Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dennyg on January 14, 2017, 10:41:45 pm
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I'm using a separate 6.3V 2A trans to supply the pre-amp heaters (2 - 12AX7's) via voltage doubler then regulated with a 7812. The neg side is wired to a voltage elevation circuit (approx 44V). I'd never tried 12V heaters nor a regulated supply before, so just built it for fun. A couple questions:
* Does it make a difference where I ground the voltage elevation circuit? Currently grounded at the first PS filter cap but wondering since it's separate supply from the PT (and it's heater windings supplying the SE power tube), that maybe it should ground somewhere in the pre-amp circuit?
* My wall voltage is high - 120V+ so the 115V tranny yields 7V. The doubler outputs 16V DC which the 7812 seems ok with, but if I plug into a lower wall voltage, thinking the doubler might not yield enough for the regulator. Is this valid concern?
Responses always much appreciated!
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thinking the doubler might not yield enough for the regulator. Is this valid concern?
since you're using a separate circuit, you could calculate the current you want for the tube, say 1.6A, 12V, come up with an appropriate Resistor ohms n watts, then "dummy load the PS n see if she sags to much.
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If your using DC heaters, you shouldn't need and elevation circuit. The ground lift, (elevation circuit), serves in most cases to, lift the heater noise level above the noise floor in the preamp section. Another purpose of the lifted heater circuit is found in a cathode follower. Since the voltage difference between the cathode and heaters is higher without a ground lift circuit, the potential for arcing between cathode and heaters exists so the heaters are lifted to keep the potential difference between the heater-cathodes lower.
As for grounding the negative side of the ground lift circuit to the negative output of the regulator circuit- I'm not sure. My "guess" is, there is the potential for a shorted regulator circuit.
silverfox.
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Elevate several volts above your cathode.
Your top tube (from 12V to 6V) is well above the 1V-2V we expect at cathodes and is already "elevated". Let this be your first preamp tube, the most sensitive.
The 6V-0V heated tube is partly elevated, but will be fine for driver or maybe post-Volume stage.
You want 3V across the regulator; you need 2V. Taking your data as given, you are OK to 112V. My house will dip there. But how much ripple?? If your "16V" is really zig-zag 14V-18V, you are already on the verge of drop-out, and ripple dips will appear on the heater lines, which run very close to precious audio signals.
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Thanks guys – my education continues. Yes, I am elevating for the CF stage in this build, but only because I read somewhere that I should ground lift 30-60V for cathode followers. In my current build, the cathode runs at around 120V relative to chassis ground (using a 33K Rk). My understanding is that assuming 12V DC heaters, elevating 44V would yield a cath-to-heater voltage of 120 – (44 + 12) = 64V. The 12AX7 datasheet states a max cath-heater of 200V. Without elevation, cath-heater would be at 108V which would seem to be well below the 200V maximum, and if that’s true, do i even need to elevate for this circuit? BTW, in my first build the cath potential was at 190V relative to chassis (used a 100K Rk) so elevating made sense.