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S-5 16LS tube amp use with 8 ohm speakers


2008-04-12 4:04 am
Hi everyone - I am new to tube amps.

I have come to own a S-5 16LS tube amp which has a speaker impedance of 4 ohms.

All of my previous experience is based on new solid state devices with 8 ohm impedance (or are smart enough to compensate). I suspect that tube amps are less forgiving.

From a practical perspective what do I need to do or worry about to use this 4 ohm tube amp with my 8 ohm speakers? Should I use 2 sets of 8 ohm speakers (parallel arrangement) to match the 4 ohms? Or, 4 ohm amp into 8 ohm speakers will just mean degraded performance.....? Just want to understand how the tube world works in this situation.

I do understand ohms law and the basics of electronics. E=IR, I got it. I just want to draw on the practical experience you guys have.

I am used to new digital devices which I take to be forginving and "plug and play" in their use.

Your kind assistance appreciated.
You probably won't get as much power (in most cases it's a definate, but the S5 kits tend to have a pretty high Zout) and the low frequency won't extend as far. It won't detonate or anything like that as an 8ohm load is more gentle on the amplifier. You could always upgrade those transformers anyways to one that will offer an 8ohm tap and give you the option to run Ultra linear. Edcor has some pretty cheap open bracket ones if your on a budget.

Give it a try with the 8ohms. If it sounds bad or isn't loud enough for you then either switch transformers or use a 4ohm load.


2008-04-12 4:04 am
Many thanks for your info. The S5 site is pretty confusing. In one place it says the impedance is 4 ohms and then in another 4-8 ohms. I found the later information after I posted the question but am still not sure which is the mistake. Is it 4 or 4-8....... At any rate, your reassurance is most helpful.

I definitely want to upgrade this. What specific model tranformers you recommend (by model number) ? What's the "best bang for the buck"? For a huge improvement in quality I'd pay a couple hundred maybe.....

Also, if someone can provide is a list of parts (by spec or part number) that I should go after to upgrade the kit overall (capacitors, transformers, etc), I would certiantly appreciate it. I have seen some info on the earlier K12 model which is specific but nothing on the new "L" design. I would definitely like to hot rod this a bit before I move on to getting a higher end amp.
The problems bruce encountered are because of the output transformers that are supplied with the kit.

If your using 8ohm speakers any of these should work well.

XPP15-8-5k (It's an upgrade, but still budget)
CXPP25-8-5k (more expensive, but better)

PT-1615 (Similar to CXPP25-8-5k performance-wise but rated at a lower power. It gives you the option to wire for different impedance loads where as the CXPP25-8-5K is fixed 8ohm out.) I use these in one of my home-built amps, they have excellent low frequency extension.


2008-04-12 4:04 am
Well, I must say that that bit of info about Bruce's findings is disappointing, but it is appreciated.

I scoured the web (I thought) for reviews and opinions on the 16LS and found noting negative, but did not think to look for the 8LS. With the same basic architecture as the 12K which everyone seemed to like, I thought it would be a good bet.

It is reassuring to know I can imporve what ever I get with the upgrade trasformers.

I guess worst case I'll use the experience to learn a bit about tube amps. Maybe I will get a better result than the Bruce......I will post an update when I get a result to report on.

I am already looking to pick a next more "from scratch" project. Back in my EE college days (25 years ago) I used to etch my own PCBs and build all sorts of projects, just never built any tube projects. Back then I'd have been laughed at by my classmates for playing with that "old technology".

I think that Claus Byrith 5-30 design looks interesting - Any opinion on that design or does anyone have a suggestion on a similar project that is well documented but not a pre-fab kit like the S5 kits?
Jeb-D. said:
The problems bruce encountered are because of the output transformers that are supplied with the kit.

Don't quote me on this, but Bruce and I exchanged several emails about this and I am quite certain he tried good Edcor iron with similar results. The problem may well be the driver stage. Hopefully Bruce will chime in on this.

123rmp said:
With the same basic architecture as the 12K which everyone seemed to like, I thought it would be a good bet.

It is reassuring to know I can imporve what ever I get with the upgrade trasformers.

The K-8 and K-16 are different beasts than the K-12. The new kits have a driver stage and the tubes and iron are different.

I don't know if anyone has shown that new iron will fix the problems.
Hi Everyone, K-8LS one of my favorite subjects. As Gio noted, my experience is not good here. I have 2 K-12s, a pp El84 and a pp KT77 the first two are modded kits the latter two are my own designs. All work well (the 77 is a pending project for Gio's site 25 watts per ch all class A ). Now the K-8LS like some have said ought to be a good thing. Well maybe mine isn't like everyone else's. I did use 25 watt Edcor iron (great stuff BTW) Auricaps, Ruby filters and lots of other premium parts. My initial measurements with a HP 331A distortion analyzer were around 5% distortion at 1 watt. I cringe at 2%. Bass distortion and poor bass response (badly tilted square waves at 1 watt and below 100 HZ) were clear on my scope. So now I am in the process of redesigning the driver. It should work, but I show about 3% distortion at anything above a very low level. I am aware that the circuit is almost identical to the K-12, but the tubes are (obviously) different. I suspect that the driver is not operating in a linear range. I have no axe to grind in this kit and would love it to behave as well as a good K-12 so I would like to hear from others about it.