Good projects for undersized power transformer

[Pixel]

Hi All,

I have a power transformer that is marked as having primary 100/115, secondaries 250@40mA, 5V@.4A, 6.3V@1A. The only centre tap is on the 6.3V.

I would appreciate a couple of ideas on what project this transformer would be suited for.

Thanks!

[Tom Phillips]

Sounds like a candidate for a classic Champ amp (Single 6V6 output tube) or a stand alone reverb unit. I suspect that "5V@.4A) is a typo since 0.4A is too low to operate any common rectifier tube heater. You could use SS rectifier diodes.

[eschertron]

Hi Pixel! Welcome to MEF.
After seeing a couple youtube videos on micro amps with a 12AT7 used as the power tube, I built one myself.* Through a 2x12 cab it's louder than I would have expected! If you want something with novelty value, or 'crank up' and not bother people in the next room, that would fit the bill. Another option (that I haven't tried) would be a tube mic pre, if that floats your boat.

*a switch on the cathode resistor allows me to put a 12AU7 in place of the 12AT7 for a little more clean headroom, if desired.

[dannythetrucker]

You could possibly do a Champ 5F1, although you'd be going over the 40mA rating a fair amount.

You could do this micro-champ project with ease ! MICRO-CHAMP

EDIT: I didn't notice the 0.4 amp rating on the 5V, that could be an issue. You certainly can't run a rectifier tube that I know of with that.

[R.G.]

One of my favorite amps ever is a single-ended 6AQ5 providing about 2W-4W. The overdrive on these is simply beautiful, if not very loud. It's great for bedrooms as is, and can easily be miked up, iso-boxed, etc. to be as thunderous as need.

Your transformer might be about perfect for 1-2 12AX7s and a single ended 6AQ5.

[Pixel]

Sounds like a candidate for a classic Champ amp (Single 6V6 output tube) or a stand alone reverb unit. I suspect that "5V@.4A) is a typo since 0.4A is too low to operate any common rectifier tube heater. You could use SS rectifier diodes.

I just tested this transformer in the hope that there was a typo. Sadly the .4A rating on the 5V is bang on or maybe even slightly optimistic.

It was nice that this "found" transformer actually had specs printed on it at the factory. But I should probably do more testing on it to see whether it actually performs as advertised...

[Leo_Gnardo]

I just tested this transformer in the hope that there was a typo. Sadly the .4A rating on the 5V is bang on or maybe even slightly optimistic.

Sounds like your 5V winding is prime to run a pilot, 47 or similar lamp, just a little dim and therefore will last near forever.

[eschertron]

5v @ 400ma would be plenty to run some relays 'n' stuff. Of course, this amp may not need that level of complexity

[Enzo]

Look at the wire on the 5v winding. Is it heavier than the 6v winding? That might indicate a typo for where 4A was intended.

[Tom Phillips]

Look at the wire on the 5v winding. Is it heavier than the 6v winding? That might indicate a typo for where 4A was intended.

Good idea.
We are assuming that the transformer is reasonably old especially since it lists a 100/115V primary. Unless it was made fro a really oddball application, a 5V / 0.4A secondary just doesn't make much sense.

[dannythetrucker]

Your post made me curious, and Google pointed me towards the 6X4 rectifier. Is anyone familiar with it ? It's spec'd for 600mA @ 6.3V, I wonder what would happen if you gave it 5V ? Some guy in a hi-fi forum claims that he uses them in all his pre-amps.

[Enzo]

Then we are back to determining if that 5v winding is really .4A (400ma) or really 4A. Clearly 600ma heater draw is more than 400ma. That precludes the 6X4 in my mind.


I don't know who your guy might be, but be aware you can find some guy on the internet to say most anything. I make speaker cones from Yeti hair myself. I was in the mountains when I found a dead one. I saw Himalayan there, and figured he didn't need his fur anymore...

[Tom Phillips]

The 6X4 was used in equipment with relatively low power needs. The tube is a small miniature 7 pin glass tube that did not require a separate 5V heater supply winding. Keep in mind that it came from a time when tube rectifiers were the only practical choice that a designer had for many applications. I don't subscribe to the belief that the 6X4 had any special mojo. When given the choice I use solid state rectifiers. It is possible to simulate the voltage drop, voltage sage and quiet performance of a tube rectifier when it is necessary for guitar amp performance reasons. Using SS rectifiers for new designs reduces the heat produced (just the heater uses 10 to 15 watts - more if you are into the dual rectifier thing). In addition, many of the current lines of tube rectifiers have performance and reliability problems.

Attached is a copy of a 6X4 spec sheet.
6X4 Spec sheet.pdf

[dannythetrucker]

Then we are back to determining if that 5v winding is really .4A (400ma) or really 4A. Clearly 600ma heater draw is more than 400ma. That precludes the 6X4 in my mind.


I don't know who your guy might be, but be aware you can find some guy on the internet to say most anything. I make speaker cones from Yeti hair myself. I was in the mountains when I found a dead one. I saw Himalayan there, and figured he didn't need his fur anymore...

Hence why I stated "some guy claims". To be taken with the appropriate grain of salt.

Pixel stated he/she tested the transformer and it is indeed 400mA, not 4 amp.

The resistance of the heater doesn't change, so wouldn't Ohm's law dictate that it would draw 79% (5/6.3) current at 5V that it does at 6.3V ?

[Enzo]

79% of 600ma is still way over 400ma.

I too suggest just using diodes.