• 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.

K-12M Enclosure + Upgrades. Finished!

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#3
Welcome to the wonderful world of valves! You are in deep trouble now, you won't be able to stop! 😀

Very nice work. I also built one of those as my first tube amp and still use it almost every day. I applied most of the mods from Voltsecond's site. The result is a very nice sounding amp.

I like your case but I also like to see the tubes glow. Tough call there. I don't know if I would add a fan, I might just cut a hole in the bottom and cover it with a piece of perforated aluminum to get good bottom to top airflow.

This is a nice amp to experiment with and the fact that it sounds so good for $140 is almost a bonus. Enjoy!
 
#5
My goal with this amp was to make it look as inconspicuous as possible... Yeah, it would be nice to see the tubes glow (I thought they might been seen glowing through the vents when the lights were out in the room, not the case). To me, it looks like a piece of test equipment. Which is exactly what I was going for.

I've already implemented the heater cap mods (check the gallery).
http://beastmasterelec.250free.com/k12m/gallery/index.html
VoltSecond's site is a great resource.

GG: Where are the "DC blocking caps" on the board? I assume the coupling caps are C5, C8 and C9. What would you recommend I replace them with?

K-12M Schematic
http://www.circuitspecialists.com/products/km12sch.jpg

Sherman: Yeah, I know. I'm in deep now. I found someone who has built an Ampeg Portaflex from the ground up (for right around $350!!!!). I think this will be my next major project. I can't find the link right now, but I'll post it when I do.... Thanks for the praise.
 
#7
beastmaster said:

GG: Where are the "DC blocking caps" on the board? I assume the coupling caps are C5, C8 and C9. What would you recommend I replace them with?
Click to expand...


alexistheo said:
the DC blocking caps are C1 & C4. i recommend replacing them with a piece of wire.
Click to expand...

Yup, a piece of wire is best! 🙂

On my site, I have some notes and pictures for coupling capacitor upgrades.

http://ca.geocities.com/gmilitano

Cheers,
Gio.
 
#9
Ground loop

Hi
You wrote:
I'm only having one little quirk with this amplifier right now. I have to isolate the ground pin on the mains plug when the amp is selected to the Television's audio monitor output, otherwise, there is this awful hum, and very little audio signal coming out of the speakers. If anyone can help me out with this, I would be forever grateful.

It looks like you need a TV ground loop isolator. Basically it is has two baluns inside, connected end-to-end. Install it on your cable TV input cable, between the incoming cable and your TV or HT receiver.
It isolates the cable TV ground (usually very noisy) from your stereo system ground.

If you do not have cable TV, use an RCA to RCA ground loop isolator on the TV audio only, like this one:
http://www.ramelectronics.net/html/GLI1-X1.html

Google and you will find several sources.
Good luck
🙂
 
#11
Great work! I hope you enjoyed the construction project, and wish you counless hours of listening pleasure.

One thing caught my attention, though:
beastmaster said:
Initially, I had some grounding problems with the RCA jack; the outer ring of the RCA stereo cables were making contact with the chassis.
Click to expand...

The chassis should be connected to the ground to make the entire enclosure a grounded Faraday's Cage.
I usually pick one of the mounting points of the power transformer as the central grounding point by putting a few lugs and connect all the grounding wires to that single point.
 
#14
Re: Ground loop

casouza said:
Hi
You wrote:
I'm only having one little quirk with this amplifier right now. I have to isolate the ground pin on the mains plug when the amp is selected to the Television's audio monitor output, otherwise, there is this awful hum, and very little audio signal coming out of the speakers. If anyone can help me out with this, I would be forever grateful.

It looks like you need a TV ground loop isolator. Basically it is has two baluns inside, connected end-to-end. Install it on your cable TV input cable, between the incoming cable and your TV or HT receiver.
It isolates the cable TV ground (usually very noisy) from your stereo system ground.

If you do not have cable TV, use an RCA to RCA ground loop isolator on the TV audio only, like this one:
http://www.ramelectronics.net/html/GLI1-X1.html

Google and you will find several sources.
Good luck
🙂
Click to expand...


Thanks for the suggestion, casouza. I bought a cable tv ground loop isolator , installed it in between my cable box and the outside line, and viola!; no more buzzing noises when I'm selected to my television input. Just crisp, clear, tube driven audio...

Again, and sincerely, thank you.
 
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