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Old 3rd April 2008, 02:54 PM   #411
chrish is offline chrish  Australia
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Time to bore you with some more progress shots.

In previous projects I wired the heaters with 1mm single core wire. With this project I purchased valve sockets that have small posts rather than eyelets. They look good, but I soon discovered that wiring he heaters with 1mm wire was going to be impossible. Only other single strand I have is 23AWG, a shade under .6mm. Mr. Jones states in "Building Valve Amplifiers" that .6mm is OK for 9 pin valve heater wiring. He did not elaborate on how many in the string can be wired with .6mm. I ended up wiring each valve heater with its own run of wire from the tag strip near the power transformer.

An advantage of having a -15 volt supply is that I could easily use a switch that has an led back light.

Hope to have it finished next week!

Chris

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And, of course, the compulsory glowing valves shot...

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Old 3rd April 2008, 03:53 PM   #412
Yvesm is offline Yvesm  France
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Bah !

Having each heater powered separatly is not a bad thing and your wire is probably large enough for that.

Easy to check remaining voltages !

But, why using small caps (?) at each socket ? Are there locally grounded ?

I can see the "usual" two resistors on the solder lug where the heater come from, no need for more cosmetics !

Yves.
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Old 3rd April 2008, 03:54 PM   #413
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Chris: Is that a Bulgin (Blue LED) Power switch? I am assuming that these are only available in a momentary config and I am wondering how you use this as a power switch.......
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Old 3rd April 2008, 09:29 PM   #414
chrish is offline chrish  Australia
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Yves,

I recall reading in one of Morgan Jones books where he mentions using a small cap from each heater pin to chassis. Did this in my last build and it did not appear to do any harm. I am new to this, so if I am doing the wrong thing I can always remove them. My first point to point effort hummed and buzzed so I went in to the books and tried to absorb as much as possible to make a quiet amp. The last two builds have been hum free, but I am a bit paranoid after that first build...

boywonder,

The switch is something I picked up one day while browsing at an electronics shop in Perth WA (Altronics). It is alternate action, DPDT, 12volt for the light. Think it was $8 or $9 here in Australia. Noticed that Rob11966 used a similar thing on his beautifully built Dynaco clone, that is what made me pick up the switch when I saw it, hoping to incorporate it in a future project.

Thanks for the interest,

Chris
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Old 3rd April 2008, 10:42 PM   #415
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Chris: Any chance that you could post any ID markings on the switch? Mfr?

Thanks
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Old 3rd April 2008, 11:35 PM   #416
chrish is offline chrish  Australia
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Here is a link to the Altronics web site http://www.altronics.com.au/index.as...=item&id=S0937

The above mentions that the switch is rated at 500mA 250VAC, however the actual switch has 3A 220VAC printed on it.

The switch is ONPOW LAS2, google showed this http://www.made-in-china.com/showroo...-LAS2-GQ-.html
However I suppose that you don't want to order 1000 units (min order)

Surely you must be able to find something locally, you guys have everything over there!

Cheers,

Chris
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Old 5th April 2008, 04:55 AM   #417
jueic is offline jueic  Taiwan
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Default LTP-CCS --Bias current

Hi, Gingertube

I am in the middle of building this amp and need to clarify the resisitor value controlling CCS bias current.

In the circuit, the RED LED gives 1.7V, and BASE-EMITTER junction takes 0.7V, this leaves the voltage across the 1K ohm resistor just about 1V, so the CCS has constant 1 ma cuurent flow..this means each side of the circuit has 0.5ma.

Could this 0.5ma be a bit low for 12ax7 ??
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Old 7th April 2008, 07:56 AM   #418
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See the test result from Chrish above. He got 1.28mA

There is a trade off between getting the 12AX7 current higher and the anode voltage dropping due to increased voltage drop across the anode resistors.

About 1.2mA (that is 0.6mA each side of the diff amp) is a good value but 1.0 or 1.1mA would be OK.

If the anode voltages on the diffamp are less than say 140 Volts then decrease the CCS current a little. If the anode voltages are above say 190 Volts then you can increase the CCS current a little.

The way to think about this is ( for example)
The Output EL84s are going to need +/- 12V signal swing, so that is what has to appear at the differential amplifier (diff amp) anodes.

The feedback voltage appears 180 degrees out of phase at the other end of the 220K anode load resistor. Say we have +/- 24 Volts of feedack signal at that point (depending on the size of that cross coupled feedback set resistor). We would then need to develop a total of +/- 36V across the 220K load resistor which means we have to swing 36/220K = +/- 0.164 mA current through each triode of the diff amp. In that case 0.5mA would be OK - the current would swing down to 0.336mA and up to 0.664 mA. Output may be a little cleaner (less distortion) with 0.6mA idle current as it doesn't swing as close to 0 current where the worst non-linearities are BUT the distortion is mostly second harmonic and the diff amp action will take most of that out, I don't think it will make a lot of difference.

So while you will nearly always see 12AX7 operated at higher idle currents than this (typically more like 1mA) in this circuit 0.5mA to 0.6mA is fine and that helps keep the anode voltage at reasonable levels.

From the above you can also see that it is possible, if you use really high values of resistor for that cross coupling feedback set resistor, that eventually the 12AX7 would try to swing down to 0 current. I've found that about 47K or 56K are the practical limits on its value but I never liked the sound with any of those really high values. Around 15K to 20K sounds good - up to 33K can sound OK depending upon how "tight" and "neutral" you want the amp to sound. At 47K the amp sounds transistor-ish (those 3 legged fuse things) and is quite dull, sterile and boring.

Hope this "explaination" helps.

Cheers,
Ian
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Old 7th April 2008, 09:42 AM   #419
chrish is offline chrish  Australia
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Thanks for the explanation Ian, and for the question jueic. When I plotting the load lines for the input triode, I forgot that each would see half of the load (doh!). No wonder I was scratching my head about 1.28mA being too much!

Hope to have my amp wired up tonight! Will post results if successful.

Cheers,

Chris
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Old 7th April 2008, 03:50 PM   #420
jueic is offline jueic  Taiwan
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Thanks, Ian & Chrish

Now the circuit makes a much better sense to me after reading your explainations... I finally settle with 0.65ma and 12ax7's plate sitting at 140V. Music sounds quite good (and right) at this point.

The circuit is brilliant.. and very sensitive to component change, I can hear the difference of music presentation for each component change.. I am totally amused !!!

JueiC
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