• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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

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#83
Just for reference, a schematic of Rv E is posted early in this thread:

http://www.diyaudio.com/forums/attachment.php?s=&postid=542461&stamp=1104387288

This is a nice kit with very good sound considering the cost.

The heaters are in series and as a result, the 48-50VAC do cause some hum. With my old speakers, you could only hear it with your ear up against the driver. However, with my more efficient BD-Pipes, you can hear it when the music goes silent. Right now it is not a problem, but I think it could be annoying if one had very efficent speakers ~97dB+, so potential buyers should keep that in mind. The snubbers did help reduce a good portion of the hum, but not all.

Also, I played around by temporarily adding more capacitance to the PSU. More capacitance does seem to help out quite a bit.

There is a lot that can be done to this amp, but making modifications is to a large extent limited by the PCB.

Cheers,
Gio.
 
#84
Why not build it without the PCB and do point to point wiring along with Edcor (or other) transformers? Would this improve the overall quality of sound it is capable of making? I'm a total newbie to this so please pardon my ignorance if I am totally off.
 
#85
Building the amp without the kit is definately plausable, but I think a main reason why this kit is so inexpensive is because of the traffos. With better traffos it would definately be better, but the cost would be quite a bit higher. I think this is a good kit for beginers because it is a PCB, point-to-point is not necessarily for a beginer.

Josh
 
#86
dj_oatmeal said:
Why not build it without the PCB and do point to point wiring along with Edcor (or other) transformers? Would this improve the overall quality of sound it is capable of making? I'm a total newbie to this so please pardon my ignorance if I am totally off.
Click to expand...

As Josh suggested, I wouldn't build that exact schematic unless I was buying the kit. However, if you did a search on 6BM8 schematics you would find quite a number of them, since that tube is very popular in Japan. Add some Edcor transformers and you should have good performance for the money.
 
#87
dj_oatmeal said:
Why not build it without the PCB and do point to point wiring along with Edcor (or other) transformers?
Click to expand...

If i was going to do it PP (which i would), i would do some variation on Eli Duttons "El Cheapo". This can be done with 6BM8s, EL84s, 6AQ5s, 6V6s.... With the cheap Edcors you could keep cost to the same or less than the K-12 and if you want to be more frugal you could look for something like these (excuse me for pointing at one of my eBay auctions, but it is a good example). To my mind the topology of El Cheapo is better than the voltage amp followed by concertina phase splitter. It does require either the CCS or at least a negative B- and a big R (this is how we did our 1st build) to get the LTP to track well.

dave
 
#89

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#90
Thanks, so impedance after the tube output is 8 KOhms. How is this number determined?
Also, if the output is 8 Ohms, does it mean that the secondary winding of a transformer must be exactly 8 Ohms, or it can be greater or less of that number?
By the way, does anyone has the specs for the secondary winding of a transformer that provides a B+? Is it 120V, or greater than that?
 
#91
I recently finished casing up my S-5 amplifier. I was reading the paper work that came with the amp and it says “Do not operate without speakers connected.” Does anyone know why this should not be done and what are the consequences of doing so?

Also, does the input impedance change if the potentiometer and input caps are removed?

I posted some pics of the amp in the Tube photo gallery:
http://www.diyaudio.com/forums/showthread.php?postid=920008#post920008
 
#92
BuzzerBro said:
I recently finished casing up my S-5 amplifier. I was reading the paper work that came with the amp and it says “Do not operate without speakers connected.” Does anyone know why this should not be done and what are the consequences of doing so?
Click to expand...

Even though you didn't include a schematic, since it's a VT amp, it's safe to assume that the speeks are coupled to the finals with an OPT. Since VTs operate as output transconductance amps (current output, not voltage output) the OPT operates as a current step-up xfmr. As such, it will try to output the same current regardless of the secondary load. According to Ohm's Law: V= I * infinity= INFINITY!.

Of course, the voltage won't get there because something will poof first, either the insulation of the OPT and/or the finals. This will probably take something else along with it: the power diodes and/or the power xfmr.

Operating with no load frequently renders the VT amp FUBAR.


Also, does the input impedance change if the potentiometer and input caps are removed?
Click to expand...

Yes
 
#93
Only remove the negative feedback resister if you want to

1 kill the bandwidth response of the amp

2 increase the output impedance of the amp.


The amp has a gain bandwidth product, if you remove any negative feedback you will lose bandwidth. To get the gain bandwidth product of an amp is the multiplication of the gain and bandwidth so by removing negative feedback the gain increases and you "don't get out for nout" as they say in Yorkshire.

With a higher output impedance an amp can not control the speaker as well. As the impedance of the speaker is closer to the output impedance of the amp. An idea amp would have no output impedance.
 
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