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Old 6th August 2013, 01:30 PM   #1
ttan98 is offline ttan98  Australia
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Default CCS on Driver tube

Hi,

I own a SE KT88 with 2 tubes per channel, 6N1 driving the KT88 power tube. The anode of the 6N1 drives the KT88. The anode is resistor loaded. I notice SE amplifier designed by Tubelab uses CCS on top of the 1st tube replacing the anode resistor. Aslo, my amplifier has a feedback applied to the driver stage(which is also the 1st tube) from the transformer output.

I believe the CCS improves the linearity of the driver tube hence reduces the overall distortion of the amplifier. My questions are:

1. Is the linearity improvements sufficient enough for me to modify my amp.?
i.e. did you hear real sonic improvements or just perceived improvements?
2. I think I should use a heatsink attached to my CCS.

Those who have experience in this mod. I appreciate a response. Any other suggestions are welcomed. Thanks
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Old 6th August 2013, 01:35 PM   #2
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What is "CCS"?
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Old 6th August 2013, 01:50 PM   #3
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"Constant current source (or sink)."
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Old 6th August 2013, 01:53 PM   #4
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That could be a good idea using a Power MosFet as a current source. Surely it will upset the logarithmic beauty of the triode valve however and produce a totally different sound?
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Old 6th August 2013, 02:01 PM   #5
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In the sense that it linearizes the stage, perhaps, depending on how high the distortion was with a resistor load. It also can maximize the gain, often a good thing.
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Old 8th August 2013, 04:37 AM   #6
ttan98 is offline ttan98  Australia
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Hi,

I don't mean to be rude, there are so many builders here who built amps based on Tubelab designs, no one can comment whether adding a CCS make any significant change to the sound quality reproduction? Is it worthwhile doing?

An alternative mod would be to replace the 6N1 with a 12AX7 in SRRP configuration? Any comments fellows? I think this maybe a better option even though it is a more complex modification.
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Old 8th August 2013, 10:18 AM   #7
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It will take you about and hour and $5 worth of parts to find out for yourself.

I can't fathom why a 12AX7 in SRPP would be in any way a good choice.
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Old 8th August 2013, 02:13 PM   #8
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Quote:
there are so many builders here who built amps based on Tubelab designs, no one can comment whether adding a CCS make any significant change to the sound quality reproduction?
Replacing the plate load resistor of a triode tube with a CCS WILL make a significant change in the way the circuit operates. Whether this change will make an improvement in the sonics in your particular circuit with your chosen tube is something nobody can accurately predict.

Most triodes will operate in a more linear fashion as the total load impedance is increased. This is the plate load resistance in parallel with the input impedance of the next stage. Simply swapping the plate resistor for a CCS will usually increase the gain of the stage and lowers the distortion of that stage. What that does to the overall amp depends on a lot of things. If there is some distortion cancellation occuring (reasonably common when a two stage amp contains two triodes) the overall amps distortion can actually increase.

The 12AT7 used in the SSE was actually designed for RF amp use, and resistor loaded it produces considerable second harmonic distortion. Its gain is barely enough to drive a triode wired KT88 to full power from some CD players. In the case of the SSE, the CCS raises the gain a bit and lowers the overall amps distortion. I tested this with about 50 different sets of tubes of various kinds during the design process. Multiple different driver tube currents were tried with dozens of tubes, and 8 to 10 mA is the best compromize for overall amp distortion averaged over all the different tubes tried, even though this is a bit hot for a 12AT7.

I have no 6N1 to test, and a 12AX7 in any configuration will distort as the grid of a KT88 starts to approach zero unless it is buffered by a follower(current is too low). Random tests on a single amp may not transfer to the next amp, so if you really want to know how it works in your amp, you will need to try it yourself.
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Old 12th August 2013, 03:49 AM   #9
ttan98 is offline ttan98  Australia
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Hi Tubelab

Thanks for a thorough response, your design is a modern day approach towards circuit design. If I implement a CCS on 1st driver and cathode follower driving the KT88 it would be a much better than just putting a CCS on the 6N1.

I have decided not to put just a CCS on the 1st driver and using your design approach would mess up my current component layout of my amp. making the mods very messy.

I will do is just replace the 1st driver current tube which is 6N2(not 6N1 as I mentioned earlier) with an 12AX7 using a large load resistor as possible. In fact 6N2(close to 12AX7 except for the heater). I would be able to find high quality 12AX7 in the market instead of using a Chinese or Russian std. 6N2 tube. At later stage I may upgrade to 12AX7 SRPP config. if it warrants it, what do you think? My guess if I use a large enough load resistor it does not really matter.
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