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PostPosted: 14 Jan 2013, 08:40 
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I'll be interested in your views of _why_ they have "gone back to the single 9V battery for both filament and B+ supply", and the impact on the sound.


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PostPosted: 14 Jan 2013, 13:34 
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I see a AA battery holder. I would imagine that AA batteries would be used for the heaters. Mark, can you confirm?

Cheers

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PostPosted: 14 Jan 2013, 15:24 
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Oh cool! I can't wait to hear your impressions.

Any idea when they might start offering them to the public?


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PostPosted: 14 Jan 2013, 18:06 
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When the K272A came out I thought it a great idea to have separate filament and B+ batteries. So I think it is a retrogressive step not include that battery holder. The battery pack in the image is just what was used to test the prototype. I think to keep the preamp portable and light a singe or two 9V batteries is the way to go. What I have used in the past is two 9V in parallel.

Without the on-board batteries the cct. brd. is definitely small and light. See image. The other cct. brd. is a K272 original with upgraded parts. You can see though it is a little shorter it is much wider than the new brd. Hope to have it going within a week. I will ask when it is to be released.

Attachment:
K272AandNew.jpg


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PostPosted: 14 Jan 2013, 21:11 
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I question the designer of the K272B (my naming of the kit so it may change) why the single supply. He envisages the kit being driven by 6 X AA cells. On my calcs. you should get better than 50 hours of play on a good 9V battery. See his reply below:

"Hi Mark:

The new K272B is powered from a single 9V Battery pack: Six series connected AA 1.5V cells.

To reduce the current consumption, the original K272/A Kits had the filaments of the two tubes wired in series. This resulted in a potential gain unbalance between the 2 channels. In the new K272B kit the filaments are powered seperately, so the two channels are electrically identical.

The total current consumption of the amplifier is around 25mA. Even inexpensive AA cells will power this kit for many hours. The kit includes the PCB and all its on-board components, and the battery holder only."

I have asked again about a kit availability date and price. Waiting reply.

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PostPosted: 15 Jan 2013, 13:44 
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Cool!
Last time I checked, a typical AA alkaline battery was capable of 1.5Ah - 2.Ah where as a 9v (transistor) battery was only capable of about 300mAh. I originally thought it odd that the 272A had split power supplies, however, understood what was trying to be achieved. The original 272, which is what I have, could be powered by a AA pack and get more "running time", or parallel 9v. to get the same operating as a AA pack, you'd actually need 3 to 4 9v in parallel. So, all in all, I'm glad to see that Oatley has gone back to the original supply method. I think having separate filament/cathode supply/bias is a much better approach. It does eliminate the biasing problem of series running fil/cathode. As I understand it, the 6418 isn't a true filamentary cathode, but rather the cathode element is connected to one end of the filament. So even though the K(I) for a given lot of tubes may be equal, inherent differences in H(I) will effect over all biasing when you take the series filament approach. Separating the heaters should resolve those issues. The other issue that I am most interested in is does that also quell the ring problem?

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PostPosted: 15 Jan 2013, 18:35 
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The ringing problem is a mechanical issue. I can't see this even being 100% resolved.

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PostPosted: 15 Jan 2013, 19:18 
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The ringing makes the amp unusable while in motion so IMO battery weight is irrelevant. AA, C and D are ideal for the heaters as you get long play life. For the B+, both the opamps and tube should benefit from higher voltage (18V - two 9V batteries).
:2c:
Cheers

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PostPosted: 15 Jan 2013, 19:43 
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Gio I agree with the higher voltage for B+ and for the buffer chip. I use the OPA2134 which I run on 18V in my HP amps. I think the only way to make the perfect K272X is to build it right from scratch. Use the front end design of the K272B but modify for 18V working and make a differential voltage network with resistors like in CMoy and make the buffer end same as CMoy. Really what you would have is a tubed CMoy. I have been thinking about this for a while and may have a go.

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PostPosted: 16 Jan 2013, 00:58 
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The commissioned job I was meant to start has fallen through. Now I can start the K272B. I thought I made up the name K272"B". Silly me "K272B" is printed on the cct. brd. I have all the parts to start but will have to mount the cct. brd. on it's side. It is too tall for my standard enclosures.

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