Screen Drive Push Pull ideas

[DigitalJunkie]

Hey Whitelabrat,

I've got my workbench set up in the garage finally so I hope to do some testing on the power transformer. I estimate about 26A in 6.3v.

If you look on Pete Millet's site,he might have some info on your transformer,if you're lucky.

http://www.pmillett.com/Tek_xfmrs.htm

[Matt.B.H.]

You guys in the US are in the great position that you can part out these wonderful old scopes.

I am kind of a TEK fan, I own a couple of more modern "transistor-ated" ones. I am looking to get a decent 5 series simply to do up and use. I know its sad but they are the best. Think about it, dual beam, masses of drift (on a few plug ins), room heating- just the ticket.

Anyway it is a shame that some go this way but I do agree that most are BER and why not strip em out. Lovely ceramic strips (use a silver loaded solder) the best sockets etc.etc. The transformers are awesome and would make the best bench supply. Use the 6080s as pass valves and combine secondarys, 600V at half an amp or more, depends how brave you are, would not be out of the question

Cheers Matt.

[tubelab.com]

but is there something else I'm missing about this topology?..... My amp idles at 3mA per output tube and coughs up 150 watts for a pair of sweep tubes.

This makes for higher overall efficiency, and since the idle current is low the idle dissipation is too. The amp only dissipates power when it is driven, so the average dissipation is much lower than a class AB or class A amp. Heat is lower so tubes and everything else lasts longer.

Since these things aren't main stream, expect to do some experimenting to find the optimum operating point for your particular tubes.

screen is low at 250 volts so i wonder these tubes could be good candidates for your screen drives... surpluss sales at about 22bucks a pop

The data sheet looks good, but as I explained above, only some real testing can determine if they will work. I found them on their web site, but I have decided that for this year at least all of my experiments will be done with the collection of stuff that I already have. I have tubes that have been awaiting "testing" for years.

[Merlinb]

Originally posted by SY
No crossover distortion, unlike similar near-class-B conditions with conventional grid drive.

Very interesting! So where's the best place to brush up on the theory of screen control? I'm having trouble understanding how it overcomes crossover so well...

[SY]

There's not much literature on it. I'd look at the Bascomb King review of the Berning EA-2100 in Audio (some output tube curves shown at low currents) and study the schematics for the EA-2100, BA-150, and EA-230 at the Berning website, Berning's Patent #3995226, but beyond that... I really can't think of any analytical papers.

[whitelabrat]

Originally posted by DigitalJunkie

If you look on Pete Millet's site,he might have some info on your transformer,if you're lucky.

He's got some measurements on the site, but he only did load testing on one model. Mine is very similar though.

Originally posted by Matt.B.H.
[B]You guys in the US are in the great position that you can part out these wonderful old scopes.

I didn't want to do it, but I was moving house and I had to lighten my load a bit. They guy selling it practically gave it away so I had nothing to lose. I did salvage some nice parts.

Bernings Patent was good reading and there are a couple of topics that discuss screen drive floating here on this forum. Tubelab has published some very insightful information if you dig around.

I don't expect to heat up the soldering iron on this project for a long time. Maybe I'll have time next year. I don't have any useful measurement equipment either for troubleshooting, especially after parting out my oscilloscope. Excuses, excuses right?

[Matt.B.H.]

He's got some measurements on the site, but he only did load testing on one model. Mine is very similar though

I read on the TekScopes forum that the TEK warranty covered transformers at the time and that they neither stocked or had made many replacements. Read into this what you like but I would say they are very over-enginered.

You could get on the TEK forum and ask but dont say you stripped it, you probably wont be too popular. Another way of finding the ratings would be to get the manual if you dont already have it. Try BAMA:

http://bama.sbc.edu/

Use the mirror, it does work.

You obviously wont be using all the heater windings so the primary will be well over-rated. I would reckon that you could run the HT secondarys at maybe 150% current without too much trouble.

The trouble with the TEK trannys would be that you cant tell any temperature rise until it has soaked and heated up the core, covers etc. I havnt tried taking the covers off one but if you can and then get closer to the actual windings you could feel if its too much. Also if you can see the wire guage used you can estimate current. I am pretty sure the later transformers of this type were fully potted under the covers so if you do try to take the top one off be careful, dont even think about the bottom one.

Cheers Matt

Sorry I cant work out how to add a quote properly. Will re-read the FAQ

[whitelabrat]

I did a search for "boat anchor" and found the bama site. I had a print manual for a different version of scope so finding the manual online was priceless. I can get an idea how the secondaries were used which helps. While moving, it was funny to see the look on the guy's face who picked up the small box that I packed the transformer in. A bit of swearing followed.

One thing I'm concerned about is voltage sag. If I were dealing with a Class A amp I wouldn't expect any problems. With screen drive turn up the volume and the B+ may be a whole new game. That's where a toroidal transformer would probably do better. Lots of capacitance may cover spikes otherwise. In any case I've got to work with what I've got for now.

[tubelab.com]

Lots of capacitance may cover spikes otherwise. In any case I've got to work with what I've got for now.

I think you have that covered. My current KT88 experiments drive my Fluke 407D power supply into oscillation at anything approaching a reasonable power level. The power supply is 100% original including the tubes and bumble bee caps.

When I add one of the 600uF ASC caps across it, the current meter sits quietly on 150 mA unless I really get carried away. Depeche Mode at the clipping level makes the current meter bounce around a little, but it never goes over 200 even though the peak currents are way higher.

The TEK transformers are overkill and I have never heard anybody say that one wasn't big enough. I have one sitting under my bench from a 500 series scope. I don't know what scope exactly but it had lots of 6DJ8's in it. It was being scrapped for the tubes, so I grabbed the transformer.

[DigitalJunkie]

Also if you can see the wire guage used you can estimate current. I am pretty sure the later transformers of this type were fully potted under the covers so if you do try to take the top one off be careful, dont even think about the bottom one.

I've got a pair of the 120-120 trannies from the 535 scopes. The bottom covers do come off,they aren't potted.It's a bit of a chore to pry the cover up after you've removed the screws,because all the stiff lead wires want to hold it down.

You obviously wont be using all the heater windings so the primary will be well over-rated. I would reckon that you could run the HT secondarys at maybe 150% current without too much trouble.

These Tek trannies are pretty beefy,and can take a bit of overload without any complaints,especially if you aren't using all of the heater windings,etc. there should be some 'overhead'.
I've run the 120-296 that I have pretty hard,and it barely got warm.