The transformers are 5K ohm. I tested 300B's with an 8 ohm load and a 4 ohm load on the 8 ohm transformer for a 2500 ohm load to the tube. I listened to the 300B's for about two weeks with 8 ohm speakers before moving on to trioded 6AV5's and finally UL 6AV5's. I breifly tried 6CD6's since I had several. They can't take the screen abuse that the 6AV5 can. Other sweep tubes will be tortured (oh, I mean tested) in the future. The point is to find the minimum dollar SE amp that sounds great.
The part number is Edcor XSE-15-8-5K.
These experiments have been chronicled across three threads:
There was a thread that introduced the Edcor XSE-15-8-5K transformer to the world:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72654&highlight=
There was also a thread where the 6AV5's were tested and abused:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=37403&highlight=
This thread is about what happened when the two meet:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=74650&highlight=
The part number is Edcor XSE-15-8-5K.
These experiments have been chronicled across three threads:
There was a thread that introduced the Edcor XSE-15-8-5K transformer to the world:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=72654&highlight=
There was also a thread where the 6AV5's were tested and abused:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=37403&highlight=
This thread is about what happened when the two meet:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=74650&highlight=
Hi Tubelab,
I measured the L at 8H on the XSE15-8-5K with my DMM. I know my meter is chinsey on larger inductances, so I'd say it's more like 10H.
I always go by my own measurements. Like any other component, the only way to test if anything is suitable in *your* circuit, is actually in your circuit
tubelab.com said:
I measured the L at 8H on the XSE15-8-5K with my DMM. I know my meter is chinsey on larger inductances, so I'd say it's more like 10H.
I always go by my own measurements. Like any other component, the only way to test if anything is suitable in *your* circuit, is actually in your circuit
This little project has gotten me thinking. 300Bs are nice because of their 40W plate dissipation, among other things. However, they are much too much the flavor of the moment, so their price is not nice at all. With that in mind, I nailed down a quad of Reflector 6550s on EBay, which have a similar plate dissipation and will probably sound pretty keen in a triode-strapped single-ended application. It's getting to be an addiction....
I got talked into the 300B thing a few years ago by a customer who just had to have a 300B amp. I have built about 5 300B amps since then. I have never heard any of the high priced 300B's, and I don't want to. Like you said, it is an addiction.
I am currently using Chinese Shuguangs, which are under $50 each. I have bought about a dozen of these with no failures, and they sound good.
I had bad luck with Sovtek 300B's, 50% failure rate, and New Sensor would not replace any of them. The ones that didn't die in the first 6 months are still working 3 years later and sound good though. They are in my 300B push pull amp. I have yet to find ANY other amp that flat out rocks the way that one does.
I also have a old quad of Sovtek 6550 WE's these also rock. I sold the amp that they were in, but kept the tubes.
I am currently using Chinese Shuguangs, which are under $50 each. I have bought about a dozen of these with no failures, and they sound good.
I had bad luck with Sovtek 300B's, 50% failure rate, and New Sensor would not replace any of them. The ones that didn't die in the first 6 months are still working 3 years later and sound good though. They are in my 300B push pull amp. I have yet to find ANY other amp that flat out rocks the way that one does.
I also have a old quad of Sovtek 6550 WE's these also rock. I sold the amp that they were in, but kept the tubes.
tubelab.com said:
I have less total 300B experience than you but my limited experience with the Shuguang 300B carbon plate has been failure free with good sound. My experience with ElectroHARMonix gold grid (aren't these made by Sovtek?) has been 66.6% failure rate as delivered, forget even getting a few minutes from a new matched pair.
At any rate, I'll be tying the 6CD6GAs first, screen driven, as I have the iron and tons of tubes. If I hanker for more single-ended power after that experiment, the triode-strapped 6550 would be next, maybe with some pricier iron than the Hammond 125 budget series. I'm rather disturbed about the failure rate on some of the 300Bs as mentioned - that's a pretty expensive fuse/paperweight.
Yes, I was really pi**ed off at New Sensor when 5 tubes died after the 90 day warantee was up. The best one lived 6 months. They basically said tough s***. Try our new Electro Harmonix 300B, it has an improved filament. This was right after they said that there was nothing wrong with the filaments in the Sovtek tubes.
The amp went into the closet and I forgot about it. A year later I got some used Sovtek 300B's at a hamfest for cheap, and popped them into the amp. They have been playing happily ever since (about 3 or 4 years). This tells me that the problem must be lot related, some are good and some are not so good! Yes they are real expensive fuses and they blow far too often. Most of my complete amplifiers cost less than I paid for these tubes.
I built a 45 amp because I got some cheap ($5 each) on Ebay before they became ultra trendy. I had a customer who liked the amp but wanted more power, so I put in 2A3's. He wanted more power, so I put in 300B's, the good Sovteks borrowed from my P-P amp. He liked it, so I bought the Shuguangs. I have made several more of these amps, including one for me. It is the Lexan amp that is on my web page. That poor amp has been offered up to science many times. In fact I connected the 6CD6's into it with clip leads for the experiments earlier in this thread. It currently has 6AV5's in UL wired into it.
I have a screen driven 6AV5 P-P amp on a breadboard that makes over 80 watts without trying hard. I may try 6CD6's and other sweep tubes in it when I get time to play, but now the tubes are not the limiting factor. I am limited by the power supply (550 volts max) and the OPT.
The amp went into the closet and I forgot about it. A year later I got some used Sovtek 300B's at a hamfest for cheap, and popped them into the amp. They have been playing happily ever since (about 3 or 4 years). This tells me that the problem must be lot related, some are good and some are not so good! Yes they are real expensive fuses and they blow far too often. Most of my complete amplifiers cost less than I paid for these tubes.
I built a 45 amp because I got some cheap ($5 each) on Ebay before they became ultra trendy. I had a customer who liked the amp but wanted more power, so I put in 2A3's. He wanted more power, so I put in 300B's, the good Sovteks borrowed from my P-P amp. He liked it, so I bought the Shuguangs. I have made several more of these amps, including one for me. It is the Lexan amp that is on my web page. That poor amp has been offered up to science many times. In fact I connected the 6CD6's into it with clip leads for the experiments earlier in this thread. It currently has 6AV5's in UL wired into it.
I have a screen driven 6AV5 P-P amp on a breadboard that makes over 80 watts without trying hard. I may try 6CD6's and other sweep tubes in it when I get time to play, but now the tubes are not the limiting factor. I am limited by the power supply (550 volts max) and the OPT.
I'm setting up to run some curves for a few 6CD6GAs in screen drive mode with the control grid grounded. I'll post curves when I get them. I got the test jig set up this afternoon with filament supply - now I need to throw together a variable HV supply that won't kill me.
On another note, an 807 or 1625 might be a nice candidate for screen drive. The plate dissipation is nice at 30W, and they'll put up with a lot of plate voltage. Filament requirements are much more modest as compared to the 6CD6GA. As I've said before, though, I'll try what I have first.
On another note, an 807 or 1625 might be a nice candidate for screen drive. The plate dissipation is nice at 30W, and they'll put up with a lot of plate voltage. Filament requirements are much more modest as compared to the 6CD6GA. As I've said before, though, I'll try what I have first.
I ran points tonight using a 6CD6GA for plate voltages from 100V to 400V, and screen voltages from -50V to 0V, control grid grounded. I haven't had time to plot points, but I have concluded looking at the data I have so far is that this tube will need to have the screen driven positive to get significant plate current at lower plate voltages, and so will need a low impedance driver. I ran the test tonight using a used GE sample - I'll try later using one of my NOS tubes to make sure I didn't have a weak sample.
In my screen driven 6AV5 P-P the screen voltage swings from about -50 volts to + 300 volts. Yes the screen draws current. I feed it with a mosfet follower. I have a bias supply on the control grid and am now running a slight negative voltage. I have variable power supplies on everything, and adjusted them all for minimum distortion at 60 watts. I ran out of time, so it will have to wait until I return from a trip in mid April. Then I will experiment further, and post the schematic.
I plotted the information I had last night, and it looks like the 6CD6 is more or less cut off at -20V all the way up to 400V. Next step will be to add a source follower with dual supplies so I can swing both negative and positive and source significant current without droop, and repeat my measurements up to 500V (max output for my plate supply). I'm also monitoring screen current, and will post plots of that as well when the time comes. At present, with only negative to 0 volts on the screen, the current is not significant - I expect that to change when I swing positive. At any rate, what this tells me is that I will need a negative supply for my cathode follower driver if I expect to direct couple it to the 6CD6.
Strange to relate, what I've plotted so far looks very similar to the screen drive plots for the Svetlana EL509 clone. Maybe not so strange at that....
Strange to relate, what I've plotted so far looks very similar to the screen drive plots for the Svetlana EL509 clone. Maybe not so strange at that....
Some observations about the curves in the previous post:
1) The plate resistance is pretty high (about 10-11k), so feedback will be necessary to lower the output impedance.
2) the curves look similar to those posted by Svetlana for the enhanced triode connection of their EL509, but appear more linear.
3) I expect a screen driven 807 or 1625 to look similar - more on that later.
4) Within the range of drive voltages shown in the curves, screen current is pretty small (< 1 mA), so I should have no probem driving the 6CD6GA screen with something like the huskier triode half of the 6CY7 in cathode follower mode.
At this point, I'm planning to use 400V plate voltage and to bias the tubes at 40-50ma. Interestingly enough, the Sylvania manual deccribes the 20W dissipation rating for the 6CD6GA as a "design center" rating rather than an absolute maximum rating. It'll be interesting to see how long the tubes last when biased at 20W continuous dissipation.
At his point, I'll be starting with 40-50ma bias, 400V plate voltage, a 6CY7 preamp/driver tube, and Hammond 125ESE output iron using the 2.5k output tap.. Next up will be a pair of 1625s using screen drive, and maybe some more ambitious output iron. After that, 6550s triode connected (more like real triodes - the plate resistance is much lower).
1) The plate resistance is pretty high (about 10-11k), so feedback will be necessary to lower the output impedance.
2) the curves look similar to those posted by Svetlana for the enhanced triode connection of their EL509, but appear more linear.
3) I expect a screen driven 807 or 1625 to look similar - more on that later.
4) Within the range of drive voltages shown in the curves, screen current is pretty small (< 1 mA), so I should have no probem driving the 6CD6GA screen with something like the huskier triode half of the 6CY7 in cathode follower mode.
At this point, I'm planning to use 400V plate voltage and to bias the tubes at 40-50ma. Interestingly enough, the Sylvania manual deccribes the 20W dissipation rating for the 6CD6GA as a "design center" rating rather than an absolute maximum rating. It'll be interesting to see how long the tubes last when biased at 20W continuous dissipation.
At his point, I'll be starting with 40-50ma bias, 400V plate voltage, a 6CY7 preamp/driver tube, and Hammond 125ESE output iron using the 2.5k output tap.. Next up will be a pair of 1625s using screen drive, and maybe some more ambitious output iron. After that, 6550s triode connected (more like real triodes - the plate resistance is much lower).
I have some 807s, and some 1625s are coming. I'll run a set of limited curves to verify the bad news and post it here as a warning... Too bad if it's true - the 30W dissipation capability would've been attractive.
For the triode connected 6550s, I'll definitely want some better iron than the Hammond 125 series. Any suggestions for reasonably priced single-ended output transformers? I don't want to blow a lot of money on expensive stuff at this point, but what can I get for about 100 dollars or so that will sound reasonable?
For the triode connected 6550s, I'll definitely want some better iron than the Hammond 125 series. Any suggestions for reasonably priced single-ended output transformers? I don't want to blow a lot of money on expensive stuff at this point, but what can I get for about 100 dollars or so that will sound reasonable?
I've pretty much figured out how to drive my 6CD6GAs. I'll post a circuit for critique as soon as I get my circuit editing program running again. In short, I''ll be using a 6CY7 vertical oscillator/amplifier tube in the front end, with the small high-gain half as a common cathode amp for gain, and the beefier low gain half as a cathode follower to drive the screen. The cathode follower and screen will be direct coupled, and I'll set bias in the 6CD6 output tube by adjusting the bias on the cathode follower grid. It looks like I'l be using somewhere around +10V screen bias for the output tube if the sample I investigated was anywhere near representative.
Just to throw everybody a curve ball, I'm also thinking of trying 6HV5A beam power triodes in a single ended application. They have a very respectable plate dissipation and high transconductance. It looks like the way to use them would be to run them at high plate voltage (800V-1kV) and to use a transformer with high primary impedance, say, 10k. I'm not sure I'd even need a preamp to drive them, maybe just a modest cathode follower. Too many tubes, too little time....
Just to throw everybody a curve ball, I'm also thinking of trying 6HV5A beam power triodes in a single ended application. They have a very respectable plate dissipation and high transconductance. It looks like the way to use them would be to run them at high plate voltage (800V-1kV) and to use a transformer with high primary impedance, say, 10k. I'm not sure I'd even need a preamp to drive them, maybe just a modest cathode follower. Too many tubes, too little time....
I finally got my main computer reconstructed and had a good, close look at the curves for the 6HV5A. The tube is very sensitive as far as grid voltage is concerned, but I'll need to run the plate voltage at 1.5kV or so to get the plate current I want unless I'm prepared to go positive on my grid voltage. In that case, 1kV plate voltage would work. I was thinking of driving the grid with a low mu, low plate resistance triode like a triode-connected 6AQ5.
Schematic is alomost ready for the circuitry to drive the 6CD6GA. I have to manufacture my own pentode symbol for Orcad, which is a bit of a pain...
Schematic is alomost ready for the circuitry to drive the 6CD6GA. I have to manufacture my own pentode symbol for Orcad, which is a bit of a pain...
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