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

[Brett]

Originally posted by planet10


Blackie,

The URL is giving me a "Your dataset has expired" message... what are you sending us to look at (i'll have to get some and try them)

dave

Dave, it's an IXYS 600V/8A FRED . Put the part # DSEI8-06A-ND into the search engine on the main page.

[planet10]

Originally posted by Brett
it's an IXYS 600V/8A FRED . Put the part # DSEI8-06A-ND into the search engine on the main page.

Thanx Brett.

dave

[noisenyc]

Originally posted by planet10


Blackie,

The URL is giving me a "Your dataset has expired" message... what are you sending us to look at (i'll have to get some and try them)

dave

plug this part number into the digi-key search engine:

DSEI8-06A-ND

it's a FRED...fast recovery epitaxial diode.

if you want to totally max out that little amp, your best bet would be tube recitification, choke, smaller caps, and triode/no NFB bridging. also replace plate load resistors with carbon film or tants...www.angela.com

compare THAT to a tripath on a sensitive full ranger!

[gmilitano]

Originally posted by noisenyc
[B]
if you want to totally max out that little amp, your best bet would be tube recitification, choke, smaller caps, and triode/no NFB bridging. also replace plate load resistors with carbon film or tants...www.angela.com

Greetings,

So I tried out some of the VoltSecond modifications. Results were good. You can read about them on my webpage.

I have read that a choke did wonders on this little amp. I will keep that in mind, but fitting a choke onto my enclosure will be tough. Tube rectification would be nice, but I don't have the real estate.

BTW - I replaced the bridge with 4 UF4007.

As for the resistors, they are already carbon film with the exception of the power supply resistors which are metal oxide. Another member of the form tried resistor upgrades but did not find much of an improvement.

http://www.diyaudio.com/forums/showt...912#post227912

Tants will cost almost as much as the kit and premium carbon film such as Riken are also pricey at $3-4 a pop. Used carbon comps could be implimented at salvage costs, but is replacing generic 5% carbon films with vintage 10-20% carbon comps a real improvement? Heat may be a problem with vintage carbon comps.

Further upgrades that I am contemplating:
Removing the DC blocking caps - as suggested by Blackie
Adding MKP caps before and after the bridge.
Upgrading the poly coupling caps with nice film-foil caps.

Cheers,
Gio.

[noisenyc]

Originally posted by GG


Greetings,

So I tried out some of the VoltSecond modifications. Results were good. You can read about them on my webpage.

I have read that a choke did wonders on this little amp. I will keep that in mind, but fitting a choke onto my enclosure will be tough. Tube rectification would be nice, but I don't have the real estate.

BTW - I replaced the bridge with 4 UF4007.

As for the resistors, they are already carbon film with the exception of the power supply resistors which are metal oxide. Another member of the form tried resistor upgrades but did not find much of an improvement.

http://www.diyaudio.com/forums/showt...912#post227912

Tants will cost almost as much as the kit and premium carbon film such as Riken are also pricey at $3-4 a pop. Used carbon comps could be implimented at salvage costs, but is replacing generic 5% carbon films with vintage 10-20% carbon comps a real improvement? Heat may be a problem with vintage carbon comps.

Further upgrades that I am contemplating:
Removing the DC blocking caps - as suggested by Blackie
Adding MKP caps before and after the bridge.
Upgrading the poly coupling caps with nice film-foil caps.

Cheers,
Gio.

those carbon films should sound fine. wirewounds might be nice in the power supply, mouser has some for not too much dough.

[chromal]

Heh, I'm new to tube amps and have been lurking diyAudio.com until today. Hey, hello.

I received and assembled my K-12M on Wednesday night. For lack of parts, this is a stock kit amp, excepting that I have foregone the plastic power switch. I'm pretty pleased with the overall sound; it compares favorably with my Alesis RA-100 solid state amp on my SPL 94dB 1w/1m loudspeakers.

Within a few feet of the midrange/tweeter drivers, I notice some 60hz harmonics buzzing quietly. I intend to try the AC snubber and UF diode upgrades this weekend if the local radio shack can give me sweet instant gratification...

I am a bit concerned that one of the four 11MS8 tubes' plates can be seen dimly glowing in one corner if I turn off the lights and let my eyes adjust. After several combinations of tube ordering, I can say with a fair degree of certainty that the problem is following the tube...

I'm wondering to what extent the 11MS8s can be matched. A rep at S-5 Electronics states "our testings show that they are all very close....at least for the kit." Hrm.

Rk for the power pentode push-pull pairs (R8, R17 - 300ohm, 3w) drops about 24.5 and 25.4v DC respectively, or 80mA and 83.3mA cathode current for the two pairs (one of which contains a glowing tube, which seems to follow the tube). The pentode plate voltage was 204 VDC on all four tubes, so the voltage from cathode to plate on the tubes is up around 180-181VDC.

The tube data ( 11MS8 ) for the RCA version of this tube says the zero-signal plate current should be 50 Ma at 120V as a class A amplifier. Now... this is a class AB1 push-pull amplifier with quite a bit more plate voltage...

[noisenyc]

Originally posted by chromal
Within a few feet of the midrange/tweeter drivers, I notice some 60hz harmonics buzzing quietly. I intend to try the AC snubber and UF diode upgrades this weekend if the local radio shack can give me sweet instant gratification...

I am a bit concerned that one of the four 11MS8 tubes' plates can be seen dimly glowing in one corner if I turn off the lights and let my eyes adjust. After several combinations of tube ordering, I can say with a fair degree of certainty that the problem is following the tube...

I'm wondering to what extent the 11MS8s can be matched. A rep at S-5 Electronics states "our testings show that they are all very close....at least for the kit." Hrm.

Rk for the power pentode push-pull pairs (R8, R17 - 300ohm, 3w) drops about 24.5 and 25.4v DC respectively, or 80mA and 83.3mA cathode current for the two pairs (one of which contains a glowing tube, which seems to follow the tube). The pentode plate voltage was 204 VDC on all four tubes, so the voltage from cathode to plate on the tubes is up around 180-181VDC.

The tube data ( 11MS8 ) for the RCA version of this tube says the zero-signal plate current should be 50 Ma at 120V as a class A amplifier. Now... this is a class AB1 push-pull amplifier with quite a bit more plate voltage...

you could install a bias balance pot to adjust matching...the mismatched condition will cause audible hum in the channel...

100 ohm wirewound pot 5 watts should do, and change bias resistor to 250 ohm 5 watt wirewound...

[noisenyc]

Originally posted by chromal
Heh, I'm new to tube amps and have been lurking diyAudio.com until today. Hey, hello.

I received and assembled my K-12M on Wednesday night. For lack of parts, this is a stock kit amp, excepting that I have foregone the plastic power switch. I'm pretty pleased with the overall sound; it compares favorably with my Alesis RA-100 solid state amp on my SPL 94dB 1w/1m loudspeakers.

Within a few feet of the midrange/tweeter drivers, I notice some 60hz harmonics buzzing quietly. I intend to try the AC snubber and UF diode upgrades this weekend if the local radio shack can give me sweet instant gratification...

I am a bit concerned that one of the four 11MS8 tubes' plates can be seen dimly glowing in one corner if I turn off the lights and let my eyes adjust. After several combinations of tube ordering, I can say with a fair degree of certainty that the problem is following the tube...

I'm wondering to what extent the 11MS8s can be matched. A rep at S-5 Electronics states "our testings show that they are all very close....at least for the kit." Hrm.

Rk for the power pentode push-pull pairs (R8, R17 - 300ohm, 3w) drops about 24.5 and 25.4v DC respectively, or 80mA and 83.3mA cathode current for the two pairs (one of which contains a glowing tube, which seems to follow the tube). The pentode plate voltage was 204 VDC on all four tubes, so the voltage from cathode to plate on the tubes is up around 180-181VDC.

The tube data ( 11MS8 ) for the RCA version of this tube says the zero-signal plate current should be 50 Ma at 120V as a class A amplifier. Now... this is a class AB1 push-pull amplifier with quite a bit more plate voltage...

howdy again!

sorry, i was in a bit of a hurry when i made my last post.

check the manual for the design max plate dissipation.

then just do the math to make sure the design maximum current is not exceeded. you are totally on the right track. calculate current per tube and multiply X plate volts for dissipation in watts.

generally speaking, well made NOS tubes are okay if the plate voltage max is a bit exceeded, sometimes a LOT exceeded, as long as the overall dissiaption is within design maximums. glowing plates is generally not groovy on AB pentodes tho...they will gas up and/or fail prematurely....

you can install a bias balance pot if you wish so that unmatched tubes wont be an issue. you will find with cathode bias arrangements there is a fair amount of leeway on the bias resistor. bigger resistor=less current draw but plate voltages will tend to rise. even more so on tube rectified circuits. sound will change...many tubes sound good low'n'hot...depends on the tube and the loading...

have fun!

[chromal]

There's a low-light shot of the tube overheating. It's not really visible in normal light, but with a 10 second exposure on the digital camera, you can see it clearly. (The right pair is also dimly glowing, though this isn't really evident to the naked eye).

check the manual for the design max plate dissipation.

then just do the math to make sure the design maximum current is not exceeded. you are totally on the right track. calculate current per tube and multiply X plate volts for dissipation in watts.

Okay; the RCA data source says the pentode max ratings are 250V 6W. No input/Idle measurments: 25VDC at cathode at 83.3mA current across 300ohm resistor for both tubes together (screen current neglected), 205VDC at plates makes for a 180v differential between cathode and plate. So... 180V * 83.3mA = 14.994w, or about 7.5w per plate at 205vdc...

Does this mean that the pentodes are dissipating 1.5w beyond the max rated 6w spec *on average*? (with the understanding that a common cathode current between two pentode cathods in a push-pull arrangement will not say a thing about indivudual pentode currents.)

I've been reading a bit about biasing here. I guess that the practical offshot of all this is that it's sounding as if I might want to consider a modification that allows me to vary cathode resistor's impendence independantly per tube, perhaps the cathode 'balance' potted approach you mentioned...

[noisenyc]

Originally posted by chromal


There's a low-light shot of the tube overheating. It's not really visible in normal light, but with a 10 second exposure on the digital camera, you can see it clearly. (The right pair is also dimly glowing, though this isn't really evident to the naked eye).

Okay; the RCA data source says the pentode max ratings are 250V 6W. No input/Idle measurments: 25VDC at cathode at 83.3mA current across 300ohm resistor for both tubes together (screen current neglected), 205VDC at plates makes for a 180v differential between cathode and plate. So... 180V * 83.3mA = 14.994w, or about 7.5w per plate at 205vdc...

Does this mean that the pentodes are dissipating 1.5w beyond the max rated 6w spec *on average*? (with the understanding that a common cathode current between two pentode cathods in a push-pull arrangement will not say a thing about indivudual pentode currents.)

I've been reading a bit about biasing here. I guess that the practical offshot of all this is that it's sounding as if I might want to consider a modification that allows me to vary cathode resistor's impendence independantly per tube, perhaps the cathode 'balance' potted approach you mentioned...

that tube is def running too hot.

when you do the calculations, use the differential plate/cathode figure, that is the true plate voltage.

otherwise, your calculations are correct.

you can also measure the actual current running thru individual tubes if you have a meter, and check matching.

set meter on Ma setting, and connect one lead to the plate, the other lead to the center tap of the output trannie.

there are a few choices of bias schemes, each will produce different sonics.

matching between tubes should be within 5 ma any way you shake it.

if you get yourself a Radiotron Designer's Handbook, 3rd edition, you have all the bases covered. pretty much everything is covered in there, except for some fun esoterica. all the basics. a fun read for the enthusiast.