problem was that the primary leads to the output transformer were flipped. An oscillating amp does sound like a squealing pig and I used to see a lot of amps come through the studio with this problem. Depending on how you wire these Hammonds, you may find that they are not wired the way that the schematic wants to see them (based on color alone). I’m not surprised. It’s always a small, simple problem. Reversing the leads made everything better. One last check of the voltages showed me that everything was operating in a satisfactory manner.

Tube selection is always very important. I had tried Sovtek 5Y3 and 6V6 tubes at first, but they weren’t all that nice sounding. I switched them out with an older JAN 5Y3, a RCA 6V6 and kept the Sovtek Groove Tubes 12AX7 in there. At this point, the amp was put back together and tested out. As expected, it rocks. There’s that Tweed rock vibe that I love. Amps with a 5Y3 rectifier have such a nice, compressed sound to them. It may not be the absolute best for some live bands, but it is perfect in the recording studio. There was one ’65 Bassman that I loved and sold unfortunately, but every amp that I currently own has a rectifier in it. They’re great!

I  also meant to follow up on my post from a year or so ago regarding capacitors. I’ll try to make that an entry in it’s own right, but here’s my quick impression of a few caps:

• Jupiter Tone Capacitors: AWESOME. These cannot be beat. I’m looking forward to trying their Red Astron caps soon.
• Mallory 150 Series: Okay. They’re small enough to fit in some microphone bodies, but I find them to have a midrangey sound in guitar amps. They aren’t spectacular, but they are fine for most uses.
• Blue AJAX caps (old ones found in Fender 60′s amps): These are the best. The Jupiter Tone caps come the closest in sound character to these ones. If you can find them, they are seriously good.
• Orange Drops: The midrange on these seems subdued. Instead, the top is nice and there’s a fat bottom. They sound best in newer circuits that get distortion from the preamp stage, like Mesa Boogies. I generally don’t use them when building or repairing old Fender amps.
• Mojo Dijons: These are really cool and affordable. They sound like the Blue AJAX and Jupiter caps. I prefer the previously mentioned brands, though I’d use the Mustard caps from Mojo as a third best option.
• WIMA: I’ve used them in microphone builds before, but rarely in guitar amps. They are super “hi-fi” and I like them a lot. However, hi-fi is not what I go for when I build an old Bassman.

Question of the Day
I saw this search come in from Google: “What is the difference between a large and small capacitor of the same value?” I’m sure that others can point out engineering materials choices and the differences in how they handle heat and everything else, but the simple answer is this: Two capacitors of the same value and of the same rating should be identical in performance. In real life, they usually aren’t but they both are rated to the same tolerance. A more likely situation is when you see two .1uF capacitors  – one is huge and the other is small. Usually, both have different voltage ratings. Their capacitance can be equal, but one might get fried at voltages above 100V and the other may be safe to 630V. That voltage rating is very important, especially in guitar amps. Look at both numbers (the capacitance and the voltage rating) to make sure that the cap in question is the right fit for the application.