MC10 update

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Jun 25, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Meister

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    That’s correct. It’s a reverse parallel set of LEDs (Red and Green) to make an amber color and light up the head shell.

    Turns out it’s pretty dim... it won’t light up much. It was an experiment! I’d probably do it again if I could find a brighter double LED.
     
  2. andrewRneumann

    andrewRneumann Tele-Meister

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    Status Report!

    The amp worked on the first try! But since I had my pots wired backwards, it was behaving strange. I thought to myself, “Self, I will just pretend 10 is 1 and 1 is 10 for now and it won’t make any difference.”

    Well I bet you are all laughing because that would work with a linear taper but with an audio taper... not so much.

    So I swapped some wires around on the pots and got that working.

    I have had zero problems with oscillations or noise—even with the leads flying all over the place.

    My biggest concern at the moment is the amount of 120Hz hum that I’m getting. I know single-ended is harder to get rid of all the hum, but was thinking with 66uF reservoir and 33uF for the screen filter, I would have pretty low hum. Turns out there is still a bit more hum than I would like. I got some more capacitors that I will clip in and see if that is all I need or maybe it’s something else.

    My first thought is wow, it actually works. I can hardly believe it. My second thought is... this amp has zero headroom! Even with all the bias set to neutral, crank the gain knob past 2 and it starts growling. I disconnected some cathode bypass caps and that really helped.

    So now the fun really begins. Getting it to sound right. Guitar in one hand, stack of alligator clips in the other.

    Thanks to everyone who has helped out so far! I’ll be experimenting a lot and I will keep sharing my results and questions.
     
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  3. James Knox

    James Knox Tele-Meister

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    Cool idea. Cool learning tool. Cool build. I look forward to hearing about your results!
     
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  4. NSB_Chris

    NSB_Chris Tele-Meister

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    Excellent!

    It looks like you have a two diode rectifier before the standby switch and then a single after. Does the single diode after the switch do anything since the rectification has already occurred? Also it seems there is a resistor to ground before the single diode. Is that intended to discharge the filter caps after shutdown? If so, will it work connected on that side of the diode?
     
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  5. andrewRneumann

    andrewRneumann Tele-Meister

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    I learned *ahem*copied*ahem* that standby layout from the Valve Wizard. I think it’s pretty brilliant. You may not see in the photo a large metal oxide resistor across the standby switch terminals. So the standby doesn’t completely cut current... it lets it trickle in. I think I calculated that it cuts power by about 90%. So there’s still some current flowing but not enough to get any sound out of the amp. I did increase the wattage on the resistor. 2W didn’t look like enough to me.

    The third diode is to keep DC from the standby switch. The switch only has to handle the rectified voltage, but no smoothing has taken place. DC is hard on switches. And the extra resistor is a 1Meg that keeps everything at 0VDC. It is not intended to discharge the caps.

    You have a keen eye.
     
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  6. andrewRneumann

    andrewRneumann Tele-Meister

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    Here’s something I noticed and need some advice on. I am running a set of matched EHX 6V6’s in parallel.

    I can clearly see that one tube filament is glowing brighter than the other. That same tube exhibits a nice blue glow that is observable when the shop lights are out. The other does not. I’m thinking to myself that the extra heat is probably just sending more electrons to the anode and it’s enough to make one give off blue emission and not the other. Does that sound about right?

    They are both running at about 6.6VAC right now. They share a cathode resistor so I don’t have a good way of checking the individual current easily. (Or is there?) The screens vary by about a volt—maybe a sign that current is different in each valve or just within the tolerance of the screen stopper resistors.

    I did have a 3rd EHX 6V6 to swap in and I didn’t notice any change to sound or hum.
     
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