How to choke load for big voltage swings?
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 28th August 2010, 06:42 AM #2 DualTriode   diyAudio Member   Join Date: Aug 2009 Location: Sacramento Hello, How is your tube biased? Do you have a capacitor bypassed cathode resistor? If you do choke or not the grid can only swing so far and there will be hard clipping. Increase the bias voltage and or remove the bypass resistor, there will be a bigger output voltage amplitude. DT All just for fun!
 28th August 2010, 04:47 PM #3 SpreadSpectrum   diyAudio Member     Join Date: Nov 2007 I haven't actually built it but for the sake of argument let's say it is biased by a perfectly bypassed CCS. I don't think you understand my question. It is not a question of there being enough bias voltage. If I make the bias voltage more negative, my plate voltage will simply shift upward(it's already right next to the rated maximum) and I will still clip 90V from my idle point due to the slope of the load line and the idle current being at 12mA. It looks to me that it is impossible to get more than 180Vpk-pk out of ANY tube idling at ANY plate voltage at 12mA with 60H load without clipping.
Arnulf
diyAudio Member

Join Date: Feb 2009
Quote:
 Originally Posted by SpreadSpectrum The problem is that if you draw the load line, the line crosses the I = 0 "x-axis" at a mere 90V above the quiescent state(V = IR = .012*7500). I was hoping to get 200Vrms out of this and am disappointed that it seems I will clip at low frequencies at levels much lower than this. Have I made an error somewhere or is this just a drawback of choke loading?
And where would it go if you used a purely resistive load ?

Note that choke has relatively low DC resistance so if you neglect it (just like in the case of OPT) the loadline can be in any point above Va = B+ (at any current, determined by bias conditions).

If you take DC resistance into account the line from Va = B+ is still almost vertical and your quiescent point will be on that line. AC loadline will then go through quiescent point with predetermined slope.

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 28th August 2010, 08:28 PM #6 Arnulf   diyAudio Member   Join Date: Feb 2009 It was my assumption that you meant that ceteris paribus. Your question (thread subject) would make no sense otherwise as the answer could be simple: use higher B+. __________________ mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
 28th August 2010, 09:04 PM #7 dsavitsk   diyAudio Member     Join Date: Jan 2005 Location: Hartford Why are you running the tube at such an odd point? It isn't linear, and it will clip. So, bias it at, say, -7V running at ~225V on the plate. You have 14Vp-p on the input without driving the grid positive, and with a mu of ~20 you should get close to 280Vp-p. Oh, and don't forget that the next stage is in || with the choke, so the choke is not a load, and the load line is not nearly as flat as you think. Edit: and Electra-Print will sell you fine chokes for less than half the cost of the Lundahls. __________________ http://www.ecpaudio.com :: http://diy.ecpaudio.com
diyAudio Member

Join Date: Nov 2007
Quote:
 Originally Posted by Arnulf It was my assumption that you meant that ceteris paribus. Your question (thread subject) would make no sense otherwise as the answer could be simple: use higher B+.
Sorry, this comes from discussions I had in other places where it was suggested that I would get better results with choke load than CCS and higher voltage. I presented my results (1.2%THD at 200Vrms) and asked if it could be improved upon.

I couldn't see a choke would be an improvement as plate load impedance would be low at low frequencies. In fact, I can't see any way to get 200Vrms out of any smallish (<10W) tube at 20Hz. I just have to start at too high of a voltage and too high of a current. Plus, the CCS is just a lot cheaper than a choke. I am willing to do whatever it takes within reason to get good performance.

So, back to my original question: Did I mess up conceptually here? This will clip at 90V swing in the positive direction, right?

diyAudio Member

Join Date: Nov 2007
Quote:
 Originally Posted by dsavitsk Why are you running the tube at such an odd point? It isn't linear, and it will clip. So, bias it at, say, -7V running at ~225V on the plate. You have 14Vp-p on the input without driving the grid positive, and with a mu of ~20 you should get close to 280Vp-p. Oh, and don't forget that the next stage is in || with the choke, so the choke is not a load, and the load line is not nearly as flat as you think. Edit: and Electra-Print will sell you fine chokes for less than half the cost of the Lundahls.
The answer is that I want 200Vrms(570Vpk-pk) or better with less distortion than I am getting with the 6SN7 if possible.

My following stage is 1M || 5.5pF(source follower), not much of a load, but it will swamp the choke's impedance on the very HF side. Load line should be pretty flat compared to how others would use a choke, driving a power tube grid resistor directly.

This was a hypothetical last voltage amplifier for a circlotron amp that may be built some day. If I were going to build a circlotron (a complex and costly endeavor) I would spare no expense on the driver as it will be the most significant source of distortion in the amp. If I have to go A2 on a driver stage, so be it. Mosfets are cheap.

dsavitsk
diyAudio Member

Join Date: Jan 2005
Location: Hartford
Quote:
 Originally Posted by SpreadSpectrum However, at 20Hz XL = about 7.5k. I was hoping to get 200Vrms out of this and am disappointed that it seems I will clip at low frequencies at levels much lower than this.
Quote:
 Originally Posted by SpreadSpectrum In fact, I can't see any way to get 200Vrms out of any smallish (<10W) tube at 20Hz. I just have to start at too high of a voltage and too high of a current.
W=V^2/R

V is 200 and R, at 20Hz, is 7.5K.

You are asking your tube that can dissipate 5W per side to output 5.3W at 20Hz.

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