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Re: Is 4.5 Vdc Heater Enough????

3/23/2005 6:25 AM
Wild Bill
Re: Is 4.5 Vdc Heater Enough????
---"In an perfect world it would :) In the real world with diode voltage drop, ripple voltage and the regulation of the transformer (1A isnít enough for a DC supply) you could end up with only 4.5V."  
Dave, we mustn't forget that we have a cap-input filter. Even if you load the hell outta it the minimum voltage is .9 x applied RMS. This only happens if you have way too little current capacity in the transformer. So you can't drop from nearly 9 volts down to 4.5. The math for using 2 diodes and the centretap however works out just about perfect for 4.5 vdc.  
I'm with Enzo on this one. Not using the centretap and going with a fullwave bridge might be the answer.  
---Wild Bill
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3/23/2005 10:01 AM
What I'm doing:  
AC is bridged (400V-4A rated), then I use a PI filter using 1ohms and 2x 10000uF. No center tap on the transformer so each AC lead have a 51Ohm resistor joinning to common point, and that common point is then elevated to a higher V+ node.  
My calculation were the following: 6.3VAC rectified should be 8.9Vdc - 2 diode drop (1.4V) = 7.5Vdc. 1.2V to high so R=1.2V/(3x300mA)=1.30Ohm: I used a 1 Ohm. Even with 10% more filament current, I would need 1.2V/(3x.330)=1.21Ohm: That is a negligeable difference.  
Anyway, I won't get any sound out at that voltage right?
3/23/2005 12:28 PM
Dave H

"Anyway, I won't get any sound out at that voltage right?"
No, it should work with a 4.5V heater supply. The capacitor only charges up to the peak voltage - diode Vf when the supply is unloaded. When itís supplying current the source impedance of the transformer and the ripple voltage will result in a lower measured dc output. Try running Duncanís psu simulator as loudthud suggested. Youíll see a fair amount of ripple. The dc voltage a meter reads will be about half way between the high and low voltage peaks of the ripple waveform.  
3/23/2005 2:23 PM

Now this is the real question to me, yes you will get plenty of sound from 12AX7's when you only heat them using 4.5V :)  
Its mainly the power tubes where you get MUCH poorer performance when the heaters are underpowered.  
If this was a complete or nearly-complete assembly and your initial readings are just disappointing you because this is the only discrepancy, I would probably put down the soldering iron and pick up the guitar for a few minutes :)  
You can always correct this problem later, here is a link to a recent thread:  
In your case I think I would try removing the 51ohm resistors and let the heaters go without a ground reference or an elevated V+ reference, just to test and see if the DC voltage increases a little without their significant current draw.  
You can also then remove the 12AX7's one at a time to see if the remaining tubes have an increase in working heater voltage. To provide 900ma DC you basically need to draw 1260ma from a 6.3VAC winding so it appears you may be asking more from the transformer than it can deliver, and that could be the cause of the low DC voltage.  
When you want to reference DC heaters to a stable ground or elevated V+ it can be done without adding noise simply by referencing only the rectified & filtered heater B- (or alternatively the heater B+) with a single wire to the reference point. This way there is no additional current draw from the reference connection like there is when some of the AC current is employed in forming a virtual center tap.  
3/23/2005 4:13 PM
Well thank for that response,  
I actually have 6.55Vdc with 1 tube, 5.6 with 2 tubes and 4.9 with 3 tubes and 4.5 with the front panel light.  
This is a preamp only assembly, 2 preamp tube and 1 tube for an effet loop. I don't have space for a another transformer in there. One of my channel is using all of the preamp triode (4) with a plate supply of 340Vdc ( witch I want to reduce to 275-300Vdc)  
What are my best option? keeping noise down is a issue for me. I can't get any sound test yet because I don't have a -12Vdc voltage derived for all of the muting JFET's gate (2SKJ30A), so every thing is muted right now.
3/24/2005 2:28 AM
Dave H

"I actually have 6.55Vdc with 1 tube, 5.6 with 2 tubes and 4.9 with 3 tubes and 4.5 with the front panel light."
Are you still using the PI filter? Shorting out the 1 ohm resistor between the two caps would get the heaters up to 5.5V  
3/27/2005 11:09 AM
I Dave,  
If I bypass the 1 ohm resistor I get 6.15Vdc on the heaters with a 0.07Vac reading on my meter.  
Is this going to do it?

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