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Old 22nd February 2012, 04:34 PM   #1281
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Stock tubes wont go below 6.6k. If you look back in the thread several people posted charts of output power vs voltage vs transformer load. The thing you need to take into account is how much voltage the tubes can take, and how much you can load them.
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Old 23rd February 2012, 02:01 AM   #1282
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The 6HJ5's or 6GY5's should handle a 4.4K primary. The 6GV5 would need the 6.6K primary (and the 6HJ5 or 6GY5 should be fine there too for more power). Since the 4.4K OT is lower power rated, the 6GY5 would seem the best fit there.

(estimates based on cathode max DC current ratings: 6HJ5 280 mA, 6GY5 230 mA, 6GV5/6JN6 175 mA; The 6HJ5 is plate dissipation rated 24 Watts and the other two rated 18W, 17.5W)
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Old 16th March 2012, 05:37 AM   #1283
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Default 300Beast

Dear George,

Just wondering if there are any developments regarding the availability of a 300Beast driver board?

I would really like to build your 300Beast amplifier.

-Steve
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Old 17th March 2012, 02:11 AM   #1284
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Quote:
Just wondering if there are any developments regarding the availability of a 300Beast driver board? I would really like to build your 300Beast amplifier.
So would I !!!!! My 300Beast died a few years ago. One of the power supply caps shorted and fried the rectifiers. Then I stole the choke out of the power supply. The rest is on a shelf collecting dust.

I planned to rebuild it someday but it hasn't happened yet. The driver is still a wild card since I keep making new ones that work better than the previous one. In truth any of the drivers that I have made in the last few years would probably handle 300B's just fine. The driver circuit that I designed for the 300Beast was the best that I had seen....about 12 years ago. I have done better and simpler designs since I discovered mosfet followers about 8 years ago.

Still, some people don't want sand in the signal path, even if it does work better than a cathode follower, so I am still working on drivers of several flavors. If I had to do a push pull 300B amp today I would use the driver circuit I designed for the 6L6GC in AB2 thread. It is a good choice for a medium drive level tube like a 300B. Pete's new stand alone driver board is another good choice. I have even wired some 307A's into Pete's big red board, but I couldn't het the hum out since all of the heaters / filaments are tied together. I also have a few new designs, and I can steal the board out of the 300Beast.

I want to start building push pull amps with all sorts of tubes, big ones, little ones, common ones, and some that no one has heard of. So, how do I decide which driver board works best for a given output tube, and how do I drive the output tube, G1 or G2, or BOTH at the same time?

I build the mother of all breadboards, thats how. It is about half finished. It is a research project only, not for use as a finished amp. It contains a power supply for ANY driver board known to man, and maybe some alien creations too. All of my current drivers will fit in, and so will Pete's new one. It can fit up to 4 output tubes of 813 size, or 8 smaller ones (like sweep tubes) or lots of little tubes. The heater supply goes up to 50 volts and 10 amps, and the plate supply goes to 1000 volts at 1/2 amp, or more current at less volts. The mosfet buffer boards can source several hundred volts of drive at an amp of current and the bias can go from -200 volts to +300 volts.

It will probably take me a year to sort through all the combinations of tubes, drivers, and operating conditions that I want to try, but the 300B and 307A are on my short list. Once the breadboard is operational I will start a new thread. It will be a few weeks though, too many things to do, not enough time.

Just have to make a 300Beast now, can't wait? I thought I had a rather novel idea back when I made the Beast. I made an LTP out of an SRPP, remember SRPP's were rather in vogue in 1999. When I posted it on my web site the email ranged from "are you stupid, you can't do that" to "how do you do that". I was poking around this forum the other day and I found a post about a KT88 amp, so I followed the links, and there it was. An LTP SRPP circuit. It is very similar to the beast. It carries a 2000 date. I never posted my schematic, so at least one other person came up with the same idea.

See post #3 in this thread, follow the link and look at the amplifier schematic. About the only difference is the choice of output tube, my B+ is 400 volts and the plate resistors of my input tubes go to a 300 volt supply. My input tube is a 5751 and my drivers are 6FQ7's.

Great sounding KT88 amp?

The 6L6GC design is here

6L6GC AB2 Amp
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Old 11th May 2012, 07:49 PM   #1285
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I just orderd the board and a majority of the components so I can start stuffing. This is going to be a build for a friend (wedding gift)! My plan is to build it as an integrated amp (seperate volume pots and selector switch) so I also ordered 100K log PEC pots as well.

My question is should the 100K log pots replace the 100K input resistor (R64) or should it be wired in series between W28 and R61 (keeping R64 as is).

http://www.pmillett.com/images/DCPP.h1.gif

I know, total newb question so TIA!
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Old 11th May 2012, 10:15 PM   #1286
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R64 remains. You should hook up the pot in parallel with(infront of) it, that will give you an input impedance of 50k, which should be easily drivable. R64 remains just in case your pot opens up. Its kinda the max size you can get away with connecting pin 1 of the input tube to ground. You can go lower. In your case your adding another 100k. If you had a really powerful source you could go really low, like 1k
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Old 21st May 2012, 02:05 AM   #1287
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Originally Posted by nightanole View Post
Im waiting for George's evil twin to show up with and all motor run cap version thats doing 600v on the red board alone O.o
I thought about it just because I have quite a few motor run caps still sitting around from another build but the size of the chassis with all the capacitors and four transformers is just too large. I am still debating of whether I want to use two of the 15lb ebay transformers or if I can make a pair of tektronix transformers I have from a 585a scope work. Either way it will be a back breaker.
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Old 21st May 2012, 02:54 AM   #1288
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In order to make my amp smaller and lighter I decided to use one 15 pounder for the main power transformer and an Antek toroid for the boost transformer. It's all still mounted to a piece of plywood but works quite well.

My amp currently has a 470uf 450volt electrolytic across each power supply output. I used them because I had them. I got some 1000 volt film caps to try, but I am currently 1200 miles away from the amp so it will have to wait.
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Old 28th May 2012, 10:34 PM   #1289
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Good evening!

IŽve started reading this thread after listening to my corner bass horns powered by a wide range of amplifiers, from 1W SETs to commercial 300W solid state amps. Awkward as it may be, the transistors won this time... Probably not by being transistors but by offering more power and lower output impedance, something that must be achievable with tubes as well...?
Push-pull connected pentodes with Schade feedback seems like an logical alternative to SS amps in this application.

The tubes and transformers that I have in my junk box are either too small or too big (6AQ5 and 15W Edcor transformers vs PL519/40KG6 and 120W Hammond transformers) but since the horns seemed perfectly happy with the 2x300W amp IŽll give the big stuff a chance...
The OPTs are 1,9k p-p and B+ will be 400-440V. Anything I should know before I start sacrificing expensive components to the gods of fire and smoke?
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Old 29th May 2012, 12:52 AM   #1290
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Quote:
but since the horns seemed perfectly happy with the 2x300W amp
I was using my red board with a set of EZ-10 horns. The red board can still sound nice with the average power under 1 watt. My horns really liked this amp. They sound nice with a SET on simple music, but if you want to ROCK build an amp like this one.

I haven't experimented with the 40KG6 since I only have 1 or 2 of them. I don't know if a pair of them would be happy with a 1.9K ohm load, but they might be OK at 400 to 440 volts. Most of my experiments were at 600+ volts.

Quote:
Anything I should know before I start sacrificing expensive components to the gods of fire and smoke?
My only sacrifices were due to dumb blonde moments on my part, like running 600 volts through a 450 volt cap and 650 volts through a 250 volt cap! No other parts met their maker.
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