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Old 6th January 2004, 04:05 AM   #1
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Default st-70 questions... tips?

hello

i plan on building my own dynaco st-70. i dont know a whole lot about tube amps. i am also buiding some speakers to go with the amp and i am aiming for about 93 db efficiency... is that efficient enough for 35 watts. also are there any good mods for the st-70 besides the PS cap mod that i should know about. i was also thinking of using solid state rectifacation instead of tube rectification, or maybe even integrading both, to switch between the 2. what transformers should i use, should i use printed circuit boards, or wire the amp point to point, any one know of a good chassis to use, i could go on forever. any input or tips would be greatly appreciated.
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Old 6th January 2004, 04:25 AM   #2
SY is offline SY  United States
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I've probably built/modified a hundred of these over the years. It's a classic. The 7199 is problematic these days in terms of availability. You may have to go with a different input stage. Check out the one used in the Eico HF87- it's a simple topology and works extremely well. It will drive EL34s easily.

Regulation can be a good thing, depending on your chosen output stage topology. Joe Curcio's regulator works very well and is reliable. His input board is also excellent. I've got one of his ST-70s (lent out at the moment) and have had excellent results with it.

What sort of iron are you planning to use? Original Dyna?
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Old 6th January 2004, 05:21 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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First off, design your amp around the iron. Do you have any output trannies in mind? They are the heart and soul of a tube amp. Good trafos are ESSENTIAL!!!!

I take it you have some electronics experieence, and can solder. That, and some basic metalworking skillss are all you need to build a stonking tube amp.

If simplicity is your goal, go for auto-bias (cathode bias). Much easier to design and more reliable. Slightly less power however. Post any questions you have here, people will give you good advice.

As for tech stuff:

I would use SS rectification in a PP amp. Much better power supply regulation and less distortion. A mullard GZ34 is running pretty close to max in an ST70 anyway. You could use 2 modern GZ34 or a couple of 5U4, that would work pretty well.

As for power output, 35W will drive pretty much any speaker on the market to decent levels... with 93dB speakers you could go as low as 2W!!!

I was driving my 89dB B&Ws with a 4.5W 2A3 amp fpr a while, it was fine, with a surprising amount of grunt available.

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Old 6th January 2004, 08:10 PM   #4
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Default ...

i work at a repair electronics shop and have no problem getting 7199's... but would it be better to use the input stage in the Eico HF87. and yes i am very experienced in soldering and fabricating(i was a machinest for 2 years). i plan on using the magnaquest irons, or the one triode elctronics sells, what do you guys recomend. also, simplicity is not my goal, ill do whatever it takes to get the best sound possible with this amp, without getting to carried away. thanx, sean
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Old 7th January 2004, 11:16 AM   #5
SY is offline SY  United States
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I'd still use the Eico's input stage. All triode, good drive and balance, simple circuit (three tubes for a stereo board). And you won't have to worry where your next 7199 is coming from!

If your goal is "the best," I'd absolutely use a regulator, and lean heavily towards the Curcio design.
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Old 11th July 2005, 01:47 AM   #6
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Hey, I finished the amp a couple of weeks ago. It is awesome, very smooth, very good frequency response and bass nextension for a tube amp. Sound clear, mids are amazing, i couldnt be any haoppier with this amp. I am pairing it with a pair of Fostex FE167E's in a TL. There is no need for a subwoofer when listening to music. The bass definition and extension will blow you away, especially when coming from only 2 6.5 inch drivers. Using an ashley spectrum anaylzer, a sig gen, and my ear, the system goes from about 30Hz to 22k, +/- 2dB with the exception of a 4dB dip at 160Hz caused by floor bounce. One thing to look out for if anyone plans to build an ST-70 using joe curcio's mods, or at least the PS mod. the schematic is wrong, the 2 bias caps are backwards and point "D" on the original shem is actually ground.
Oh yeah and about the tranys, i got a used pair of original cloth leaded OT's off ebay for $150, and bought the PT from triode for $100. All 3 are very good transformers. I highly recommend this amp and am very proud of the results, ill post pics next week.

Ive got the itch now... what'll my next system be?
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