New OT for 5e3 with 6l6 AND 6v6

[calcam]

Hi,

I'm a complete newb the amp building world but i have managed to put together a bare bones 5e3 kit that actually works and sounds great! So i decided to build another with all the best parts (IMO). So when it came to an OT i wanted a little more juice and something that works well with both 6l6 and 6v6's. I found an OT from MM that has dual 8Ω taps (one optimized for at 5k for use with two 6L6s the other optimized at 8k for use with two 6V6s). Model is a FTDO-59M. Then you just add a switch and you have both options that will in theory sound better than just swapping tubes. Right??

Problem is i have no idea on how to wire the switch and the speaker jacks!

There are three wires coming from the secondary side. Yellow, yellow/white, and green. Can anyone please help me out on how to hook this up. I've attached the tranny sheet and a pic of what i have so far. Any help is appreciated.

Thanks!

[tubeswell]

Hi calcam

I don't think this is going to be quite as simple as you were hoping.

The 6L6s take suck more current from the PT's Heater and High Tension windings than 6V6s do. And if you want both sets of output tubes running at the same time (in order to be able to change them with a flick of a switch - and you probably should at least have the heaters running all the time of both sets of tubes), that will be more yet. (By my guestimate, that latter option would want a PT rated at least 4A on the heater winding at the very least. And if you wanted to leave both sets of output tubes running, then you would also need about 300mA on the High Tension winding). Don't know how you are intending to tackle that part of it.

What's more, in order to have the switchable OT, you need a load resistance on the output tubes that are switched 'off' if you want to keep them running while the other ones are switched 'on' - unless you simultaneously either lift the B+ from the redundant set of output tubes, or disconnect the cathodes of the redundant set of tubes (when you switch the OT over). This implies a multi-pole selector switch that does both jobs at the same time.

And what were you intending to do about the signal grids on the 'off' tubes? (These may need to be grounded when the tubes are switched 'off')

The output jacks however, would be simply connected the same way that they would be in a conventional 5E3 (to my way of thinking). Others may have more practical suggestions - like why don't you just build a straightforward 5E3 for 6L6s instead of 6V6s?

Don't go getting yourself electrocuted.

2CW

[calcam]

Hey tubeswell,

Ok, here I went and made things sound more complicated than they are.

I'm not looking to run both 6l6's and 6v6's AT THE SAME TIME. I merely bought a transformer that allows you to use either a pair of 6l6's OR 6v6's and optimizes the impedance for each at 8 ohms. The 6l6's would use the 5k for 8ohm's and 6v6's on the 8k for 8ohms.

I'm just concerned about how to wire the simple switch to choose between the 8k and 5k and how it's connected to the speaker jacks.

How would I wire the MM's two yellows and single green?

My first thought was this
Weber Black = MM Green
Weber Green = MM Yellow; Yellow/white

That's why I started wiring the MM's yellow and yellow white to each side of the switch with middle lug connected similar to the Weber green. Then I was going to wire the MM green the same as the Weber black.

Tubeswell thanks for your reply and also thanks for your contributions to this forum. I've come across your posts many times that have been very helpful.

[tubeswell]

Hi calcam - I couldn't find an OT schematic for that MM FTDO-59M, so I can't help you with the colour scheme (unless you want to test the different leads on that OT with a variac).

I'm now guessing the taps they talk about are secondary taps (but for some reason the first time around in your post I thought it had 2 sets of primary taps). In which case its just a simple matter of connecting one of the taps to one of the output jack tip connections and the other tap to the other jack tip connection, and the common return wire to the sleeves of both jacks, and swap your speaker between them when you swap tubes.

Make sure your PT's HT winding is rated for about 180mA min, and the heater winding at 4A min for when you run the 6L6s. You just need to plug in a GZ34 to bump up the voltage for the 6L6s and I wouldn't bother even changing the 250R cathode resistor (but I would install a 10W one to begin with to be on the safe side) and plug your 8R speaker into the '5k' tap.