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PostPosted: 30 Dec 2010, 15:56 
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After great success with the K272 tube preamp / headamp kit from Oatley Electronics in Australia, Branko, owner and designer, has come-up with a tube buffer kit. After some discussion with me on what I thought DIYers wanted in simple tube kits Branko decided a buffer rather than a preamp may work better in hi-fi.

With the preamp came gain. With gain some hissing on some preamps was evident. With the K295 unity gain means noise in the cct. is not amplified. This is the first of the prototypes Branko sent me.
Attachment:
K292ss.jpg

The later K295 kits have only one tube per channel. Also notice on this cct. brd. rectification etc. The brd. will take AC or DC adapters. In fact Branko will sell the kit with a DC international SMPS or drop on your own AC power pack. Good thinking for those who don't like SMPS.

I'm about to enclose this one and give it to a Rocker Billy double bass player who is looking for a different sound from his instrument. The latest K295 has only single JAN 6418 tubes. I have that prototype too. It should be released as a kit soon.


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PostPosted: 30 Dec 2010, 20:30 
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Cool! Any idea what the price of the kit will be?

If the board has not been finalized, suggest that they leave more space around the film capacitors for those who would like to use better caps. Also, suggest that the pcb can also accept the sealed multi-turn Bournes trimming potentiometers.

Cheers

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PostPosted: 31 Dec 2010, 01:42 
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The kit is the Oatley Electronics Kit: K295 Stereo Tube Sound Line Buffer Amplifier and can be ordered for $36 (plus postage) from: http://oatleyelectronics.com//product_info.php?cPath=81&products_id=922.
Image
Attachment:
k295notes.pdf

I have all I need to house this PCB in a plastic case with 6.5mm guitar plugs and power socket. Plastic because the tubes ring like a church bell for Sunday Mass and I have found when encased in deadend plastic this dose not occur as much.


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PostPosted: 01 Jan 2011, 01:01 
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Here is the finished early prototyped K295 (beta). Once more I made it hard for myself by selecting too smaller enclosure. But with careful cct. brd. placement I managed to get everything in. The hook-up wire is a a thin tinned multi-strand. This wire was used throughout. There are two red leds on the PCB. The idea is you have the tops of the tubes exposed and a red glow issues from the tubes.

There are trim pots on board where gain can be set. I originally thought the K295 had no gian but that is not so. By cranking up the trim pots more volume can be achieved. The strange part is with the K295 turned off signal still passes through but at about -3db lower. Not sure how this happens but it may have to so with my earth wiring.

Why a plastic case I hear you say? Well I have felt in the past when working with these tubes (JAN6418) that in metal enclosures they ring a lot more. In this build, even with the preamp sitting on my guitar amp and speaker, the tubes DID NOT RING AT ALL. Even when I tapped down on the case they barely made any ringing noise. So your next JAN6418 based build will be in plastic? YES!!

I have tried the K295 with my guitar and Fender Champ 5F1. When switched on there is about a 3db lift in the signal level. More tubey sounding or not I don't know. I am already playing through a tube guitar amp. Tomorrow I hand over the preamp to Ray, the double bass player, for a evaluation.
Attachment:
K295CasesSS.jpg
Attachment:
K295Cased2SS.jpg


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PostPosted: 02 Jan 2011, 02:08 
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Ray, who asked me to put together the K295, has had a short opertunity to listen to the preamp via his electric bass. He says it is giving him everthing he wanted in this type of tube preamp. He was after a heavier thicker sound from both his acoustic and electric basses.

Ray also has dual pickups on the double bass. One on the bridge and one at the bottom of the neck. The K295 has two channels. Ray will be able to pass both pickups through the K295. Or, as I suggested to him, pass one pick-up through both preamp channels in series. Extra warmth and extra thick bass.

I was complimented on my build (Ray pulled the lid for a gander). I confessed to Ray that it was a slap-up job and I just tossed it together for proof of concept. He disagreed and thought it was a very professional job. Ray needs to lift his standards. He was also vey impressed with the bitumanised Al foil which lined the lid and bottom. He thought it was a great idea for shielding and resonance control.

The plastic case appears to work very well with these tubes. As everyone who has built anything with the JAN6418s knows they are shockingly microphonic. And yes you mount them in a metal enclosure they will ring with just a hand clap. BUT in a tough ABS plastic case, with the bitumanised Al foil lining the lid and bottom, it is a much quieter build. I couldn't get the tubes ring, even tapping down hard on the case, only gave a very short very quiet ring.

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PostPosted: 02 Jan 2011, 11:20 
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Hi Mark, do you also have the latest version of the kit? I see that the PCB is slotted. Does that allow breaking the PCB in two so that the PSU can be located away from the preamp circuit?

Good notes on the enclosure damping for microphonics. Are you also using soft absorbing feet?

Cheers

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PostPosted: 02 Jan 2011, 13:19 
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mwhouston wrote:
...you mount them in a metal enclosure they will ring with just a hand clap. BUT in a tough ABS plastic case, with the bitumanised Al foil lining the lid and bottom, it is a much quieter build. I couldn't get the tubes ring, even tapping down hard on the case, only gave a very short very quiet ring.

That's interesting. There has got to be a "secret" to keeping these tubes from ringing - I can't believe they were designed to allow that to happen in the intended application for which they were designed, whatever that was. Perhaps you've stumbled on to something. The only way I could control the ringing was to feed the grids with a low impedance source, but everything was on a breadboard unshielded, and even then I don't understand why the tubes would stop ringing with a low impedance source.

One thing I also noticed (very strange but perhaps a clue) was that when the tubes are ringing, if the heater supply is disconnected the ringing goes up in frequency until it either dies or goes ultra-sonic. A momentary push button N/C switch in series with the heater supply will kill the ringing in about 1 second or less while still allowing the signal to pass for the most part unless the button is held down too long, but during that 1 second you can hear the pitch of the ringing go up, up, up - gone! I would expect the ringing to stop with no heater, but the pitch change while the heater cools down is strange.

I bought a good supply of these tubes for a guitar/bass preamp and it would be nice to make it all tube, but right now because of the ringing I put SS op-amp buffers (not likely to degrade the sound of a distorted guitar) between each stage to 1) stop the ringing, and 2) allow inversion of the signal (or not) so that distortion can be applied asymmetrically (mostly even order products) or symmetrically (odd order "power amp PP type" distortion). It look GREAT on paper - I just hope it works like I expect it to. If it does I'll design a PCB and post it. Ideally this would actually fit inside one of the the guitar body cavities or clip onto the guitar strap.

I guess I'll have to move the breadboard to a plastic case and see what happens, but shielding is an absolute must.

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PostPosted: 02 Jan 2011, 14:24 
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Just about everything I make, metal case, wood or plastic, I line it with the bitumanised Al foil. But I rearly earth it?? I picked up on the plastic case and not metal case for these tubes when I made the two K282 phono preamps. I noticed my phono preamps rang even when there were just loud passages in the music.

Yes Gio I have thick soft silcon feet on the plastic case for the K295. I'm hoping to get to Ray's in the next few days to take listen to his set-up with the preamp in place. Ray's amp is something like a 400W SS bass guitar amp. I helped him set-up a horn with the amp to enhance the high frequency of his double bass. Ray likes to hear the string "thwack" thus the neck pickup. A horn will do this. But Ray kept burning them out so I suggested a large piezo horn and a 2.2uf 100V Xover cap. All of which we sell at work. This has work all year under some very heavy and loud playing.

For this build I went back to plastic. Also I only gave myself an afternoon to get the whole project complete so I wanted to work with something which was easy to drill etc. Also cost came into it.

Yes Gio the production run K295 dose have the option to break the PS away from the preamp brd. Another nice little touch from Branko to offer flexibility to the kit builder. I have one of the later brds. (still prototype but complete and final design). The onboard tranni is tiny. If Ray would like I could case this one (single tube per channel) too. Also I have asked Branko if there are any more twin tube brds.

I have suggested that Oatley may like to put this kit out in two forms: K295 single tube and K296 twin tube. I think the PS should be common to both. No need to change.

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PostPosted: 02 Jan 2011, 15:04 
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What is the difference between this kit as a line buffer and the 272 as a preamp. With the 272, I used the trimpots so the gain is relatively fixed. The 295 uses AC power. Is that the only difference?

My 272 begins to make some noise after about 10 - 12 hours on the battery, so a kit with AC power would be nice.

-- Chuck


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PostPosted: 02 Jan 2011, 18:13 
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The front halve of the K272 is similar to the K295. The K295 doesn't have the OPAMP buffer. Branko felt originally the JAN6418 would not drive the next stage well but later found it would.

The AC PS is a good idea and can be 110 or 240v driven.

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