Author Topic: V-M 557 Plate/Screen Grid Voltage Questions  (Read 4041 times)

ed from Baltimore

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #15 on: January 15, 2019, 01:07:39 PM »
          Thanks.  Its good to be back, electra, and I almost feel like I have met or at least talked on the phone with you, Bill, TC chris, harbourmaster, and other regulars. It's sad to think of how far away from each other we are mileage wise and unless we have a Magnavox console convention every 5 years or so, we may never meet, but there'd be a lot to talk about if we did.
          I have learned a heck of lot from you all too. I actually don't have that much experience doing the rebuilding and refurbishing of the equipment especially in the last twenty years or so. If it's the last thing I do, (which scares me to use the phrase as I get older  !! ), I am going to set up a comfortable table/bench with a chassis holder, good lighting, now with magnifiers as I age,  radios nearby tuned to good stations, soldering irons on variacs, nice old boat anchor test equipment,  push carts on  wheels, a coffee percolator and fridge close by and a good digital camera that will let me share current activities and projects with the "gang".
         How's that for a dream ?  The projects are all here and waiting and so is most of the boat anchor test equipment, thanks to some recent ebay buying binges.  oops, post time again !!

voxACthirtee

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #16 on: January 15, 2019, 02:54:02 PM »
Well, no, or sort of. There are different types of designs in ultralinear, but as they apply to these basic designs where the pentode is used.....You start out wrong and then do delve into the basic concept.If the Plate and Screen are at the same voltages, or close, you have not achieved what is called "ultra-linear".Now, one does not HAVE to achieve Ultra Linearity with an amp, but if you want to , there is an actual patented design......And you can of course, just have screen taps at any level if you want. Again, doesn't make it ultra-linear.There are plenty of manufacturers/winders of tube amp output transformers, many who sell Ultra-Linear designs. They will NOT get you equal plate and screen voltages....why? because they understand the concept. Its not a difficult one. While the ratio does vary by tube, it seems the smallest screen to plate "difference" is more like 20%, not 5. The specifics of this i didn't just "hear from a guy". Its well documented if you want to spend a couple minutes learning ACTUAL FACTS, not just believing that guy on some forum who "thinks it might be this....."-----Again, google is your friend
            The DC screen voltage is same as plate DC voltage in an ultra-linear power amp unless the screens connect to an entirely  separate screen which usually  fed by a lower voltage DC source as in the ACRO transformer made for 6146 output tubes. In that circuit the plate winding connects to 600 VDC, screens 300 VDC..
          The term ultra-linear, which was I believe first used in the Audio Engineering article by Herb Keroes and Dave Hafler, the partners  in ACRO products  in their patent application from 1952 (patent granted 1955)  refers to any power amplifier circuit in which a only a partial amount of the plate to plate load is shared by the screens. Percent refers to the portion of power, not voltage, fed to the screen. It has nothing to do with screen DC voltage. In their patent application the inventors mention different tubes having an optimum percentage power ratio, 18,5 % for 6L6, 5881 etc, only 5 % for 6V6. I remember reading in another article somewhere that 43 % was optimum for 6550 and that is the same number mentioned by VOX ac30 so the person he was quoting must have seen the same test. 
        Google Dave Hafler Herb Keros ultralinear patent, and ACRO transformer catalog for more info. Patent app has interesting graphs showing variations in di=storton with percent loading from  0 to
- 100

ed from Baltimore

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #17 on: January 16, 2019, 08:46:56 AM »
              Well, I may have got into trouble because I use old time terms when newer ones have became more popular, like Megacycles instead of MegaHertz for frequency.
              When I use the term DC voltage, I mean the average of the waveform, if there is a waveform present, as read on a voltmeter set in the DC volts position. If the output tube is connected to the plate winding of the transformer it doesn't matter where the screen grid taps are on that winding, except a slight difference due  to the low ohmic value of the copper wire in the winding, at least not to a DC voltmeter.
              I was quoting the numbers from  the original patent application from the early 1950s by Dave Hafler and Herb Keros and I hope your later source was too. They spoke in terms of impedance ratio, not voltage ratio. Voltage ratio would be more common when talking about windings in power transformers. Audio transformers seem to use impedance ratio. 18.5 % impedance ratio is the same as 43 % voltage ratio. Also 5 % impedance ratio is the same as 22 % voltage ratio.  The 43 % number I saw years ago, must have been referring to the voltage ratio, not to KT88 tubes. The original patent mentions 18.5 % for 6L6 5881 KT66 etc which KT88 would belong to but they hadn't come out with KT88 at that time.
             Anyway, we are both saying the same things, but I am locked into the distant past cuz that's when i did most of my reading. 

TC Chris

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #18 on: January 16, 2019, 07:52:44 PM »
                   If it's the last thing I do, (which scares me to use the phrase as I get older  !! ), I am going to set up a comfortable table/bench with a chassis holder, good lighting, now with magnifiers as I age,  radios nearby tuned to good stations, soldering irons on variacs, nice old boat anchor test equipment,  push carts on  wheels, a coffee percolator and fridge close by and a good digital camera that will let me share current activities and projects with the "gang".
         How's that for a dream ?  The projects are all here and waiting and so is most of the boat anchor test equipment, thanks to some recent ebay buying binges.  oops, post time again !!

Chris grins.  Occasionally now I calculate what age I will be in 20 years, a period of forward looking that never seemed especially daunting before.  Geez, that's scary!  I am closer to 100 than I am to 40 (scarier still...).  And I have the same "some day" plan to have an ideal bench with good lighting, convenient access, lots of time.  But in the real world, I was out in the shop last night working on the 1935 GE, which is sitting on top of the table saw, propped up with a bock of wood, and illuminated (sort of) with a clip-on reflector lamp hanging from a rope.  The little soldering iron is clipped to the saw's table with a spring clamp so the cord doesn't pull it skittering off onto the floor.

It's important not to let the perfect be the enemy of the good enough, or even the barely acceptable, of you want to get things done. 

Chris Campbell

TC Chris

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #19 on: January 16, 2019, 07:58:36 PM »
Well, no, or sort of. There are different types of designs in ultralinear, but as they apply to these basic designs where the pentode is used.....You start out wrong and then do delve into the basic concept.If the Plate and Screen are at the same voltages, or close, you have not achieved what is called "ultra-linear".

I had posted this link:

http://www.oestex.com/tubes/ul.html

Scroll down to "Table 1" and note the plate and screen grid voltages shown for EL84/6BQ5.  It's a Mullard table.  They're equal. 

Chris Campbell

ed from Baltimore

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #20 on: January 17, 2019, 02:48:31 AM »
          My  last two employers had the "dream" set-up as far as test equipment, carts with wheels, soldering stations, and lighting goes, even pleasant radio stations available to listen to, but the percolators and fridge and microwave oven were in the next room.  I could leave my own projects set up and just turn in less hours when we weren't strapped for time on work projects. So it really spoils one when compared to working at home under poor conditions.
          BTW, the Voice of Music circuit has quite a few unique circuit features, especially for a commercial manufacturer who all seem to copy each others circuits. I'll have to print the schematic and refresh my memory (I told you it was bad, at least for recent things anyway) but one feature I remember was the hum control. The 6.3 volt filament winding is center-tapped to ground, but there is also a hum balance pot that has its wiper going through a high value  resistor to the cathode of the voltage amplifier triode. I've only  seen that a few times, actually  injecting some filament voltage into the circuit, and hose times the injection was to the control grid instead of cathode, and the value of resistor was much higher, like 4.7 Meg.
          And......yes, how can the DC voltages be anything but equal when they all come from the same piece of copper wire. The screen DC voltage has to be somewhere between the DC voltage on the center tap and the DC voltage on the plate. However,  I do think the web site everyone's referring to is careless in it's use of the term voltage without specifying  AC or DC.
           

electra225

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #21 on: January 18, 2019, 08:29:36 PM »
My stepdad had a long bench with three three-legged stools.  He did radio in the middle, record players to the left end, and TV on the right end.  My shop is three sided, in a "U" shape.  The front bench is 12' long and each return is 8 feet long.  It could be 40' long and not be long enough.  I have so much junk on my bench, I typically need to clean something off in order to be able to work.  I have plenty of light and each "leg" of the bench is on its own 20 amp service.  I have electric outlets just under the edge of the bench and on the wall.  My service equipment is set up on shelves over the benches.  I like the setup and find it convenient to work.  Clutter is my problem.  I simply can't keep my bench neat and organized.
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #22 on: January 18, 2019, 10:24:04 PM »
Clutter is my problem.  I simply can't keep my bench neat and organized.

Sounds like my garage shop.  Every horizontal surface has filled up with future projects, things I'll surely need some day, containers that I'll need as soon as I recycle them, and other important stuff.  The GE radio cabinet stands on the Workmate bench so my service on the chassis is being done on the table saw.  If and when it's working and reassembled there will be a crisis about where to put it, because my house is stuffed already.  Don't get me started on the Victor radio highboy cabinet that I took to make sure it didn't get trashed/surely somebody would want it, or the Silvertone console radio that looked lonely in the roadside trash pile, or the outboard  motor stand for the sailboat motor, or the sturdy (heavy) footlocker/trunk that my Dad shipped his stuff back from Europe in after WW II.  A POW painted his name and address on it in very nice letters.  Geez, you can't just toss something like that out. But where am I going to work?

Chris Campbell

electra225

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #23 on: January 18, 2019, 11:07:37 PM »
My brother is the organizer of the family.  He could organize a landfill.  He worked a week in my shop organizing and storing things.  Naturally, NOTHING could be tossed.  When he got done, it was beautiful.  His car tracks were not dry in the driveway until I had it all cluttered again.  I can't find anything when it is stashed.  I can find things in the clutter.  I toss junk tubes, caps I have removed, bits of wire and other junk toward the back of the bench.  That stuff grows and migrates toward the front.  I keep tools I commonly use laying on the bench.  I have leads to test equipment laying about.  I have projects I have been working on for eons, like the bakelite chassis Zenith that I have been working on for probably ten years.  My stepdad always told me he did not trust a workman with clean tools or a neat bench.  He would really trust me, given that criteria.   ;)


I would LOVE to have a complete set of Sams' manuals.  I have had the chance to have a couple sets, for free.  I have no place to put a set. 
I don't need Google.  My wife says she knows everything.

Motorola Minion

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #24 on: February 01, 2019, 01:40:45 PM »
I was fortunate enough to inherit a near-full set of sams photofacts ;D. They make for great reading at busy intersections on my 2 hour daily commute. I pull one at random and look at all the fabulous and cheap stuff offered by the RETMA manufacturers. Makes me thankful I did not have to work on anything that comes along...like in my TV shop days.

I had to give up the 400s-500s (1958-1962) as part of the deal, but I was only missing about half of those. I would really like to help more of you out with schematics but my problem is finding scan time and a readily-available connection to send ("not at work"). I do have them stored and fully organized in 8 vintage heavy-steel file cabinets.

Speaking of several "bench" locations like Chris does, I can understand that. I have a TV test jig/equipment and wood shop in my warm basement, an audio bench and metal shop in the frozen garage :( (yes, I like it loud). An old radio bench that was in the attic with the majority of my radio collection, got moved to a semi-heated summer kitchen where I could watch movies and play records, until I lost my late-nights due to changing family routines. :'(  The old radio bench is being moved again since I can actually make money and there is demand for all tube and early transistor restorations. This I can spend 30 min/day on before going off to work, after everyone thinks I'm "gone".  ::)

I have a record player and HiFi bench shoe-horned into a rented, semi-heated garage where my car is stored. That is where I have been working for the last 2 years about an hour every day after work, and almost always come home in a good mood 8) But most of my best tech moments happen in the mornings, after Ive been away from the bench.

The best discovery I have made with limited time to organize is this:
When you're hot on the trail of a problem set or phono, you often need to STOP and come back a few times. When, during one of those sessions, you feel like you're getting NOWHERE, you STOP: Gather up all the scattered parts, tools, test leads and wires and put them "back". Organize your stuff again then go home. It clears your mind, and you will probably have a breakthrough the next session.

My moments of clarity come and go like Jerry Reed famously sang in '71: When you're hot, you're hot and when you're not, You're not.https://www.famousfix.com/topic/jerry-reed-when-you-re-hot-you-re-hot

 
Tubes - Magical - Tubes

Dave

Harbourmaster

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #25 on: February 01, 2019, 03:07:00 PM »


It's important not to let the perfect be the enemy of the good enough, or even the barely acceptable, of you want to get things done. 

Chris Campbell


I appreciate what everyone has said here in this discussion but I had to stop and laugh when I read this Chris!


While I generally subscribe to this idea of not fixating on perfection, I also have this quote posted on the bulletin board over my work desk.


"Good is the Enemy of Excellent,
Talent is not necessary for Excellence,
Persistence is necessary for Excellence, and
PERSISTENCE IS A DECISION"
-- Aloha, Ken

No Console Left Behind!

TC Chris

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #26 on: February 01, 2019, 07:35:16 PM »
Well, I'm about to figure out whether I've accomplished good, good enough, excellent, or a face-plant on the old GE radio.  Night before last I installed the last caps needed to test it out (one more electrolytic, the high-voltage one at the output transformer, and the line filter safety caps) and then poked around to see what I had missed.  Oops, a wire from the volume pot to ground had come unhitched where it was supposed to be attached to a ground tab after passing through the chassis.

Excellent would be cool.  Good enough will be good enough.  Not exploding will be satisfactory.  I'll try it first with the dim bulb tester just to make sure there are no gross errors.  On an eighty-two year old device, especially one that has seen some fiddling-with along the way, sound coming from the speaker will be a victory.

Chris Campbell

electra225

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #27 on: February 01, 2019, 09:00:03 PM »
"Not exploding" is the criteria I have set for my TV repair project.  If, when I hook the chassis up to the tuner and the CRT, and I plug it in, then turn it on and if nothing smokes, the CRT lights and there is some kind of noise from the speaker(s), I'll be picking in tall cotton.   ;) :)


If I make it that far, it is not too far from there to "good."  Excellence is not something I'm even thinking about.   :-[
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: V-M 557 Plate/Screen Grid Voltage Questions
« Reply #28 on: February 01, 2019, 10:29:48 PM »
As noted elsewhere, the GE did not explode and did not even smoke at me.  And noise came out.  You're being more careful and thorough than I was.  Yours will turn out closer to the "excellent" end, I bet.

Chris Campbell