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voltage drop?

8/25/1999 10:13 AM
voltage drop?
I was noticing that everytime someone asks a question about getting voltages down at the plates you guys talk about a zener diode on the B+. How come nothing is ever said about increasing the value of the resistor under the filter cap cover of a fender? Wouldn't this be just as easy, if not easier, than the zener trick? I really don't know but am going to change the 10k to a 15k to try to drop my voltage down from 438 to 415 in a SFDR. It will run at 415 with a 5u4 in it but with a 5ar4 its up at 438. So far I have done a lot of mods in the tone circuit and not messed much with the power supply end so this is new to me. I am also not an electrical wizard like most of my gurus in this page. Can someone tell me if I am on the right track to get the voltage down? TIA  
8/25/1999 12:32 PM
J Fletcher
Well a zener will drop it's rated voltage no matter how much current is drawn through it, while a resistor drops a voltage depending on how much current is drawn through it. Specifically, V=IR, Ohm's law.A zener gives better voltage regulation. The plate current in a Bassman ,for example varies between, approx 70 and 200 ma, so a zener is better suited to the task.The resistors under the cap pan are decoupling resistors in series with the preamp and driver stages, so they don't affect the output tube plate voltage. Unless you're refering to the 220k resistors across the 1st filter cap network.Varying these will have very little effect on the B+ until you get into very low values that will draw excessive current from the power transformer. You don't want to do this...Jerry
8/26/1999 3:15 AM

I am really a babe in the woods on this. Can you explain the zener diode? Et al...Does it come in different voltage ratings? Would I be hooking this up on the B+? Where can I get them? I need to drop 23 volts to get to spec. Thanks.  
8/26/1999 6:40 AM
Steve A.

    You would mount a reverse voltage K-type stud-mount power zener in series with PT CT connection to the chassis. For a Fender style amp this would also reduce the bias voltage tap. They do come in a wide variety of voltages.  
    As for bringing the voltages down to "spec" remember that all of the old schematics specify +/- 20% (so for a nominal 400 volts that could be anywhere from 320 volts to 480 volts).  
    In copying an old design you might want to aim at the voltages on the schematic, but if you don't hit them exactly there are other ways you make the amp capture that vintage sound (speakers, cabinet construction as well as modifying the circuit itself). Unless the plate voltages exceed the ratings of the output tubes, I'd say keep the high B+ right where it is and then fiddle around with the B+ voltages going to the PI and the preamp stages. The current draw from the PI and preamp tubes is low enough that you can use 2 watt resistors freely to set whatever voltages you want. And if you are modding an existing production amp, I believe that the PT, OT, and output stages should be fairly well matched. IMHO without changing the B+ going to the output tubes and just diddling with the preamp/PI voltages you can really get a lot of different sounds...
8/26/1999 12:27 PM
J Fletcher How 'bout a 5Y3 ?
You might also try a 5Y3 rectifier tube, or perhaps a 5V4. These both draw 2 amps for filament current vs. 3 amps for the 5U4. This might give you the B+ you're looking for. They'll have more voltage drop than a 5AR4. I've never done the zener thing myself...Jerry
8/26/1999 12:37 PM

5Y3's work, but the amp gets really mushy. A DR (stock) has a very stiff power supply (despite having a tube rectifier). The 5Y3 undoes this pretty well.. I'm not crazy about the effect, but, as always, YMMV. :-)
8/27/1999 1:25 PM

A 5Y3 most likely will give you that "mushy" or "slugish" response on the lowend. A 5V4 keeps the lowend very "tight" or "solid." 5R4's are really nice sounding but hard to find and expensive when you do find them. They are often times to tall to fit in most amps but they sound fantastic!  
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