cheap tube amp
This amp is based on a design by Frank Hughes, his very informative article can be found here.
Credit for the original M1115/6CM5 design goes to Grant Wills, although its heritage can be traced even further back. The main difference is Frank's has a 6AU6 pentode stage on the front end, Grant's has a triode.
I used an 0B2 as the regulator and made a few changes to the power supply circuit, not improvements, just so I could use parts that were lying around
Big Thanks to Frank for helping me with this!
So, why cheap? The output transformers are M115 100v line output speaker line transformers from Altronics, cost about AU$12 each, not bad compared to over $100 for Hammonds.
Used, tested 6CM5 tubes cost me $1 each. Maybe the anode caps were the 2nd most expensive parts at about $2 apiece.
The chassis and power transformer was an old lab power supply. The transformer is completely overkill for this amp but as the saying goes -'you can't beat cubes'
Lots of recycled resistors in there, all wiring is recycled as are most of the caps and switches. Experience has taught me to be fussy about recycling caps, but one skanky one slipped thru and blew its guts 5 minutes after starting up the amp. It was just decoupling the power, so nothing else was damaged.
Starting the amp....after debugging 50 various stuff-ups. I wasn't expecting much as my previous efforts had all been more hum and noise than music. Turned it on, Crappy Akai test speakers start humming, touch the volume pot and hum increases...oh well. Plug in the Cd player (New Order - Confusion (Pump Panel Reconstruction Mix) ever seen Blade?)....speakers goes dead silent....hit play....beautiful clear sound....turn it up....beautiful clear loud sound!!!
Guess I better chop up some wood to wrap around it. Don't expect to see photos of the wiring, its nasty under there. Probably should remove the extra sockets and cover up all the holes somehow, plenty of bitey volts in there, or shall I say it is ventilation :-)
has wood, found it will need a pre-amp to use with a turntable, so that is next on the agenda.
11/09 - This is basically the same design except using 6CW5 tubes (thanks Frank!!!).
The only other differences are the bias voltage is -30V, a 110V zener instead of an 0B2 (simply because I am out of 0B2s), also the dual triode is a Telefunken ECC81, the 6AU6s are also Telefunken. These tubes, the case, the power transformers, pots, knobs, sockets, caps and wiring all came from recycled junk.
This amp cost me about $35 for the M1115 transformers, resistors and caps.....and a LOT of time. It sounds great and once I discovered the benefits of star grounding, hum-free.