What is the meaning of max dc ma ratings on single ended output transformers?

[Austin]

I bought two 3k 10w output transformers last week with a 100ma max dc rating, but at how many volts? I have connected them to one 6L6 each at 200 volts with 2k screen resistors and -9v bias and they sound very nice and with deep bass but I plan to upgrade the power transformer to a bigger and better one that I already have that is rated 280 volts CT @ 265ma and run 2 6L6 per opt at about 50ma each in parallel. Obviously 100ma at 200 volts is 20 watts and 100ma at 300 volts is 30 watts but how do know when to start backing off on the current or voltage? When the opt starts getting warm?

here are transformers like I bought, Pair 3k Transcendar Audio Output Transformers for SE amp 8 ohm 10 watt | eBay And they also have a website they sell other transformers too. Home - Transcendar Audio Transformers

Thanks

[Enzo]

If you have tubes conducting 100ma through a transformer with 200v on the tube plates (and assuming grounded cathodes)then you have 20 watts. But that is 20 watts of plate dissipation at idle. That is not 20 watts of audio anything.

100ma transformer rating means they don;t want you running more than 100ma through it. Your example would be running right at the limit for that part.

The voltage rating for a transformer would be how much voltage to ground the insulation on the winding wire can sustain without arcing over.

[Austin]

If you have tubes conducting 100ma through a transformer with 200v on the tube plates (and assuming grounded cathodes)then you have 20 watts. But that is 20 watts of plate dissipation at idle. That is not 20 watts of audio anything.

100ma transformer rating means they don;t want you running more than 100ma through it. Your example would be running right at the limit for that part.

The voltage rating for a transformer would be how much voltage to ground the insulation on the winding wire can sustain without arcing over.

So are you saying the specs of the transformer frequency response will be the same so long as the current doesn't exceed 100ma no matter if the primary has to endure 20 watts or 60 watts?

The specs say "Each transformer has the insulation Hipot tested to 3000 Volts DC"

[exclamationmark]

Single ended transformers are built with a fairly large air gap in them to step them saturating from the DC offset they experience. The 100mA current rating refers to the fact, that at idle, you should not have more than 100mA running through them or the core will saturate and do some unintended things to the signal. The voltage is largely irrelevant.

[Austin]

What about an 813 tube at 800 volts on the plate at 100 ma then? Why not right? They cost about the same as a 6l6.

[Steve Conner]

The transformer has a volt-second rating as well as a DC idle current rating. If you apply too much signal voltage at too low a frequency, it will saturate and cause distortion.

You can estimate the volt-second rating from the low frequency limit given in the datasheet, and the B+ voltage the transformer would normally be used with.

The onset of distortion will be quite gradual because of the air gap, and it'll be mainly low-order. So you can probably overdrive the transformer quite a lot in a musical instrument application.

[Austin]

It makes sense that the output transformers will get hot with too many watts power idle in the primary. Although they are not even getting warm now, I was curious how far I could push them. It sounds like there is no easy way to know without asking the makers directly or rasing the voltage in gradual increments and monitoring the temperature right?

In my ears it sounds very nice even with zero negative feedback but I have only been listening to mp3s at reasonable volume, however it can get surprisingly loud even with the lowish volt power supply I am using and 87db Epi m100 acoustic suspension speakers.

[Enzo]

Measure the DC resistance of the primary. If you know the current through it or the voltage dropped across it, you can use basic Ohm's Law formulae to calculate the power dissipated in the winding DC. That is where any heating will come from. What the tube dissipates will not likely be the same as what the transformer dissipates.

[Dave H]

It makes sense that the output transformers will get hot with too many watts power idle in the primary. Although they are not even getting warm now,

It won't get warm at idle. Don't confuse the idle dissipation of the power tubes with the idle dissipation of the OT. In your ebay link it says the transformer resistance is 300 ohms so it will only drop 30V at 100mA. With a 300V supply there's 30V across the transformer but 270V across the tube. At 100mA that's 3W power in the transformer and 27W in the tube.

[loudthud]

Although they are not even getting warm now, I was curious how far I could push them. It sounds like there is no easy way to know without asking the makers directly or rasing the voltage in gradual increments and monitoring the temperature right?

You should do a quick calculation of the peak voltage required to produce 10W (or whaterver power you want) at the primary impedance. Add about 100V to account for voltage across the tube and DC primary loss and that should be your B+.

[mooreamps]

What about an 813 tube at 800 volts on the plate at 100 ma then? Why not right? They cost about the same as a 6l6.

Perhaps...but the filaments for an 813 are a little thirsty for power ; 10 volts at 5 amps....

-g

[Bruce / Mission Amps]

On a curious note, are you using this to play prerecorded music through or are you building a SE Class A guitar amp.
The specs look very good. 20Hz to 90Khz +- 1dB!!
I bet you could get an honest 10-12 watts out of the OT with a properly biased 6L6GC at higher voltages then the wimpy 200vdc you are discussing.

[Austin]

On a curious note, are you using this to play prerecorded music through or are you building a SE Class A guitar amp.
The specs look very good. 20Hz to 90Khz +- 1dB!!
I bet you could get an honest 10-12 watts out of the OT with a properly biased 6L6GC at higher voltages then the wimpy 200vdc you are discussing.

\

Yes prerecorded music, listening to it now. I have an older stancor power supply to try as an upgrade but I still need to enlarge the hole with the grinder to get it to fit.. The new supply is 260-0-260@280ma, so I guess it could get probably close to 600 volts with a full bridge but since I have 3k:8 impedance opt and my speakers are only 4 ohm I think it might be best to drill two more holes and do parallel output tubes at 300v. Not even remotely close to hifi I'm sure, but to me it sounds great so far. Chassis and power transformer are an old Bogen Chb 30 mono p.a. head. This smaller power transformer gets pretty warm after a few hours.