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Old 23rd July 2012, 08:04 PM   #1
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Default Which OP transformer and materials?

After my recent fiasco of the non arriving but take my money amplifier from J. Sound Lab and not being able to find anything I can afford or "like", I am going to build my own 300b SET integrated amplifier. I have decided on the schematic (J E Labs 300B SE), that I am going to use. I am given to understand that there are other designs that may be better but I really would like to keep it simple as this is my first big project. Being a bit in the dark I have a few questions that I would really appreciated getting some feedback on...

I was thinking of using 10mm black acrylic for the top and bottom plate with a dark mahogany frame. Will the acrylic be able to take the heat generated from the valves?

Should I use Teflon or ceramic valve bases and is there any brand that I should keep clear of?

Which OP transformers to Use? I have looked at the Sowter SA08, the SA07, Audio Note group B and the Transcendar Transformers TT-017-OT. I am really in the dark here and I don't want to pick the wrong one. I have heard good things about all of the above. The Sowter SA08 I think might be overkill but I am probably justifying picking the cheaper SA07. I am note sure if the Audio Note reputation is swayed by the Brand Name. The spec looks decent on the Transcendar though they are the most reasonably priced of the lot and I think the lightest in weight.

Any help would be great!
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Old 23rd July 2012, 09:03 PM   #2
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What is the budget?
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Old 23rd July 2012, 09:25 PM   #3
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For the whole amp or just the transformers? If it is the latter then no more than 500
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Old 23rd July 2012, 10:53 PM   #4
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top plate: somehow the heat needs to get out, and a metal chassis or top plate is so much easier to work with. Copper is easy to make black, but it's expensive.

transformers: Sowter give enough data to enable a sensible decision, and will provide more if you ask. If Audio Note and other options provide the same data, you can compare, otherwise you have to trust to reputation. I trust Sowter's no-nonsense approach, but they are not cheap.

Given you have enough data, you then have to decide on your priorities, because every transformer is a compromise. To some extent you should take into account the kind of speakers you will use, and whether you will be using global feedback. The relationship between valves, transformer and speakers is quite complicated, especially in the bass region. OTOH, it is unlikely that enough data will be available to support a fully reasoned decision, so you may as well not bother.

If you are choosing between transformers of equal quality, dc current rating, bandwidth and power rating, then the biggest is probably the one to go for. This is because full-power bandwidth is not the same as that quoted for small signals. If you want full, articulate bass, you need a transformer that remains as linear as possible at low frequencies at high power. The bigger the core, the better in this respect. If I were uncertain or befuddled, that's how I would decide.

You've been sensible to choose a simple circuit and concentrate on the quality of the transformer.
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Old 23rd July 2012, 10:55 PM   #5
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Quote:
Originally Posted by bottletop View Post
For the whole amp or just the transformers? If it is the latter then no more than 500
Well, that allows lots of possibilities for top-shelf iron, like Hashimoto, James, Electra-print, Tango, Tamura, etc.

What's the budget for the entire amp?

On another note, have you considered any other 300B SE designs?

Are you planning on using an 83 rectifier? You may want to consider a non-mercury rectifier like a 5AR4, GZ34, GZ37, 5U4 Etc. Any of these should work as well with little (or none) power supply tweaking, and eliminates bringing mercury vapor into your living room.

Using the 83
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Old 23rd July 2012, 11:31 PM   #6
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Several years ago I snagged a pair of Transcendar TT-015's on eBay for about $110. (3K-100mA-15W SE) This was before they began jacking the starting price up. I made a lash-up with an external Kepco lab supply and was impressed with the sound quality. But I got busy with something else and never made a finished amp. Maybe someday. But even at todays price, they're still a good deal.
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Old 24th July 2012, 10:32 PM   #7
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Boywonder- I will be using 5U4G as I have several SED and Faico versions of these. The faico version are virtually identical- I can hear no difference and the faico is a fraction of the price

Hollowsate- Cheers for sharing your experience and yes they are a bit more pricey now.

Plasticisgood- Thank you for you rational input. I did wonder about copper. At work (teflon extrusions) our electrical engineer pointed out to me that copper is good blocker of RF. I think you are probably correct on the sowter transformer. I have looked at James as well. But I keep coming back to sowter. I need to decide which to go for the SA07 or SA08. Again showing my ignorance but trying to be sensible- the SA07 seems to show a flatter response but I am wondering if the overkill of the SA08 might be better in the long run. IE- Better to spend once on the expensive transformer than to spend less then end up buying the one I really wanted but talked/justified myself out of. I did not; but should have guessed that like with valves, circuit design and even caps there is no one ultimate perfect one for every job. Like you say- it is a compromise.

I need to think more on this and spend more time comparing the stats.
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Old 9th August 2012, 12:13 PM   #8
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I remember a fire at work where there were acrylic turnings in the tray of a lathe and hot metal chips had landed on top. Plastic has a tenancy to crack where cut or drilled unless you use special tools for cutting, would require rubber washers under the screws or bolts to keep from over compressing the transformer mounts. I would have serious reservations from the flammability, if a component fails you could loose everything!
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Old 9th August 2012, 06:09 PM   #9
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Roline, That is a good point about the cracking. I could imagine that would happen at some point down the line. Flammability is an issue which I did not take into consideration. A very good and concerning point. I think ally plate is looking like the winner so far.
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Old 9th August 2012, 07:14 PM   #10
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The best OPTs I have used out of Lundahl, Audio Note and O-netics is O-netics. Bud Purvine makes them in the USA in small quantities. There may be a waiting list but it's worth finding out what the situation is. hpurvine@gmail.com I'd prefer these to Sowter. I'd also seriously consider James and Hashimoto instead of Sowter.

But I'd put my resources into the input section. I'm using 4P1L in filament bias into either Lundahl LL1692A/18mA or Hammond 126C interstages. This is a cheap and wonderful directly heated tube. If you're anywhere near the London area you can come and have a listen. I'm in Hammersmith.

Andy
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