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Old 12th September 2012, 05:20 PM   #2021
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ps these things are SO microphonic. You guys are nuts! .."

We're worse than nuts. I. for one, have gone over to the dark side and am using 4P1L instead of 26. That's just after stocking up on 50 nice 26s!! I blame Ale....

The 4P1L is even more microphonic, but it's quiet - much quieter than the 26.

Andy
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Old 12th September 2012, 05:55 PM   #2022
euro21 is online now euro21  Hungary
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Quote:
Originally Posted by CSlee View Post
Firstly THD was quite high - 0.1%, but that settled after a while to 0.07% - seems reasonable.
Bad tubes.... or Cunningham CX-326s? :-)))
My five pairs #26 tubes (at 6Veff output) has 0.026-0.041% THD... except CX-326s (0.097, 0.124).
Furthermore CX326 has mediocre sounding. :-(
For me the best sounding pair is Majestic G-26 tubes.

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Originally Posted by CSlee View Post
10R 50W filament resistor got to about 110 degrees C
I think your heatsink undersized... or no heatsink.
I use passive (HP computer) CPU heatsinks.
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Old 12th September 2012, 06:29 PM   #2023
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Default 4P1L

Quote:
Originally Posted by andyjevans View Post
We're worse than nuts. I. for one, have gone over to the dark side and am using 4P1L instead of 26. That's just after stocking up on 50 nice 26s!! I blame Ale....

The 4P1L is even more microphonic, but it's quiet - much quieter than the 26.

Andy
With a qick scope around I found a 4P1L Tube Price of a staggering 1.60 each!

If that a real deal, I can see the attraction already

How much quieter? Anyone got a real mV or uV comparison?

Not so pretty though

Best,
Susan.
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Old 12th September 2012, 06:47 PM   #2024
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Originally Posted by Susan-Parker View Post
Not so pretty though
Military people don't like prettiness. They like robustness and other specs that matter.
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Old 12th September 2012, 06:51 PM   #2025
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
ps these things are SO microphonic. You guys are nuts! .."

We're worse than nuts. I. for one, have gone over to the dark side and am using 4P1L instead of 26. That's just after stocking up on 50 nice 26s!! I blame Ale....

The 4P1L is even more microphonic, but it's quiet - much quieter than the 26.

Andy
With filament starvation (i.e. 500mA if filaments used in parallel) and proper socket suspension, 4P1L microphony is barely noticeable.
Knowing your building layout don't expect it to be quiet if you bolt it to you 4mm aluminium top plate
Ale
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Old 12th September 2012, 08:49 PM   #2026
CSlee is offline CSlee  United Kingdom
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Originally Posted by euro21 View Post
Bad tubes.... or Cunningham CX-326s? :-)))
My five pairs #26 tubes (at 6Veff output) has 0.026-0.041% THD... except CX-326s (0.097, 0.124).
Furthermore CX326 has mediocre sounding. :-(
For me the best sounding pair is Majestic G-26 tubes.


I think your heatsink undersized... or no heatsink.
I use passive (HP computer) CPU heatsinks.
I am not sure what type my valves are, nearly all the markings have gone. But it is good to know that they are normal results.

As for the heatsink, they are mounted straight on the 4mm aluminium chassis, so no purpose heatsink.

Here is a photo I took earlier, it isn't that clear but it shows the large instability issues. y axis shown in 2dB steps. Again, didn't have much time to continue after I fired it up but I am glad that it is almost up and running, it's only taken a year....

Charlie

edit: the frequency response is a multiple tone response for rapid results - so this isn't acurate
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Last edited by CSlee; 12th September 2012 at 08:54 PM.
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Old 12th September 2012, 08:54 PM   #2027
mogliaa is offline mogliaa  United Kingdom
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Better 10ohm 20W dale wire wound resistors as Andy suggested before. I'm using 4 (2 parallel pairs in series) and they do get hot anyway.
Check out the 26s. Some are very noise and do pick up hum when you put your hand close to them. Other good ones are very quiet!
Ale
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Old 12th September 2012, 11:20 PM   #2028
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Knowing your building layout don't expect it to be quiet if you bolt it to you 4mm aluminium top plate...>

I'm not getting any problem with microphonics in my solid chassis, but my room is well damped and speakers are about 89db. I even used to use 1G4G and they're horribly microphonic in other rooms and systems.

By quiet I mean they don't hum at all or make funny noises when you put your hand on them!! They also all test pretty much the same - quality control is excellent.

Andy
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Old 18th September 2012, 09:41 PM   #2029
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Default One year on...

OK so worked on the preamp this evening and the end was in sight so I carried on. I have no way of testing properly at home but I was able to make sure I had the right voltages in the right places. And managed to get it to a working state - still a lot of work to be done tweaking etc... making it safe for my cat..

But it is working, and I am currently listening to Joni Mitchell Blue as I type this. In fact it sounds like she is sat right next to me. The tone is superb, very soft yet still retaining all clarity. Unfortunately I just moved and am not yet used to my new listening environment, but I am looking forward to listening more and more.... it is very intriguing!

I will hopefully take it into work tomorrow and get some proper results and eventually .... some day... make it safe! I could not get the full 1.5V across the filaments but I think this is down to a slightly low PS.

Thank you to all for your help. Ale, Rod, Thomas, Andy and DIYers here and in the London Circle - thank you for being so patient with my questions...

.. Andy I hold you fully responsible for getting me into DHTs and may never have money again because of it, but at least i'll be happy!

Cheers

Charlie
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Old 19th September 2012, 07:09 AM   #2030
mogliaa is offline mogliaa  United Kingdom
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Hi Charlie,
Well done! DHT is one way route to financial breakdown, but as you say, happy days!
You don't need to have 1.5V across filaments, dht can perform better in preamps with slight filament starvation. I run mine at 700mA which should put filaments around 1.5V...
Cheers,
Ale
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