Hey everyone,
I finally completed the build of my first tube amp, it's a ax84 Hi-Octane (rev. 10.04.04).
Here's what I used for it:
- 1/16th inch aluminum chassis
- Switchcraft input and output jacks, isolated from the chassis with plastic shoulders
- alpha pots 24mm
- Alphawire in 20awg / 600V for everything except the heater filament
- green cloth-covered 18awg heater wire
- 0.75mm2 cross section shielded wire for some of the connections
- 1W carbon film resistors for most places except a bunch of metal films here and there
- transformers are PrimaryWindings', based in the UK, John there designed a 10W 5K SE output transformer in my ordering process and I went for that instead of an equivalent of the Hammond 125DSE rated at 5W. the power transformer has a secondary for the heater with center tap and a 190 and 270V secondary. I'm obviously only using the 190V secondary as I'm only feeding a single EL84.
I used the headphone trick to find the best spot for the output transformer but found out that using my oscilloscope gave me a finer result.
The original schematic calls for a wire connecting one heater pin of the EL84 to its cathode, I imagine to elevate the heater wiring but I wired the heater wiring center tap to that cathode instead instead.
The original schematic also requires a two-phase rectifier but my transformer didn't have a center tap on that winding so I had to go for a bridge rectifier.
I wired the pilot light in parallel to the 220V instead of the 6.3V as my pilot light was rated for that much voltage.
I went for a stand-by switch, I read Merlin's article on the subject some time ago
http://www.valvewizard.co.uk/standby.html and since then I always flipped both switches at the same time on my amps, but I don't know, maybe a fear deep inside of me to stand against decades of amp building tradition, I gave in to the stand-by switch
There is another switch on the front for a future lo/hi-gain mode, I wanted to see if I was not encountering any problem during the build of the amp before I start implementing mods.
The amp is working as it should and that is an amazing feeling (I built quite some pedals and they were not all that kind with me).
I fired it up following Paul Ruby's first power up procedure
http://paulrubyamplifiers.com/info.html#FirstPowerUpAnd my measurements gave me consistent voltages against the schematic (keeping in mind my power transformer is giving me 20VAC less out of the HV secondary than the Hammond counterpart).
It's VERY quiet, no hum, no hiss or anything funny but it sounds very good and is quite surprisingly versatile. I measured a little less than 7 clean watts of output and it's biased a little on the cold side.
Now, I added pictures of the inside and I would like to know what I should have done differently/better, I didn't run into any issues but I know that does not mean I was not lucky.
I believe it is a very well designed circuit that probably gives some leeway to people like me to mess up without paying a high price in hum, hiss, unwanted distortion or oscillation. So if you see big no-no's in there, things to avoid when wiring, details I should have paid more attention to, let me know!
I'm posting this to get constructive criticism!
Topic: First amp build - AX84 Hi-Octane, building and layout advice (Read 6268 times)