UX-1501 Output Transformer Impedance

[astronomicum]

This amp is four tubes using 6550s. I have seen this model labeled 4-8-16, and 2-4-8 ohms on the impedance selector. The unit I am working on measures 5595, 5080, and 4956 ohms, labeled 4-8-16 ohms respectively. They should be about half that for 4 tubes. I think this selector should be labeled 2-4-8 ohms. Is this correct? Thanks.

[tubeswell]

This amp is four tubes using 6550s. I have seen this model labeled 4-8-16, and 2-4-8 ohms on the impedance selector. The unit I am working on measures 5595, 5080, and 4956 ohms, labeled 4-8-16 ohms respectively. They should be about half that for 4 tubes. I think this selector should be labeled 2-4-8 ohms. Is this correct? Thanks.

All depends on the Pri:Sec turns ratio (or squaring that to get the Pri:Sec impedance ratio)

[sluckey]

The unit I am working on measures 5595, 5080, and 4956 ohms

Is this DCR? Be specific. What measures 5595, 5080, and 4956 ohms?

[astronomicum]

Sorry. Here are the data and calcs:

The 4 ohm tap measures 37.4 turns, squared, times 4 = 5595 ohms
The 8 ohm tap measures 25.2 turns, squared, times 8 = 5080 ohms
The 16 ohm tap measures 17.6 turns, squared, times 16 = 4956 ohms

[tubeswell]

Sorry. Here are the data and calcs:

The 4 ohm tap measures 37.4 turns, squared, times 4 = 5595 ohms
The 8 ohm tap measures 25.2 turns, squared, times 8 = 5080 ohms
The 16 ohm tap measures 17.6 turns, squared, times 16 = 4956 ohms

Doesn’t sound right.
Where did you get the information on the secondary turns? And what about the turns on the Primary?

[astronomicum]

Sorry. Here are the revised data and calcs:

The 4 ohm tap turns ratio measures 37.4:1, squared, times 4 = 5595 ohms
The 8 ohm tap turns ratio measures 25.2:1, squared, times 8 = 5080 ohms
The 16 ohm tap turns ratio measures 17.6:1, squared, times 16 = 4956 ohms

[tubeswell]

Well, if your information about the OT turns/impedance ratio is accurate* a 5k reflected load is ok for a PP OT for 2 x 6550s. You want 1.75k to 2.5k plate-to-plate load for a quad.

https://frank.pocnet.net/sheets/127/6/6550.pdf

(*But still not certain about where you got the turns ratio information. Do you have a schematic or spec sheet for the OT? Or did you measure actual Primary and Secondary VAC to work it out?)

[astronomicum]

(*But still not certain about where you got the turns ratio information. Do you have a schematic or spec sheet for the OT? Or did you measure actual Primary and Secondary VAC to work it out?)

Thanks for the confirmation. Yes. I test using VAC to determine the ratio. I collect this data. In this case, it also confirms this amp is not labeled correctly.