Tearin' it up!
Well...I have to say things have been so busy for me. Nothing major...just life.
But...back to the subject at hand. What's bein' tore up? What did that terrible 2-year-old break now? Alas, it wasn't her....it was me.
My prized amp, the Fender Hot Rod Deluxe, died one day. Just up and quit...I plugged in, let the tubes warm up and then switched it on and...
...nothing.
Couldn't figure out what it was. The pre-amp was getting power (the groovy red jewel light lit up), I could plug into the 'power amp in' and get good sound. So it wasn't the power-amp...something in the pre-amp just quit. Was it a blown plate load resistor? A blown output transformer? A power resistor that got a cold solder joint? It could have been literally hundreds of things.
So...where to begin? Well, I grabbed the schematics for the Hot Rod Deluxe, gathered my electronics components, got my multimeter and soldering iron and began tearin' it up!
But wait...there's more...
MORE...
Since I was going to take the amp apart and get into it's guts, I figured I'd take the time to do some maintenance and upgrades while I was in there. I had a list of about 6 jobs that I wanted to do and figured I'd do them while I was troubleshooting. I wanted to replace the 5W cement power resistors (they come out of the factory with too high of a resistance and tend to char the solder joints), replace the master volume control pot with an audio-taper pot, do a cap-job, replace the plate load resistors, mod the reverb circuit and generally clean up some solder.
It's not too hard getting the amp apart...just take your time with it. First, before anyone attempts to dig into the guts of their stack, they have to know that the power filter capacitors have to be discharged! There's plenty of information on the web about it, and I won't go into it here. JUST DO IT...they can hold hundreds of volts in them even if the amp has been unplugged for a week. And, it goes without saying, unplug the amp before you open the back.
Upon initial inspection, nothing seemed wrong with any of the components or joints...then I removed the printed circuit board (PCB) to look at the back:
Yeah...it's charred. Well, I've read enough about the 5W power resistors and expected some charring...but GEEZ...not the whole board around the joints. So...I re-soldered all the joints in the charred area and replaced the power resitors with 360ohm cement resistors (it's recommended to go to 330ohm, but where I was ordering from 360ohm was the closest I could find).
Okay, that was easy enough. I moved on to the filter capacitors. I chose to replace the 22uF 500V IC filter caps with 30uF Sprague Atoms (the big blue battery lookin' things...ya can't miss 'em):
I wanted to add a bit more power filtering to cut back on the amp noise, and since Sprague doesn't make a 22uF 500V capacitor, I just picked the nearest value they offered. The main thing I had to keep in mind is to use no less than 500V ratings since my HRD is usually passing 430+ volts at any given time. Don't want the caps to blow! Anyway, there's enough room on the HRD's PCB and the 30uF Spragues are just the same length as the ICs, although a lot thicker. Here's a good look at the two side by side (along with my prepped Sprague Orange Drop caps):
The next part of the cap job was the first stage of power filtering. The amp comes out of the factory with a 47uF 500V IC cap. My first challenge was to find something close with a 500V rating, as I did with the 22uF IC replacement. Only problem here, is that the closest thing to 47uF 500V I could get was a 30uF or a 60uF. Well, I didn't want to go with LESS of a capacitance...who knows what that would do to the amp. I didn't want to go too HIGH a capacitance, because it would make the amp feel dead and limp. So I had to get crafty.
There are things, I learned, that you can do with combining electronic components to achieve custom values that aren't in individual parts. You can wire them in parallel or wire them in series. With capacitors, I learned that when you wire them in series, you can combine the voltage rating values while maintaining the capacitance values (i.e., a 350V and 200V cap in series gives you a 550V rating). Wiring them in parallel, the voltage ratings are maintained but capacitance values get combined. As I've seen done in a lot of Fender clone PTP rebuilds, the first stage of the power filtering has two power filter caps wired in parallel to create the large capacitance filter stage...that was the ticket; Sprague makes a 30uF 500V, as I've already used, and a 20uF 500V filter cap which I can combine to make a 50uF 500V cap. That's barely a bit more than the stock 47uF 500V cap that comes standard. It just so happens that the 20uF Sprague Atom is the same length as the IC 47uF and nearly as thin. I just needed to wire them together in parallel, which is much easier than in serial, as it involves a simple connection of leads (matching polarity, of course) rather then end-to-end with appropriate resistors between the two. Here's my parallel-wired, 50uF 500V Sprague Atom fix:
Okay, things are looking good now! After the filter caps were in, I added the Orange Drops in the tone stack as I planned. Here's how the circuit looked after this stage:
Here's another view that shows another angle of the parallel-wired Sprague Atoms along with the new cement power resistors (small, white bricks) to the right of the beefy-blue Atoms:
While I was in there, I looked at the phase inverter (tube V3) and it's surrounding components. The previous owner of the amp had some work done, or worked on it themselves, and replaced a resistor, but put it on the back of the PCB (see above pic; look to the right of the white cement power resistors and under the thin ribbon cable wiring...on the separate tube socket PCB), not on the front printed side where it belongs. No big deal...it would still work, but the soldering job was sloppy. So I took my time and unscrewed the PCBs with the tube sockets to get to the parts under there. I had some extra Orange Drops, so I figured I'd try and fit them in, too. Well, it's tight in there and I didn't want to short out any connections, so I had to be crafty again and twist the components so that they were reasonably clear from the surrounding parts. Yeah, I'm sure Mallory caps would have been a perfect choice in there as they're of comparable quality to the Orange Drops and much more compact, but I figured "What the hey!", I can try and get them in before I have to solder, so no harm in trying. Well, I did...but barely...not much room in there since the lip of the lower shelf of the chassis is in the way:
*PHEW!*
That was a lot of stuff to figure out. I was pumped, though...I knew that once I got it all assembled and powered up, I'd be in tone nirvana (well, maybe not...but I'd be jazzed about how much better this thing would sound). Plus, it sure looked purty!
Yep! I was feeling pretty good about myself. Outside of wiring my own Strat...this was my first REAL electronics jobs I've done. It's a lot of fun and I can see why this has been a hobby for folks since the '50s.
Alas, I had everything put back together and warmed it back up and got ready to jam...but...STILL NO SOUND! GAH! What went wrong?!!
Turns out that not only were some connections bad...a pre-amp tube went out, then a power-tube which gave me a nasty distortion rumbling sound even without the guitar plugged in. I spent another 10 days in what little spare time I had trubleshooting all my connections, putting in jumper wires (some soldering pads were bad on the PCB), re-tubing the amp, and going over every connection according to the schematic! I got perfect power input readings...the jewel light was lit...the amp was humming...I had all good parts in there...but was nowhere better than before I started.
Finally, I got it to work. What was it? Well, I got a bit too happy with the solder and inadvertently soldered two of the very first resistors of the input circuit together, effectively routing all input signals to ground! GRRR!
I'm happy to report that the Hot Rod Deluxe is back in action and even better than before! With high-quality caps and resitors on the front end, a totally new set of tubes (Sovtek Russian military surplus, not new stock), a bias adjustment to 68mV, and I've got me a great sounding box and found the sweet spot on my axe! Just a click on the Drive channel on the foot switch, and on my guitar volume I control the level of textured tube saturation.
So cool, and I have yet to want or need an effects pedal...but I have my eye on one or two with some mods in mind...so stay tuned, and I'll be sure to let you in on those projects when they happen!
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