Author Topic: Buck/boost transformers  (Read 907 times)

electra225

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Buck/boost transformers
« on: January 12, 2019, 05:06:45 PM »
Have any of you guys had experience with one of these things?
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TC Chris

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Re: Buck/boost transformers
« Reply #1 on: January 12, 2019, 06:58:53 PM »
Fill us in... is that the practice of wiring two transformers so they either increase or partially cancel their output?  I've read about that.

Chris Campbell

electra225

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Re: Buck/boost transformers
« Reply #2 on: January 12, 2019, 08:53:45 PM »
A buck/boost transformer, as I understand, is a device that will increase or decrease line voltage to a device.  In our case, it would allegedly cause the power transformer to run cooler.  Many that I have seen are kinda spendy.  I would like to tinker, but I'm not gonna spend the big money just to tinker.  I have felt in the past that these devices are kluges.  Our line voltage here, now that nobody is running air conditioning, is north of 120 volts some days.  Magnavox power transformers are rated 105 to 125 volts, so this is withing their design perameters.  But, boy, mine get awful hot after about two or three hours of operation.  I will admit that the two big rectifiers sit in close proximity.  If I could lower the input voltage to, say, 115 would that actually lower the operating temp of the transformer, or would it just take longer to get hot?  I reduced the voltage with a Variac in the past, and that is what happened then.  I thought I'd get some input from you guys to see what your experience has been.  A hot transformer does not seem to hurt the stereo any.  It's been running like that for a long time.  We run the Imperial 10 or 12 hours a day, every day.
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TC Chris

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Re: Buck/boost transformers
« Reply #3 on: January 12, 2019, 09:47:52 PM »
Here's a link to a DIY version.
http://www.dms-audio.com/bucking-transformer-diy

Here's another.
http://sound.whsites.net/articles/buck-xfmr.htm

Ther's an ARF thread here.
http://www.antiqueradios.com/forums/viewtopic.php?t=143607

The expense is finding a transformer with the appropriate current capacity. 

I wonder what would happen if you put an aluminum heat shield between the rectifiers and the transformer?

Chris Campbell


danrclem

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Re: Buck/boost transformers
« Reply #4 on: January 12, 2019, 11:37:50 PM »
I've read that inrush current limiters would decrease the line voltage and give a softer start.  They limit the voltage while starting up and keep it a little lower the rest of the time.  I don't know how much they would lower the voltage but it's probably not a whole lot.

electra225

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Re: Buck/boost transformers
« Reply #5 on: January 13, 2019, 09:08:07 AM »
I have considered rigging up a heat shield.  There isn't much room.
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TC Chris

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Re: Buck/boost transformers
« Reply #6 on: January 13, 2019, 11:53:41 AM »
I've seen some cramped quarters that demanded tight fits.  There's an old Harman-Kardon c-300 mono amp with two 5881 output tubes (a 6L6 variant) and two 5Y3 rectifiers squeezed into the rear by angling them to reduce height, and the Heathkit AA-100 with 4 7591 outputs and one 5AR4 rectifier.  Talk about room heaters.....  I ran a small fan on the Heath to keep it cool and avoid cooking its innards.

A shiny heat shield reduces heat radiation to the transformer and can also direct convection air currents up and away.  Might be worth a try.

Do you have one of those infrared thermometers, the little guns that give a digital readout?  My brother gave me an inexpensive one from Harbor Freight or some such, and it's useful for getting data and seeing if changes have any effect.

Chis Campbell

electra225

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Re: Buck/boost transformers
« Reply #7 on: January 13, 2019, 12:34:15 PM »
A shiny shield, you say?  Hmmm.  I could maybe do that.  How about if I get a hunk of aluminum then polish it?  We have a welding shop back home that has the prettiest aluminum I believe I've ever worked with. 
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firedome

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Re: Buck/boost transformers
« Reply #8 on: January 13, 2019, 02:29:14 PM »
Chris - you're smart to run that AA100 with a fan, they are notorious for cooking their PCBs and those 7591s do put out some major heat! I had one some years ago and liked that light bar over the controls. The AA-151s and AA-32s I've had didn't have the heat problem since they are all point to point and used smaller and lower power 6BQ5 and 6GW8 output tubes.
Happy Motoring! from Roger in NY

TC Chris

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Re: Buck/boost transformers
« Reply #9 on: January 13, 2019, 04:01:33 PM »
It was the old compromise between function and design--performance and looks.  The big Magnavox amplifiers at least get to operate without covers, in the open, where convection could help.  Shortly after I built the Heath (long ago), I put a glass-tube alcohol thermometer on top and forgot it for a few minutes.  The alcohol column reached the top and the pressure blew it off.  The alcohol stained the vinyl overlay on the case and in short order it peeled off in that location.  I guess it was hot....  I always worried about the capacitors back there among the output tubes.  One of these days I'll resurrect it and those caps will be replaced.

Chris Campbell

electra225

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Re: Buck/boost transformers
« Reply #10 on: January 13, 2019, 05:17:47 PM »
Magnavox cabinets are notoriously poorly ventilated.  This adds to heat build-up.  I made a new back for the Concert Grand that really helped transformer temp to run lower.  Perhaps a combination of fabricating a heat shield between the rectifiers and the power transformer and improving cabinet ventilation would be an option. 
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19and41

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Re: Buck/boost transformers
« Reply #11 on: January 14, 2019, 09:42:27 AM »
Could you post a picture of the affected chassis in place?  I have some ideas, but i would need to see where the problem lies.
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Motorola Minion

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Re: Buck/boost transformers
« Reply #12 on: January 14, 2019, 01:20:35 PM »
I've read that inrush current limiters would decrease the line voltage and give a softer start.  They limit the voltage while starting up and keep it a little lower the rest of the time.  I don't know how much they would lower the voltage but it's probably not a whole lot.

I agree that both a CL-90-2A (CL-60-5A for a larger console) surge limiter is the best start, especially if your line voltage is 123+ like mine. The hot resistance is low as you say, like 0.25 to 0.5 ohms.

Below, the AMP175 on my Mag 1ST208 gets a break from the shock of a power-up. The CL-90 is 120 ohms cold and makes it warm up a minute slower and the two 2.7 ohm /10 watt resistors drop the line just below 120, but add a bit of heat. Its fun to see how slow the 1847 pilot lights begin to glow when I switch it on.

A 120-6.3V secondary at 3 amps filament transformer - is better than my resistors - costs about $15 via Allied
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Dave

electra225

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Re: Buck/boost transformers
« Reply #13 on: January 14, 2019, 06:27:40 PM »
Rex, here is a picture of the amp chassis installed in the cabinet in my Imperial.  The MPX goes vertically on the right side of the cabinet, behind where the external speaker wires are dangling.  The floor in this enclosure is solid.  There are holes in the back cover, but not many and they don't provide much ventilation.
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Motorola Minion

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Re: Buck/boost transformers
« Reply #14 on: January 15, 2019, 09:37:54 AM »
I the 5U4 tubes were removed and 1N4007 used instead, you could save 30 watts on each chassis.

I can see the challenge getting convection heat removal. A fan would need to have a filter but if you run this like I think you do, its the best solution.
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Dave