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PostPosted: May 24th, 2018, 6:17 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 62
That's interesting. I'm not up for that amount of change on this amp, but I could regulate the driver stage better. The input stages are a 12ax7 amplifier that drives two 12au7 phase inverters. The 12au7s use the same 325V as the screens. The 12ax7 uses 300V which is just made from a resistor hanging off the 325V. If I regulated the drop between the 325 and the 300 do you think that would be worth it?


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PostPosted: May 25th, 2018, 7:50 am 
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Joined: December 14th, 2013, 2:19 pm
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My choice for isolating the 12AX7 and improving it's linearity would be a cascode current source for a plate load. 12AX7s, with their high Rp can't drive much. Assuming that your 12AU7 inverters (common cathode?) are DC coupled, replacing the 12AX7 plate resistor with LND150 (IIRC) in cascode form should completely unload the tube, maximize gain and improve linearity. At least in my experience. Others WILL disagree!

Just watch your plate voltage, it will go up, necessitating a cathode resistor change to get correct grid voltage to the phase inverter, again, assuming it's direct coupled.

Stuart

BTW, regulators DO sound different. My mentor tells me that current sources and shunt regs sound better than series regulators as a general rule. I try to follow his advice, since he hasn't steered me wrong yet.


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PostPosted: May 25th, 2018, 2:33 pm 
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This site has some interesting discussion on screen treatments

http://oestex.com/tubes/index.html

I don't build with pentode power tubes much anymore, but when I did I had excellent results with adding chokes in the screen lines back in the day. I kept a dropping resistor and original bypass cap then added an additional LC section.

Even a voltage divider is a step beyond most vintage designs. Some didn't even use a bypass cap on the screen resistor.

Never experimented with hard regulation on power screens so can't make any suggestions there.

One suggestion I will make is that most series regulators have lousy impedance vs. frequency curves, which is why they so often lend a mechanical robotic sound. A good tactic to employ is use the series reg for hard voltage regulation then a MosFET capacitance multiplier for impedance buffering. The cap multiplier is a lousy voltage regulator but it will give you a nice flat low Z output.

A lot of shunt regs, even VR tubes, also show rising impedance with frequency. Gotta watch those regulators!

It seems more important to watch the AC characteristics, possibly more than the DC characteristics beyond a certain level of DC stability.


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PostPosted: May 25th, 2018, 3:05 pm 
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Joined: February 28th, 2013, 3:31 pm
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Most regulators have a decreasing ability to reject changes as the offending signal goes up in frequency. What I do with most of my supplies is use an active regulator followed by a choke and lots of capacitance. That forms a 12 dB/octave low pass filter after the regulator that compensates for the poorer rejection as the frequencies increase. Then the following sections essentially do not see anything except for the last section of the supply. The regulator controls the DC voltage and the LC network removes the higher frequency changes that might occur. At that point the regulator is essentially non-existent as far as audio frequencies are concerned.


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PostPosted: May 25th, 2018, 4:10 pm 
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Location: Parkville, Maryland
tomp wrote:
Most regulators have a decreasing ability to reject changes as the offending signal goes up in frequency. What I do with most of my supplies is use an active regulator followed by a choke and lots of capacitance. That forms a 12 dB/octave low pass filter after the regulator that compensates for the poorer rejection as the frequencies increase. Then the following sections essentially do not see anything except for the last section of the supply. The regulator controls the DC voltage and the LC network removes the higher frequency changes that might occur. At that point the regulator is essentially non-existent as far as audio frequencies are concerned.



And yet -- my experience has been that when you have a large amount of C after the regulators you interfere with the regulators ability to deliver DC from a low source impedance. What does it do to the sound -- it can be sluggish and flat. The secondary effect may be a smoother sound but its smoother for the wrong reasons.

A case in point. I did a mod to a DAC for a fellow audio-nut and it drove me crazy because all of the upgrades that have been extremely successful with past projects just wasn't geting it done. I was stubborn and did not want to turn the piece over that way. I put it aside for a couple of days. When returning to it I looked over the power supply. The original design had 4,700-ufd caps before and after the regulators! I yanked the ones that were after the regulators out like a pair of bad teeth (it was a +/- 15-vdc supply) and replaced them with a pair of 50-ufd polycarbonate caps that just happen to fit the original PC board holes. Fired it up and WOW!!! My upgrades were finally doing what they were intended to do.

Certain high-end manufacturers also recognize that issue. Spectral and Mark Levinson comes to mind as I have had experience with both.

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Walt


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PostPosted: May 25th, 2018, 4:35 pm 
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That is reverse thinking. By having large capacitors after the regulator the following stages see an essentially infinite supply of current. The problem is that only having large electrolytics can slow down the delivery of current at high frequencies. The same holds true for batteries which need to be bypassed locally. In my OP Amp supplies I use 10,000 uf electrolytics bypassed by 450 uf HF electrolytics and then have 4.4 uf tantalums and .1uf monolithic ceramics at each OP Amp, right on the power terminals. That gives the best of line isolation, isolation from any perturbations from the regulators and HF supply capabilities for the circuit.


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PostPosted: May 26th, 2018, 3:25 pm 
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Tom, if memory serves, Dave Berning does the same as you suggest in his amps: regulate, then decouple. Also filters any noise or "sound" of the regulator. With big electrolytic caps it is essential to add good film bypasses. A look at the caps inductance and ESR tells the story.

Stuart


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PostPosted: May 27th, 2018, 12:11 pm 
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David is just one of many who follow that philosophy. A mistake that is often made when doing this is to have just the large electrolytic and one small bypass capacitor. You really need to have stages where each cap is optimized for a certain function. For example the large cap can supply lots of current at lower frequencies and a small bypass can provide current at the highest frequencies. But you have to fill in the middle as well. You could stretch the analogy and say it is similar to a speaker crossover where each section feeds a frequency range.

I have mentioned this before but for the highest frequencies, monolithic ceramics do the best jobs. You would not want to use them in precision timing or filter circuits but for HF bypass they are unsurpassed. There is no one type or device for all things. You often have to use combinations to achieve the right results.


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PostPosted: May 27th, 2018, 4:24 pm 
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Joined: July 15th, 2016, 10:02 am
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Thanks for all this great info, everyone!


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