No CT On Filament Windings

[plexi50]

I have been watching another thread on a heater filament with no CT.
I have the same problem
My voltages though are 16.50VAC & 5.25VAC per lead
How on earth can i get each lead to 3 volts?
Should i just buy a 6 volt filament transformer?

[Willabe]

        From Doug's library;


How to add an artificial heater supply center tap.
 The two 100 ohm 1/2 watt resistors are connected to the lamp assembly and then to ground. Solder a 100 ohm 1/2 watt resistor from each green winding on the bulb holder to the chassis ground lug next to the pilot light and you are done. In other words, you will have a 100 ohm resistor soldered to each green heater wire and the other end of each resistor is connected to the chassis ground.
  The 100 ohm resistors create an artificial center tap for the heater system. If you do not have a heater supply center tap, you will get a 120 cycle hum. Most people confuse a 120 cycle hum with a 60 cycle hum. You must listen carefully. A 120 cycle hum sounds more like a buzz than a low pitched hum.


                   Brad      

[plexi50]

With artificial ct to ground i have 6.25VAC per lead
Isnt 6 volts per lead still to high?
I have Xmas tree tubes lit up very bright still and a hum

I am wired in series now. Should i wire in parallel?

[Willabe]

I am wired in series now. Should i wire in parallel?

Why are the heaters in series? What PT do you have? What is the PT heater secondary voltage susposed to be, not what your getting as a reading?

With no referance to ground the voltages will be off.

This might help also;

http://www.freewebs.com/valvewizard/heater.html


                  Brad     

[plexi50]

The amp is a November build. I used the 1958 Mathes Console amp i had. I do not know the actual factory specs for the PT
I think im going to have to rewire and raise the filament wires up off the chassis.

[Willabe]

Connect the 2@100R, 1/2w's (MF/ flame proof), 1 from each heater leg to ground and pull all the tubes, then take a voltage reading. 
Now what do you get? 

[plexi50]

6.25VAC with tubes in and 7.11VAC without tubes

12.90VAC across the leads with tubes in

[sluckey]

12.90VAC across the leads with tubes in

It's a 12VAC winding. Didn't you measure any voltages before you pulled it?

To use this PT you'll have to wire the 12AX7s for 12 volt operation and wire the two EL84s filaments in series.

[plexi50]

No i didnt do any measurements before i built it. I didnt pull the PT, i used the entire chassis setup as it was. The power tubes were wired differently before. I recall Pin 9 was not used on the preamp tubes when it had the stock filament wiring. The power tubes i believe were wired i thought in series

[Willabe]

The power tubes were wired differently before. I recall Pin 9 was not used on the preamp tubes when it had the stock filament wiring. The power tubes i believe were wired i thought in series

That was wired for 12.6V. With 12A_7, pin 4,5/9 for 6.3V, and pin 4/5 for 12.6V.   

[plexi50]

Ok so i have the preamp tubes wired 12AX7 pins 4,5 & 9

So refresh my mind. How are the power tubes wired?

Right now they are wired pin 4 to all other power tubes and pin 5 to all other power tubes and then on to pins 4,5 and pin 9 of the preamp tubes

I have 6.11 volts AC per pin 4,5 &9 on the preamp (meter ground to each pin) and 12.60 VAC across 4,5/9

The tubes are very bright and im affraid to push my tubes filament

Maybe i misunderstand the preamp tubes filament rating, But in any case there is a very present hum

Let me go back over my work again and make sure my hum isnt from another problem though i recognize filament hum

[sluckey]

Ok so i have the preamp tubes wired 12AX7 pins 4,5 & 9

You're gonna kill'em! Pin 9 connects to nothing. One filament lead connects to pin 4. The other lead connects to pin 5. That's all.

Right now they are wired pin 4 to all other power tubes and pin 5 to all other power tubes

Gonna kill them too!If you only have two EL84s connect the filaments in series across the 12VAC leads. One filament lead to pin 4 of one tube, pin 5 of that tube jumpered to pin 4 of the other tube, and finally pin 5 of the other tube to the other filament lead. If you have four EL84s, repeat for another pair.
 

[Willabe]

Ok so i have the preamp tubes wired 12AX7 pins 4,5 & 9

No.

That would be for 6.3V.

You have a 12.6V heater wind. The 12A_7 tubes have 2 heaters in them, 1 from pin 4 to pin 9, and the second from pin 5 to pin 9.

If you tie pins 4/5 together and wire them to 1 heater leg, then wire pin 9 to the other heater leg you are wirerd up for a 6.3V heater wind, heaters wired in paralell.

For a 12.6V wind pin 4 goes to 1 heater leg and pin 5 goes to the other leg, _no_ conection on pin 9, heaters wired in series. Pin 9 is the CT of the 2 heaters and is not used for series/12.6V.


                           Brad      

[plexi50]

Thanks for the excellent info. Im going at it now. I should have just left the DM thing alone. LOL!

[plexi50]

Thanks a mill Steve & Willabe. Have it wired and working correctly. Actually it is back to the way it was wired before i got my hands on it. Shotgun Time!

[sluckey]

Nah. You don't need a shotgun for a cat biased Plexi style amp. You can do those with eyes closed!

[birt]

Thanks a mill Steve & Willabe. Have it wired and working correctly. Actually it is back to the way it was wired before i got my hands on it. Shotgun Time!

why did you change the heater wiring in the first place? :p
if you think something is unusual (strange heater wiring, strange voltages...) check the datasheet of the tubes. they're very helpfull. you're lucky your tubes can take some abuse.

could it be that the 12V for the heaters didn't have a CT because this amp probably used one of the 12AX7's to create an artificial CT? this means one of the 12AX7's had pin 9 connected to ground. (two 100 ohm resistors is better)

[plexi50]

Nah. You don't need a shotgun for a cat biased Plexi style amp. You can do those with eyes closed!

Yes i do!  Whoops! Wheres my amp?

[plexi50]

Thanks a mill Steve & Willabe. Have it wired and working correctly. Actually it is back to the way it was wired before i got my hands on it. Shotgun Time!

why did you change the heater wiring in the first place? :p
if you think something is unusual (strange heater wiring, strange voltages...) check the datasheet of the tubes. they're very helpfull. you're lucky your tubes can take some abuse.

could it be that the 12V for the heaters didn't have a CT because this amp probably used one of the 12AX7's to create an artificial CT? this means one of the 12AX7's had pin 9 connected to ground. (two 100 ohm resistors is better)

Birt i got ahead of myself when i was gutting and changing parts to build the November amp. I assumed the heaters were already running 3.2VDC + -  per filament so i removed the way it was wired. I learned something again. SLOW DOWN.