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Bucking transformer?

OK....finally got a 4A 120 to 12V tranny and hooked it up tonight. Wall VAC is 124.

Question is - this configuration brought the VAC to ~110. Running the Magnavox 8802 through it now. Is 110VAC too low???

Checked the filament V and measures 6.04VAC.......very much reduced from the previous 6.84 when plug directly into line.

Seems to sound OK and will check heat in about 45 minutes.

Main reason to do this is to prevent that goo from spilling out of by big maggies - as posted on another thread.

I see others playing in this window of volts.....any disadvantages?

v/r, HH
 
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Hi, Hornhead. As I stated in post #19 I did a power transformer temperature test of a HH Scott 222C integrated amplifier. You may be interested in the results of that test.

It is to be noted that a HH Scott 222C is not the same thing as a Magnavox 8802. But, my tests showed there was more to be gained in lowering the exterior temperature of a 222C's power transformer by improving ventilation.

There ARE reasons to lower running voltage, as well. There have been many discussions about line voltage today vs. line voltage when this equipment was designed and built. But, based solely on my aforementioned tests, I believe voltage and exterior PT temperature are two separate issues.

Checking your filament voltage was a good thing to do- I only checked B+ and bias supply in my tests. In your situation, I think I would like to split the difference between 6 and 6.8 volts. Maybe you should try the Radio Shack (or equivilent) transformer with the 6.3v tap, rather than using a 12v tap. The latter seems that it's bringing your filament voltage lower than what I would consider ideal.

I have read many threads on many forums about guitarists attempting to recreate a certain amplifier tone that Eddie Van Halen used on early Van Halen albums. There are experimenters using Variacs to drop line voltages, they say, to around 90vac. There is opposition to this method- that by reducing filament voltages, cathode stripping of the output tubes will occur over time.

I have never actually tried this. These guitarists state that the drop in line voltage improves what is called the VH "brown sound". In short, this is a certain 'warm' distortion that some guitarists are fond of. To capitalize on that there is at least one "boutique" guitar amplifier manufacturer that claims they can produce this sound by dropping B+ voltage, so they include a switch on the front panel of the amplifier. But it is important to note that, despite the, roughly, 100v drop in B+ with the front panel selector switch, they are maintaining both filament and bias voltages.

My point is that, tube life aside, too much of a drop in all voltages may have the effect of lowering power output and headroom, and/or raising distortion levels. With may be fine for a guitar sound but less than ideal for home audio.

So, (back to home audio) I decided, based on my tests (see link in post #19) and many discussions on AK (and guitar forums) that that I would lower line voltage to bring various circuit voltages of my 222c to those stated in the schematic. AND, I would improve ventilation to put my mind at ease of just how hot should a 45 year old power transformer be running.

I will be interested in how altering line voltage affects running temperature of your 8802, Hornhead, and look forward to your results!

Brian
 
Hey guys, volts times amperes does not equal watts when you are dealing with AC. Don't forget the power factor. So if you are drawing 120 Watts from a 120 Volt line with a piece of electronics gear you might have as much as 1.3 Amperes flowing.

Compounding the issue is the fact that the current won't be sinusoidal. That will result in voltage drop from the line that is also nonsinusoidal, and thus the voltage to the equipment won't be sinusoidal.

The best solution is to use an ammeter.
 
Bob91343 said:
Hey guys, volts times amperes does not equal watts when you are dealing with AC. Don't forget the power factor. So if you are drawing 120 Watts from a 120 Volt line with a piece of electronics gear you might have as much as 1.3 Amperes flowing.

Compounding the issue is the fact that the current won't be sinusoidal. That will result in voltage drop from the line that is also nonsinusoidal, and thus the voltage to the equipment won't be sinusoidal.

The best solution is to use an ammeter.
Click to expand...

Thanks for the correction...

Anyway my 2A seems to be fine as the bucking tranny doesn't get hot. And I guess that if the amp was drawing more than 2A, it's fuse should blow.
 
Can you do the same thing on 120 volt line useing a 240 volt to 24 volt 40va transformer? I got some stancor T40-24F2 trannys sitting here and am thinking of useing them.
 
COol, thanks I am also getting a few old ups units with good trannys in them and will see about useing them for this and also mybe build some isolation/ bucking units all in one case, like three trannys in one as all of them have 120 primarys and 12 volt secondarys at about 5 amps each.
 
Good thread. I've known about bucking autoformers for a while now, and even posted some tips on building them (never having made one myself). This thread motivated me to finally cobble one together.

I used an old dead Belkin 550VA computer UPS. It had a nice big fat transformer sitting in the middle of a nice safe plastic box. For added convenience, there are six outlets on the top and a 15 amp circuit breaker too. After gutting out the rest (UPS circuitry, lead acid battery, etc) I connected the transformer's primary to my 126VAC line. I measured 16.3VAC on the heavy gauge, center tapped secondary winding. The DC resistance of the secondary was very low (<<1 ohm) and the transformer was physically large(ish) which suggested it could handle considerable current.

I wired the transformer's primary and secondary together in series. Line goes across the two series'd windings. The neutral of the outlets on the top of the unit is tied to the line neutral, and the hot I wired to the center tap of the secondary. At first try the autoformer was boosting instead of bucking, so I reversed the polarity of the secondary. Now my 126VAC line comes out at a nearly perfect 118VAC.

Wish I had done this a long time ago. I bet my Mark III are going to be a lot happier with the gentler voltages.

edit: Here's some before & after schematic diagrams, if they are useful...

 
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Blast said:
Hi, Hornhead. As I stated in post #19 I did a power transformer temperature test of a HH Scott 222C integrated amplifier. You may be interested in the results of that test. I will be interested in how altering line voltage affects running temperature of your 8802, Hornhead, and look forward to your results! Brian
Click to expand...

Brian, Read your test results when you posted them....and had been doing same with fans for some time. Was just trying to get away from the fans.

Ran the 8802 at about 110 VAC for about 4 hours and it was mildly warm - not hot. Pleasantly surprised.

Without any bucking transformer I can keep that amp down to luke warm with a fan, but wanted to go natural.

After the bucking xformer I think we're there. But perhaps a 12v fan will keep it ultra-cool.

Will do a test to see what the temp is with the new xformer at 110VAC. And will check the other high voltages to see if the change made a change favorable to the life of the power tubes.

Fun stuff. Just trying to get max life out of these amps. Not trying to change tone, etc.
 
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Well a APC UPS 280 converted to a bucker works like a charm 121.5 line in and i get 112.5 out at 3 amps and it has a power switch too.
Next one will be a two in one unit, it will take three ups units to make though, a isolation/ bucker unit for testing hot chassis units and small table radios.:thmbsp:
 
Any transformer that has multiple windings can be wired for boost or buck. You need to know the "Dotting" of the transformer, that is whether wiring windings in series will boost or buck. You also need to know the transformer winding insulation rating. You don't want to put 120V on a winding rated for 60V.

Armed with this knowledge you can dive in.
 
So true, i always test the output befor plugging in an amp to one, these trannys are good sized in the old ups units and i just retested it with a load and it holds at 114.5 Volts. with 121.5 in, i found one meter i got reads a little low compared to the other four.
 
probly not old puter parts, they are to low on the amps, I would use min 2 amp on the secondary side of a tranny.
 
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