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PostPosted: 30 Oct 2010, 14:14 
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Thanks for the welcome!

I'm down in Ottawa, and yeah I've already ordered a set of six Sprague Orange Drops from the Tube Shop in Hamilton. What I need to source now is some snubbing caps for each of the tubes. I've seen many different brands and many different uF's used... from 0.47uF to 1.0uF. What's the optimal here and should I spend the cash on quality caps for snubbing the tubes or just use whatever I can find locally?

I was also thinking of putting bleeder resistors on the three power caps. Some people have said to put a bleeder on each cap and some have put just one bleeder on the largest of the three caps saying the other two would drain back to this bleeder. And is there an advantage to either putting the bleeder across the cap terminals or from cap positive to board ground?

Lots of questions :) I'm a good builder but I'm trying to get a handle on understanding what I'm building :)


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PostPosted: 30 Oct 2010, 15:46 
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I was also thinking of putting bleeder resistors on the three power caps. Some people have said to put a bleeder on each cap and some have put just one bleeder on the largest of the three caps saying the other two would drain back to this bleeder. And is there an advantage to either putting the bleeder across the cap terminals or from cap positive to board ground?

There is no need to add additional bleeder resistors; one is sufficent. Practically, all that adding more does is lower the voltage regulation characteristics of your power supply.

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PostPosted: 30 Oct 2010, 15:57 
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DisasterArea wrote:
from 0.47uF to 1.0uF. What's the optimal here and should I spend the cash on quality caps for snubbing the tubes or just use whatever I can find locally?

Generic film caps are fine for the heater snubbers. I used 1uF.
Cheers

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PostPosted: 30 Oct 2010, 16:25 
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thanks for the advice! Is it best to wire the bleeder between the leads of the largest cap on this amp or from the largest cap's positive to the boards ground?


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PostPosted: 30 Oct 2010, 21:08 
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It really doesn't matter. I usually put my PS bleeder at the output of the last filter section on the main power distribution terminal strip. The bleeder is just there to ensure that the filter caps all drain to zero charge in a reasonable amount of time after turning off the amp. Where you place it is really up to you.

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PostPosted: 31 Oct 2010, 12:15 
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how would I go about wiring an led in with the bleeder resistor so I could have a visual indicator of when the power is drained? The chassis I'm putting the amp in has a port for an led and I'd like to wire one in.


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PostPosted: 31 Oct 2010, 14:02 
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That's simple. All you need to do is select the bleeder resistor that sets the appropriate current for your LED. :confused:

For example, lets say that you have a B+ of 215v. Depending on what type of LED you're using the typical forward current is between 1mA and 20mA. First, make sure that you have the headroom available in your power supply. A few mA will be no problem, but if you want a high brightness LED that draws 20mA, you may need to take a look at your power supply to make sure you have the excess current available. Lets say that you choose one of these, http://www.alliedelec.com/search/productdetail.aspx?SKU=4310136#tab=Overview, LEDs. They are specified at 2mA and can take up to 7mA. So the bleeder resistor R=215v/0.002A=107.5k. So choose a 100k and check the current (Id=215v/100000=2.15mA) and you're good to go. :up:

The only hick-up is to keep the total time constant in mind. Your circuit has 330µf of total capacitance in the PS filter. This puts the discharge time constant at (330e-6)(100e3)=33seconds. At turnoff the tubes will pull current until the heaters cool down and this will pull the majority of the charge off the caps. But the final time to discharge the last of the capacitors' charge may be up to a few minutes. And the LED will not be on during this time.

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PostPosted: 31 Oct 2010, 17:01 
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Thanks so much! That`s pretty much what I thought I`d have to do, just wanted to make sure :) It`s been a while since my Into to Circuits class in University so I`m trying to brush the rust off. I`ve usually been content with just following the schematics I`m given, but concidering the voltage I`ll be dealing with in a tube amp I figured I should have a good understanding of what all the bits and bobs do this time around :)

Cheers!


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PostPosted: 07 Nov 2010, 17:30 
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Is there a "normal" amount of hum in the K12 amp kit? I have not been able to eliminate hum in my kit. Because this is the first project I have built, I would certainly accept the explanation that I probably have some bad solders, etc in the build. I just replaced the wires from the RCA plugs to the PCB with some shielded cable. There was no change in the level of hum.

The hum does not change with volume; hums whether I have an input source connected or not. I'll stop chasing a solution if they all do this.

-- Chuck


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PostPosted: 07 Nov 2010, 20:23 
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Chuck;

This shouldn't be the case. Properly built, a tube amp should be quiet as a church mouse. Because the hum is constant regardless of inputs or volume settings, I am assuming that you have aground loop somewhere. I recommend you begin tracing your signal and power return lines to see if you can find it.

I am assuming that you used some standard practices like twisting the heater feeds to reduce induced fields, separated the AC and DC portions of the power supply, etc. Post some picture of your build. Sometimes just putting other eyes on a project :geek: can help a lot.

I hope this helps.

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