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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Some basic questions about s-5 amp kit mods

fenpark15

Member
2007-12-12 8:01 pm
Sorry this post is long, but I have a lot of small questions. I've been doing research on the cheapest (but not completely crappy) way to build my first tube amp. I've tentatively decided on the new s-5 kit for the 16Wpc stereo amp.

http://s5electronics.com/l16stereo.html

I am assuming that many of the typical upgrade modifications will be the same even if the circuit is slightly different from the older 8W version. I intend to replace some capacitors with Auricaps and add gold binding posts.

Also, I want to replace the pot but a stepped attenuator is a little out of my price range. Would replacing the pot with a better pot make some improvement? Any suggestions for what "good" pot I should order?

For the power caps, I've heard that increasing the capacitance can help. How much would be a good amount to increase by without throwing things out of balance? Also, what brands/types of capacitors are the best for this application?

When adding "snubber" capacitors to the tubes, what values are good and where do these actually solder to?

Finally, if I wanted to add some LEDs and a cooling fan, would it be possible to add another power transformer for some 12V DC on the wall-jack side of the PT without messing things up? If that's a bad idea, how else might I go about it?

Sorry so many questions, but this will be my first "physical" experience with electronics outside of studying textbooks and web forums so I want to make sure I get it right. Thanks in advance for your valuable advice!
 
First, be careful! I don't know if they added safety bleeders on the 16W one, but the 8W didn't have them and the caps would hold a charge for days after power was removed from the amp.

If you want to change the potentiometer try an ALPS. I believe the lead spacing is the same as stock, so it should be able solder into the original position. And they can be had for like $10 on ebay.

If you want to upgrade the power caps. You may want to leave the first one in the chain (the one tied directly to the rectifier) its original value. To avoid any risks in overloading the transformer or rectifier on power-up. For the other electrolytics, put the largest value that will fit. Get a caliper and check the lead spacing. Find the largest capacitance value, of equal voltage rating that has the same lead spacing.

As for snubbers, try a .01uF 1-3kV ceramic cap on the secondary side of the transformer, before the rectifier. Some people snub the filament winding too, but I don't recall the value they use. I'd check your results to see if you’re satisfied before snubbing the filament. I think it kind of defeats the purpose of buying a kit, if you get too carried away with modifications (like cutting traces and re-routing ect.).

Let us know how it goes. I don't know of anyone that has tried the 16W kit yet.
 

fenpark15

Member
2007-12-12 8:01 pm
How about a power on LED (and resistor)? I realize this may be a dumb question for those who are practiced in designing circuits, but I just want to add one light for when the power is on and I know that if I add this to the wrong place it can step the voltage down and mess something up.
 
I know the 8W kit is a real glower, so it's easy to tell it's on with no other indicators. But if you want a power indicator
I would suggest a small light bulb from a flash light or a lamp instead of an LED. It looks old-school and doesn't require you to have DC power for it. Radio shack has quite a variety. If going for a bare bulb look, it's good to get one that is rated for a higher voltage than what the circuit has. That way it will glow a dim orange.

You can either use a high voltage one and run it off mains power on the primary side of the transformer, but after the switch. Or use a low voltage bulb and run it off the fillament supply. I'd recomend the first because who knows if the transformers fillament current rating is already being maxed out.

If you want a LED you will have to tap some wires off the fillament, rectify it, add a cap to filter ripple, then an LED with a resistor in series.

The amp will probably sound fine with the stock output transformers. Or they probably wouldn't sell it as-is.
Although you will probably want to upgrade eventually.
 

fenpark15

Member
2007-12-12 8:01 pm
I'm powering a pair of Klipsch Heresy IIs. They're not audiophile speakers, but they're the best I have and I think their sensitivity is about 96dB, so not bad.

As far as impedence, my speakers are 8 ohm, but the S-5 site is a bit confusing as far as what the amp output is made for. On the 16W page, the say something like "4-8 Ohm output impedance." However, I think the real problems happen when you put too low impedance load on the amp, i.e. a 4 ohm speaker on an 8 ohm output. This would increase the current by double what the amp is made for. (I=V/R)
I think my 8 ohm speakers should be fine whether s-5 actually means 4 or 8. If the amp IS safe for 4 ohm speakers, you could get more power from the amp with lower impedance (P=IV=I^2/R)
 
Hi Everyone, I have read the posts with some interest. I would like to start a dialogue on this amp. I completed a K-8 recently. It was unsatisfactory. (see the blog on the diyaudioprojects.com site for details) I don't like to say negative things about stuff but this amp has the worst sound of anything I own.

The K-8 and K-16 use the same transformers. The impedance needed for push pull parallel tubes in the k-16 is half that of the k-8. Do the math if you want. To achieve this, the impedance of the load has to be halved. So if the original was rated at 8 ohms then the load on the 16 must be 4 ohms to properly load the tubes. That's the good news. My build used Kimber Kaps with Ruby filters and the usual bunch of snubbers together with DC on the heaters. The amp did not perform nearly as well as my similarly modded K-12. At first I tried it with Edcor iron in UL mode then switched back to the originals to see if it made a difference (just as bad really). It distorted way to early and just didn't have the sound quality I expected. I strongly suspect the K-16 will behave in a similar manner. As I say in the blog, I would love to like the new amps, but .... There is at least one other post (of an unmodded amp) that had similar issues. If someone out there has had success, I would like to know.

IMHO, the older kits are better. Cheaper as well
 

fenpark15

Member
2007-12-12 8:01 pm
Well, I'm going to go through with my original plan anyway and cross my fingers that it will be better than the k-8. I really wanted to mount my PT inside my chassis and use a cooling fan to relieve the heat inside. I found a fan that says it is 115 VAC, 10W, 130mA. Can I hook this up to the power from the wall before I run wall power into the PCB?