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Old 19th August 2008, 07:11 AM   #1
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Default Noob questions about a K-12/K-502 kit

I guess I should start by introducing myself considering this is my first post! I'm Sam, from Australia, and I've been interested in DIY audio for a while although I'm yet to dip my toes in

I'm really keen to have a go at building a simple valve amp and was taken by either a K-12 or K-502 kit. Although my Chemistry doctorate won't help me much here, it can't be rocket surgery, right?? And yes, I do know which end of the soldering iron (and also the MIG and the gas torch) gets hot!!

First question- is this a good starting point for me? I am on a budget so things like the S.E.X. amp are a little on the high side for me.

Do these kits, as supplied, suit a 240V / 50Hz input or will I need to source a replacement power transformer if they're suited to North American juice??

I've got a set of Coral speakers patiently awaiting new cabinets (2H-21A tweeters 20w/8ohms; 5M-12 Mids 40w/8ohms; 15B-25 (?) woofers 50w/8ohms)- would these run OK? I'm not sure how sensitive they are (prob not very...) but I'm not looking to shake the roof off.

Any thoughts/comments are greatly appreciated

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Old 19th August 2008, 09:06 AM   #2
chrish is offline chrish  Australia
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I have not buiult that exact amp, but a similar circuit from scratch. It is OK, but I much prefer my simple se http://www.tubelab.com/SimpleSE.htm

Search for Simple SE on these forums and you will see plenty of people happy with the result!
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Old 19th August 2008, 02:41 PM   #3
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Hi poynterama,

I second what chrish said if you have some experience with electronics, can read schematics, and have soldered a few circuits before.
Otherwise the kits seem a good starting point and you definitely get some bang for the buck.
The K-502 is 115V only AFAIK. The K-8LS or K-16LS from S5 Electronics seem to work with both (115V and 230V, 60/50Hz).
I only have experience with the K-502 which is a nice sounding amp, very simple to build and the few issues that might be a bit annoying after a while (e.g. some hum) can be easily overcome by a few mods.
I built gingertube's "Baby Huey" recently which is an excellent amp (search the forum) with enough power for normal speakers (~90dB).
But comparing the amount of work that went into both amps, the kit is far more easy to build.
IMHO it really depends on what you are looking for: A nice project that keeps you busy for a while, with the fun of trouble-shooting if something is not working, lots of learning experience and a superior amp at the end -> go for gingertube's "Baby Huey", SY's "Red Light District", or George's "SimpleSE".
If you want to create a working amp in one or two weekends, which sounds nice (actually really nice) and that leaves you some options for improvements down the road -> go for the kit.
Actually the "SimpleSE" might be somewhere in between since there is a PCB available and after sourcing all the parts, which should be readily available, it is quite similar to the above mentioned kits in terms of assembly.
If you don't have a lot experience with electronics there is one thing you want to avoid: Getting lost in an over-ambitious project that will never be finished. Go for a proven design that can be reproduced in a short time and gives the satisfaction of DIY. In my experience the most time-consuming part is the chassis anyway. Having a PCB to start, so you can assemble the amp on a breadboard and already listen to it while you do the drilling, punching, screwing, and painting, helps a lot.
My $0.02 worth.
Martin
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Old 20th August 2008, 06:40 AM   #4
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Chris and Martin- thanks very much for your supportive comments

I hadn't come across the Tubelab site until you pointed me in that direction and have had a good read! SimpleSE sounds perfect for what I want (some experience, on a budget, not too complicated, excellent instructions but some customisation potential and highly probable completion!). It's still left me with a few questions though- there seems to be a few gaps in the build instructions between finishing the board and testing (ie. valve and transformer choices, off-board wiring) Although I feel pretty confident that I can get the board completed, I'm a little worried about wiring up the transformers (and volume pot) without some hand-holding

Chris- is that your amp on the tubelab site (user's page)?? I'll go for the EL34 power tubes I think, but what about the other 2?

Re. transformers, Edcor's fit in the budget nicely (GSXE) and I'll take your advice on the Allied power transformer. Might as well add the choke to totally pinch all your ideas hehe!!

Did you source your parts in Aus or the states? The idea of shipping heavy transformers around sounds expensive....

Building the chassis is probably going to be the easiest part for me with a background in classic car restoration and rally prep!! I can't wait!!!

Oh, any thoughts on those speakers by the way?
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Old 20th August 2008, 06:40 AM   #5
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oops- I just noticed your recommended Allied 6K7VG power transformer is for 110-120VAC primary... Any suggestions for 240VAC?
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Old 20th August 2008, 08:46 AM   #6
chrish is offline chrish  Australia
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Yes, my amp is the first one on the 'user amps' page.

I used the Hammond 374BX power transformer. Sourced from tubesandmore.com

Output transformers, I would recommend Edcor GXSE15-8-5K if you are on a budget. http://www.edcorusa.com/products/tra...se15-8-5k.html I am using the slightly smaller XSE15-8-5K with good results, the GXSE series were not available when I built.

I got my original valves through tubesandmore.com as well. JJ ECC81 is good, JJ E34L good also (and cheaper than KT88s). Had a JJ rectifier fry, probably due to standby switch implementation. Have used Sovtek since with no problem.

I got the coupling caps with the same order. You may wish to try ebay russian paper in oil. Cheap and have had no problems with shipping from them.

Don't bother with the standby switch, it fries diodes and rectifiers.

I have not been able to source the constant current chips in Australia. If you ask George at Tubelab, he might be able to sell you some with the board, otherwise you will probably have to order from DigiKey.

Sorry, am in the middle of moving house at the moment, so not much time to answer. Will be done in a few weeks and will be able to assist online then if you decide to go ahead. This really is a great amp and will whet your enthusiasm for valve amps. After building this as my first last year, I have done 5 point to point since!

Cheers,

Chris
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Old 20th August 2008, 10:54 PM   #7
gofar99 is offline gofar99  United States
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Welcome to budget tube audio. There are several sites devoted to the K-12/K-502 with various modifications that can be made to improve it. I have two k-12 and a K-8 (plus lots of other stuff). The k-12 can have an excellent sound, but will not compare with more costly units. A good starter kit and many folks will help you if you run into problems. The biggest problem in modding the K-12s is cost. You can easily turn a $200 project into a $400 one. BTW my K-8, has never achieved the quality of sound as the K-12. Some posts have indicated the K-16 is Ok though.
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