Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mwelch55 on January 25, 2019, 11:42:15 am
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My question is about voltage reduction.
I am building a stand-alone 2-channel preamp. The clean channel is going to be a typical fender preamp such as the 65 Princeton. The High Gain channel will be based on a hot rodded Marshall.
I have a 175-0-175 60ma Edcor Transformer I want to use. If I use a Bridge Rectifier across 350 volts, I should get around 490 VDC on the first filter capacitor, which would mean I would need to reduce the DC voltage to around 325 VDC.
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If I use full wave rectification the way I see on a lot of order Marshall amps, I would get around 245 VDC on the first filter capacitor. That’s a little less than I wanted.
I have seen a Zener from Ground to center tap of the transformer or between the negative terminal of a bridge rectifier and ground to reduce B+.
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I saw another version of DC reduction that I need to understand. In this drawing, do the Zener diodes reduce the DC? If so, then how much? If the diodes are 47 volt zeners, what would the result be? Would it be around 94 volt reduction or 47 volt reduction?
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You also want a LOT of ripple filter.
Assuming 5mA-10mA total draw, 490V to 325V is 165V drop, you want 32K to 16K of resistance. A preamp needs at least three R-C filter stages, so get a pack of ~~5K resistors and 3 or 4 40uFd caps. Start with about five 5K resistors series, with caps at the taps, and see where you come out.
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The fact is you have the wrong transformer for this purpose. If you can't live with 245VDC, then you must use a bridge rectifier. This requires you to drop a whopping 170V to get from 495 > 325. Zeners can do it. But that's a lot of heat production and wasted energy.
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The transformer is one that I have had for a while. I was hoping to be able to use it for this stand-alone preamp. I know that is a lot of voltage to drop, but I was thinking it would be OK since the current will be low.
I have some 1n3011 150v 10w zener diodes. Could I attach one of those between the bridge rectifier and ground to get it around 340 VDC? The watts dissipated would be around 1.5 watts, which is well under the rated 10 watts.
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This is an excellent candidate for a "fixed voltage" VVR.
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Listen to PRR. Perfect opportunity to drop voltage while reducing ripple.
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Maybe VVR is best. :dontknow:
With a zener, you're right -- not much wattage dissipation with only preamp tubes. N.B. I think you need a reverse zener in the CT. However, some scouting on the net seems to indicate that zeners are noisy; maybe not good for a preamp.
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I second the suggestion PRR made and for good fender clean you don't have to drop the voltage that much. 65 super reverb has got 385VDC on the preamp node.
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Thanks for all of the responses. I will probably take PRR's suggestion and uses a bunch of resistor/capacitor nodes to get the voltage down.
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I have a 175-0-175 60ma Edcor Transformer I want to use. If I use a Bridge Rectifier across 350 volts, I should get around 490 VDC on the first filter capacitor, which would mean I would need to reduce the DC voltage to around 325 VDC.
There is another easy option... Use a choke input filter after your bridge. Your voltage should be about .9 x 350Vrms = 315Vdc
The small Fender choke that Hoffman sells would be sufficient.
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Thanks Sluckey. I like that idea. The choke would reduce a lot of the ripple and reduce the voltage significantly.
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The whole point of a choke (instead of a cheaper resistor) is that it does not reduce DC voltage much.
I recall a Heath which started at 490V on the power tubes and drop drop drop dropped to 90V at the 1st phono stage.
It would help to actually draw-out your plan and estimate the current demand.
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I think the idea was to use a choke input filter instead of a capacitor followed by a choke
Franco