I did plan on building a known design, I'm off-topic here. Also point taken on tubes, except I have some stuff from the 50s and 60s which probably makes them a higher percent of cost. Actually tubes would be 2nd largest cost I'd think in any case caps, chassis, semiconductors, resistors, wire (in that rough order) would be cheaper if you are not buying anything specialized.
If you are referring to your question at post #1006, a schematic will be needed for reference. In your question, you refer to G2, G2l, and g2. Are these all the same point?
You have shown only the power supply. Please show the complete amplifier schematic order to answer your question.
Schematic
http://www.diyaudio.com/forums/instruments-amps/190738-hundred-buck-amp-challenge-92.html
I did not answer because I do not have a good answer. Now if you asked easy questions...
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Ditto, I have never seen a GU50 in real life.
If I wanted non linear behavior, I would insert a non linear element in series with G2. My choices would all involve sand based life forms, which you ruled out, so I did not answer.
I suppose you could use tubes. Here's how. You want linear response up to a point then increasing distortion....Feed G2 from a suitable stiff source through a CCS. The CCS would stay in saturation up to its set current then restrict the G2 current to the set value. The effect is abrupt with a solid state CCS which can be made more subtle with a resistor across the CCS. A tube based CCS should be more subtle since they are far from ideal CCS's.
I have been experimenting with a circuit that I call the "Sagulator". It uses mosfets zener diodes and other sand based life forms. The "sagulation" knob goes from regulated G2 to anti-regulated G2 with an output for the preamp B+ too. It won't be used in anything related to this contest and is still very much a work in progress.
Sadly my tubes have been cold, dark, and lonely for nearly a month. Life's little interruptions have gotten in the way of fun again. It is entirely possible that I may not finish either amp by the deadline. Minor cancer surgery last Wednesday has put me behind at work again. I will be stuck there late nights for a while and I must make a 2 week trip in Oct.
Whatever happens, both amps, and a few more (bigger) will be finished and posted here.
Hint: Many people have read about my exploits with Pete Milletts big red board. It has seen 525 watts of power in paralleled mono. Yes, I have wired a 12AX7 based preamp into it and created a most extreme sound. It was running at a conservative 250 watts!
Ok, the idea is that this GU50 is a transmitter tube and is rated at 250V max G2 voltage. Ath this operating point average G2 power is 1,5 mA. So how to get this, from a ~ 500V supply?
1st experiment: Through 68Kohm resistor from main HT supply. Voltage is OK, BUT the resistor has an inverse signal of signal voltage on plate. Even at the lowest signal levels. This is not compression, this is just feedback! So it just reduces the output power.
2nd Experment: Add a 22uf capacitor to stabilize the voltage on the dropping resistor, then 1Kohm carbon series resistor to G2. Now we have more power (around 9,5 watts) but still some feedback, as seen on scope.
So what you guys tell me is that to design compression needs a more specific circuit than just a series R, correct?
Also, Am I right to assume that this high feedback of experiment 1 made it behave like a triode?
1st experiment: Through 68Kohm resistor from main HT supply. Voltage is OK, BUT the resistor has an inverse signal of signal voltage on plate. Even at the lowest signal levels. This is not compression, this is just feedback! So it just reduces the output power.
2nd Experment: Add a 22uf capacitor to stabilize the voltage on the dropping resistor, then 1Kohm carbon series resistor to G2. Now we have more power (around 9,5 watts) but still some feedback, as seen on scope.
So what you guys tell me is that to design compression needs a more specific circuit than just a series R, correct?
Also, Am I right to assume that this high feedback of experiment 1 made it behave like a triode?
The deadline is still in the FUTURE?
I've been away since 9/12 because the cable guy came out to fix my neighbor's system and disconnected mine. (I'm not allowed to mention why I was away before that, so don't tell the guys out in the hall that I said this.) I bought a new modem which didn't work because there was no cable connection, cleaned my house for a visit from the cable guy, and took a perfectly good new modem back to Fry's because I already had one that worked just fine.
I had thought that the deadline was August 1, 2011 or something like that. I thought all the recent discussion was just post-mortem or something, all just academic rehash for the sake of everyone's passionate interest in this topic or interest in finally getting some useful results to prove it could be done after all, and after the deadline had come and gone without a clear winner, or after the thing had somehow morphed into a goal with no specific goalpost.
So there's STILL a competition going on to build the best 2-5 watt all-tube guitar amp?
And I STILL have two or three more weeks to get my design before the tribunal?
Hokey mokey, Rip Van W.! I've got to stop fooling around. I have work to do!
I'll talk to y'all again later.
--Tubekit
I've been away since 9/12 because the cable guy came out to fix my neighbor's system and disconnected mine. (I'm not allowed to mention why I was away before that, so don't tell the guys out in the hall that I said this.) I bought a new modem which didn't work because there was no cable connection, cleaned my house for a visit from the cable guy, and took a perfectly good new modem back to Fry's because I already had one that worked just fine.
I had thought that the deadline was August 1, 2011 or something like that. I thought all the recent discussion was just post-mortem or something, all just academic rehash for the sake of everyone's passionate interest in this topic or interest in finally getting some useful results to prove it could be done after all, and after the deadline had come and gone without a clear winner, or after the thing had somehow morphed into a goal with no specific goalpost.
So there's STILL a competition going on to build the best 2-5 watt all-tube guitar amp?
And I STILL have two or three more weeks to get my design before the tribunal?
Hokey mokey, Rip Van W.! I've got to stop fooling around. I have work to do!
I'll talk to y'all again later.
--Tubekit
I'm visualizing this challenge as a race. Some people were sprinters, those are all gone; then we had some good Ethiopian medium distance runners but they got hungry along the way and are also gone. Among the truly marathon men I can only see Tubelab, maybe Printer - being here a little generous. Tubelab has shown great consistency since day one and he had a great advantage over others but now (sadly) his body needs some attention and probably his amps won't be ready on time . Then we have some idlers with very irregular pace (me included) who enjoy talking more than doing and you can never be sure about those....But the greatest surprise so far has been Tubekit. He came out of nowhere (we suspect he took a lift) and now is in a very good position to win this race. The tortoise and the hare challenge. On top of that, possibly just to add a little bit of drama, the host seems to have run away with the prize money. A very crazy race indeed.
Actually since it was tubekit's amp that first spurred the race I see it more like his amp being the benchmark to beat rather than being in the race at all.
I will not be racing but I still hope to have a design in place before the time is up. I used up valuable time in the summer in the basement fiddling with parts where I should have been doing repairs. It also did not help building a guitar and having a second close to being finished, might even spend a few hours getting it ready to string up tonight.
The marathon analogy is not a bad one. I used to work as a tech for a technical program at the local college. They got themselves into a race and needed to have someone do the electronics for the vehicle. We had a few instructors with great ideas (one actually put parts to board but it never worked) but in the end it came down to me sticking it out to get us qualifying for the race and keeping things running till we rolled into town with cameras flashing. You know the t-shirt that says, and all I got was this t-shirt? Well on top of that I got a small plaque with the car's picture on it. Well there went a half year of my life.
Hopefully I will get back to the amp soon. I may just have to refinish the kitchen cupboards next year (really sucks when you only have a microwave to cook with). It may be rough built, since the chassis I started seemed to have morphed into alternate designs. It is to be a Fender Harvard sort of clone with a pentode front end with a pentode/triode control on the first stage. If I leave out some of the bits it may be a good candidate for this contest.
I will not be racing but I still hope to have a design in place before the time is up. I used up valuable time in the summer in the basement fiddling with parts where I should have been doing repairs. It also did not help building a guitar and having a second close to being finished, might even spend a few hours getting it ready to string up tonight.
The marathon analogy is not a bad one. I used to work as a tech for a technical program at the local college. They got themselves into a race and needed to have someone do the electronics for the vehicle. We had a few instructors with great ideas (one actually put parts to board but it never worked) but in the end it came down to me sticking it out to get us qualifying for the race and keeping things running till we rolled into town with cameras flashing. You know the t-shirt that says, and all I got was this t-shirt? Well on top of that I got a small plaque with the car's picture on it. Well there went a half year of my life.
Hopefully I will get back to the amp soon. I may just have to refinish the kitchen cupboards next year (really sucks when you only have a microwave to cook with). It may be rough built, since the chassis I started seemed to have morphed into alternate designs. It is to be a Fender Harvard sort of clone with a pentode front end with a pentode/triode control on the first stage. If I leave out some of the bits it may be a good candidate for this contest.
I fell into a similar situation too once. I started college at age 37 and was in my final year of an engineering degree as a full time student while also holding down a full time engineering job.
The small college with 60 engineering students had entered an IEEE robotics competition involving autonomus vehicles. After 6 months of work under the guidance of two instructors they had built a nice track that mimicked the actual course, modified a RC car to be suitable for the track but failed to make it move at all.
With 4 months left before the competition the dean of engineering showed up at my front door begging me to take over the program. I demanded to be relieved of taking a few classes (I would get 9 credits for succesfully producing a robot), and both instructors that got nowhere would be removed from the program. The new rules would be explained to the student volunteers which would each be given tasks to complete and 3 college credits for success. Those who didn't like the new plan could leave. The dean finally caved in to my deal with 3 months left.
I ripped out their electronics wired up two 68HC11 processors on perf board and wrote the (simple) code for the slave chip. The other students wrote the master chip code and did all the testing.
The list of schools that entered the competition read like the "A" list of engineering schools. UF, MIT, Georgia Tech....... The robot worked and the school bought all of us plane tickets to Charlotte to enter the contest. Despite the impressive list of engineering schools, only 3 entrants could succesfully complete the course. We took second place.
I did a lot of work but I left the school with a 4 year bachelors degree in 3 years.
I can't do much right now since my left arm is immobilized from the surgery. As soon as the doctor says I can load up the trailer and drive I must leave on a 2500 mile trip so it is entirely possible that the two designs that I have posted so far are it for me. I will post a complete BOM for each as they exist now as soon as I have time. Both work good as they exist, but there is room for some improvement in the big one. There have been no changes in the little 4 tube amp since I built it. It just needs a suitable cabinet.
