trade off higher HT current for lower filament current ?

[Bigun]

I want to pull a bit more current out of the HT secondary than the rated amount but I will be under-loading the filament windings.

How safe is it to do this based on the idea of keeping within the total rated power for the trafo ?

Specifically: I have the Hammond 270FX and I want to boost my B+ by putting a capacitor in front of the power supply choke - Duncan Amps simulation says that the rms current draw through the HT winding will be around 200mA whereas spec. is 173mA.

But I have a 5V 3A winding feeding only 1 rectifier and a 6.3V 5A winding which will feed 1x6SN7 and 2x6B4G for a total of 2.6A.

Advice ?

(should I bite the bullet and get another higher voltage trafo that I can use choke loaded ?)

[Sch3mat1c]

Aren't Hammonds rated in terms of DC output, anyway?

They're designed to run cool, so it's no big deal. I've got a 250V, 50mA winding ("125-0-125 100mA") running a 6L6 SE, it runs on the warm side but it's clearly not going to do anything for the next few decades.

Tim

[bob91343]

I agree; you aren't that much above specification. The wire size in the high voltage winding will be the limiting factor. If you can see the winding and estimate the gauge you can assume worst case 300-400 circular mils per Ampere. A good transformer is more like 600 or more so the safety factor is good enough.

The primary of course will run cooler than specification due to the total power being less than design.

[Bigun]

Thanks !!!

[tubelab.com]

They're designed to run cool, so it's no big deal.

A Hammond 200 series run cool? Who are you kidding? All of the 200 series Hammonds get hot. Can you violate the specs a bit and let them get hotter? Well I have been using the Allied house branded transformers which are made by Hammond. One of my Simple SE's uses a 6K7VG which is rated at 150 mA DC. It feeds 2 X 12AT7's at 10 mA each and 2 X KT88 at 75 to 100 Ma each for a total of 170 to 220 mA. Does it get hot? YES. Does it blow up? Not yet. It has been cranking along for almost 4 years now.

The only Hammond that I have seen fry was one that got wet during a hurricane, and it still worked for almost 2 months after I dried it out. In fact the amp was still playing when the stink coming out of the transformer convinced me that something was wrong.

[Poindexter]

Whut he said. I have operated Hammond 2xx series up to about 70% of rating, and they get hot enough you don't want to touch them. This Otemp also will cause them to start humming mechanically down the road. If you're operating this piece anywhere near 100% rating, you got too small a transformer.

Get a bigger rating, and get the 3xx series.

Aloha,

Poinz

[Bigun]

Sounds like this is a case where I can stay on the small side but likely end up with a hot trafo which will probably be OK but it's not exactly an example of good design practice and I'll need to use a cap loaded supply too. The bigger trafo sounds like the preferred option, that way I can stick with a choke loaded supply too. The Hammond 373X and 376X look to be the best match.

Thanks again.

[Michael Koster]

http://www.hammondmfg.com/pdf/5c007.pdf

Hammond has a design guide that describes how to use their so-called "AC current" rating with different rectifier and filter topologies. It turns out that the current multiplier for the FWCT cap input is 1.0; that is the nameplate rating multiplied by 1.0 in this case.

Note that the output voltages in the guide may not all be correct but the current ratings seem to be correct.

[tubelab.com]

Sounds like this is a case where I can stay on the small side but likely end up with a hot trafo which will probably be OK but it's not exactly an example of good design practice......Hammond has a design guide

There is an old "measuring stick" that says that you should be able to hold your hand on the lams of a power transformer that has reached thermal equilibrium for an indefinite amount of time without the need to use bad language. That is safe for old power transformers.

I have modified that yard stick for modern Hammonds. They have used higher temp materials, but reduced the iron and copper content to make a lower cost transformer. The result is that they get hot. I have been pushing the limits with the transformer in my industrial Simple SE. That amp was built on a chassis that is really too small, and I didn't intend to run KT88's at 100 mA when I built it, but it sounds nice with them, so I decided to let it go until it blows. That was almost 4 years ago. The PC board is discolored, but the amp runs great. This transformer is a rule of 3 transformer. After it has been on for a few hours, you will yank your hand away after about 3 seconds! 5 or 10 is probably OK for a Hammond.

I built a Tubelab SE for use with 45 tubes on a tiny Lexan chassis. It uses an Allied 6K56VG (Hammond) which is rated for 125 mA. It has about a 110 mA load when used with 45's and gets Hammond kind of hot. I have used 300B's in this amp which really irritates the power transformer, but it hasn't fried yet.