combining multiple 6.3v taps

[deci belle]

Thank you for telling me how to measure for current, g. I guess I figured out this PT isn't what I thought it was. That's a good thing, because it has a bias supply— not two heater taps!

I mentioned this on a parallel thread about this PT …I'm sending the PT to an amp factory to have the current output of the taps (especially the 6.3v tap) measured.

[olddawg]

Why don't you just use it the way it was done before. Run the preamp heaters off of one winding and the output heaters off another. Use twisted pairs for the filament run with no center tap or virtual center tap. It was a stereo amp. If you are making a low powered mono amp you should have plenty of current.

[g1]

Why don't you just use it the way it was done before. Run the preamp heaters off of one winding and the output heaters off another.

The "other" tap turned out to be 55V rather than 6.3V. It could not be determined whether it had worked with that transformer.

[eschertron]

Current can only be measured when the taps are hooked up to something and doing work. The current would vary depending on how much load you put on the tap. The current ratings on the taps are what it can safely produce. If you overloaded the tap beyond it's rating the transformer would likely overheat and fail.
Not really sure what you are trying to measure. You could load the tap with a power resistor (6.3V at 2.5 amps would be around 16 watts) of whatever value gives the current you want to test at. I guess you could check that the voltage is not dropping too much at the rated max. current of the tap.

Good point. To hijack the thread only slightly, I am currently trying to figure out how Hammond comes up with the heater current ratings on the PTs that I've ordered from them. Unloaded, or lightly loaded, the 6.3v tap seems to be higher than desired - near 7 volts. As I load them, the voltage droops. When the voltage reaches 6.3v (assuming the residential line voltage is in spec) I should be at 'rated' load, right?

If I add another 12AX7 and the voltage droops by 5% or so below the 6.3v, I'm *assuming* that I won't cause a fire.

The other extreme is if I lightly load the filament winding and the voltage stays above 6.9 - 7.0 volts, should I be concerned enough to add a 'dummy' load in order to bring the voltage into spec? I've used the CT on the winding, but I could use two resistors (and size them appropriately) to ground-reference the winding and bring the voltage into spec, all at the same time. Anybody else do this?

[g1]

if I lightly load the filament winding and the voltage stays above 6.9 - 7.0 volts, should I be concerned enough to add a 'dummy' load in order to bring the voltage into spec?

What is the Hammond primary voltage spec and what is your line voltage? Can you bring down the line voltage to the Hammond spec with a variac and see what the heater voltage is?
If all the secondaries are high I would be more inclined to use a bucking transformer on the primary, rather than just knocking down the heater voltage.

[deci belle]

You are right, olddawg— and by the size of the PT (5F6 bolt centers), and the fact that it's got the taps of a regular guitar amp PT, the heater tap should have plenty of current for my purposes …but I couldn't pass up their offer to test it for the cost of shipping— I sent it UPS today!!

Ya, g-one— I can't say if it was a "working" stereo amplifier when I got it… But I got it!! haha!!

I understand that too low a voltage for the heaters (when loaded) is better that too high, escherton. This PT is 6.9v unloaded, like yours— when I set the variac to 120v.

Someone mentioned using a 200mA choke right after the the PT, g-one. Would that be a "bucking transformer"? — or do those go before the PT?