Questions about PT loading on a Conn Repurposed PT

[vmazz]

I have an amazing sounding amp project. Using a PT from a conn organ it has two HT taps one puts out about 420v, other 520v to a pair of EL34 tubes(dont use the 520v setting much as I dont think current production tubes last long at that voltage). Its wired much like a modded marshall with an extra tube for a clean channel. So 2x el34 and 4 x 12ax7. The PT was originally powering 2 rectifiers 2 x 5v(not being used) 4x12v6, 6x12au7 and 2x6l6, has a center tapped filament winding . I had a challlenge with wiring the filament tap. The voltage was too high, not loading down enough..So my solution was to use the center tap as one end, so utilize only half the winding. THis gave me about 6.5v Perfect. Everything sounds great! Voltages seem right. Only concern is the PT gets a bit hot after 45min. Ive never had a PT get hot berfore. Worried it might have something to do with how I am using the filaments taps. Suggestions on how I should test to be sure its within a reasonable level? Thanks for any input.

[mozz]

Was it wired for 12v filaments originally?

[Malcolm Irving]

6.5V from half the filament winding suggests that the winding might actually be 12.6V with a centre tap. (In other words, the Conn used a 12.6V heater supply). Putting all your heater load on half the winding may be overheating it.

One solution could be to modify your amp to run with a 12.6V heater supply. The 12ax7s can take 12.6V from pin 4 to pin 5 (leaving pin 9 disconnected) and the heaters of the EL34s can be wired in series.

[vmazz]

I was wondering that too. Is it more efficient to run 12v ac filaments or 6.3 parallel? Just wasnt sure how to wire the 6ca7 filaments with a 12v supply.

[vmazz]

Read another forum thread about this particular pt . Looks like the filaments are 12.6. Using half the winding, am I pushing the limits of the PT. With 2xEL34 and 4x12ax7?

[vmazz]

I should have slowed down so I wouldnt have had to rewire. A little bit of a headache. But , Ill figure it out. Thanks for the help.

Here is a thread about this PT

https://www.thegearpage.net/board/in....882701/page-2

[vmazz]

[Malcolm Irving]

In the original Conn circuit, at 12.6V, we have:
4 x 12v6 = 4 x 0.225 = 0.9
6 x 12au7 = 6 x 0.15 = 0.9
2 x 6L6 (in series) = 0.9
Total current = 2.7 amps

With your existing circuit, at 6.3V,
4 x 12ax7 = 4 x 0.3 = 1.2
2 x EL34 = 2 x 1.5 = 3.0
Total current = 4.2 amps

So the existing circuit is probably overloading the filament winding. (We don’t know the rated current of the filament winding, but it is probably only 3 amps or so.)

To put the heaters of the EL34s in series (for 12.6V) connect one side of the heater supply to pin 2 of the first tube, connect pin 7 of the first tube to pin 2 of the second tube, and pin 7 of the second tube to the other side of the heater supply.

[Mick Bailey]

And that 4.2A is pulled on just half of the heater winding. No wonder it's getting warm.

[vmazz]

Yes. Wow. Thanks for that breakdown. I'm having an ahaa moment. Or more like Homer Simpson doh moment! One more question about the wiring. After the el34 how do I connect winding to go to preamp. I understand they will go just to pins 4-5 on preamp tubes. But does one come from first el34 and other from second? Just want to ensure I keep it quiet too. I am referencing elevated D.C. For these filaments but I want to keep ac wires together. As u can see it's a little tight in the chassis. Thanks a ton for the help.

[Malcolm Irving]

Let’s call the two wires of the 12.6V heater supply ‘white’ and ‘black’ to keep track of which side is which.
Pin 4 of preamp tube, using a white wire, to pin 2 of first EL34. White wire carries on from pin 2 of first EL34 to one side of the PT filament winding.
Pin 5 of preamp tube, using a black wire, to pin 7 of second EL34. Black wire carries on from pin 7 of second EL34 to the other side of the PT filament winding.
Black and white wires twisted together all the way, as usual.
Now we need a third wire (blue?) from pin 7 of the first EL34 to pin 2 of the second EL34. This blue wire should be twisted in with the black and white or at least kept close to them.
There may be other ways to do it, but this seems a good way to me.

[eschertron]

Would it be bad form to take the 'blue wire' (above) to the filament CT?

[Malcolm Irving]

Would it be bad form to take the 'blue wire' (above) to the filament CT?

The 'blue wire' would make a good 'artificial centre tap', but if there is already one at the PT then better not to (I think).

[vmazz]

A big thank you to Malcolm and all those that chimed in! The heaters are running perfect and no more audible hum from the PT. Looks like I did not do any damage . Now It will run quite efficiently thanks!