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Re: Is 4.5 Vdc Heater Enough????

3/29/2005 8:29 AM
Dave H
Re: Is 4.5 Vdc Heater Enough????
Yes, that should do fine.
3/24/2005 12:52 AM

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  
That's not what I found! Some may light up and work with that voltage, but not all. And some may sound very weak and very noisy.
3/24/2005 5:18 PM

Well, I didn't say plenty of quality sound ;)  
mainly just about any time I have an assembly with a logical stopping point where the unit should be complete enough for even partial audible testing, I want to test it with the guitar.  
With that in mind, I generally design, assemble, or modify a circuit intentionally to meet stepwise milestones for cumulative testing.  
Not much different than when you are tweaking a good sounding complete amp just to make it a little better at a time.  
Except if either one sounds lousy at the time you've got a lot more tweaking to do.  
No doubt a reading of 4.5V on the heaters should take priority as a deficiency to be corrected without needing to audition the sound.  
But it can be educational to hear the difference before & after anyway.  
I think my experiences are like yours where most 12AX7's will not sound very good with too weak heater voltage, but some are much worse than others.  
You should hear something though, and it should be plenty enough for an initial confirmation of correct assembly, or give insight about needed troubleshooting at this point in the assembly.  
3/23/2005 1:23 PM

maybe use a 6v regulator. will require less current than a standard DC supply and less capacitance, which eats up current. good luck!!  
3/23/2005 3:06 PM

I remind everyone that Duncan's PSU Designer II works just as good at 6V as it does at 600V. It'll give you the actual RMS current in the transformer winding and the ripple current in the capacitor. Play around with the cap value and see how at some point transformer losses increase as you increase capacitance and output voltage goes down.
3/23/2005 4:12 PM

FWIW Carvin's rectifies the filament supply for the first two pre-amp tubes in most of their current tube amps. I've seen this problem several times in the MTS in particular when running EL34's. On one amp the voltage was only 4.7 at the the first tube. I did find a poor spade terminal connection at the pcb causing part of the problem, and once corrected the voltage increased to 5.3, but that was the best it got. I found that some tubes would work fine at that voltage while others would not. My general advice to MTS owners is to not run EL34's, to stick with the 5881's or 6L6's. IMO those tubes sound best for the high gain heavy palm mute stuff those amp owners play anyway.
3/24/2005 6:08 PM

Yeah I agree with that too using less heater draw supply tubes like 6L6's and since your going to have to derive a 12 volt supply why not just run your pre-tubes at 12 volts in series. This would quiet things down a tad on the front-end.,  
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