I have been wanting to build my own tube guitar amp for at least 2 years. Having no previous experience with tube circuits, I spent a great deal of those 2 years looking at schematics, looking at gut shots, and trolling guitar amp forums. I finally decided to take the plunge and start my amp. I had actually bought some parts to start the marshall 18watt clone but decided I should first try something a little simpler at first, just to get more aquinted with tube circuit layout. A lot of guys start out with little single ended amps such as the "High Octane" or the "P1 Extreme". These amps look pretty cool, but to me, useless. I needed more power than the little single ended designs provide. I also did some research and found that the cost of a push pull amp is basically the same as the single end amps, since a good single end output transformer (Hammond 125ESE) cost about twice what a tweed deluxe output transformer cost, and the single ended amps require about twice as many (and twice as large!) filtering capacitors since the output stage does not cancel out hum as it does in a push pull amp! I finally decided upon building a fender 5e3 Tweed Deluxe. It is a very popular amp for its great response and tone quality, used by artists such as ZZ Top and Neal Young.
Instead of buying a kit, I searched ebay for old amplifier chassis using a 6v6 pair for the transformers and other components I'd need. I found this Zenith Phono-amp chassis for $40 which was perfect! It had a nice power transformer, and used a 5y3gt rectifier tube, exactly like the 5e3! Here is the zenith chassis as I received it:
The chassis was quite clean. To my own surprise, I hooked up the amp and connected it to a discman as input, and it worked right away! Of course that was short-lived, after about 5 minutes the large filter can-caps started to overheat and sizzle, probably due to not being used in 20+ years.
Always being one to do things the hard way, I decided to create my own amplifier chassis and layout. I had some steel sheet metal that was recovered from a dumpster, just enough to weld up a 12x8x2 chassis. I cut the chassis using an angle grinder (not the best idea I know) and welded it together. It turned out surprisingly square considering my construction method. The chassis was painted with black hammertone paint.
I designed the chassis layout and turret board layout using AutoCAD. This made it really easy to preview how things would fit before actually building the amp. You can download my AutoCAD chassis and turretboard layout from
here:
I built the turret board using supplies from http://www.turretboards.com. Here are shots of the turret board before and after installing the components.
I didn't think to take any pictures during the final construction phase, but here is a gut shot of it as it stands now.
The orange drops are from the 1950s phono-amp.
After taking some voltage measurments, I found that the filament voltages were way too high. The 5y3gt was running at 6volts and the other tubes were running at 7.1volts! This was caused by the old Stancor power transformer that came from the phono-amp. Being designed and built in the 1950's, its primary is wound for 117VAC. Current day wall voltages are much higher (around 123vac here), this causes the secondary voltages to also increase. To remedy this, I have installed resistors in series with the filament chain. I found that it needed about .24ohm on each side of the filaments to lower the 7.1 to 6.3vac and .25 to lower the 5y3gt filament voltage to 5volts. I used cheap ceramic 5 watt resistors in parallel to provide 10watts of disappation without excess heat. You can see them in the picture above mounted on the terminal strips on the sides of the chassis.
Hows it sound? Amazing! After sorting out some ground loop issues, this amp is dead silent except at full volume there is the usual preamp tube hiss. The tone is amazing and seems to improve the more I play it. I run the amp usually with the tone at about 3 o'clock and the volume at midnight. This provides me with plenty of crunch, while backing off the guitar volume cleans it up nicely, but not
too clean. Using a 12ax7 in the first preamp, it is hard to get this amp clean. It always has a bit of a growl to it. I've experimented with serveral preamp tubes and I've found using a old tung-sol 12ax7 in the first preamp, and a old sylvania in the second preamp sound best. Very nice, rounded tone. Swapping in a 12ay7 tube in the first preamp results in the amp being a little cleaner sounding, but my favorite clean tube in this amp is an old RCA 12au7 I found in a junkbox. It is very clean sounding but still has sustain that lasts forever!
Below is a shot of the ground terminal I built using a piece of 12GA wire soldered into a crimp lug that services as the ground for the entire amp turret board. Below that is the final schematic showing the grounding scheme I ended up using to eliminate all ground loop hum. Each ground wire is lettered A-F and labeled on the schematic, along with my measured voltages.
