Over on the
KT120 Oddblock thread came this little statement:
poty wrote:
Suncalc wrote:
... you can put together a really inexpensive amp that actually has pretty good sound. Two channels, two tubes (plus a rectifier for purists)....
If It would be possible to find inexpensive and small DIY acoustic systems anywhere - I would make the amp for my office desktop computer for sure.
So I decided to take up the challenge. I've had a little design for a 6CY7 amp bouncing around in my head for a while and this afternoon I decided to try it out. Now the 6CY7 is of that peculiar class of dissimilar triode tubes which were manufactured for television sets. This particular one was supposed to be a combined vertical oscillator and vertical deflection amplifier. Well, driving a vertical deflection coil is just like driving an audio output transformer and oscillator triodes are generally pretty linear and have good gain. So this tube gives you both a preamp and output stage in one small 9-pin package. Now I could bore you with load lines and such but instead I'll just jump right to the schematics.
This is a pretty simple circuit with very few extraneous components. The preamp has a gain of almost 25 and the output stage is biased at about 29v so you can drive this amp to full power with a 1.2v peak input voltage. This is good for most portable CD players, iPods, and computer sound cards. Here is the main amp schematic.
Attachment:
schematic.jpg
And the power supply is also very simple.
Attachment:
ps_schematic.jpg
So I decided to prototype up the power supply and a single channel. I noticed a few months ago that Antique Electronic Supply had a no name 5kΩ:8Ω 8W output transformer for $13.95 each so I ordered a couple just to try them out. Here is the prototype with the inexpensive output transformer.
Attachment:
Amp_1.jpg
The amp faithfully produced a nice 1.5W peak power output into an 8Ω load. Now I will say that the output transformer (when driven with a pure sine wave) seemed to kind of fall apart below about 90Hz. However, when driven with normal music (I used Holst's "Planets" suite) it sounded much better than I would have thought given the single tone performance of the transformer. I also tried out the exact same amp with an Edcor GXSE10-8-5K just to compare.
Attachment:
Amp_2.jpg
Now I'm not going to say the amp didn't sound better with the Edcors. In fact, the bass was somewhat tighter and more controlled. But since the Edcors are twice the price of the little AES transformer, this really shouldn't be much of a surprise.
Now the name of this thread is "
Inexpensive Dissimilar Triode Amps" and we need to address that whole affordability issue. One of my intents was to see if I could put together a nice little stereo amp for not too much money. Now, I don't know how to define "affordable" with respect to DIY tube amps. Mark's little 300B amp over on the
300B SET Design Project already has over $4000 sunk in parts so I know that that's not it, but I would be happy if I could bring this one in for around $100 (American). So, given that I had already chosen some output transformers from AES, I thought I'd see what it would cost to buy everything new from them for this stereo amp. Here's the damage...
Attachment:
20110417 Price List.png
So at $147.18 I missed my target by more than a little. However, it's still not too bad for a complete stereo amp. The big ticket items in this list are the power transformer, the filter choke (I refuse to build a power supply without one), the output transformers, and the amp chassis. This parts list also does not allow for a volume pot but you could throw one in if you saw fit. It also is a little more expensive than it could be because I stuck with all new components from one supplier. If one had a decent parts bin you could probably eliminate the binding posts, phono jacks, fuse holder, switch, sockets, and most of the piece part components. Diodes are also cheaper then the 6AC4 but you'd need to add a standby switch. If you just purchased the tubes and the iron you would pay just $102.45 (pretty close to my target).
There you go. One relatively inexpensive 1.5W/channel tube amp for use with a computer or iPod. It may not have a lot of volume or headroom, but it's a fun little project to throw together.
Does anyone else have an example of something like this? Maybe something with a nice octal 6EM7?
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then the end product will never be great. For someone who wants a modest amp (pp) a GXPP10 at 10K and a pair of Russian 6GV8 replacements can make a cute 
and very excellent sounding baby oddwatt and the cost is really low. I would think that you could build one for under $200 in parts easily. Maybe even under $150. About 3 watts output with lots of gain.