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In Reply to: RE: DHT OTL Linestage - Tram 2 posted by bcherry on December 16, 2009 at 00:36:09
I'm about to build a similar preamp. Unfortunately without the flexibility that fixed bias affords. But it all did cross my mind before to do something as you propose here. This should become a real hit...
To infinity and beyond!!!
Follow Ups:
Hi Bas
Yes I'm quite excited after our first listen - it definitely raises the bar with the biggest boldest and sweetest sound I can remember.
Yesterday I tried about 10 different sets of NOS/new 2a3 and 45 (globe and ST). I found only 1 pair that didn't have a problem with microphonics in this linestage: NOS 45st. These were also the smallest tube and had the least mechanical resonance when dinged with a fingernail. The next best were NOS 2A3st.
The worst were the globes, except the baddest of them all (rang like the Liberty Bell) - VA 2A3B single plate. That one has coil springs across the top holding the filaments.
We've got the hum tamed and now just trying to track down the source of some sizzle.
regards
Brian
DIY - Done Right!
Hello Again Brian,
I just ran across an article in 6Moons about Emission Labs producing a run of 45's in case you are interested. Their 300B XLS tubes are fantastic IMO. The microphonics in the DHT's are a real problem. I mounted mine inside a tall case to help in this regard. I realize that some methods are a little difficult for a production run but I had good success with the following.
1.Herbie Audio Labs tube dampers. 2. Mounting the tube sockets using rubber grommets or washers. If you can make up some washers made of sheet sorbothane it should help even more. 3. Last and perhaps most effective in eliminating both microphonics and RFI may be to have the tubes mounted inside a small size transformer cover (1 each) with vent holes. As the 45 does not generate that much heat a few holes at the top and bottom rear of the cover would do the trick and keep aesthetics intact. No, it is not the same as having them exposed but you still have the rectifier tube there and the possibility of removing the cover and mount if the customers' system will allow it. A good test for RFI seems to be my cell phone. I hope this helps in some way.
Regards,
Tony
BTW, with inputs shorted would you know output noise? With NOS 45 I'm getting 0.3 - 0.4mv. Anything else is more. TJ Globe 2A3 and VA 2A3B are the worst > 2mv.
So far all the discussion i'm seeing on DHT linestage only references subjective noise levels.
regards
Brian
DIY - Done Right!
I just measured the output noise and it comes in at 0.2 to 0.3 mV. Not too bad I guess. I should have measured it previously but the system is pretty revealing of any noise. The LadyDay amps are dead quiet now so they contribute pretty close to nothing in the way of noise.
I wish that more folks could really see what it takes to get some of this stuff right. It's not just the parts! The moment when it all comes together is pretty satisfying though. Thankfully your DIY guys know first hand what any change can bring about. I can't wait to see what your end result is!
Happy New Year,
Tony
Hi Tony
Thanks for that. Gives us something to shoot for.
You're right about it being more than the parts. How many times we build something and take it apart and build it again with the same parts, just to get it right.
Brian
DIY - Done Right!
Thinking along those lines, what's the state of development of the LD Mono Max?
Are you any closer to a final design and its pricing?
TIA
Yes! Power transformers due to arrive Monday so we'll have the new pricing a day or 2 later and should be ready to go by the end of the week.
Thanks for waiting.
regards
Brian
DIY - Done Right!
Thanks for the tips. We are testing with NOS 45. So far I've found, the bigger the bulb, the worse the microphonics. I've mounted the tube sockets with 2 very compliant rubber mounts and seems to handle chassis pickup quite well. Also been trying various tube covers. We're getting there.
regards
Brian
DIY - Done Right!
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