freestompboxes.org

Even if I am able to get everything up and running, through one way or another, I can't test it through a cab as I don't have at school with me. Theoretically, my Fender 212R can function as a cab, but that's at home. Would it be safe to run this line in to my speaker system just to get some sense of what's going on? (http://www.amazon.com/Coby-CSP96-300-Wa ... 178&sr=8-1 ...definitely did not pay that much for it though...)

Also, how would I go about altering a 15 or 30w schem down to ~3w? Is that just by virtue of the tubes, power supply, and output transformer?

It is designed for 600 ohm loads (minimum), your speaker will be somewhere between 4 and 16 ohms. If you connect that to your amp you might just hear it, or maybe not (Due to impedance mis-match).

It is 3W (max) mainly due to the absence of an output transformer to match the high output impedance of the tubes (several k ohms) to the 600 ohm (or higher) load. Max power transfer is achieved when the output impedance of one stage is equal to the input impedance of the next. So there will be a mis-match between the high (several k ohms) output impedance of the output tubes and the 600 ohm load.

Feeding directly into your speaker would be an even bigger mis-match that would yield even less power. Add an output transformer which has a high input impedance and low output impedance and you can achieve near optimum impedance matching hence power transfer into your speaker.

If you have access to a scope use that to check it. You will be fine running it without a load as there is no output transformer to damage.

The tubes (6W6GT) in push pull with an output transformer will probably give about 6 - 8w (at a guess) compared to 10 - 12W for a pair of 6V6GTs (IIRC, could be a few watts out here).

Something like this schematic is what I would be aiming for but using your tubes. If you want more power swap them for a pair of 6V6GT.
Your amplifier is designed for low distortion so is quite a lot more complex than the Fender 57 which has low parts count (cost) with little or no regard paid to distortion. Probably why we are still using tube amps today.

I believe the light bulb has finally flickered on! I'll keep you guys updated. Should know about the power supply by Monday or so.

Lemme see... A very non guitar amp-ish low distortion high bandwidth circuit, old caps, no power supply, no transformers, no guitar interconnects, etc. To make it any useful as a guitar amp you would need to totally rebuild it. Basically, what you have there as a "nest egg" for the purpose are only a few good tubes and their sockets, that's pretty much all, and it's not much. What it still needs are actually all the most expensive parts and hours worth of work that turns it into something completely different. It's no easy task and it surely won't be very inexpensive. Believe it or not, retrofitting all the modifications and the parts they'll require into the existing chassis is likely going to be far more difficult than making a completely new chassis and the result will easily look amateurish and untidy.

IMO, just for acquiring the tubes and their sockets it's not worth the time and money to tear the thing apart. Additionally, even to make it work reliably as what it is (a preamp for signal generators and alike) it would likely require some work and moderately expensive parts. If you seriously need a DIY guitar amp project then I would invest all the money you'll be spending to a good kit amp instead. It would get you going right from the start. Yeah, you could pull out the tubes and their sockets to recycle them but those seriously don't cost a lot even as a new and are only a small fraction of the price of a complete amp.

As for the unit... I'd just leave it as it is, sitting on the bookshelf and to impress people who love the look of anything tube + vintage. I have bunch of old military tube raditation meters and they look super cool but I don't dream about building guitar power amplifiers from them because it just wouldn't make sense. This unit is, unfortunately, pretty much comparable. As a more realistic idea, maybe you could turn it to a hifi preamp, perhaps a mic preamp or even a phono preamp, at least that stuff wouldn't require excessive modifications, complete rebuild and $$$ worth of extra parts as the core of a good preamp is pretty much there already. Even the input and output impedances might match the purpose. It would still require a power supply and proper connectors - not to mention a complete overhaul to find out the condition of all those old parts - and being a non-transformer coupled, high feedback, low distortion, high bandwidth design it wouldn't sound any different from your average $10 solid-state amp for the job... I don't know if I'd bother.

teemuk wrote:... I don't know if I'd bother.

Yes you do......you wouldn't.

Damn, don't you just hate the voice of reason! But, when you get down to it, you have to please yourself.

Forgot to mention, the thing puts out voltage levels of 260V peak-to-peak without load and 120V peak-to-peak to 600 ohms. Even a typical "line level" input signal of 1V will generate an output of about 80 volts peak-to-peak. This will be WAY too much for practical applications utilising generic consumer hifi or studio gear that operates on WAY lower signal magnitudes.

Alas, the power supply was not meant to be mine. Guess this is going to be one of those gradual side projects...

If you just got the tubes and sockets from the chassis, you'd still be able to make a good little guitar amp from them, all you'd need to do is mount them on a new chassis with a suitable output and power transformer, then add a couple more 12AX7 tubes and 9-pin sockets, there's a really good website which has lots of good information about building guitar amps, I'm not sure if you've checked it out yet so if you haven't already done so, just click on the link below (note that it's well worth registering as a member, it's free):


http://www.ax84.com/