DC Heaters

[garyboy]

I am building a Hot rodded 100w JCM800 2203 w/ 12VDC heaters for my preamp tubes. i am using a 12VAC 30A tranny for the filament heaters, 2 12VDC fans, an LED, and a 24VDC relay to switch 2 seperate filter banks and a sag resistor plus change my bias for for the tubes when i sag the voltage. I was wondering can i run my power tubes 12VDC or am i going to hafto step it down to 6.3VDC or AC? I am using a transformer from a Crate GT-80 tube/mosfet for my power. Its red/red wires are 468VAC, its yellow/yellow wires are 171VAC, its orange/orange wires are 60VAC, and its green wire is the center tap. I'm using a 24VAC 30A tranny for the Heaters and accessories to take the load odd of the main tranny. It'll be running 4 JJ KT-77s in ultra linear tied to a Hammond Marshall replacement 100W OT. I'm also need to step the voltage down to 12V after the relay. Here is a view of the circuit.

[garyboy]

I meant 24VAC 30A tranny in the second sentence sorry.

[Willabe]

Its red/red wires are 468VAC

Are you sure about that measurement?

If you use a SS FWB you get 468acv x 1.4 = 655.2dcv! Ouch.    


             Brad      

[HotBluePlates]

... I was wondering can i run my power tubes 12VDC ...

If you wire 2 of the output tubes so their heaters are in series, then you can apply 12vdc to that series pair (6.3v + 6.3v = 12.6v). Place the other 2 output tube heaters in series, then take that series pair and place it in parallel with the other series pair and the rest of the preamp tubes.

[garyboy]

Ok thanks. As for the red/red wires on the PT from the CRATE i just plugged into a 1000V 30A rectifier bridge an measured  the AC on one side and DC on the other w/out any filter caps.

[Willabe]

I just plugged into a 1000V 30A rectifier bridge an measured  the AC on one side and DC on her w/out any filter caps.

????    

So was the 468acv before the bridge? Can't be, a crate amps not going to have that high of a voltage from the PT. You must mean DCV?  

Once it's rectified it's DC but you won't get an acurate reading unless you add a filter cap (20uF will due) from the rectified B+ dcv to ground.


                 Brad    

[Willabe]

If I'm looking at the right schemo (Crate GT-80) the PT from that amp will not support 4xKT77. That amp only has 2 @ 12 _ _ 7 tubes in it.


               Brad     

[garyboy]

When i mesured the voltage i just plugged the tranny in and got a reading from the red/red, yellow/yellow, and the orange/orange wires. Then i measured them again w/ the green which is its center tap.

red/green 240.5VAC
yellow/green 72.4VAC
orange/green 33.25VAC

Then i plugged a rectifier bridge w/out a cap and check the DC which was alittle lower than the AC.

I didn't think it would cause the KT-77 heater draw 1.4A each x4 =5.6A heater draw. Plus the preamp heaters, 12V fans and relay would draw too much current from the PT. That why i was going to use the 24VAC 30A tranny to take the load off of the main PT. So it can run the plates and screen grids for the power amp and preamp. Here a spec sheet for pdf for the KT-77.

[garyboy]

If I'm looking at the right schemo (Crate GT-80) the PT from that amp will not support 4xKT77. That amp only has 2 @ 12 _ _ 7 tubes in it.

Yeah this one had a 12ax7 and 12au7 in the preamp and a 12au7 for the reverb.

[Willabe]

When i mesured the voltage i just plugged the tranny in and got a reading from the red/red, yellow/yellow, and the orange/orange wires. Then i measured them again w/ the green which is its center tap.

red/green 240.5VAC
yellow/green 72.4VAC
orange/green 33.25VAC

Then i plugged a rectifier bridge w/out a cap and check the DC which was alittle lower than the AC.

The PT you have it's B+ high voltage winding (240 - 0 - 240) has a CT. It's not made to be used with a FWB. You also CAN NOT ground the CT and the ground connection on the bridge. It will kill the PT. You might have got lucky because you had no filter cap/load on it.   

So with a PT that has a CT you take the voltage from only 1 leg of the wind to CT for the math.

240.5vac x 1.4 (SS diodes) = 336.7vdc. Which would be fine. Except.....

Still the problem with wanting to use that PT to supply 4 x KT77 or any 4 x power tubes is it only has enough plate current for 3 x 12 _ _ 7. They only need 2 to 4 mA of current the reverb 12AU7 might need 5 to 10 mA? KT77's will probly need at least ~30mA each. So 4 x 30 = 120mA + all the 12 _ _ 7 tubes. That PT is probly at least 100mA's less than what you need for the B+ current.


                
                 Brad    

      

[garyboy]

I see what your saying now. Thanks for shedding some more light on that. Rats oh well maybe i'll use this PT for a rack mount preamp instead. Was just looking at all my salvage parts i had laying around for an amp build to save a few bucks.

[Willabe]

Rats oh well maybe i'll use this PT for a rack mount preamp instead. Was just looking at all my salvage parts i had laying around for an amp build to save a few bucks.

That might work.


                 Brad