"Killer" Screen Driven Push-Pull Amp Initial Queries - diyAudio
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Old 30th October 2008, 05:10 AM   #1
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Default "Killer" Screen Driven Push-Pull Amp Initial Queries

I've been heavy into SE tube amps for the past year or two, and am edging slowly into push-pull territory. Inspired by Tubelab's torture of 6BQ6s with shockingly high output power, I have in mind an amp I'll call "Killer". It will feature screen driven 25LR6 output tubes. I'm looking for around 100W per channel, if not steady RMS, at least for an extended burst. The most important part will be the output iron. .I don't want to spend a fortune, so I'm initially looking at budget options. Option one is using one of the E-I based transformers with 5k primary impedance. Classic examples are made by both Hammond and Edcor. Option two is using a toroidal output transformer. This is where screen driven push-pull amps may especialy shine, as they operate well with low levels of bias current, making it much safer to use toroidal output transformers. Antek has just released a series of push-pull toroidal transformers that appear to ape the Plitron offerings at about 1/3 the price. The high frequency response is inferior to the Plitrons, perhaps implying less interleaving/more leakage.. Does anyone have any comments (based on experience) for usin g these transformers in a 100W amp? Any other suggestions for powerful iron that won't require me to take out a second mortgage?
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Old 30th October 2008, 11:32 AM   #2
SY is offline SY  United States
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Naive question: how much LF bandwidth do you need? If that can be compromised, you can greatly decrease transformer cost and potentially improve HF performance.
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Old 30th October 2008, 04:49 PM   #3
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In my current prototype of a bass guitar rig I use industrial control transformers for mid-hi section, I will report my results, they may help you.
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Old 31st October 2008, 03:26 AM   #4
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I don't need to go all that cheap. By the same token, I don't want to be paying $300 per transformer, either. That (I guess) describes my zone of comfort . Yeah, Stew, bass is important - so are the highs, but if they get to be a problem, I'll have a slew of little SE amps with crystalline highs waiting in the wings to do tweeter duty in a biamp rig. This is an exercise in raw Neanderthal (well, maybe Cro-Magnon) power.

I did some back of the envelope calculations this afternoon. Information regarding the 6LR6 is rather thin on the ground (I've got a few numbers in my old, tattered RCA handbook, but no curves anywhere). The guts and ratings look very similar to the well-characterized 6JE6/6LQ6. First assumption - the tube will pull to within 50V of ground at peak excursions. The 5k transformers (1.25k/side, really) have a 12.5:1 turns ratio into an 8 ohm load. Assuming no other losses, if you use a 550V plate supply and pull down to 50V, you get 40V peak at the output. This is 5A peak into an 8 ohm load. Reflecting bacwards, it's 400 mA peak plate current. This is well within the peak rating of the 6JE6 and 6LR6 and ok with the average cathode current, too. This would disappoint George (no red plate? - darn...). More importantlly, with somewhere between 75-100V on G2, a 6LQ6 will pull down to 50V, 400 mA. This gives me great expectations for the similarly sized 6LR6. Putting it all together, with a 5k transformer and 550V plate supply, I get about a 98W RMS output capability into an 8 ohm load. Losses will eat some of this, but I can always bump up the plate supply a bit to compensate. I'm going to use a switching power supply, so I'll have a lot of freedom in terms of what voltages I can deliver.

The 100W Edcor iron is less than $100 each. The Hammonds are about $20 more expensive, and are no doubt huge (but ugly (butt ugly?)) clunkers with good bass, The Anteks are about the same price as the Hammonds, but with different impedance. Are there any other transformer candidates out there that aren't priced out in cloud la-la land?
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Old 31st October 2008, 03:46 AM   #5
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The Antek OT data doesn't specify what driving impedance was used to derive the data curves. If a simple 50 Ohm generator was used, then the data are no better than random wound power toroids. Just that they may have used enough turns presumably for the required inductance, which they also don't list.
They need to post more info, like primary inductance, and distributed capacitance or resonance frequency, and use appropriate source impedances for deriving the freq. response curves.

Progressive wound units should have much better specs than those listed. It's also strange that the curves given suddenly improve with the lowest power unit, after consistently degrading with each drop to a lower power/ higher Z from the highest power unit.

I have given up on commercial toroid OTs, I am building a shuttleless toroid winder. With one of these, its easier to wind toroids than E-I s. Then I can do progressive winds, and any turns ratios I want. 500KHZ bandwidth typical.

Don
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Old 31st October 2008, 03:50 AM   #6
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Lundahl LL1620 @ 6k would do it at $235/ea, on the higher end
Hashimoto has something suitable but shockingly expensive
James I think has something, but don't know how much it costs to import them now that euphonia is out.
I had Bud Purvine of Onetics made me some 100watt OPT's that were well under your threshold for pain.
Doug likes Heyboer, maybe they have something in that range.
Electra-print is probably worth looking into.
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Old 31st October 2008, 03:57 AM   #7
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Smoking amp, I thought the Antek data looked a little slim and skanky, too - that's whay I was asking about any real-world experience.
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Old 31st October 2008, 04:10 AM   #8
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http://freepatentfetcher.com/GetPate.../US4127238.pdf



Hmmm, looks like that won't download it directly. It's patent # 4127238

Darned if I can get that dumb website to download it.
Try this one:
http://www.uspto.gov/
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Old 31st October 2008, 05:04 AM   #9
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Found the problem, try this one:
http://free.patentfetcher.com/GetPat...7/US4127238.pdf

Geez, still doesn't work, at least not from this web page. Works for me if I copy it into the address line and hit GO --- if I'm already on that website. Maybe its some kind of protected or keyed access.
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Old 31st October 2008, 07:41 AM   #10
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Well, that didn't work either, whatever you were trying to do. Rather than mess with toroids (though I have some), I would wind a c-core transformer. That is, if I wind instead of buy...
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