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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 05:13 
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Joined: 07 Mar 2017, 03:14
Posts: 103
Bruce,
You use LM317HV in an unseen way. Can you elaborate on this? How does LM317HV provide constant current to the input leg, I mean to the cathodes of power tubes?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 08:22 
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ozapaydin wrote:
You use LM317HV in an unseen way.
Unseen? Look into any 3-leg regulator datasheet and you'll find the constant current usage.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 16:20 
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I didn't write it to offend. I just want to understand how it really works. I already had looked into the LM317HV technical sheet and other internet resources before posting it. I've never seen an example demonstrating the way you use LM317HV: An adjustable resistor in Vin pin of LM317HV and Vout - ADJ is grounded with a resistor between Vout and ADJ. In the examples I've seen, a constant current is delivered to the load connected to the circuit after Vout-ADJ (with resistor).


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 17:05 
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ozapaydin wrote:
In the examples I've seen, a constant current is delivered to the load connected to the circuit after Vout-ADJ (with resistor).
Bruce's approach works the same way except it's puts the load on the other side of the source. You described above, the situation in the data sheet illustrated below:
Attachment:
Current Regulator.png

But this circuit is essentially unchanged, from the load's perspective, if the load is moved to the input side of the circuit and the adjustment pin and the reference resistor are tied to ground. The only limitation is that whatever is on the input must have enough potential above ground to make the regulator work. This would be the regulator dropout voltage plus the adjustment voltage. In the case of an LM317 this is a few volts at these current levels.

A better diagram to help understand is this one:
Attachment:
LM317 Current Source.png
Here there is no ground reference. The only requirement is a minimum voltage differential between the input and the output. This is the circuit that Bruce puts in the tail of the output pair (without the variable resistor). The tube characteristics set the eventual cathode voltage based on the current drawn by the regulator. But at startup and before current starts to flow, the cathode voltage is high enough for the regulator to begin functioning. Once it does, the current rapidly approaches steady state and the cathode voltage settles where it does based on the tube characteristics.

It is actually not uncommon to form a current source in this manner. The fact that Bruce applies it to tube amps is just another application of the constant current source. The same thing can be accomplished with a simple NPN transistor, but the biasing becomes a little more complicated.

Does this make the function a little more clear.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 17:54 
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Joined: 04 Jun 2008, 20:59
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Hi, Glad you got to the answer before me. I was a bit slow today. Anyhow the reason for the CCS is that it provides what would essentially be the bias voltages for the tubes in a rather non-obvious way. Both tubes have their grids at essentially ground potential. To prevent run away (and other things) the cathodes need to be raised high enough above ground to set them properly. This can be done in several ways. A simple resistor to ground from the cathodes will do it. The voltage drop across it will provide the needed voltage. Not very linear IMO, but often done. Usually there is a capacitor across it to minimize the AC signal voltage effect. The CCS does it differently. The static voltage (no signal) will be just enough to set favorable operating conditions for the tubes. The actual voltage can vary depending on the individual tubes. However it will be exactly what is needed for the target current flow. Now if you apply an AC signal to either grid the voltage will change on the cathode circuit in phase. It will be a mirror of the signal and in doing so will change the value of the voltage between the grid and cathode of the second tube. A see-saw arrangement. As one conducts more the other will conduct less. The sum however will be very constant. This is a sort of simple description of how the circuit works. It looks like it ought not work at all, but is actually quite linear. Careful selection of the current, the B+ voltage, and transformer load impedance can (and in the amps does) easily obtain very low distortion without NFB and wide band resonance There is a little NFB applied in the amps to be sure, but it is designed to prevent potential above audio resonances. This intentionally limits response above 30K HZ Inside the 10HZ-25KHZ band it does pretty much nothing.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 18:28 
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It started to make sense. But, if you explain it by sticking the Bruce's circuit, I'll probably understand it completely.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 03 May 2017, 21:34 
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Hi, What doesn't make sense? Please describe. Then I can be more helpful.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 04 May 2017, 08:06 
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Bruce,
I didn't get how we get constant current in the input of LM317HVs. In the sources I've seen, it is mentioned how to get constant current on the output.

Another thing: You use a potentiometer in the input of LM317HVs and between cathodes of power tubes. This way, you get the ability to provide different constant currents to each cathode which are actually idle currents on each tube. At the end the whole purpose is to get a limited cure for tube mismatch. Am I right?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 04 May 2017, 12:51 
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ozapaydin wrote:
I didn't get how we get constant current in the input of LM317HVs.

http://diyaudioprojects.com/Technical/C ... Regulator/

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 Post subject: Re: KT120 Oddblocks
PostPosted: 04 May 2017, 15:54 
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Still constant current is in the output...


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