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In Reply to: RE: It may not be the tube posted by Frihed89 on July 24, 2013 at 01:35:27
Thanks for posting a reply. My conclusion were drawn from both measurements and consulting several people on different forums, and also speaking to a very experienced audiophile who has built many amps. Even the people at Sophia Electric recommended I switch to the Carbon Princess, which I did. The problem is not the lower current operating point in itself, it is trying to get an optimum operating point at a lower current using Kit-1. There are many factors that determine where the tube can and should be operated, including the B+ voltage and the output transformer load. Not just the cathode resistor. The AN Kit-1 is a self biased circuit and B+ is fixed at close to 450V with a 5U4GB rectifier. The output transformer has a DC resistance of about 130 ohms and a musical impedance of 2.7K ohms. Given these limitations just changing the cathode resistors does indeed increase the distortion at full power. If I could change the output transformers, or the B+ than that would be a different story. A higher output transformer impedance and lower B+ will indeed work very well at lower currents with lower distortion. If you have an suggestions on how to achieve a lower current without having to make major (expensive) modifications to my kit-1 I would be very appreciative. Thanks again for your guidance.
Follow Ups:
Sorry, didn't know what else you knew. I wish i could answer your question since you pinpointed the two things I was thinking about. I am not that familiar with the kit. A suggestion: post a schematic on the DIY forum and ask the same question.
Observe, before you think. Think before you open your yap. Act on the basis of experience.
I believe I will just keep my system running where the original designers intended it to be run. There is a reason why this kit has been around for so long :-) There is an easy modification that I could try if I wanted to lower the operating current. Simply put a series resistor in front of the choke to drop B+ to a lower level. I don't believe this will be compatible with the output transformers however. If I decide to build another amp I may try using custom parts and target 70mA or so. As for my kit one, I will enjoy it as it is using solid plates. I really would like to hear a pair of KR 300Bs in this system. The KR tubes are rated at 50 Watts and 120mA, so they should work very well. Thanks for all the help and comments.
Hi
I am confused by this thread. The Kit1 has been sold for years, with many happy customers. Are we now saying that it is a tube eater? Or is the issue with the Sophia 300b tube?
I would have thought the circuit design of the Kit1 is correctly figured for optimum standard fair 300b tubes to run within tolerance.
I am wondering if your particular Kit1 build has an issue? It might be worth doing some voltage an current checks and pinging Brian for advice on that. Also bear n my (in my experience) that tolerances can go out of wack if feeding speakers with wild impedance swings or none tube friendly 3 way designs.
And I presume you have the amp in a cool environment with no shelf above to reduce air flow. Tubes amps don't like getting too hot or beyond there normal operating ambiant temp of 30°C max.
The kit one is great! The problem is that the mesh plate tubes can't handle the same dissipation level as solid plates. I bought the mesh plates because they had a good reputation and they looked cool, but I have since discovered that they must be run at a lower bias power. The bias points of kit one are indeed optimum and I have very happy with my kit-1. The folks at Sophia upgraded me to Carbon Princesses and they sound excellent. The bias point of Kit-1 is:
B+ ~450 w/ 5U4GB
B+ ~430 w/ 5U4G
V-plate ~360 w/ 5U4GB
V-plate ~340 w/ 5U4G
I-plate ~85mA w/ 5U4GB
I-plate ~82mA w/ 5U4G
V-cathode ~77V w/ 5U4GB
V-cathode ~73V w/ 5U4G
All these measurements were made with Sophia Carbon Princesses. JJ 300B's gave a slightly lower I-plate. If you calculate the power dissipation at these levels you get:
W-Plate ~31 Watts w/ 5U4Gb
W-Plate ~28 Watts w/ 5U4G
Notice that I included numbers for both 5U4G and 5U4GB. The 5U4G is a better choice in my opinion but the GB version is usually included in the kit, so I included these numbers for reference.
From what I have read, the mesh plates can't handle this continuous power. All the solid plates can, and I have had no problems with Shuguang, Psvane, JJ, and Sophia Carbon Princesses. I am currently running the Carbon Princesses. They seem to open up the sound a little more.
kit-1 is indeed a great circuit and I am very happy with it. I only ran into trouble with Sophia Mesh Plates.
here are a few pics of my build. Notice that the mesh plates are indeed running a little hot:
https://dl.dropboxusercontent.com/u/109856281/17-Finished%20A0.jpg
https://dl.dropboxusercontent.com/u/109856281/17-Finished%20A2.JPG
https://dl.dropboxusercontent.com/u/109856281/AN-K1-2.jpg
https://dl.dropboxusercontent.com/u/109856281/IMG_0255.JPG
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