Heaters and centre tap

[duderduderini]

Hi There
I am wiring up my Preamp which is a Hammond 370FX. I want to wire up a Rectifier(5V) and also a regulated dc supply which uses a 6080 tube as well as a 12AT7(6.3V)
I have never used a PT that has centre taps for the 5V supply and the 6.3 Volt supply.. Do I ignore the Centre tap? The other pre's I have built never had the CT so what if any advantage/difference should I deploy
Thanks
Nick

[dsavitsk]

For the 5V rectifier, if you use a directly heated rectifier, then you can take the output from the center tap. Or you can ignore it. For an indirectly heated tube, ignore it.

For the 6.3V, if you rectify this in to DC* then ignore the center tap. If you use AC, then you can connect the center tap to ground, or to a voltage divider from B+ to bias up your heaters a bit.


* in general, you'll never get a clean 6.3VDC from a 6.3VAC winding. This is particularly true for a current hog like the 6080 heater.

[vangogh-hk]

May I ask a similar question.

If I have a power transformer with 5Vac (no centre tap), how can I use it on a rectifier with 6.3v heater (e.g. 6ca4) .
Or I have to use a direct heat rectifier.

I ask this question because I want to use my power transformer (0-250-280V, 5V, 6.3V x2) to drive a 5687-dc-5687 power amp (with 20 ma plate current in power stage of each channel)

Thanks a lot

[Merlinb]

Originally posted by vangogh-hk
May I ask a similar question.

If I have a power transformer with 5Vac (no centre tap), how can I use it on a rectifier with 6.3v heater (e.g. 6ca4) .
Or I have to use a direct heat rectifier.

Running a 6.3V rectifier from 5V is not a good idea. You simply need to provide it with 6.3V, either from a separate winding or even a small, separate 6V transformer, and forget about the 5V winding.

[duderduderini]

Thanks for the help
I will ignore the ct for the rectifier as I am using an indirectly heated one.
As for the 6080 i have used AC for that before and since its in a regulator circuit, it doesnt seem toaffect the sound much if at all..
Thanks
Nick

[vangogh-hk]

Running a 6.3V rectifier from 5V is not a good idea. You simply need to provide it with 6.3V, either from a separate winding or even a small, separate 6V transformer, and forget about the 5V winding.

Thanks

Will it be a good idea to rectify the 5v ac following with capacitor to give a dc volatage of about 6V (5x1.4V minus 1V rectifier drop)for a 6CA4 rectifier heater?

Regards

[Merlinb]

Originally posted by vangogh-hk


Thanks

Will it be a good idea to rectify the 5v ac following with capacitor to give a dc volatage of about 6V (5x1.4V minus 1V rectifier drop)for a 6CA4 rectifier heater?

Regards

If you're really desperate, then yes, provided the 5V winding is rated for at least 1.5A

[minhaj]

Dear Vangogh,

We are DIYer, let us keep the spirit up.

Should you use 5 volt tap for 6.3 volt you could do voltage doubler and tune it for 6volt (better than using 6.3 volt for many reasons given the plate is not at its 100% dissipation).

You can not get 6.3 volt DC using 5 volt AC tap because of current loss in the conversion system. So pls forget it and think in the right way.

I only give you idea and right ridections pls check the net for details.

Regards

[Brit01]

Have a question regarding the 12xxx tubes with center tap.

for example the data sheet for a 12AU7 states 300mA heater at 6.3V or 150mA at 12.6V .

Also the 12AU7 has a center tap pin.

Do I use a 6+6 transformer and connect to center tap to pin9?

Or a 12+12 transformer and center tap to pin9?

[DF96]

You can either use a 6.3V winding, connect to pin 4+5 one side, pin 9 the other side, or use a 12.6V winding, connect to pin 4 one side, pin 5 the other side leaving pin 9 unconnected.

The 12Ax7 series has two heaters, one for each triode: pin 4 to pin 9, pin 5 to pin 9. Each heater is 6.3V at 150mA, so you can connect them in parallel or in series.

A separate issue is what you do with any centre-tap on the transformer winding. This has been dealt with in earlier posts.