Flamenco-Pete :)

Won't Get Fooled Again, Flamenco style :)
Recorded at The Troubadur, LA, November 7 2008
You have to stop Gary Stocktons' Radio Station while you play this video! Hit the square-shaped button in the upper left corner of the player.



And yes Pete, we feel very good about it!

Guitar amplifier prototyping

Remember I was showing some bits and pieces of an amplifier project-to-be last spring?
Well now that the nights grow longer and the weather is constantly crappy I have been taking some few hours every now and then to work on my project.
I started to work on the schematics in the spring and decided to squeeze in two channels on the same chassis (which really wasn't a good idea because it got so cramped underneath the hood...) taking advantage of the very compact output tube which is a double-beam-tetrode (=two tubes inside the same glass-envelope) known as QQE 06/40 in Europe or 5894 in the USA, the 829A can also be used as a substitute if biased correctly.
Well, one channel is now ready but I think you wonder why there are two output power-tubes connected when channel number 2 is otherwise unpopulated.
The reason is that because of a calculation mistake I made, the impedance matching ratio of the output transformer was set way too low. And it didn't help that the guy who wound the transformer made it even lower by mistake.
With one tube attached I could not get much power transferred to the speaker before the tube-plates started to glow red.
So I put two output-tubes in parallel to get a better match and really carry out some serious tests.
This first channel has two pieces of 12AX7:s (hi-gain double triode) the first half of the first is a low-distortion pre-amp with some amount of local-negative feedback followed by a gain control potentiometer dumping the signal into the other half which has a switchable gain which can be doubled with a flip of a switch and easily driven into asymmetrical clipping (aaahhh.... the real tube distortion...) when set in hi-gain mode. Then follows a Marshall-type tone-stack, bass treble and mid-range adjustment, which is recalculated to fit into my topology, in the Marshall version it is driven at low impedance from the cathode of a 12AX7 while I tap my signal in from the anode.
After the tone stack is another main volume control that dumps the signal into another 12AX7 where the first half is a replica of the first stage with slightly less gain and provision to tap in global negative feedback from the loudspeaker outlet to reduce distortion in the PA and to provide for better damping in the bass-region. The second half is a cathodyne (squeezbox) phase-splitter which splits the signal into two signals of equal strength and opposite phase, this signal is then amplified in a 12AU7 which has enough current dumping capacity to drive the low-impedance gate1 circuit of the QQE 06/40. This QQE 06/40 is a rugged little bastard capable of doing the work of two EL34:s or 5881:s in one small case.
I have used this tube before on 145 MHz Amateur radio band and it could take a lot of abuse.
In the photo I'm testing the amp in our laundry-room. I found a really nice oscilloscope-software which turns your computer into an oscilloscope, a frequency analyser and a signal generator. Very handy I may say!
I then tested with the guitar in the garage and wow!
Despite that I could not get but a fraction of the output the sound was good and the tube distortion function worked like a dream.
A new output transformer is ordered and the old is going back in return to be rewound for another project that involves 2 pcs of KT-100 (or 6550) but that is a story to be told later.


Last Sunday the bloody amp jumped off the desk and bit my hand...
Just kidding, I just pulled another stunt, being stupid and not taking the right precautions (like falling of the ladder last summer) when trying to turn the beast (it weighs about 25 pounds) upside down while plugged in and working.
When I grabbed the handles, I also touched the soldering lugs on the power transformer in the front. That winding is connected to the rectifier for the high-voltage for the PA-tubes and that high-voltage (about 350 V DC during that test) was shorted to the chassis by my left hand (and some also via my right hand through my body) and made me see blue, red and yellow dots and stars appear inside my eyeballs while 25 V AC passed through the path between the lugs burning my skin severely.
Luckyly I could let it go and drop some inches down on the desk but my left hand was very numb for the rest of the day.
Building amps is very painful.

Congratulations to USA

...and to the rest of the free world!
Obama for president and The Who on tour in USA
I envy you!
:)
It is not every morning you rise and find that the world has changed.
Lets' hope for the change to take place, really!
We won't get fooled again!