Help figuring current capacity

[jon]

I have a pt from a fisher integrated amp. Wondering what the current capacity of the transformer is?

[Firestorm]

Enough for four 7591s plus preamps. You were expecting numbers?

[jon]

Duh!!!


I'm looking for a rough estimate.

[martin manning]

Can you get the DC resistance and unloaded VAC across the HT leads? Separate the leads with strips of tape and measure voltage from CT to each end to minimize the risk of something bad happening.

[cbass]

I like old no name amps and they use 7591's a lot.Most of them have been rated around 125mA for a pair.Thats guitar amps though.

THere is a chart on here somwhere where you can figure VA based on the Ohms of the secondaries

[martin manning]

That's where I was headed, CB. You need the two things above to use that method.

[jon]

I had assumed the knowing the plate voltage, screen voltage and cathode voltage for the four power tubes one could make an educated guess on the current capacity.

[martin manning]

Well, you know what they say...

;^)

[cbass]

That's where I was headed, CB. You need the two things above to use that method.

Sorry Martin didn't mean to but in .BY all means listen to Martin.He knows

[martin manning]

No worries; I should have explained myself better!

[jon]

Transformer #1
181.3VAC leg to leg (the HV has no CT)
5.3 ohms

Transformer #2
79.0 Ohms
197VAC leg to leg (the HV has no CT)

[martin manning]

Two transformers? And, they are very different... what's up with that?

Per the Duguid method:

Tx #1 at 5.3 ohms and 181 VAC would be 380 mA, but Duguid's curve is for a FW rectifier. Hammond uses a factor of 0.62 for IDC FWB/FW, so I'd adjust that to 240 mA for a FWB. If it's used with a FW doubler like in the circuit above, I'd cut that in half (120 mA), but then that doesn't sound like its enough for the tubes that are in it.

Tx #2 at 79 ohms an 197 VAC would be 80 mA as a FW, 49 mA as a FWB. Is the 79 ohms correct? 7.9 ohms would make it 200mA as a FWB.

Note I'm using IDC(FW) = 0.046*(DCR/VAC)^0.6, which reproduces the capacitor input curve in the article.

[jon]

TX one was the power transformer for the integrated amp. As it was a 25-30 W per channel amp I think the numbers are a little off.

TX two powered the FM tuner.

Bold transformers were used in a voltage doubling circuit.

[martin manning]

I wonder if your measurements could be a little off on the PT- short your meter probes together on the "ohms" setting and see what you get. It is pretty tough to measure single-digit ohms without specialized equipment. I think the 49 mA for Tx2 with a FWB is probably pretty good. That one would work nicely for a tube preamp power supply.

Bold transformers were used in a voltage doubling circuit.

True that- 1/(0.62*0.5) = 3.2 times the current rating as a FW CT transformer at the same secondary voltage.

[cbass]

According to this page http://tubes.nekhbet.com/power.shtml

A PT rated at 380mA would be reduced to 105mA when used with a Voltage Doubler .Barely enough to run a pair of 7591's

.38 / 3.6=.1055555