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This Fender Princeton Amp circuit (5F2-A) was built into a Peavey Audition 30 practice amp. It was a friend’s amp that had seen many years of playing, abuse, and beer stains. It wasn’t used all that often – his larger amp played the shows and practices. This amp didn’t sound very good, so it probably didn’t even get much use at home. When he brought it over to get refitted as a Tweed Fender Champ, I thought it would be a chance to build an old Tweed Princeton circuit into it instead. As you’ll read, our initial ideas morphed into something different by the end.

There was a twist in this build, though: like most problem-plagued amp projects, this one initially went together quickly and without issues. Often, that smooth build process means that you turn the amp on and it works. Other times (like this time), it means that some odd problem will confound you to no end.

The old Peavey practice amp ready for disassembly.

Peavey Practice Amp Deconstruction
It’s always smart to take a look at the chassis and circuit of the existing amplifier before removing anything. If you can reuse the jacks, lights, switches – anything, you might consider leaving those if they are known to be in working order. I left the mains power switch on this one only. Also take note of the transformer placement: it is all too easy to lay out the transformers, then bolt and wire them only to find out that they are in the way of the speaker. It happened to me on this build, since I hadn’t made one of these for a while. Thankfully, I had the foresight to leave the transformer leads long. You may also find that you can reduce hum in the amp by positioning the transformers at certain angles to each other. You’ll need some lead length to be able to try this out. Despite your best attempts to reduce noise this way, you’ll often find that chassis space limits what you can do with the transformer positioning.

Initial Tweed Princeton 5F2-A Circuit Layout
I had built this 5F2-A circuit board over the last winter with some spare parts I had purchased and not used from other amp projects. Some of the parts included some very large coupling caps from the 60′s and some old RCA tubes. I usually prefer to use turret boards for the scratch builds (or amp conversions) and eyelets or turrets for the replica builds, depending on whatever was used in the original. This DIY 5F2 circuit used a turret board, like the other Fender Champ DIY amp project I had built. That Champ amp was a bit of a test bed, so the wiring wasn’t as neat as it could have been.

I positioned the power transformer (both transformers were built by Magnetic Components – I really like their stuff) in the usual spot for Fender amps, near the mains plug entry. The output transformer first went into the front of the amp – a beautiful spot – until I realized that it was right in the way of the speaker. Forgot about that! Always remember that the chassis lives in the cabinet and the transformers must clear the speaker. The output transformer ended up between the 12AX7 and the 6V6GT. It produced very little hum and was one of the few available spots in the chassis.

The initial layout idea for the amp - it changed a bit before the Princeton was finished.

Changes, or mods, to the Princeton circuit included a planned variable adjustment to the amp’s negative feedback loop. I canned this mod when I was troubleshooting. Rather than keep the pot at the front face of the amp, I decided that it was neater and if I used a simple switch at the rear of the chassis – it simply would break the negative feedback loop with one throw, giving the amp a much grittier character. This negative feedback (NFB) mod also brightens up the sound a bit during all volume settings and is very useful for studio work. Next time I build one of these, I’ll try to use a resonance control to affect the bass frequency NFB only.

The tone controls in the Princeton amp are very interactive, unlike the Fender Blackface tone controls. I love this quality, as playing becomes a bit more about listening to what works rather than focusing on number settings that you think should work.

I used a couple of old RCA tubes in this Princeton amp and they sound great as they always do. The preamp tube is a Groove Tubes 12Ax7 – nothing special in itself, but it worked well for being able to give the amp just the right amount of brightness. I tend to find the original RCA 6V6GT tubes to be round and somewhat dark sounding, in the best way possible. In something like a Princeton Reverb, they go extremely well with that circuit’s round sound. The RCA 5Y3 rectifier is very solid and seems to last a long time.

It’s done… no it’s not.
I completed the amp and tested it at low volumes on the bench – it worked and sounded fine, so I took it out to the studio to turn it up and see what it could do. That is when I noticed the horrible hash and squeal when you hit certain notes, or went above 4 on the volume knob. My first thought was DC on the pots (I did measure some), or a bad pot, so I replaced the .022uF coupling caps after confirming that the voltages in each stage of the amp were correct (they were). I don’t have an oscilloscope for testing and am probably equipped similarly to most people reading this blog. I next poked and prodded the amp with some chopsticks – nothing that I could notice except a buzz when pushing the leads to some connections on the 12AX7 (this was the step in hindsight that I should have paid more attention to. For whatever reason, I dismissed this buzz and moved on. If something produces a crunch, buzz, or some crappy noise when you push it, take special note of that and don’t ignore it.)

I then replaced the pots, because I had repaired several original Bassman amps that had faulty pots and the symptoms sounded a lot like this amp’s problem – squealing, rumbling, oscillating madness. That did nothing. I took out my variable feedback control and returned that part of the 5F2-A circuit to stock configuration. That did nothing. I was starting to get frustrated, because the time spent troubleshooting this amp had surpassed the amount of time that it took to build it in the first place. I could also find no fault with my layout or wiring and that was probably the most frustrating part. These old circuits always work when they are made correctly. Usually a problem can be attributed to a faulty build – that makes it easier to troubleshoot.

Next, I started looking into the wiring layout – sometimes this is the cause for oscillation. I had kept the wires braided where possible and used perpendicular crossing points. I reorganized some wires, resoldered others, and cleaned up the rest. This all had no effect on the general status of the amp except to allow the amp to run without howling at high volumes and unloaded. I wondered if this was going in the right direction, but after playing it at high volumes, the same problem persisted.

With nothing to go on and after having rebuilt most of the preamp section to rule out bad components, I had a beer and finished some other project unrelated to amps. Later in the day, I again went back to the chopsticks because I was getting to the point of ripping it all out and calling this build a complete failure. That is when I noticed that the wire from the volume pot to pin 7 (or pin 3? I don’t have the schematic in front of me) was intermittent. Aha! Pushing on it would cut the volume by a lot, or bring it back up significantly. But wait – wasn’t this the same thing I had noticed before? Uhh…yeah. It was.

Getting back to the story, I replaced that wire and cautiously turned the amp up on the bench…sounded okay so far. I took it back out to the studio and expected the worst as I plugged in a Telecaster and…it worked. Finally. It actually did better than work – it sounded amazingly awesome. It was a longer than expected amp journey, but the end result was worth it.

That one intermittent wire had caused me to spend three times as much time troubleshooting this amp as it did to build it. Despite that, I felt that the whole experience gave me a chance to revisit some core troubleshooting skills and add “partly broken wire” to my list of possible problems to look out for.

The final amp. Not the prettiest thing in the world, but a hell of a lot prettier than the original!

The wire was partially broken right near the connection point at the volume pot. I was right to suspect the pots and the preamp section, but I was wrong in replacing everything but the problem component in a pursuit to fix the issue. Should have paid more attention to the problem when I first had found it! I have found this happens a lot in amp building or fixing: If you are looking for a problem and already have a list of possible cures, there is a tendency to overlook things that don’t fall into your list of solutions. Keep an open mind and try to make sure anything that looks odd, even just a bit, is looked into. When you push on something and it makes an unexpected noise, it could be a bad connection, a bad wire, or a bad component. Most of the time, it is #1 or #2.

Princeton 5F2-A Amp
Now that the amp was done and sounding amazing, it was on to the cabinet construction. Since this was a dirty old Peavey practice amp from the 80′s or early 90′s, it needed some cleaning up. I wanted to surprise the new owner with something to be proud of, but still had some DIY charm to it. Those old amps have aluminum side panels and the material doesn’t go all the way to the sides of the baffle. The panels on this amp were all marked up, so I painted them a vintage white and had a heck of a time trying to refit them back on the baffle. I ended up cutting them down a bit and gluing them on, rather than using the brads that were originally securing them on there. The brads made it difficult to install or remove the baffle into the cabinet, so now the panels can be more easily sanded down and repainted if they get marked up in the future.

I used some straw-colored fabric to cover the baffle – it looked great and I had used it on some recent Princeton Reverb and Vibrolux Reverb builds. Instead of the Peavey logo plate, I used an old Saab “turbo” badge from a 900 series car. It looks pretty sweet.

And finally, I polished the top hardware that was originally chrome down to a nice shine. I wasn’t sure if it would come out in the end because it was so rusted and pitted. However, a Dremel 400 took good care of it with the polishing wheel and some compound.

Conclusion
A practice amp with a 10″ or 12″ speaker is a great candidate for a DIY Fender Princeton amp project. Any smaller and you start to find that the speaker isn’t up to producing the full tones that you’d expect from this amp (just plug it into a 2×10 or 2×12 cab to see the difference – these can be HUGE amps with the right speakers). Having more room in the chassis also makes the build easier. Transformer layout and board positioning are all helped by having more space between them, to a point.Now, this old practice amp has a new lease on life and will be appreciated in a way that it hadn’t before.

The backside. The 5Y3 in this photo is an old Magnavox brand.