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Old 29th October 2014, 06:43 AM   #1
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Default Silicon Chip mag "Currawong" amp

Details and cct in November issue.
10+10W, 6L6, 12AX7
Looks way easy to build.
Interesting to see what a kit costs.

Edit - can't spell "Silicon" ^^
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File Type: jpg Currawong smaller.jpg (217.4 KB, 557 views)
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Old 29th October 2014, 07:44 AM   #2
Keit is offline Keit  Australia
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Not to worry.

The WA newspaper The Sunday Times consistently spelt it "silicone" every single time for years. Perhaps they thought transistors and IC's were made of oil or rubbery potting compound.
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Old 29th October 2014, 01:27 PM   #3
TheGimp is offline TheGimp  United States
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If that Is the OPTs in back of the tubes, FR is going to suck. They look smaller than the Edcor XPP10 series.

So much effort and then cut corners on OPTs does not make sense to me.
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Old 29th October 2014, 01:44 PM   #4
Keit is offline Keit  Australia
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It may suck more than you think.

If you read the article in Silicon Chip, it appears to have been designed to use parts that are standard stock items of one of their major advertisers.

The article (October 2014 issue, pages 24 thru 26) states that the transformers are standard 100 Volt line transformers used for building public address, of the sort that have "power" tappings on the primary side. They use the Gnd as one plate load, the 20W tap for the other anode, and the 5W tap to connect to HT.

Last edited by Keit; 29th October 2014 at 01:48 PM.
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Old 29th October 2014, 09:28 PM   #5
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There must be a zillion people that would love to make a valve amp but the cost and availability of output transformers is a major stumbling block. This amp might just be the thing that gets them going. And the OPTs might be smaller than on some other amps but the proof is in the pudding - it may sound better than some people expect. Anyway, nothing a sub amp wouldn't fix. Besides, there is nothing stopping you from putting on a different set of trannies if you really wanted to. Heck, I've seen some stuff around here modded to the point that there is hardly anything left of the original design.
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Old 29th October 2014, 10:28 PM   #6
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Besides, there is nothing stopping you from putting on a different set of trannies if you really wanted to. Heck, I've seen some stuff around here modded to the point that there is hardly anything left of the original design.
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Old 30th October 2014, 07:03 AM   #7
Keit is offline Keit  Australia
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The info I put in my earlier post about transformer tappings was what I deduced from the intro article in the October issue, and I got 2 tappings reversed by mistake anyway.

The November issue of Silicon Chip has now appeared in newsagents. It has the full circuit and some performance graphs.

The distortion performce is not that good for a an ultralinear amp with 6 tube functions and the low power output of 10 W per channel. THD is claimed around 0.1% to 0.2% depending on level and frequency. Neville Thiele did better than that in a HMV (Australian EMI) design using just two 6BM8's in a Quad-style circuit.

It is interesting what they did for an output transformer. It is a Soanar M1115 100 Volt line transformer intended for building public address. This has tappings of 1.25W, 2.5W, 5W, and 15W.

The Com (100% of one half winding turns wrt to HT) is connect to one anode, the 5W terminal to HT, and the 1.25W tapping (100% of one half winding turns) is connected to the other anode. The 15W tapping (42% of one half winding) is used for the first screen, and the 2.5W tapping (41% of one half winding)

It was found by the inventors of ultralinear, Haffler & Keroes, that tapping around 43% is optimum from the point of view of distortion, for tetrodes, and this was been confirmed by others, eg the Amalgamated Wireless Valve applications laboratory.

However, using 43% tappings puts stringent requirments on the transformer for stability, and many commercial UL amplifers used tappings at lower percentages eg 20%, resulting in slightly more distortion and marginally more power output.

Unfortunatley Silicon Chip have included no waveforms or any information that can be used to assess stability. The maplifier uses lag compensation, which is a simple effective way to address stability, but it may cause transient intermodulation.

The output stage screen resistors are 47 ohm. Many people think that screen resistors are just there to prevent parasitic oscillation. They do prevent it, but it was found by researchers in GEC a long time ago, that as screen gm is curved, you can optimise the screen resistors to futher reduce distortion. The Silicon Chip authors seem unaware of this, and their chosen values are quite low.

The construction technique is "naked PCB on display", which may offer visual appeal to some, but is a hazard for family use. It will collect dust, and be a hazard to children and even adults who are not electronics trained.

Last edited by Keit; 30th October 2014 at 07:11 AM.
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Old 1st November 2014, 11:23 AM   #8
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Hi

I was very exited when I saw this new kit, then very disappointed with the output transformer choice.

There are huge choices of output transformers they could have chosen, but I suppose we should make it 1st and see .. suppose we could use as a headphone amplifier?

But lets see if we can do our 1st mod, change out the transformer, I probably will not make it as I have just completed my K8011 90w valve mono block, so I don't see the point

regards

Nick
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Old 1st November 2014, 04:32 PM   #9
Keit is offline Keit  Australia
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Never in a million years would I spend all the money required to building an amp with 6L6's and all the supporting solid state stuff they put in, delivering just 10 watts, when I could build an amp with just two television triode pentodes readily available as NOS, that will deliver more power and less distortion. And most likely much better stability. And less transient distortion. Sure you can put in a decent transformer. Put it in a better circuit.

Like I said before, the Silicon Chip is not designed for quality performance. It is designed to use up parts (6L6's, the PA transformers, and the rest of it) that are in stock at their major advertiser Altronics.
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Old 2nd November 2014, 10:40 AM   #10
Mitty is offline Mitty  Australia
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I was pretty excited to pick up the November issue to learn more about this amp. However as I look at the required parts, I begin to worry about the cost for something with such low performance specs. While it may offer decent bang for buck, it would be a bit risky for myself as a bit of a beginner to put that kind of cash down on something I could fry if I were to make a simple mistake.

Keit; do you have any links to schematics/diagrams/guides that would help educate a beginner on how to construct something like what you have described?
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