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Old 18th December 2012, 02:03 PM   #71
JimW is offline JimW  United States
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Hi Mr2racer, yes it is negative feedback, a positive signal at the grid reduces the amplitude at the plate by making the tube conduct more, a negative signal has the opposite effect, the reason it is applied to the plate of the preceding stage is to utilize the blocking cap to keep the dc plate voltage from the grid. I went to a larger resistor than the usual 5x plate resistor of the driver stage because the gain of the 6n15p (sv83) is higher than the gain of an el84, which is the tube for which the RH84 was optimized, and I though I may have had a little too much feedback (I thought I may have been oscillating on one channel, raising the value seemed to stop it, may have been coincidental). Read Broskies article on tube cad for a good explanation of partial feedback.
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Old 29th December 2012, 09:17 PM   #72
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Jim,
I still have way too much gain after changing to the 150k feedback resistor. And with a square way its fine at the output of the 12AT7. But on the other end of the cap at pin 8, (I used pin 8 as a tie pin.) the square wave goes nuts. I was looking at the load lines for the two tubes and if I read them correctly At 200 volts and -1.5 bias voltage The 6P15P is in the range of 45ma and the EL84 130ma? These tubes are much more different than I thought. I was thinking of lowering the Rp for 12at7 to decrease the gain. I know how to calculate for given gain but what should be a good target? If my math is correct the original RH84 gain for this tube is about 40 before feedback?
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Old 30th December 2012, 04:42 AM   #73
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hey Jim if I haven't tried your patience? What did you do with G3? did you ground it? with a wire or a cap?
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Old 31st December 2012, 05:30 PM   #74
JimW is offline JimW  United States
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Hi, glad to comment. I am going to assume you don't have the cathode of the 12at7 bypassed. If you do, remove it - this circuit is dependent upon the higher plate impedance resulting from the unbypassed cathode. Raising the value of the cathode r will have the effect of raising the plate impedance, which is a good thing in a plate to plate feedback arrangement, as long as it doesn't put the 12at7 in a part of its plate curve where linearity might suffer. If the amp performs well other than having excessive gain, you could build a simple resistor divider on the input, or switch to a different driver tube. I don't use 12at7 with the 6p15p, instead I use a 5965, which has lower gain. On the 6p15p I connect g3 to the cathode. The decware people like to ac couple it to the cathode with a small cap. One thing to keep in mind with the 6np15, is with the lower bias voltage, it is easier to clip the output stage than with an el84. A 6p15 running 300 v on the plate, 200 on the screen will bias at about 35ma with about -4 volts, where an el84 will probably be closer to 8-9 volts. I actually bias mine at about 30ma and 5 volts. Now the 6p15 will have higher gain (I measured a gain of 27 in circuit) than the el84, so it will minimize this to a limited degree. If you are running 150 volts on the screen, I am going to guess you are biasing up a little lower. In my amp I set it up with a removable jumper to change the cathode r from 270 (el84) to 135 (6p15p), and I change one of the vr tubes (stacked vr105) to a vr150 to give higher screen voltage on el84. Also keep in mind that the 6p15p has higher transconductance than an el84 (which is high too) and might oscillate if given a chance, so use stoppers on g1 and g2 and pay attention to your layout. I had a buzz I cleaned up by shifting a few wires a bit.
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