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Old 28th April 2009, 08:52 PM   #1
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Default Screen Drive Push Pull ideas

I didn't want to dig up an old thread on the same screen drive topic so I've started a new thread. I've done a lot of reading on it (probably not enough) and I find it fascinating. I fear I'm about to dip my toe into this puddle.

Mr. Tubelab seems to be cooking up something interesting. Berning's site also is interesting to poke through.

This is what I've got brewing in my head:

Voltages will be high!
It seems that oddball horizontal sweeps tubes are ideal for the output. 6GB6's, 6AV5's, etc?
The next trick is the driver which has to swing some serious voltages. I'm thinking *CG7's, or *UA7's since *SN7's are too pricey for me. Heck, perhaps even the 6N1P?
It seems at though having a *FET is useful to provide more current at the end of the driver stage?

So I'm thinking of a 60w push pull output is a reasonable target, mainly to keep the OPT requirements sane.

Looking at the parts pile I've got a Bogen mx60a that may be a iron donor, a Tek 531 scope with a huge power transformer, and other miscellaneous odds tubes and components.

I'm just wondering what folks out there have done with this, and if I'm on the right track?
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Old 29th April 2009, 02:15 AM   #2
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I just found and interesting thread here.

It doesn't appear there was any conclusion to the last schematic posted.
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Old 29th April 2009, 06:52 PM   #3
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I suppose I could attempt a copy (with modifications?) of Bernings EA-230 . My intended output tube has similar ratings to the 6JN6 used. Perhaps I can hot-rod it a bit to up the output where I don't need the output tubes to last 10,000 hrs or whatever.

I do have a spare Hammond 274BX at my disposal for testing and may be fine for the 30W of output.

I haven't heard a peep from anyone, so I'm going to quietly continue researching. I'm just curious if anyone out there has anything working that they can share details on?
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Old 29th April 2009, 09:24 PM   #4
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Quote:
Mr. Tubelab seems to be cooking up something interesting.
Unfortunately my universal driver board looks about the same as it did in that thread. Work and family commitments have again forced the postponement of most Tubelab activity.

The schematic that I posted in the thread you mentioned does work and is the place I keep starting from with all of my screen drive experiments. I have built 3 different versions with several types of tubes. One used a pair of 6SN7's. It is the tag board design seen in the same thread. I had another tag board version with 9 pin tubes. I tried several different tubes and most worked. The best choices for input tubes were 5751's, 12AY7's, and 12AT7's. The best choices for driver tubes were 6GC7, 12BH7, and 5965. There are plenty of other tubes that should work, but I haven't tried them yet. Based on previous experience and stash on hand, I plan to use a 5751 in the first stage and a 6CG7/6FQ7 as the driver in the PC board version. I have made the following undocumented changes since that thread was published.

I would add resistors from the screen grids to the negative supply to insure complete cutoff of the output tubes. I used some big 20K 5 watt jobs in my later experiments. The 6AV5's worked fine without them, but some output tubes liked to oscillate during the moment of cutoff without them.

There is now only one cathode resistor for both driver tubes. Tie both cathodes together and use one resistor to ground. There is a 330 ohm in there now, but that may change. I have done some experiments using a CCS chip here. It improves the balance at high frequencies.

Point labled "SG supply" is now a 500K pot from B+ to ground. It sets the idle voltage for the screen grids.

Point labeled "- bias supply" is a 50K pot from the -20 volt source to ground. It sets the control grid idle voltage.

The negative supply voltage is not critical but needs to be high enough for the CCS to work. -12 votls to -50 volts or more should work.

Most builders of screen drive amps claim that they work best with the G1 voltage set to zero and the G2 voltage adjusted for an idle current of about 5 mA per tube. I have better luck with a small negative voltage on G1 and G2 set for about 10 - 20 mA per tube. Each output tube works somewhat different.

Idle current is low and so is the idle dissipation. The output tubes will see very little average dissipation since music has a fairly high peak to average power ratio. The tubes may get hot when the amp is cranked up to maximum power with a sine wave, but it will run cool with music.

The output impedance is somewhat high, pentode like. Some GNFB will be needed.
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Old 30th April 2009, 04:11 PM   #5
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Thanks George. I figured I'd just sit tight since you seem to onto something good. I need to spend a lot more quality time with the Book of Jones, and the basics of Art of Electronics before I can make any entertainingly misguided contributions on an engineering level.

How about a 6J6 for the input? The cathode is already tied together, medium mu, and... dirt cheap. I think I have a few. I'm also wondering if a 6BH6 pentode with a CCS for voltage gain is sane at all. I've got a pile of 6CG7's which is my only interest in using them instead of the precious few 6SN7's I've got. I don't care for *AU7's but 9AU7's can be gotten cheap.

Do you think I could squeeze 60w for a single channel from a single 274BX power transformer at 450v? The great big Tek 531 (120-037) power supply could probably supply a lot of juice at over 400v too, but a bridge rectifier would be needed along with some experimentation with the different secondaries to get the right voltage. I figure I could tie some of the Tek's 6.3v secondaries together in a series to get a negative supply.
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Old 30th April 2009, 05:25 PM   #6
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6BH6 into 6CG7 was good enough for Saul Marantz...
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Old 30th April 2009, 05:25 PM   #7
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Quote:
How about a 6J6 for the input?
It works. I tried it for an LTP phase splitter in early Simple P-P experiments but there wasn't enough gain to allow the use of GNFB with only a single stage. You will find it in the Book of Jones with an LM124 CCS chip. The 6J6 and the LM124 combination is one of the few times that a negative voltage source is not required, although I thought that it worked better with some negative voltage. I was probably driving it harder than Morgan ever imagined.

Quote:
I'm also wondering if a 6BH6 pentode with a CCS for voltage gain is sane at all.
Pentodes generally work better with a resistive load, while triodes like a CCS. I have found exceptions to this and some pentodes can be made to crank out some serious gain using a CCS with a resistor across it. I have some 6BH6's and a zillion other IF amp tubes but haven't tried them yet.

Quote:
I've got a pile of 6CG7's which is my only interest in using them instead of the precious few 6SN7's I've got. I don't care for *AU7's but 9AU7's can be gotten cheap.
I have more 6SN7's than 6CG7's but they are fairly similar tubes. Either should work well. I have several hundred 12AU7's so I am obliged to try them too.

Quote:
Do you think I could squeeze 60w for a single channel from a single 274BX power transformer at 450v?
I built dozens of "50 watt" guitar amps using 274BX's and Allied 6K7VG's about 15 years ago. None of them ever fried a power transformer despite some heavy abuse. A screen drive amp runs at a much lower idle current but can draw some serious B+ current on musical peaks. Fortunately the peaks are infrequent, and usually supplied by a big fat capacitor on the B+ line. Many Sweep tubes have a serious thirst for filament current, so make sure that you are not overloading the 6.3 volt winding with your tube choices.

I have heard that TEK power transformers are nearly indestructible, and I have one from a 500 series scope, but again I haven't had the time to experiment with it.
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Old 30th April 2009, 07:45 PM   #8
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Quote:
Originally posted by Tom Bavis
6BH6 into 6CG7 was good enough for Saul Marantz...
I've got an 8b on my bench (for listening) which is where I got the idea. No complaints here...

Quote:
Originally posted by tubelab.com


Fortunately the peaks are infrequent, and usually supplied by a big fat capacitor on the B+ line.
I have three monster 600uF 500v film caps that I can borrow from my Cheapfield project...

I'm just in a planning phase right now, since the wife and I are tied up will selling our house (in a good way). I'm trying to pack up the workbench, but things keep hopping out of boxes. I can justify taking time to part out the TEK 531 though.

I think I may have a plan. I'd like to follow Tubelab's screen drive circuit, but with a 6J6 for voltage gain and a 6CG7 for the other end. I've got almost a whole set of parts including sockets, 10M45's unless I have to use LM124's instead for the CCS. Just need a set of 2SK2700's (STM STF3HNK90Z). I'm hoping to drive 6AV5 equivalents.

The Bogen's OPT I think has a 3k ohm primary, but I'm not sure. I could use one of the secondary taps to bring it down to half of whatever it is, hopefully near 1500 ohm which is close to what SY mentions he uses.
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Old 30th April 2009, 09:15 PM   #9
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Quote:
10M45's unless I have to use LM124's instead for the CCS.
I prefer the 10M45 if a negative voltage is available. The LM123 will work with 2 or 3 volts across it. The 10M45 needs 10 or 15 volts. All of my screen drive experiments have used 10M45. The LM124's are tiny and blow up easily!

Quote:
Just need a set of 2SK2700's
The 2SK2700 is extinct, a lead dinosaur. Killed by ROHS. I am using the 2SK3563 now, it works better, but is only good to 500 volts. You need a mosfet with a low Crss, that remains constant over the range of applied voltage used. The 2SK3563 is a flat 7 pF for anything over 20 volts. This is less than many tubes!

Quote:
I can justify taking time to part out the TEK 531 though.
Yeah, since they are large by huge by heavy! I was at a friends place who is a scrap dealer. He sold a large TEK scope on Ebay. The winning bidder said to remove the tubes and send them (lots of 6922's) and toss the rest. I removed the power transformer, but never used it yet.

Quote:
I'm hoping to drive 6AV5 equivalents. The Bogen's OPT I think has a 3k ohm primary
That is where I got the best results with screen driven 6AV5's. I was using a 6600 ohm OPT with an 8 ohm load on the 16 ohm tap. SY used a 1250 ohm load with considerably bigger tubes.
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Old 5th May 2009, 05:01 PM   #10
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I think I've got a solid plan together and I figured I'd bounce off the forum here.

I'll draw it up later here, but I'm looking at using a pair of Bogen MX60A's as iron donors for their transformers and chassis. The OPT's have an 82 ohm secondary, a secondary with taps for 16, 10, 8, and 4 and then has an interesting secondary that appears to be used for feedback. Cool. The power supply uses a doubler to provide 615v and 295v B+ supplies while using filter caps rated for 350v. Interesting. I'll have to use real iron to filter a B+ over 450v instead of a gyrator.

I only have four 0.47uf's in the stash, but only two are rated for 630v. I have lots of 0.22uf which I think will be OK as coupling caps. I haven't done any math yet so I'm guessing. I also think I can drop in the 6J6 in place of the 6BQ7's, but some tweeking may be in order once the B+ is determined. I would target about 200v and 7-10ma for the little guys.

Since 8 coupling caps will be needed for two channels can I mix 0.22uF with 0.47uF? Not optimal, but I gotta do what I gotta do.

How about IRF840B instead of 2SK3563. They have a Crss of 35pf and a 8A, 500v rating. I've got a few that I haven't fried yet.

Voltage doubler a bad idea? The tube's plates should be good over 700v for the high B+ and the lower B+ supply can be filtered to heck for the driver stages.

On an upside, I got a contract on my house super fast. I say the economy is picking up for housing. On the downside I'm going to be house poor soon, so I'll likely need to work with as much as I can from my parts stash.
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