4A on a 3A 5V Winding?

[David Root]

Hammond 290DX, would like to use two 5Y3GT instead of one 5U4GB.

Is this too much over rated current? Otherwise I need to buy a 5V 2A transformer and wire it in.

[Cliff Schecht]

I think what you'll find is that the power transformer will start to cook pretty quickly and may sag more than you expect. It will also spew out more noise than it would if normally loaded. The tube heaters may draw more than 2A as well, not even factoring in the initial inrush current. You can try it though, it might work fine or might not.

[selloutrr]

My personal preference to add the 5V heater transformer and use it run both Rec. tubes, and not tax the PT at all. I figure If i'm using it for one i might as well use it for both.

[David Root]

Thanx guys. I'll be running 4x6V6 in cathode bias, roughly 76mA each pair quiescent, this is basically a 5E3 w/an extra pair of 6V6. The 6V3 winding is 4A rated, will be handling 2.4A. I doubt the HT at max outputwill be more than 200mA as the amp is biased in high AB1. HT is rated 230mA.

I also want to be able to pull the 6V6 and put in two 6L6, and change the rectifier to 5AR4. So I need to run the 5Y3s in true parallel so I can pull both 5Y3s and put in one 5AR4.

Didn't think about inrush current, thanx Cliff, I do have a few suitable thermistors I could apply.

[Cliff Schecht]

Honestly you're probably alright, especially with thermistors. It's not hard to wire in a transformer later and you'll at least be able to just use a 5AR4 worst case scenario.

[overtone]

When adding the 5V transformer do you have to pay attention to phase polarity of the two transformers?
It is a question that I have always meant to ask, hope that you don't mind me asking here.

[Cliff Schecht]

Yes. Especially with higher current, lower voltage filament transformers, they spit out the most noise.

[David Root]

Interesting. I have frequently used auxiliary PTs for generating 5 or 12V DC for relays, and never even thought about phasing, and had no problems.

Maybe after rectification and regulation it doesn't matter, but I can see it could be important when dealing with AC output as you have the two windings in parallel, feeding into the same string.

Thanx for pointing that out! Likely I would have discovered it the hard way, but who needs that?

I'll try it first with a thermistor to control inrush current, I know I have two sizes. I forget the calculations though, will have to look that up.