Tube rectifier question.

[el rockyo]

I am planning to build a little SE amo with 2X12AX7 and 1XEL84. I wanted it also to have a tube rectifier and I thought the EZ81 would be a good option. My power transfer has 250-0-250V @ 130mA, 0-4-5-6.3 @ 3A, 3.15-0-3.15 @ 4A.
Ok, so I looked up the datasheet and for 2X250V it says that the Rt minimum has to 2X150 ohm. Is this the winding resistance? Is is dc ohms or ac impeandence?
I measured the winding resistance with a multimeter and I read 2X60 ohm.
Is it ok to use the EZ81? Must I make a modification to the design or is it unviable?

Thank you for your time and effort.

[costis_n]

No, it is the internal equivalent resistance of the EZ81(i think). In any case, an EZ81 is fine for this kind of amp, connect its heater to the 3.15 -0- 3.1 5 wiring and ground the central (0V) connection, to minimise hum.

[kevinkr]

No, that's the dc winding resistance of the high voltage secondary, and based on your measurements it is on the low side. You could choose another rectifier or just use a pair of 91 - 100 ohm resistors in series with the EZ81 plates.

I just realized you have a multi-tap filament winding which you could use to power any rectifier you like. You might also want to consider an octal type such as the 5Y3 or 5V4, 5U4 etc..

I assume with a single EL84 and two 12AX7A this is a small guitar amp, otherwise did you mean 2 x EL84 and 1 x 12AX7A for a stereo SE? If for an axe an octal rectifier would give you some choices to fine tune tone through rectifier choice.

[Hi-Q]

The 150 Ohms refers to the manufacturers recommended limiting resistance for normal operating conditions for each anode at 250v input. The resistance is increased for higher voltages e.g. 230 Ohms for 350V and 310 Ohms for 450V.
Most people do not bother, relying on the transformer winding resistance but you may want to consider adding a couple of 100 Ohm 3W resistors.
The above assumes you will be using the conventional capacitor input filter on the HT (B+) line.
You should be using the windings 0-4-5-6.3 for the rectifier heaters, the 3.15-0-3.15 to be used for the other valves with the centre tap grounded as suggested earlier.
The rectifiers heaters should not be grounded and doing anything here will not reduce hum. The 0-4-5-6.3 winding is provided purely for the rectifier and allows the optional use of a 5V heater rectifier such as a 5Y3.

[el rockyo]

It is for a simple guitar amplifier. OK, so I should add 100 ohm wire-wound 5W resistors in series with the rectifier plates, I thought of this solution to begin with but, won't that drop quite a bit of the B+ voltage?
Has anyone tried this solution?

[kevinkr]

Originally Posted by el rockyo

It is for a simple guitar amplifier. OK, so I should add 100 ohm wire-wound 5W resistors in series with the rectifier plates, I thought of this solution to begin with but, won't that drop quite a bit of the B+ voltage?
Has anyone tried this solution?

I have, and it will drop some voltage dependent on the load on the rectifier. Since the current flowing is non-sinusoidal it is a bit hard to predict the drop. If you have a windows based computer I would download Duncan's Amps PSUDII here: PSUD2 - this will allow you to determine the effect of the resistors by changing the transformer dcr to reflect with and without the resistors. Some learning curve is involved in using the software.

I would consider an octal rectifier for the considerable flexibility it will offer.

[el rockyo]

Ok, so when i edit the transformer-rectifier block, should i set the trasformer voltage to be 500v or 250v?
60 ohm or 120ohm?

[el rockyo]

Thank you for the help guys, I really appreciate it.