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In Reply to: RE: Schade feedback or RH amp with fixed bias? posted by DAK on January 23, 2016 at 00:44:04
No reason why fixed bias can't be used on the output tube. Other than the resistance coupling to the grid, there is no path to the rest of the circuit that does not go through a cap.Read the Blog attached.
These types of pentode based feedback circuits were developed as a cheaper way to get linear power.
Edits: 01/23/16Follow Ups:
Hi Chip, yeah, I got that on bookmark and refer back to it when I need. But I don't recall reading anything about him using fixed bias on any of his amps. Do you recall a specific reference?
I am really trying to avoid the cathode bias setup in higher powered tubes. thanks a bunch, Dak
Here he uses a CCS in the cathode
It seems to work fine on amps up to 8watts but I have been having failures in higher powered amps over 10 watts. I have also read about other builders running across this problem. I am guessing that one needs a fairly large heatsink to keep the 317 from overheating once you get tubes with outputs over 10watts.
So, I am thinking of going back to a negative bias voltage on the input grid. Do you know how that would impact the RH type amp? cheers, Dak
"I am guessing that one needs a fairly large heatsink to keep the 317 from overheating once you get tubes with outputs over 10watts."
Good grief, the TO-220 version is rated at 1.5A. Even in free air, it should dissipate a watt or two, and it's thermally protected. Are you using it with a bypass cap to be sure instantaneous current variations don't cause its terminal voltage to exceed the maximum allowable? Even at that, it probably needs either a Zener or a TVS to be sure the rating isn't exceeded. Really, this sounds like a voltage breakdown to me.
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Buy Chinese. Bury freedom.
Hmm, let's have a look at this. The LM317 has a temperature rise of 55-65 degrees C per Watt dissipated (depends on whether you get the insulated one or not). I like to assume that an amp chassis will be on the warm side (40C), and that anything over 100C is asking for trouble, so 1 Watt of dissipation without a heatsink is sort of reasonable.
I don't know what the bias voltage would be in the RH amp, but I suspect it could be around 20V, in which case the 317 is going to get very hot and fail in relatively short order.
hi TK, how would you employ a zener or tvs to protect the lm317? regards,Dak
I'd like to see the circuit you're using around the 317 before making a recommendation. Can you post it?
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Buy Chinese. Bury freedom.
Here is a link to the website. http://rh-amps.blogspot.com/2013/01/rh-universal.html
BTW, I wrote Alek and he explained that the lm317 needed a heatsink. I did not realize that and consequently left it unfettered. But, in the amp the chassis I used is unventilated and I am not sure if even if the lm317 were heatsinked if it would have survived long term.
Well, ambient temperature does play a big role in all this. As you say, if the temp is too high, even a heatsink might not help much.As a side note, Have you considered using a voltage regulator instead? That would allow you to more easily adjust bias from tube to tube. The "enhanced fixed bias" scheme was originally designed for push-pull, but it should work for SE as well. Here's a representative schematic:
About protecting the LM317 from excessive voltage, a TVS (transient voltage suppressor) acts like a Zener, but has a much steeper knee. It would be connected across the LM317 with the anode to ground and the cathode to the "IN" pin. A device rated for 28-30V should work OK.
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Buy Chinese. Bury freedom.
Edits: 01/24/16 01/24/16
and it died after less than 100 hours. That ic was mounted on the chassis with a proper insulator. So, for higher powered applications, I am not convinced that a transistor is as reliable, and I mean as bulletproof as a suitable resistor. If one is building an amp for someone I would think that would be the standard to achieve.
I'm pretty confident about applying analog ICs to control vacuum tube circuitry. However, they need to be protected in ways that wouldn't be necessary in their native environment. The user has to examine all the possible "what if" scenarios for potentially failed parts surrounding the IC and look for ways to prevent those failures from exceeding its ratings. This is all more trouble than it appears at first glance.
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Buy Chinese. Bury freedom.
If you are interested, I have been using a cathode CCS on my PPP KT120 amps that supplies ~3oomA@-70V (needs a large heat sink) that is based on a Fairchild KA431 (low tempco variant of the TL431) and a power N chn MOSFET. Variation in current between dead cold and hot is +/- 0.25mA. So far has proved itself reliable with no oscillation.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Hi jkt, thanx for emailing I tried to email you but I got a "mailer daemon". As far as I can tell I reproduced the email you gave me. regards, dak
Hi JKT, how is availability on that chip? I find that it is sometimes hard to purchase some transistors. Also how big a heatsink are we talking about? I have been getting away with using any but I think I need to use them to increase reliability. Thank you for the tip and if you can give me details of what you did I would like to check that out. best regards, Dak
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