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I've been reading in the archives about bucking transformers, and want to try one for a pair of Dynaco MkIII amps and a PAS I just took out of storage. They seem simple enough to configure, at least the example below does.
I've got a filament transformer I want to use, but it's not quite the two primary, two secondary wire variety shown above. The specs on the secondary are: Series: 12.6V ct@6.3A, Parallel: 6.3V@12.6A Here's the diagram:
When the primary is wired via the parallel 115 v, I measure 7 VAC between output taps 7 & 8 and 11 & 12, the parallel connections. Good so far. But what is throwing me is Note #2 from the spec sheet:
I asked one of our electrical techs at work today, and he seemed confused by note #2 as well. He drew up what he thought was the correct connection using both secondaries, but I'd like a few more opinions
My question is: How should this be wired to buck 6.3 V @ 12.6A?
Link to complete spec sheet is below.
Thanks.
Follow Ups:
Hi Shovel,I read through this thread, and want to suggest ONE other possibility that I like for several reasons. It eliminates two of the "less than stellar" design choices being discussed in this thread: (1) NO buck or boost winding to degrade the PT's inherent regulation and (2) NO ultra whimpy 5R4 rectifier.
Add a Traid C-56U (as an INPUT filter, fed DIRECTLY by the rectifier tube) onto the back of the chassis, vertically mounted, and PSUD what follows after that, to get it right.
I would also use a directly heated rectifier tube, but NOT the "ultra drop under load" ultra-whimpiest 5R4. How about a 5U4 ??? Maybe a 5V3A.
The C-56U is 2 A. rated, and .79 Ohm in DCR. It should reduce B+ by at least 10% compared to a cap input filter, and maybe as much as 15% less than a cap input, just about right for a more conservative VDC level, with NO whimpy / kluged design parts added.
L2 in my simulation is just a small sized EDCOR 1 Hy 21 Ohm DCR open frame choke, didn't try the Dynaco stock unit, because it is well over 20 Ohms, 60 Ohms DCR as as I recall.
Then you have very nice sounding L1/C1/L2/C2 B+ filters to the Finals, a near ideal and doable design. 482 VDC B+ with 5V3A. Parts cost is quite reasonable.
Cheers,
Jeff Medwin
Edits: 05/22/12 05/22/12
How's that C-56U going to fit into an already crowded Dyna MK3 chassis? Never mind the ongoing controversy about non critical choke I/P filtration.
I suggested the 5R4 because it, like the 5AR4, draws 2 A. of 5 VAC filament current. A 5U4 draws 3 A. and I was uncertain about the power trafo's ability to provide 3 A. It seems the trafo can provide the requisite 3 A. and, specifically, the 5U4GB is a reasonable option. Of all 5U4 variants, only the 'GB is rated for 250 mA. of B+ current, like the 5AR4 and 5R4 are.
If the OP sets up the 1st position capacitance with 5R4 limits in mind, he can try both the 5R4 and the 5U4GB to see which of the 2 yields the best result.
Eli D.
My idea was to mount it on a 1/8th inch AL plate, with the plate attached behind the power trannie, vertically, on the rear of the MK 3 chassis.Eli, you must not forget, as soon as you put ANY decent choke in the position of L1, be it L Crit. or Non L-Crit., it is usually superior to a C1 input filter, in low end sock, PLUS, it sets up the amp for a BEST possible B+ filter configuration, which I have always sonically found to be L1/C1/L2/C2, PLUS it eliminates a rectifier that is whimpy - a 5R4, worse dynamic Z of all the choices we have discussed.
Cheers,
Jeff
Edits: 05/24/12
I have bookmarked your post. If I decide to take the MkIII 's to another level, I will refer to it, as it offers some interesting suggestions.
The point I was attempting to make was that I wanted to restore function to the Mk III amps, then either use them occasionally, or sell them. Adding to them externally, if I'm going to keep them, won't be an issue. But if I'm going to sell them, it will be. I'm rebuilding them with a NTC CL-90 in the primary, and will use a variac to keep the voltage down, for now. The buck idea was to have a "voltage dropper in a box" for any vintage equipment I might be trying. Much easier than a variac, since I have mine built inside a capacitor reforming unit.
I never thought much of the Dynaco MkIII sound when I used them with my Quad ESL-57 's (a ST-70 sounded better), but it will be interesting to see if I like them any better with my single driver Horn Shoppe speakers.
Oh, and you were close with the Dynaco C-354 choke. I just checked one of mine and it measures 63.7 ohms.
Your reasoning sounds fine to me. I think they sound best as a Paoli 60M variant, with a totally separate G2 supply, which I have engineered already on paper, and not built.
I am not so sure the 50-60 watt rated MK3 will be a good match on the efficient speaker load you have. Maybe a real good SE amp would kick butt.
Used eBay prices on MK3s are good now, sell em when the public is relaxed, before the next stock market CRASH, which is VERY CLOSE to being fully set up IMHO.
Jeff Medwin
n.t.
Read Eli's note - you need to know what the voltage is under the intended load before you can make any other decisions.
That trafo has a 25% regulation spec - you'll get significant voltage drop when it's loaded!!
I was hoping for a healthy 6 V drop for the over-voltage MkIII's. I'll follow Eli's advice.
Thanks Jim.
Primary in parallel for 115Vac.
If you wire the secondary in parallel you get half the voltage and twice the current rating vs. a series connection.
What is your desired "buck" goal?
Since your wiring up all four coils, note 2 is of no concern.
Do you have a variac to bench test it?
I do have a variac. I'll try wiring it up per your instructions, and go from there.
Thanks Wheeze!
FYI I always have an inline fuse after the variac - just in case.
Wire the trafo with both primaries in parallel and both secondaries in parallel, per Triad's diagram. Load the setup down with a power resistor that draws close to the rated current. Then, you can measure the voltage. You are ignoring regulation effects!
Eli D.
Thanks Eli, that's clear enough. This may be "too much" transformer for my intended application, from what has been said here. If I remember correctly, I read you weren't a big fan of bucking, is that correct?
Both bucking and boosting are fine, in an appropriate place. For the record, a Triad VPS24-1800 is the current recommendation for "12"
V. heater power and B+ boost, in scratch built "El Cheapo". ;> )
If you use a secondary winding of the Triad trafo you have to buck the other secondary down, you can access only 1/2 the current of the 2 secondaries in parallel.
If that trafo (with the secondaries in parallel) comes in too "tall" install a bleeder resistor across the composite to get the total draw right. Yes, you generate waste heat, but the tube heaters will not fail prematurely.
Eli D.
That was my thought as well, along with the over-spec B+ the Dynaco MkIII 's are noted for. I'm installing SDS cap boards with two 450V caps in series on the first leg, so an over voltage condition won't be an issue. At least not on the cap bank, anyway.
The amps haven't seen the light of day for about twenty years, so I want to get them working, and go from there. I don't expect much, which is exactly why I put them away 20 years ago. But still, they should be used once in a while, or sold.
Thanks Eli.
It's easy enough to bring the B+ rail voltage down. Replace the 5AR4 with a "potato masher" 5R4. The 1st filter cap. can't exceed 20 μF. So, replace Dyna's crappy choke with something better, like a Hammond 159T, and pile the capacitance up after the choke.
At a 250 mA. B+ draw, the forward drop in a 5R4 is a whopping 67 V. ;> )
Eli D.
Perhaps somewhat oddly enough, finding those low µF values are becoming an issue.
As I mentioned, I ordered a couple of SDS cap boards for the MkIII's, which run pairs of caps in series to cover the high voltages. I found it difficult to locate low µF caps to use, even though values are halved. The lowest I could find in the Nichicon KX were 56 µF @ 450V.
I could use a 20µF oiler, but the MkIII chassis doesn't leave much room for big caps, chokes, etc.
The bottom line is that there is a lot I could do with these, but I've read a many Asylum posts regarding these amps. Unless I'm willing to move the output transformers to a larger chassis, add a new input circuit, and completely re-do the power supply, it seems pointless.
As one poster opined some years ago: "I've done about every mod there is to the MkIII's. Sometimes I think I liked the sound of them better when they were stock."
Unless they overwhelm me when I get them running, I'll either use them occasionally, or sell them.
Thanks for the ideas, Eli. They are always appreciated!
You didn't dig hard enough. ;> ) Mouser stock # 598-81LX330M450H012 is a 33 μF./450 WVDC snap in part that happens to be 105 o C. rated too. :> D Two of those in series will be just fine for the 1st cap. position, when fed by a "potato masher" 5R4.BTW, if you add a NTC inrush current limiter between the rectifier and the 1st filter cap. position, you rate to be safe with 2X 47 μF. parts in series. Start up arcing has to be prevented. Once that's been accomplished, a few extra 1st position μF. are just fine.
edit: added NTC remark
Eli D.
Edits: 05/21/12
I had a feeling you might find caps I could use, Eli, thanks! Technically they are too tall to use with the SDS board, but I doubt 0.4mm will be an issue. I'll know more when I assemble the boards.
I did some 5R4 reading last night. The first question is the first filter cap. The data sheet and many posters say 4 µF is the limit. I linked to one of your posts where you said 20 µF with "real" potato mashers. Jim McShane lists more than a few 5R4's. Which one's "real" and safe to run with the extra 16 µF?
I've already got CL-90 's ready to go on the primary. I'll look at adding one to the secondary, if I go the 5R4 route. I did that to the secondary of my SCA-35 (as well as one on the primary) per your recommendation, and it worked great.
The RCA 5R4GYB data sheet shows 20 μF. as the max. sized cap. allowed. So, that variant is definitely safe. IIRC, AA sponsor Jim McShane has some RCA 5R4GYBs.
The "potato masher" gets its moniker because it slightly resembles a WW2 German hand grenade. The actual types are 5R4WGA and 5R4WGB made by Chatham and Cetron. The variant was developed for use in (sic) B52 bomber avionics systems. :> D Oddly enough, the data sheets I looked at show 4 μF. for the "masher". However, I've never heard of the "masher" arcing into 20 uF., at the voltages we routinely work at. It's a very tough hombre.
You can look at a photo of the "potato masher" by using the link that follows immediately.
http://tctubes.com/images/products/detail/36_5R4WGB_800.1.jpg
Eli D.
I did some more homework last night. I read that you ran them with 20µF, and at least one guy used 30µF, for five years with no issues.
I will get these MkIII amps up and running, and then decide exactly which route to take.
Thanks for all the food for thought!
Number 2 means use all windings for full current capacity.
Ok, thanks Neff. So would I connect #8 and #12 to the primary, while #7 and #11 in parallel would be one leg of the output? I do understand that it might boost, in which case I would reverse the connections to the primary.
Do I have that right?
.
1&5 will be one leg of the primary and 2&6 will be the other leg of the primary. Same for 7&11 and 8&12 for the secondary.
From there, follow the Hammond setup.
If the voltage goes down, you've bucked the tranny.
If the voltage goes up, you've boosted it. Flip the secondary leg to primary connection and the voltage will go down.
How about two thermistors in series on the primary side? I saw in Quad II forty thermistor on HF straight to 5u4GT rectifier (this one was added for saving chinese recifier tube I guess)
Putting two in series is the same as two resistors in series. x2$
Yes to answer your question.
Most put them on the primary or secondary or both - one leg or both legs.
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