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.
8 comments:
electric things are scary! What a project, looks very complicated for me.
:)
you have to stop playing with such scary and dangerous stuff.
Yeah... well it may look a little complicated, I reused some stuff out of my junkpile and thats why there are two really big power transformers where one would have been OK if I could get all the different voltages needed.
Transformers are the most expensive pieces and thats why I happily reuse them when appliccable.
Otherwise it is just a plain amplifier without any extra gadgets, bells and whistles.
Yes, I got a little scared because it is a really long time since I subjected myself to unexpected electric shock treatment.
I've got a little sloppy the last years but this incident put me back on track again.
Everywhere I visit I'm hearing Gary's song, Ellen's Eyes! It's good, isn't it?
Sorry about your hand, Hans...
You must be a marvel with electronics - far beyond my range of knowledge. I have an uncle whom you would most definitely get along with. Aside from having been an avid HAM radio operator, he built his own TV back in the early 1970's.
I can just imagine you in your laboratory - lightning flashing, thunder rolling, electicity leaping from one electrode to the next (or something like that) with an image resembling Mr Townshend, glimering shadowlike, in the background...
how do you get that link to Garys music to your blog?
have a good weekend
Yes Dale, Gary Stocktons stuff is great, not only "Ellens' Eyes", all of it!
So you have an uncle who is a HAM ?
That's nice! What's his Callsign ? Some VE-something I suppose. My callsign is OH6MY, give him my regards or 73:s as we say.
You make this amp designing, amp building and tampering with electricity and electronics sound very romantic. :)
"Thunder rolling..." LOL!
The closest yo come to that might be when I'm cursing out loud when burning myself on the soldering iron or dropping some delicate component worth $$ and breaking it...
And my "laboratory" is a corner on the desk in the laundry-room where I have to fight for space with clothes, tees, stockings, panties and stuff while the "glimmering shadowlike image in the background" more resembles my wife.
:)
Grace, if you log in to Blogger and go to the Dashboard and then to Layout you will find a link to "Add a new Gadget"
Klick on that and you get a window where you can paste snippets of code from other websites. Go to the Iacmusic and to Garys' Radio Station and follow the link that deliveres you the code to paste into the Blogger gadget!
Hans, you are correct. Our call signs around here are usually VE7 or VE6 something something something...
Alas, my uncle has grown too old and has given up HAM operation.
He was quite the eccentric and I loved listening to his stories.
I believe he has moved to the West Coast, but I have lost contact with him over the years.
Hmmm...I might just look him up.
Ouch Hans! That must have been painful. I have to confess I haven't read the whole post yet, but will do so tomorrow..or very soon, but couldn't help being caught by this!
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