S-5 Electronics K-12M (11MS8) 8 wpc PP Tube Amp Kit ($139)

[gmilitano]

I have purchased and completed the $139 tube amplifier kit by S-5 Electronics.

The amp was purchased from S-5 Electronics. The folks there are very friendly and helpfull and I recommend them. The kit was well packaged and arrived intact. Canadian duites, GST and fees was exactly $20CDN.

I have posted photos of my finished amp in the photo section:
http://www.diyaudio.com/forums/showt...315#post541315
http://www.diyaudio.com/forums/showt...318#post541318

Fitting the kit into an enclosure was much tougher than I had originally imagined. However, I think that it turned out rather nice. The front controls are a power switch, input selector and volume control. On the back are the speaker outs, RCA inputs, fuse and power cord.

The kit has been reviewed by AudioXpress, http://www.audioxpress.com/reviews/m...hansen2145.pdf

Since that review, it is important to note that changes have been made to the kit. Most impotantly, the tube sockets have been upgraded to ceramic to address the scortched socket problem. In addition, the circuit has been modified (addition of two capacitors) to reduce noise and hum.

I plan to listen to the amp more critically, but have the following initial impressions to share:

The amp is quiet and the transformers do not hum as others have reported.

Based on limited listening, the performance seems good and is a clear improvment over my mass produced SS amps (i.e. Sony etc.).

The power is only 8 watts per channel, however, 8 watts is much louder than I expected. It is loud enough on a pair of ~91dB speakers to make conversation impossible and get me evicted from my apartment.

As reported by others, the tubes run very hot, so do take that into consideration when fitting the kit into an enclosure. The power transformer also gets hot. Any comments as to why the transformer is that hot?

There is no safety bleeder on the power supply. However, keeping the speakers connected after power down seems to drain the capacitors sufficiently.

I have some tube dampers I want to try out. I am a little concerned that the tubes may be too hot.

I also have some polypropelyne caps that I will try in place of the metalized polyester input coupling caps.

I will also likey try the mods from http://www.siteswithstyle.com/VoltSe...Push_Pull.html

I will post my results.

Regards,
Gio.

[audiousername]

Originally posted by gmilitano
It is loud enough on a pair of ~91dB speakers to make conversation impossible and get me evicted from my apartment.

LOL But I hope that was a joke...

Originally posted by gmilitano
As reported by others, the tubes run very hot, so do take that into consideration when fitting the kit into an enclosure. The power transformer also gets hot. Any comments as to why the transformer is that hot?

The power transformer may run a little hot because it may not be sufficiently large enough (or not sufficiently overdone ). You'll probably have to live with it unless you replace it with something larger. Another problem that people sometimes run into is putting the valves too close to the transformer, heating it. It doesn't look like that though from the pictures (a little hard to tell though)

Anyway, how hot is it? So hot you can't hold your hand against it?

Originally posted by gmilitano
There is no safety bleeder on the power supply. However, keeping the speakers connected after power down seems to drain the capacitors sufficiently.

This is a transformer-coupled amplifier, and since the OPT can't pass DC, the speaker wouldn't be able to act as a bleeder. The B+ probably goes down because the valves remain conducting for a little while after the power is off (they can conduct as long as the cathode is hot), or through leakage in the B+ caps.

Anyway, congratulations is in order. The more valve amps out there the better!

[Geek]

Originally posted by audiousername
LOL But I hope that was a joke...

Like, Canadian made apartment, eh? We put R31 in the outside walls and nothing from apartment to apartment

Anyway, congratulations is in order. The more valve amps out there the better!

Agreed, congrats!

[audiousername]

Gio, I had a look over the link you posted about mods to the amp, and I must say, it looks quite well thought out. I doubt you would go wrong with following those modifications.

It said at the end of the site about adding a bleeder to the supply. Well how about this idea... using the unused contacts of a DPDT power switch to switch in the bleeders only when the power is turned off. It would reduce the additional strain on the power supply caused by the bleeder current (I admit this may be negligible), but more importantly, will eliminate the production of heat in the bleeder when the amp is on. It seems that this amp is too hot already

Originally posted by Geek
Like, Canadian made apartment, eh? We put R31 in the outside walls and nothing from apartment to apartment

Muhahahaha Australian apartments (well mine anyway ) have 13mm (1/2") plasterboard, something like a 50mm (2") airgap filled with fibreglass insulation, 150mm (6") or so of concrete blocks - and the same plasterboard and insulation on the other side. So the whole wall is like 300mm (12") thick, and caulked all around for fireproofing! To top it all off all the windows are double glazed and sealed, along with the gaps around the doors. So as long as the music stays above 100Hz and I'm not trying to destroy my hearing it's for the neighbours. But darn, those bass notes are pervasive

Edit: Whoops, I forgot to attach the file. Doing a lot of that nowadays As you can see, using the generally unused switch contacts was not my idea.

[gmilitano]

Originally posted by audiousername

The power transformer may run a little hot because it may not be sufficiently large enough (or not sufficiently overdone ). You'll probably have to live with it unless you replace it with something larger. Another problem that people sometimes run into is putting the valves too close to the transformer, heating it. It doesn't look like that though from the pictures (a little hard to tell though)

Anyway, how hot is it? So hot you can't hold your hand against it?

I will have to measure the temperature when I get a chance. I don't think I would want to touch the tubes. I can hold my hand against the transformer, but is does get uncomfortably hot.

I think the valves may be a little close to the transformer, they are about 2-3 inches away.

Should I worry about the heat?

Originally posted by audiousername

This is a transformer-coupled amplifier, and since the OPT can't pass DC, the speaker wouldn't be able to act as a bleeder. The B+ probably goes down because the valves remain conducting for a little while after the power is off (they can conduct as long as the cathode is hot), or through leakage in the B+ caps.

When I finished the kit, I powered up (with no load) to make sure there were no shorts and I waited till the tubes started to glow. After power down, voltage on the B+ was just shy of 200V. I waited 1/2 hour and there was only a 2 or 3 volt drop.

Then I tested the amp for a while with speakers. When you kill the power, the amp continues to play music for a while (30 seconds) with decreasing volume and eventually just a faint music. Then I checked the B+ voltage and it was only 3.5 volts. So I guess the cathode conducts while it is hot as you mentioned.

Anyway, this has been a very fun and rewarding project.

Cheers,
Gio.

[djQUAN]

Originally posted by gmilitano



When I finished the kit, I powered up (with no load) to make sure there were no shorts and I waited till the tubes started to glow. After power down, voltage on the B+ was just shy of 200V. I waited 1/2 hour and there was only a 2 or 3 volt drop.

Then I tested the amp for a while with speakers. When you kill the power, the amp continues to play music for a while (30 seconds) with decreasing volume and eventually just a faint music. Then I checked the B+ voltage and it was only 3.5 volts. So I guess the cathode conducts while it is hot as you mentioned.

Anyway, this has been a very fun and rewarding project.

Cheers,
Gio.

since you were playing the amp when the switch was turned off, the sound playing was the load that dropped the B+ voltage. if you still connect the speaker but did not play anything, the B+ will still stay at the high voltage.

[audiousername]

Originally posted by gmilitano
I don't think I would want to touch the tubes. I can hold my hand against the transformer, but is does get uncomfortably hot.

I think the valves may be a little close to the transformer, they are about 2-3 inches away.

Should I worry about the heat?

Please don't touch the valves. They will probably burn your hand!

Um... if you can hold your hand against the power transformer, it's probably around 60-70ºC or so. So around 20-30º above ambient (actually, not so sure how cold it is in Canada this time of year ). I don't think it's anything to worry about.

[noisenyc]

hi gio

the bulb temp of tubes is quite high, over 200 degress....normal.

i would suggest as mods...

bypass input caps (not necessary).

replace diodes with hexfreds.

disconnect screen supply to power tubes and tie the screens to the plates with 100 ohm resistors.

remove negative feedback resistor from the output tap.

[gmilitano]

Originally posted by noisenyc
the bulb temp of tubes is quite high, over 200 degress....normal.

i would suggest as mods...

bypass input caps (not necessary).

replace diodes with hexfreds.

disconnect screen supply to power tubes and tie the screens to the plates with 100 ohm resistors.

remove negative feedback resistor from the output tap.

Hi Blackie,

Why are the input caps not required?

The diode is a single bridge. I was considering the UF4007 mod as proposed Voltsecond.

What would be the purpose of removing the negative feedback resistor?

I will post the new schematic.

Cheers,
Gio.

[gmilitano]

The schematic for RV-D (10/5/01) can be found at:

http://lib1.store.vip.sc5.yahoo.com/...cs/km12sch.jpg


I have scanned RV-E (5/7/02) and attached it below.

The only difference I see between RV-D and RV-E is the addition of C10 and C11, which are 33pf ceramic capacitors.

Cheers,
Gio.