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Old 9th September 2014, 04:15 AM   #1
Rob43 is offline Rob43  United States
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Join Date: Mar 2014
Location: Richmond VA
Default K12G For Mom & Dad

Hi everyone, I've decided I to purchase a S-5 K12G for my very first tube amp build. The main reason for purchasing now is to get it built so I can trouble shoot it by December at the latest. This way I can give it as a Xmas gift knowing it's all sorted out, my wife's mom & dad should really enjoy it. My overall electrical knowledge is low, my soldering skills are very good, and my ability to follow directions as to building a kit are reasonable. A few months back I tested myself by building a small (5a) motor speed controller kit, it turned out well & worked great. So far I've spent hours and hours reading as many threads on this K12G subject as I can, which of course leads me to a few questions.


1) OPT: would an Edcor CXPP10-MS-10K, or a CXPP30-MS-10K be a good upgrade for better sound & a little more bass ?

https://www.edcorusa.com/cxppseries#...!#-!279!##!241

2) Will this 8W amp work well with bookshelf speakers rated at 90dB ?

3) I need to add a two way input switch so they can switch back & fourth between the tuner & cd player, I'll need help to wire this in properly.

4) I'm probably going to buy the S-5 K12G case that they offer, will this metal case help get rid of any "hum" because it's easier to ground compared to just using the wood block ?

5) I'd love to hear what everyone thinks is the Top 3 modifications to do to this amp to make it work & sound great.

Here are two pics of that speed controller I built.

Thanks,
Rob43
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Old 9th September 2014, 04:27 PM   #2
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It's this one, yes? http://www.diyaudio.com/forums/attac...chematic-2.png

1) Maybe. Probably.

However the cheapest and most effective improvements (in my experience) are elsewhere in this circuit. For example, put a CCS tail on the power sections. This will improve sound quality a LOT, and also probably improve bass response a bit.

The better the balance, the better the OT can function properly. A bigger and beefier OT will tolerate more imbalance in the power amp. Imbalance will hurt power output and bass response first, also linearity.

2) Depends on what "working well" means. Available volume will surely be sufficient.

3) http://www.learningaboutelectronics....ch-diagram.png

Top and bottom rows are the inputs, the middle row is the output.

4) Probably yes.

5) It's best to provide a schematic if you want these kinds of comments, but I assume it's the one I linked above.

My top 3 would be:

1) CCS tails for the power section. Cost only few bucks, big sound improvement.

2) Gyrator plate load for V1a. Cost very very little, big sound improvement here also.

3) Regulator for the power pentode screens. Can be done for a few dollars with - again - surprising results.

I would do all these 3 before thinking about a new OT.
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Old 9th September 2014, 04:30 PM   #3
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Also, tube amps do make an excellent gift for music lovers!
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Old 9th September 2014, 11:32 PM   #4
Rob43 is offline Rob43  United States
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Quote:
Originally Posted by MrCurwen View Post
It's this one, yes? http://www.diyaudio.com/forums/attac...chematic-2.png
Yes.

1) Maybe. Probably.
Which one would be better ?

However the cheapest and most effective improvements (in my experience) are elsewhere in this circuit. For example, put a CCS tail on the power sections. This will improve sound quality a LOT, and also probably improve bass response a bit. That certainly sounds good, now I just need to figure out what it is.

The better the balance, the better the OT can function properly. A bigger and beefier OT will tolerate more imbalance in the power amp. Imbalance will hurt power output and bass response first, also linearity.

2) Depends on what "working well" means. Available volume will surely be sufficient.

3) http://www.learningaboutelectronics....ch-diagram.png
Thanks.

Top and bottom rows are the inputs, the middle row is the output.

4) Probably yes.

5) It's best to provide a schematic if you want these kinds of comments, but I assume it's the one I linked above.

My top 3 would be:

1) CCS tails for the power section. Cost only few bucks, big sound improvement. Is this Link what you are referring to ?
CCS Amplifier & Aikido Active Long-Tail Phase Splitters

2) Gyrator plate load for V1a. Cost very very little, big sound improvement here also. I don't know what this is.

3) Regulator for the power pentode screens. Can be done for a few dollars with - again - surprising results. Again, I have know idea what this is.

I would do all these 3 before thinking about a new OT.

I know I can build the "base" kit the way it's shipped from S-5 Electronics, its all the modifications that I'd like to do that will prove challenging.

It appears from these simple questions that I'm going to need more help from this community than I realized.

S-5 Electronics K12G:
Untitled


Rob43
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Old 10th September 2014, 01:26 AM   #5
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Quote:
Will this 8W amp work well with bookshelf speakers rated at 90dB ?
Only in a limited way. A small Jazz combo or string quartet could be OK. Forget large scale symphonic or "head banger" stuff.

You want honest 94+ dB. sensitive speakers, whose impedance curve is "flat", as mates to an 8 WPC tube amp.

Several "gotchas" exist in the speaker arena. Manufacturers will state nn dB. sensitive with 2.83 V. of drive. If the speakers' impedance is 4 Ω, 2.83 V. of drive consumes 2, not 1, W. and the "true" sensitivity is nn-3 dB. Another concern is the impedance curve of the speakers. Take nominal 8 Ω speakers rated as 90 dB. sensitive. If substantial dips, particularly in the bass region, exist in the impedance curve, the speakers have to be treated as 4 Ω when mated to a tube amp. Once again, derating by 3 dB. is appropriate, leaving the sensitivity at 87 dB.
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Old 10th September 2014, 03:39 AM   #6
Rob43 is offline Rob43  United States
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^ Thanks for the reply, that's very interesting info & good to know. As I already mentioned, this whole endeavor is a Xmas gift so I'm really hoping one of these two sets of bookshelf speakers I already own workout for this. If I have I'd probably buy some Klipsch RB-61 II or a pair of BIC Acoustech PL-28 II & run them as a left and right, but I don't want to spend that money unless absolutely necessary.

The two bookshelf speakers I have are:

1) Polk S4, 91db
"The S4 employs the new SL5000 Dynamic Balance 1" drive unit in a vented cabinet. The smallest bookshelf model in the S series, the S4 offers big performance. Julian Hirsch, writing for Stereo Review, said: "The S4's measured performance was exceptional, especially for its size and price. In particular, its frequency response over most of the audio range was among the flattest we have ever measured from a speaker."

This Ebay add shows all the S4 specs:
Polk Audio Model S4 Studio Monitor Audiophile Bookshelf Speakers Nice Set | eBay

2) BIC America DV62si, 90db
BIC America - Top Rated Since 1973

Amazon.com: BIC America DV62si Bookshelf Speakers (Pair, Black): Electronics

I'll definitely A/B test these two against each other to see if they work with this amp once it's completed.


Rob43
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Old 10th September 2014, 04:03 PM   #7
Rob43 is offline Rob43  United States
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Is there Any difference in where I order this K12G amp kit from ?

TubeDepot price is $199.95 shipped. I can add four (4) 10GV8 tubes at $4.95 each to the order & still come in for less than S-5 Electronics kit.

S-5 Electronics price is $225 shipped


Thanks,
Rob43
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Old 10th September 2014, 05:46 PM   #8
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Rob43,

While I agree with most of what others have written, I would suggest getting and building the kit as is, and then listening to some speakers with it. I have built several low-power amps and found them adequate with normal bookshelf speakers, even down to 87 dB. Like Eli wrote, you just won't be able to crank it up and maintain the headroom needed for major instantaneous demands on power.

Once you have the kit done, you can decide if you want to improve it for your parents' enjoyment and for your further learning. You may not be able to do it all before Christmas, so by building the kit and putting it in a nice case, you have something to offer on time.

The CCS tail, the regulated screen and other ideas are worth exploring, and most can be built in small spaces using solid-state components. If you want to try any ideas, let people know on this board and you'll get some good help. The thing is, the case you are planning to use may not be easy to modify for adding a negative bias supply for the CCS tail if it doesn't have much room inside. You'll also find that you'd be altering the pcboard components.

You might be better off using this kit for starters and then looking at another more flexible project design that incorporates some of these suggested improvements from the start.

As far as input selection, consider a simple RadioShack input selector box. Your parents may be able to use it for a DVD audio or other input as well, and you won't have any fixed limitations.

All this is said in a spirit of "Christmas comes sooner than you plan, and things always take longer than expected." If you want to go whole hog, that's great, but I'll bet your parents will like the kit as is.

--Jeff
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Old 10th September 2014, 06:23 PM   #9
Rob43 is offline Rob43  United States
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Jeff, that is certainly solid advice, I've decided to build it "as is" as soon as I get it. I've toyed with the idea of buying a second K12G for myself & modifying that one. Can you give me any input on my question in post #7 ?


Rob43
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Old 10th September 2014, 10:23 PM   #10
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Rob43,

I think people on this forum have ordered from both places with good success. They appear to have the same contents and the same kit. Other than that I have no experience with either place.

Have fun,

Jeff
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