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Drake L4B summertime project




 
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Author Topic: Drake L4B summertime project  (Read 307 times)
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w9jsw
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« on: August 11, 2019, 08:43:01 AM »

A year ago I bought a rather rough looking Drake L4B at a hamfest. It was without PS but had 2 3-500 tubes with globes. It seemed relatively intact and unmolested except for some rust on the cabinet from sitting in a humid midwest garage for a number of years. I gave 2 Benjamins for it. I thought the tubes were at least worth it. I had planned to remove the tank and use it for an AM transmitter.

Well I just could not do it. I did not want to tear it apart. I had the tubes tested in a known good amp and found out one is just fine and one is gassy. So not a good of a find as I thought -  Undecided

The power supplies for these are hard to find and expensive when you do find one. I could easily drop 4 Benjamins or more. So it sat.

This spring I started helping a guy make some PCBs. He said he could take that amp and get it working well with a PS he had. So I shipped it to him and he fixed it up nice! New paint, grounded grids, soft key upgrade, etc. Ran full power with his tubes.

So - what to do about a PS...  

See attached. I am taking a Antek 8T650 and a Harbach doubler board and making a power supply to run the 3-500 at 3500V. Hammond makes a enclosure that looks almost identical to the L4-PS enclosure. I think this will work FB with my single tube. Perhaps it may even have enough current to run two if I decide to drop more Benjamins on one. Transformer is a 800VA unit that makes .62A on the secondary. Pretty close to the OEM supply. I think it will drive one tube just fine at 3500v @ 400ma.

I will let you guys know when I fire it up.

On edit - running Duncan's amp PS program, using the actual winding resistance of the transformer, I think I will be ok. The output voltage is estimated at 2800V at 560ma with a 5K ohm load. Running the load as constant current with steps of 400ma and 800ma, it will make 3200V@400ma and will do 2800V@700ma. I think it will sag if I drive it to full power with 2 tubes. With 1 tube this might just work FB!

John


* L4B-Antek PS Schematic.jpg (1007.69 KB, 3269x2062 - viewed 45 times.)

* IMG_0280.JPG (625.1 KB, 1836x2448 - viewed 34 times.)

* IMG_0281.JPG (505.89 KB, 1836x2448 - viewed 58 times.)
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KD6VXI
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« Reply #1 on: August 11, 2019, 03:07:44 PM »

That xformer is a beast.

It will be OK at full power ssb if you want to run legal limit.  1.5kw pep.  The factory SB220 xformer is 600 or so VA.  The factory L4B xformer is appx 800VA.

You have to input it as a .96kva xformer.  Uts designed to run at 50 hz, and on 60 it's about 20 pct better, give or take.

They do make a better one.  950 volts AC output.  It's almost twice as much however.

--Shane
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w4bfs
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« Reply #2 on: August 11, 2019, 07:15:24 PM »

I like those Antek toroids and have bought quite a few over the years ... be sure to test them on receipt ... have had 1 or 2 bad small ones but Antek made good on them with no hassle .... until we find out more on manufacturing methods (like a doa dissasembly) I would only run one hv toroid with the secondary windings in parallel not in series .... afaic we had a discussion on here a few years without getting to a final answer ... I would use 2 smaller toroids stacked

as far as the L4B is concered it is a good rf box that needs more cooling air .... suggest cascading centrifugal blowers by adding a flange on the air input, a connnecting hose and a supply blower .... this can be cherry
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
KD6VXI
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« Reply #3 on: August 11, 2019, 09:38:07 PM »

I've run that one in series.

And the secondary isolation is 3.5kv rated.

--Shane
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WZ1M
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« Reply #4 on: August 12, 2019, 03:28:29 AM »

Just so happens I have an LB-4 plate transformer. Cusyomer sent it in for a rewind and never paid for it. For some reason, after closing up shop, I hung on to this beast. Heavy to ship so its going to be a pick up only near Bangor Maine.
Regards,
Gary
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w9jsw
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« Reply #5 on: August 12, 2019, 06:47:13 AM »

This one is hard to beat at $115 shipped. A bargain.

Got it mounted this weekend along with the harbach board, will do the under chassis wiring next. I was able to get the exact Drake power cabling on the bay as well as a connector so this is going to look almost OEM.

Only 1 day off this weekend so limited shack/bench time.

John
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w4bfs
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« Reply #6 on: August 12, 2019, 09:57:20 PM »

I've run that one in series.

And the secondary isolation is 3.5kv rated.

--Shane
KD6VXI

Shane ur not getting the point just yet .... the 3.5kV isolation rating is from the whole secondary to the whole primary not from secondary winding to secondary winding ... if the hV toroids are made like the lower voltage models then the primary is bifiliar wound and the secondaries are bifiliar wound ... this means that the secondary to secondary insulation is  the 2 coats of enamel between the windings .... now do you see my concern ?
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
KD6VXI
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« Reply #7 on: August 12, 2019, 10:57:52 PM »

I did miss that it's primary to secondary isolation.  But that is what I'm most concerned with when I build....  Not blowing every MOV in my house.

As I've said, I've built with that xformer.  I know a couple other people using either that one or their beefier ones.

Sure, it's anecdotal evidence, but it's what we have for now.

--Shane
KD6VXI
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