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what CCS for a 6sn7 driver tube

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Posted on January 9, 2017 at 16:20:27
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I have built a SE KT120 amp with 1/2 of the 6sn7 powering each kt120. the driver tube is biased with a resistor but if i use a CCS i should get better performance? So, i am wondering which of the kits should i get ? As with all tubed amp there heat will be an issue during the summer and so i want to make sure that CCS would be bulletproof and i guess immune to heat stress. Cheers, Dak

 

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RE: what CCS for a 6sn7 driver tube, posted on January 10, 2017 at 05:35:39
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 Your description indicates that you have built a parallel SE KT120 amplifier, is that correct? If so, the 6SN7GT driver triodes are operating as SE drivers, so a CCS in the cathode will only work if the cathode is bypassed to ground with a capacitor and the CCS "negative" terminal is connected to a negative supply of a few volts. It's a bit more complicated than I expect you realize and won't really improve performance much at all. The better place for the CCS would be as an anode load for the 6SN7GT drivers, where the performance will really improve. The anode load CCS will not have a thermal problem if you choose the heat sink properly.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: what CCS for a 6sn7 driver tube, posted on January 10, 2017 at 09:41:11
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Kevin, i apologize for not being clear. The 6sn7 is driving just a single kt120 in each channel. And you are correct to say that i should put the ccs on the anode to replace the resistor, which is what i was trying to say in my original post. I wanted to know if there was a particular kit that you sold that was more reliable than the others? Also if it was necessary to mount the heatsink on the top of the chassis where the heat could dissipate in a relatively cooler environment or can it be mounted under chassis? thanks again

 

RE: what CCS for a 6sn7 driver tube, posted on January 10, 2017 at 11:39:07
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 You will need to estimate the dissipation (in Watts) of the CCS. To do this you will need to decide what idle current (bias) you want for the 6SN7. E.g. 8mA. The current source "sets" the current and Kevin's are adjustable. Next you will need the grid bias voltage. If you are using a cathode resistor use Ohm's law to calculate (V= resistance x bias current). Now go to the plate curves for the 6SN7 and determine the plate voltage that corresponds to that grid voltage and bias current. This is an estimate as this will vary from tube to tube. Subtract the plate voltage from the B+ voltage of the PS that will connect to the + terminal of the CCS. This is the voltage across the CCS. Multiply this voltage by the current through the CCS in Amps and the result is the dissipation in Watts. Be conservative in choosing a heat sink for above the chassis and very conservative for under. If under, provide vent holes in the bottom of chassis under heat sink and vent holes in top for heat to escape.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: what CCS for a 6sn7 driver tube, posted on January 10, 2017 at 11:47:06
JKT
Audiophile

Posts: 612
Location: Midwest
Joined: November 26, 2002
 If you are using an Aluminum chassis and the dissipation is under a Watt you could purchase the attachable CCS kit and use the chassis for a heat sink. For performance considerations I would recommend the cascode version. You will want to allow ~30V headroom for the CCS plus the anticipated P-P signal voltage swing on the output of the 6SN7. So make sure your B+ supply is sufficient to allow for that.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

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