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In Reply to: Re: Resistor as parafeed plate load? posted by str8aro on October 28, 2001 at 21:06:53:
OK...Dave Slagle says there are no dumb ideas, so here's what I have been pondering. What about a VR tube (or paralleled pair) in place of the resistor or plate choke? I know you are looking for a slightly greater voltage drop than say a VR150 will give you and I'm betting there will absolutely no inductance-that is what we want/need in parafeed right? How about doing this in the driver circuit or in a linestage? Keeping in the same vein, how about stacking rectifier tubes to drop voltage? I have been playing with a 5Y3 in place of the cathode resistor/cap combo in a 2A3 amp. Again, I don't think you will have any inductance, but do you get any from a resistor?
i think you may be confusing a few different ideas...
for parafeed on the plate of the tube we want infiniate inductance in the plate so the VR tubes probably are not ideal as a load.
but we could look at VR tubes in the cathode to bias the tubes but this is a different issue than parafeed and will work similarly for both parafeed and direct feed... there are issues with doing this, but i have never tried it so i don't know if any of the "issues" are really issues..
now onto the good stuff...:-)
with that 5Y3... there are a couple of cool things you could look into...
first, be a man and use a 280 :-) looks count for something.
seriously, what voltage drop are you getting the book says 60V @ 125ma so run 70ma and you might be close to -40V???
also you can use one tube for a pair of outputs, somehow there has to be beauty to that approach...
and finally... try tweeking the filament voltage to adjust the bias... you should be able to trim the filament voltage by 10% or so and get pretty close to the book points for the 2a3, and the rectifier should still be operating linearly... it might be a very easy way to build a simple amp??? also using an indirectly heated rectifier will give you a nice slow turnon... but as it looks now the 5Y3 80 5Z3 family looks best someday i'll scour the books a bit more and see what turns up...
also how about an 83 to bias up a 12B4 linestage for a nice fixed 10-15V bias??? a friend said some industrial applications used mercury tubes for bias due to the huge currents, a resistive load would dissapate too much heat..
just food for thought.
Forgot to mention that I'm running a traditional SE OPT amp so the parafeed inductance issue doesn't apply for me, I just tossed it out as a thought.
Ahhh...the 280! Hard to come by in these parts because all of the old radio restorers seem to hoard em. Found 1 at the last Hamfest but it resides in one of my linestages...you are right, it's a great looking tube! I had not even though of that one for this application. I do have some NOS 80s that I'll have to try but do you think I might be pushing the current limits using a single tube for both 2A3s? Series 5AR4s seems kind of excessive in my little budget amp and I'm not sure the voltage drop would be right.
I don't have enough B+ to get direct coupled on this amp. That led me to using a rectifier tube on the cathodes instead of a VR type. The 5Y3 drops 35 volts for 115ma for the pair of 2A3s. Neg leg of the 5vDC supply gets grounded, right...that accounts for the reduced hum on the output?? Sounds very fast! Now I have to play with the driver stage and coupling caps to get a little more low end. It's nice and tight but I would like a touch more of it.
But how about a VR tube from B+ to plate in a cap coupled output or WOT linestage...or in the driver section of an amp?
i'm confused here...
are you using a single 5Y3 per tube, or are you connecting both plates to the respective cathodes of the 2a3's and the filament of the 5Y3 to ground???
you say 35V @ 115ma, but how are you getting 115ma??? i am asuming its the combined current through both tubes, but you have to consider the current for each rectifier plate, which is just under 60ma and a 35V drop which sounds about right...
you also mention a 5VDC filament... is that for the 5Y3??? did you try ac... it just seems odd rectifying the filaments of a rectifier... just curious... i never considered the filament as cathode of the directly heated rectifiers... i suppods that could be an issue.
Sorry, too many variables without drawing up a schematic.
Yes-1 5Y3 but with the plates tied together. Plates to the CT of the common 5v AC filament tranny for both 2A3s. I tried AC to heat the 5y3 but it was a 60Hz hum machine! I thought...try a DC supply to heat the 5Y3, you have a big ol' Lambda taking up space that puts out tons of current. Voila! No cap bypassing the 5Y3 at that point. Lacking bass but otherwise a great improvement over a cathode resistor. I even switched back to hear if I was just thinking the rectified cathode was really better....yep, it was...errr...is, at least to my ears.
I have been playing with different caps bypassing the 5Y3 to bring up the low end. Maybe this "shouldn't" be done, but I'm just in a mood to try anything that sounds feasable. There really isn't anything expensive in this amp if it goes up in flames...except the OPTs. But they are really in little danger here...the main rectifier(an old EZ80) will fail before I get anywhere close to the current rating of the OPTs. This has been an interesting experiment...and it's sounding better!
Any thoughts or advice? I'm all ears tonight.
not too many variables... i just wanted to be clear...
since the plates are tued together, i would think you would be getting the equivalent of 30ma of current per 1/2 since they
are in parallel, you could try just using 1 plate and your bias should go up because you will be drawing double the current...
its worth a try.
i never thought about the 60hz hum when using an DHR... so thanks for that pointer... maybe thats the problem with the 83
how about tryong a hum-balance on the rectifer??? i think its probably step in the wrong direction, but it would be
interesting to try i would use gordons trick of a 100 ohm pot bypassed with 10 ohm resistors and maybe bypass them with
you could also try bypassing just the filament with caps... again wrong direction i suspect but something may be found
i don't much care for the filament supply for the rectifier... so maybe the search must start for that elusive inefficient
IDR.... damn.... i opened up the book and out jumped the 6853... looks like an IDR same specs as the 5y3 but it shows a cathode (filament is connected to the cathode) ...probably as easy to
find as the 8212 and the 8213...
also the 6106 looks like a possibility, but they are listed (by AES) as a 5Y3-wgt... so you would have to be sure you got the right one... but they may be easier to find...
I was talking to one of our older engineers (who probably thinks I'm nuts..."just use a resistor") and he suggested a hum pot also. He thinks that with a good quality CT 5v tranny, there might not even be need for it. The tranny I was using had no CT and that's why I simply grounded on side and had all that hum.
Gotta run again,
ahhh that changes things a bit :-)
another thing to try is a pair of say 5 ohm WW resistors in series across the winding and ground the "virtual" CT... you could even size theseresistors to load down the trannie a bit and drop your filament voltage below 5V and get a larger bias???
lots of things to explore here.. have fun!
I used an 866A under the cathode in a preamp I was fiddling with a few weeks ago. It looked really cool, but there were problems with noise. I'm not sure if it was from that or something else - it was a new design that I just sort of did for kicks and I've disassembled it since. It only dropped about 10 volts, though.
was it the same 60hz noise tom mentioned from the 5Y3???
Well, there was 60hz hum. Plenty of it. However, I was using SV572-10's as a linestage, so that could have been the cause of that. The reason I lost interest was that there was a ton of higher frequency hash. When I turned on my speakers, there was just solid noise. I don't have a scope, so I couldn't look at. I didn't really ever track down what the problem was. I just unclipped it all and moved on to other things. It looked cool, though - the bright white 572-10's and the blue 866a's (I had them in the supply too).
Reading your other posts now, I did try a hum balance pot on the 866a filament - it helped some, but not enough. I had a huge amount of voltage present on the outputs initially and tweaking the hum pot got it low enough for me to actually turn on my speakers to listen to. Well, I never actually listened to anything through it...
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