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Thread: Choke experiment

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    Choke experiment

    I've built an amp that is pretty high voltage to try to get a lot of power so the PS has to branch off in some ways to deal with the high voltage. I do not use a choke and instead use a 1K 20W screen dropping resistor for the screen supply. Building the same amp but with more regular PS voltages allows me to arrange the PS in a more normal way, and truly, the amp sounds a bit better. It is more tight and responsive which I guess makes sense since the screen supply sags less. But also I think part of the thing is that the preamp sags less as well because it's not drawn directly off the B+. I haven't actually tested this by measuring voltages yet but it makes me wonder if I could add a choke to my HV PS for the preamp only and somewhat mimic the better result of the lower voltage PS.

    1. If I added a choke after the 18K resistor is there any particular value that would be good? I know they say higher inductance is better filtering but everyone seems to alwasy be talking about chokes for screen and preamp, this would be preamp only. I figure low DC resistance and 500V rating would be good, but maybe inductance value doesn't matter much?

    2. If I added a choke after 18K resistor should I add a power supply node between choke and 18K resistor?

    3. any other thoughts on if this is would be an improvement? I am trying it anyway because I have a hammond 193H laying around. This would not be an ideal choke for this application because it's a big monster of a choke (5H, 600VDC, 200mA, physically large), but i figure if I clip it in and out of series with an alligator clip I can at least preview the effect.

    Thanks for any comments

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    Last edited by nsubulysses; 07-19-2019 at 07:22 PM.

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    How about a VR tube? Only a couple ma right? You can stack them to get higher voltages. Other than that, I think you would need higher henries than 5H.

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    Who makes them?

    Thanks but it seems like these tubes are from the 1940s and 1950s. I would not add an obsolete old tube to a new product design. Also it seems like I would need 3 to deal with my 415V

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    As an LC ripple filter is more effective than an RC filter, a choke makes sense where high ripple rejection and low voltage drop are required. (Doubling inductance has the same effect as doubling capacitance of the following filter cap.)

    Preamp stages run in class A and thus don't cause additional signal amplitude dependent supply voltage sag. Apart from that, preamp voltage gain is not much affected by supply voltage.

    But why not test it and let us know how it works?

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    Last edited by Helmholtz; 07-19-2019 at 10:06 PM.
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    You didn't say new design. Anyway, i don't think a OA2 is 40's, its a 7 pin. Used in many pieces of Tektronix test equipment. You can get hundreds of them cheap. Instead of daisy chaining all your nodes, just take your preamp node direct from the transformer 1st stage. If you have enough current capacity, it won't sag at all. Low resistance is not going to be a requirement, few hundred ohms will be fine. If you need it that clean and sag free, you should be running pentodes.

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    Continuing to mess around with all this choke and power supply variations for the high voltage 580V version. Using a choke for the preamp power supply nodes did tighten up the sound but it added too much bass. I still need to mess around with the values but a 3H choke was pretty good, 5H choke was worse because of too much bass. Maybe 1 or 2H would be perfect to make the amp slightly more tight on transients but not change the sound

    For normal B+ version another thing I tried was a different PT. It lowered B+ from 480V to 460V. I guess in retrospect this was a negligible change to only drop it 20V. I still get screen grid glow and also my 1K screen grid resistors are darkening with the 460V B+ and choke. With the high voltage version with 580V B+ and 1K resistor in place of choke the screen grid resistor does not darken. With 480V B+ version and 500R in place of choke the screen grid resistors also do not darken. Is there any value of B+/Screen HV that would make it easier on the screens or will it just always be like this because screen voltage is barely less than plate voltage? For instance, if I dropped my B+ to 400V and still cranked the amp, would it just react the same? I think I will try 1.2K screen grid resistors to try to tame it a bit and still try to retain the edgy sound. 1.5K screen grid tames it too much and the amp sounds kind of subdued and more boring.
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    My other thought is, with 460V B+ this is just the typical power supply arrangement of a JMP or JCM800 or many other amps that have B+s in the mid 400V with a choke so should I just not worry about it? Oranges and many other amps have B+ in the high 400/low 500V and have a choke. All those amps have been cranking away for years and occasionally they need new tubes. Oh well, is that just how it is and I'm overanalyzing it?

    preamp is: Input jack 1 -- gain stage 1 -- input jack 2 -- gain stage 2 -- gain stage 3 -- CF for tone stack -- LTPI -- power amp -- OT is 1.7K primary

    Power tubes are 6CA7

    460V B+ arrangement with the choke and 1.7K OT primary has the amp doing 21.5V into 4 ohms for 115W at clipping. Amp has been cranked full volume for band practices (master volume on about 6) for about 20 hours or so. 1K screen grids have darkened but amp still does 21.5V into 4 ohms on the dot. Another question, does excessive screen grid current wear out the tube and make it do less power or does it just fail or not fail? Would excessive plate dissipation rather than screen grid dissipation be the only way the tube would wear down quicker?

    Anyway, thanks for reading and thanks for any insights.

    Chuck H has commented on lowering grid drive. What is the method for this other than lowering grid leak resistors in the gain stages. I tried that but it changes the sound too much, not in a good way. Would you lower grid leak resistors in PI, or lower control grid resistors for power tubes or ??? It's a never ending battle of trying to make it easeir on the screens but not sound worse! It sounds raging and pissed off, in a good way. probably beceause the power tubes are being tasked kind of hard.

    Heres these screen grid resistors. They are 1K 5W
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    Last edited by nsubulysses; 08-07-2019 at 09:40 PM.

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    Have you tired the 5H and a smaller cathode cap?

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    Screen dissipation under heavy signal conditions is partly dependent on the load resistance on the plates. A higher load resistance has a shallower load line, putting the line below the knee of the Vg1=0 grid curve making the screens more likely to enter heavy Ig2 for longer (in the part of the cycle where plate voltage drops). So you can either lower Vg2 (which pulls the Vg1=0 curve down to where the knee is below the load line), or decrease the load resistance (which steepens the load line (so it remains above the knee of the Vg1=0 curve), or add bigger Rg2s - or an unbypssed Vg2 supply resistor -(which makes the screen voltage sag more under heavy signal conditions), or bias the output tubes colder (which reduces overall tube current - however colder biasing has a trade off with Vg2, in that the colder the bias is, the higher Vg2 has to bee to keep the tube ‘on’).

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    Quote Originally Posted by nsubulysses View Post
    1. If I added a choke after the 18K resistor
    With an 18k resistor feeding the screens you'll get some interesting compression on the output tubes. I don't know if that's your intention because the amplifier will feel more creamy and not "hard" when strumming or plucking bass notes.

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    I appreciate the replies and sorry I have not replied yet but I am still messing around with all this.

    Mozz, my cathode cap for V1 is .6uF so it's already pretty smalll. I guess I could reduce it further but I think if I tried a 1H or 2H inductor to feed the preamp I could possibly retain the frequency same response but get it to be a bit tighter sounding since the 3H seems to add to much bass and make it tighter sounding. Just geussing.

    Jmaf, the 18K feeds the PI. the screen is tapped of the B+ in a parallel way I guess you would call it rather than a linear power supply.

    Best thing I have done so far sound-wise is to do a lower voltage amp (screens still glow anyway) with 460V B+ and use a choke with the typical linear style power supply that most amps have where it's B+ -- > choke -->screen supply --> dropping resistor -->PI HT --dropping resistor-->v3 HT etc.

    i raised screen grid resistor from 1K to 1.2K and it still sounds good but not 1.5K because it starts to sound less good than both of the others.

    In amp with NO choke and B+ in the 580V range I used the other tap on the PT (it's a hammond so it has 115v and 125V primary) and lowered B+ to about 530V by using 125V primary tap rather than the 115V tap. 580V amp has 1K screen dropping resistor and 1K individual screen resistors. 530V version has dropping resistor reduced to 500R and 1K individual screen grid resistors. It sounds better and is probably a little harder on the screens. I wonder if I could change 1K screen grid resistor to 1.2K in this scenario and it would still sound good but be a bit less hard on power tubes.

    So anyway, a choke feeding the preamp helps a bit and is probably worth it to make it have a tighter response but my main source of looseness is not using a choke for the screens and the sag/compression effect that comes with it because of using a 500-1000 ohms resistor instead. Kind of a duh moment but it had to be explored.

    Make the power supplly less saggy and it has a harder attack

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