Sunday, March 16, 2008

Tube amp - design

Months away from a soldering iron gave me the itch to start working on something. In my "past life", I had a hobby of building audio amplifiers and speakers, and I never got a chance to work on a tube project back at home. Now, I figured, is a good time to start.

Hi-fi tube designs are necessarily expensive, the tubes involved can cost quite a bit, and the designs aren't exactly for the beginner. The next best thing, IMO, is a guitar amp, so that's what I'm going to try. Given that I don't play (oh the irony!), I enlisted the help of a friend, S., who will also be building the speaker cab in addition to providing guitar input :)

So anyway, I started off by hunting for existing guitar amp designs, and there are literally thousands to find. I settled on the VOX AC-15, since it's a well-known and proven design. It's also relatively low-powered, which is good, since S. plans to play in his apartment, not a stadium. Also, the output tubes are EL84s, which are not terribly expensive. Finally, it is a push-pull design, which means that the output transformer isn't too expensive, and won't saturate easily.

I did do some tinkering with the design, though. First, I took out the tremolo and reverb parts of the AC-15. These are a bit difficult for me, as a beginner, to work with. Besides, S. has a bunch of fancy effects pedals which would do the job. I also decided to do away with the rectifier tube of the classic AC-15, and use silicon diodes instead, with a series resistor to allow the supply to sag a little when heavily loaded. Caps across the diodes kill some of the switching noise. There's also the usual filter cap and bleeder resistor.

There were several variants of the AC-15 design floating around. I'm using two 12AX7 dual-triode tubes as pre-amps, since they're cheap and plentiful. I might also try an EF86 input stage later. There's two sets of channels, a "normal" and "brilliant" channel, the difference being the "brilliant" channel uses smaller coupling caps between stages, which decreases bass response. Also, the tone controls only affect the "brilliant" channel. A third 12AX7 acts as a phase splitter to drive the power tubes. The power tubes are EL84 beam power tetrodes. Given the relatively low power involved, there is no power tube bias control.

Another change from the AC-15 design is the inclusion of a switchable 12-dB attenuator. This will permit S. to drive the amp really hard without too many complaints from the neighbors :)

Some more hunting around lead me to TubesAndMore, a nice online store that lets you buy, as the name suggests, tubes and a lot more stuff that you can use to build hi-fi and guitar amps.

So anyway, that's what I'm going to try. I'll post here when I get some results. Anyone with tube experience, feel free to give this beginner a hand :)


Andrew said...

Hi Jim,
I build guitar amps for fun and profit, mostly fun thought right now.

I've build quite a few and am just about ready to light up for the first time, my latest build, a pair of Marshall 1974x clones. These are 18w EL84 cathode biased amps. I am going to run them into 2 12" 20watt Celestion speakers. The 2x12 speaker setup is not usually done and I don't know of anyone since the early sixties that has done the 20watt speaker. It is about double the price per speaker but I like to play blues (yes, I also do the guitar input :)) so I like an early breakup.

Anyway, as you can tell, I love to chat about amp building so if you have any questions or would like to see any pics of my latest build, let me know. Like I was saying, I am just a few wires away from lighting the first of the pair of 18w up so it is a very exciting time in the build process.

Oh, by the way, I use JJ tubes mostly, especially when I first turn on a new amp even with my Variac, but I also have tons, well not tons, but thousands of dollars worth of NOS tubes. NOS tubes really are the shizzle and I have some amps that some players won't leave the house without that tone....okay...blah blah...I really hope you have a lot of fun building your amps and I hope you post your progress on your blog.

Jim said...

Being a first attempt, I'm using a relatively inexpensive speaker (a Celestion Tube10), perhaps I could build a cab with a better speaker if all turns out well. Thanks for reminding me to use a Variac, I have one at work I can borrow.

I'm getting JJ tubes for this build, will try other types later and see how it goes. Thanks for the comments!

Karthik said...

Hi Jim..writing this from work, could not find your email address. Need help. Can you find out about anti-barking devices, asap? Our dogs are creating a lot of noise when Archana and I are both not at home. Please let me know at the earliest. Very important. send me an email. thanks.