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I'm going to build a push pull 2a3 amp using the UTC LS-19 as an interstage transformer. It's specified as 15K:95K (1:2.5) with no DC allowed on the primary. I'm thinking about using a 12B4 loaded with a current source. I'm a bit confused as how to select the driver as I'm not certain how the impedances work out and how low an Rp I need to avoid roll off due to Miller capacitance. I'd like to use a DHT as the driver but all of the possibilities I've look at seem to have a very high Rp.
I'd appreciate any thoughts.
Follow Ups:
Thanks to gilmorneau and danlaudionut for your replies.
I'm not concerned about the high frequency response too much, at least not enough from stoping me from trying the 76; My hearing stops at about 15 KHz now. The gain of the 76 should be about right given that I'll be driving it from a Fisher 80-T preamp into an EV Georgian speaker.
I'll be reusing transformers and chassis from an old homebrew mono amp which was never completed and looks to date from the late 1930s or early 1940s. I've wondered about the history of this piece. The purpose of the project is to get a period mono system to play back mono LPs and 78s. I'll also play around with some mono reel to reel tape playback at some point.
I'll think try a choke/parallel feed for the driver to eliminate any possibility of silicon spoiling the vintage vibe.
Check the link below. It would be a good place to start. Sub a 76 for the 27, sub the LS19 for the A-18, and use whatever OT you have (presumably UTC?). You may have to adjust some of the component values, and for the sake of getting it up and running you could use single components where Jeff has paralleled some, but it should work as drawn.
I'm on board with using a choke load on the 76, but you're opening up a can of worms. The math gets more complicated.
Normally, the LS19 would be used with driver tubes from about 5k to 10k Rp, a plate load resistor about 3-4x Rp, and about a 0.5uF parallel feed capacitor. Think 6J5, 1/2 6SN7, 76, 6C5, paralleled 6N7, triode wired 6J7, etc. Even using the transformers this way, as UTC intended, you may have to put loading resistors across the secondary (as opposed to running it open) to avoid resonances. Also, the impedance that the driven tubes (the 2A3's in this case) 'see' will end up being 6.25x the Rp of the driver (turns ratio squared). All that said, the LS19's can sound excellent like that. Just not much BW in ultrasonic territory.
If you want to use DHT's, there are several that could work. 26's & 10's come to mind if you want to mess with proper filament supplies for them. There are others.
If you want to explore the 'DHT drives interstage transformer to PP 2A3' topology, you might be better off with some nice Lundahls, Tangos, Tamuras, or something else that was specifically designed to do what you're looking to do. I understand the impulse to use what you have (I'm assuming you have a pair of the LS19 already), but you could always sell them to fund the purchase of some new production transformers that would be easier to implement successfully in your project. If you decide to sell the LS19's, let me be the first to ask that you contact me before throwing them to the wolves. ;-)
LS 19 was designed around tubes of the 6J5 family, so Rp around 7-8k. 12b4 is much lower. It might work, but you may get some FR artifacts that will need attention. My experience has been that transformers work best when used as intended. Can't hurt to try what you propose, but you might want to consider either using the ls19 as utc intended, or getting a transformer that is designed for what you intend.
I have built amplifiers using ls19 and other similar utc transformers with great success. Feel free to contact me through asylum mail if you have any questions.
Consider also the 12A4, its mu is 3x the 12B4. Or try to scaare up an HY40...I love the irony of using a signal tube that is bigger than the final.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I've often considered using the 12A4 in other projects but I've never bought any. They seem to be fairly rare although not too expensive when you can find them. Thank you for the suggestion.
If that is so the 6GK5
with a rp of 5.5K
and a gain of 78
would be awesome.
It is proven to be
great with 2A3s too.
DanL
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