• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Rectifier Capacator (voltage adjust)

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I've got the tubelab se. I've got a hammond 350-0-350 transformer. c4 on the schematic, (http://www.tubelab.com/AssemblyManualTubelabSE/Schematics_TSE.htm) the capacitor immediately after the rectifier, was 47uf. I was getting 410 volts b+

I decreased the value of that capacitor to 15uf, and now I'm getting 412 volts!

I thought a lower value capacitor would lower the voltage.

Am I wrong about that?
 
I am curious about that as well. I can easily test the current the entire amp is pulling, but my clip around inductive meter is at a friends house right now.
But would that help? Or should I test only what b+ is pulling?
Unfortunately this is on a PCB, so it will take cutting a trace, or pulling a component to test.

Something is wrong, though. Or, at least I think there is.
I've used everything from 4.7uf up to 47uf and its ALWAYS 410 volts (+/-3v w/ power fluctuations)
Last night I was looking for failed components and ended up replacing all the caps, other than the nice paper/foil/oil coupling caps. While the caps were out I measured the resistance of all of the resistors I could.. everything measured fine. Put in all new caps, and still the same results!!!!!!! I've basically rebuilt this amp.
The only components that are questionable (no way for me to test them) are the IC's in the tubelab design, but and IC should either work or not, right?
The amp still works. I have been noticing way more distortion at mid/high volume levels, but thats up for debate as to whether something in the amp has failed, or if Im just getting better at hearing distortion.
I've also tried different output tubes w/ the same results.
 
I asked about the current being pulled as a choke input PS only works if a minimum current is pulled: if you only had a bleeding in the circuit pulling 1mA a choke input PS will act like a capacitor input, increasing the voltage dramatically.

From your description I understand that you have the amplifier playing, so it is pulling the expected current. If you want to have it working more or less as a 'choke input PS' you will have to decrease the value of the first capacitor 'way more'. Ideally a choke input PS doesn't even have one, though a small value (less than half a uF) will not affect the 'choke input function' and even help stabilizing the whole thing. Start with about 220nF and keep adding more (I would say a 100 to 200nF a time) until you reach the desired voltage. This way you can play with your output voltage, adjusting it between 0,9x VRMS and 1,4x VRMS.

Most of these recommendations are taken from Valve amplifiers: I would seriously recommend you to read his extensive description of choke input PS.
 
In answer to the original question, yes; using a smaller capacitor as the input cap on a cap-input supply will start to shift the filter over to a choke (or, in this case, resistance) input filter.  This is usually done with an inductor (choke) as the second element, so I'm not sure how a CRC filter would react, but presumably similarly.

The thing is, this mostly happens when you take the input cap below 15µF or so, and so you're not seeing the effect.  In essence, you are developing the same voltage with the 15µF cap as with 47µF.  Two volts at 400v is half a percent, not significant.  I can think of two mechanisms for this; either the line voltage from the wall going up half a volt or so between tests, or the larger cap having a little more ESR than the smaller one. Try putting a 4.7µF or smaller in there.  The change due to that first microFarad is larger than all the rest; it gets less and less leverage up to about 10µF or so, where it levels off.

As the gentleman said, you can model all this stuff in PSUD2 very quickly and easily. The 'ware requires you to plug in the winding resistances of your transformers and other ephemera, so that without knowing that stuff your output may be a little off, but it will still show changes in output due to changes is filter values and configuration very accurately.

Aloha,

Poinz
 
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