PT for 7A heater current, reliably power 7.2A?

[SolidStateCircuitReject]

Hi Folks.

My thinking tells me it is dangerous enough to run a tranny so close to it's rated limit without adding an additional 12ax7... but for a JCM800 hybrid-style amp, the PT is a hammond 290HX, rated for 7A. Typical JCM800 with 4xEL34 and 3x12ax7 is already cutting it super close since the tubes need 6.9A!!! So is throwing an extra 12ax7 in there going to really stress it out or is there a little padding here when it comes to the ratings?

Thanks much!!

SSCR

[sluckey]

I'd try it.

[PRR]

1) The added heat is about 0.18 Watts (6V at 10% loss is 0.6V, 0.3A more current, 0.18 Watts). A 100+W PT already has 5W-10W of waste heat on normal load. Another 0.2W is not going to kill it quick. It may reduce calculated life from 5,000 Hours to 4,900 Hours.

2) Hammonds are very conservative. You can flog them worse than this. They use very old designs with much better modern materials and are tougher than ever.

[SolidStateCircuitReject]

I'd try it.

Oh, I tried it before I posted  but wanted to know if it would be stable!

1) The added heat is about 0.18 Watts (6V at 10% loss is 0.6V, 0.3A more current, 0.18 Watts). A 100+W PT already has 5W-10W of waste heat on normal load. Another 0.2W is not going to kill it quick. It may reduce calculated life from 5,000 Hours to 4,900 Hours.

2) Hammonds are very conservative. You can flog them worse than this. They use very old designs with much better modern materials and are tougher than ever.

Just what I was hoping to hear - thank you!!!

However, when you say "flogging" - how far can one be pushed... say another 12ax7 for a tube effects loop or addition of a second channel?

[DummyLoad]

measure the secondary load with specified input voltage to the primary. if your load drops that secondary voltage below it's rating, (6.3V in this case) then you are over stressing the transformer. i'd run it at 7.5A without concern.


--pete