External 12V heater supply conversion question

[luvdunhill]

hi!

I have a headphone amp that uses 6SN7 and uses a simple DC heater supply followed by a CRC filter. I'd like to modify this amplifier to allow me to hook up an external wall-wart style (eventually a much nicer supply) to allow me to use 12SN7 tubes. I know there are DC jacks that allow this sort of automatic selection (i.e. when the wall-wart is inserted, this supply is selected). Any ideas on the best way to go about this? Should the jack be between the regulator and existing CRC filter, or after?

Thank you!

[ArtG]

I think I'd go ahead and build a simple 12 volt regulated and well filtered supply, which is pretty easy to do. Some of the Wall-warts that I've looked at, on a 'scope, put out some pretty nasty looking "DC". I don't know if all of them do this, but I'd certainly check it out before you use one, as a headphone amp should have a clean DC supply for the filaments. If the filter in your amplifier is set up for 6 volts, it may not survive 12 V, anyway.

[luvdunhill]

ArtG:

thanks for the reply. Sure, I can plan on doing this. The important thing is:

a.) it's external (not enough room in the amp)
b.) it's easy to switch between both supplies.

[Eli Duttman]

The "barrel" power jack should switch between the O/Ps of the 2 supplies. That is it should come after the internal DC heater supply filter.

A Triad VPL 24-400 trafo from Allied Electronics is suitable for powering the external 12 VDC supply. Phase up and connect the dual secondaries into a 12-0-12 arrangement. FW rectify with a MBR20100CT common cathode twin Schottky diode. Filter with a 4700 muF./25 WVDC 'lytic. Follow the filter cap. with a 7812 3 terminal regulator IC. Attach a clip on heatsink to the regulator's TO220 case.

[luvdunhill]

Eli:

Ok, well I have all the parts ready. I ended up going with a different transformer, namely a Hammond 183K12. It is rated at 12.6VCT @ 4.4A / 6.3V @ 8.8. Is it possible to add a 12V / 6.3V switch so my external supply can easily be configured for either heater voltage? I have never placed a switch across transformer secondaries like this, is it ill advised?

[ArtG]

I can think of three different ways of doing this, but since you asked this of Eli, we'll wait for his reply. However, meanwhile, how many 6SN7s will you be powering, i.e., what will be the total current draw?

[luvdunhill]

Originally posted by ArtG
I can think of three different ways of doing this, but since you asked this of Eli, we'll wait for his reply. However, meanwhile, how many 6SN7s will you be powering, i.e., what will be the total current draw?

I'm up for all suggestions! I will be powering 3 6SN7 tubes, but wanted a heater supply that I could use for other designs, up to something crazy like 4 EL34. I just asked Eli first, as I already purchased all the parts he recommended.

[ArtG]

With further consideration, of the three possibilities mentioned, I now believe that only one would really be practical. I'd suggest a variable regulated supply based on an LM 317. I found the circuit in the link below that's pretty close to what I have in mind.

This circuit with your transformer, and of course, the full wave rectifier, as Eli mentioned, would replace the bridge circuit shown, giving you a maximum output of a little over 12 volts. The input cap (C1) can be reduced to a 25V item, but I'd stick with the suggested 4700mF. The output cap (C3), while not absolutely necessary, is good idea, and can also be reduced to 25V. The "on" indicator LED ( L1, R1), and voltage meter ( R3, P2, V) are also optional, although, again a very good idea, IMHO.
While a continuously variable output is more than what you may need, replacing P1 with a two position switch, and calculating the correct resistor values, would only complicate the supply, and getting the correct values for the resistors, might involve some trial and error. Calculated values, from my experience, seldom produce the exact desired voltage.

The above will work for your three 6SN7s, but it's not enough to power four EL34s. If you want to do this, you would need a "larger" 3 pin regulator (if one is available--I haven't looked), or a slightly different circuit with a separate pass device.




http://www.uoguelph.ca/~antoon/circ/vps.htm

[Eli Duttman]

I ended up going with a different transformer, namely a Hammond 183K12. It is rated at 12.6VCT @ 4.4A / 6.3V @ 8.8. Is it possible to add a 12V / 6.3V switch so my external supply can easily be configured for either heater voltage?

When you cap. I/P filter, approx. 1/2 of the AC RMS current is available as DC. So, figure on 2.2 A. at "12" V. and 4.4 A. at "6" V.

Setting up an adjustable supply overlooks the fact that heating in the regulator IC will be GREAT, when a "6" V. O/P is selected. Remember, the available regulator ICs pass all the current. They are series regulators. If some ingenuity is exercised, the heating in the regulator IC can be controlled. A DPDT switch will be used to select between the 2 OP voltages. One leg of the switch will control how the PSU rectifier is grounded. A full wave bridge rectifier using 4X HIGH current rated Schottky diodes will be placed across the entire 12.6 V. The junction of the 2 SS anodes gets connected at an end of the ground control leg and the other end gets connected to the CT of the trafo. The wiper gets connected to ground. Please observe you bridge rectify for "12" V. and FWCT rectify for "6" V. The 2nd leg of the DPDT switch is used to select the resistor network needed for a particular O/P voltage. Use a pot. initially to work out the fixed resistances needed for the 2 selectable O/P voltages.

The MBR20100CT is rated at 10 A./100 PIV X2, with a common cathode. I did not think currents as large as currently under discussion were going to be present, when I made the suggestion. We are talking serious amperage, now. I'm a belt and suspenders guy. Use diodes rated for at least 30 A. I HATE parts failure.

Where a 5 A. rated 3 terminal adjustable regulator in a TO3 case and the MASSIVE heatsink to support it can be sourced is a question to be answered. It may be necessary to use a lower current rated IC combined with external pass device.

BTW, add an additional 10000 muF. of filter capacitance to the circuitry. As the DC draw increases, so must the energy storage of the filter.

[andyjevans]

LM338 and especially LM1084 are good for 5A. Need tidy heatsinks.

The DC connector range goes up to 5A in 2.1mm and 2.5mm sizes, and the chassis socket should have a switch so it disconnects the internal supply when inserted (usually batteries)