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Old 27th August 2011, 04:00 PM   #21
mogliaa is offline mogliaa  United Kingdom
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Again. Have done some testing as I posted in the 26 pre-amp thread.
Attached is my modular test bed using Gluca's board for the IXYS Mosfet CCS. Filament and HT supply is regulated from my lab power supply.
So, after a testing several operating points I came up with the best one in terms of distortion:

Ia=25mA, Va=77V, Vg=-5.8V (LED array combination of amber and red ones)

THD = 0.019%. Slightly better compared to my best 26 valve measure. I just tested one 4P1L which I had at hand, may need to check if there is a more linear one, however, as it stands the result is really good compared to other valves!

Any suggestions around trying a different op point? What about biasing around -15/-20V and increasing the anode voltage? Will improve distortion?

Thanks
Ale
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Old 27th August 2011, 04:16 PM   #22
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Ive played around with the 4P1L....

Wow! Incredible Microphonics! That things ring like a bell if you make any harsh vibration, and if you tap 'em they ring for many seconds!

I was running about 80V anode and around 25mA with a MOSFET on top, and std cathode bias un-bypassed.....
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Old 27th August 2011, 04:24 PM   #23
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by Alastair E View Post
Ive played around with the 4P1L....

Wow! Incredible Microphonics! That things ring like a bell if you make any harsh vibration, and if you tap 'em they ring for many seconds!

I was running about 80V anode and around 25mA with a MOSFET on top, and std cathode bias un-bypassed.....
Hi Alistair,
:-( that doesn't sound very encouraging though. I did tap it whilst driving it with a 1kHz sine signal and couldn't see any traces of ringing in the scope...
Well, I got very close with the operating point anyway!
Will have to build and test this on a real amp...
Thanks
Ale
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Old 27th August 2011, 04:51 PM   #24
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Maybe its the tubes I was using, I have 4 'svetlana' ones and 4 of some other make, They all rang like bells!--Although slightly different tunes!...

I had arranged the test as the first stage in an amp just to see what they would be like.

I was using a SLA batt to supply the heaters via current limiting resistor, and had it triode-strapped....
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Old 27th August 2011, 05:00 PM   #25
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Quote:
Originally Posted by Alastair E View Post
I had arranged the test as the first stage in an amp just to see what they would be like.
I am afraid it is overkill to use power output tube for the first stage in an amp, unless it is also the last one.
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Old 27th August 2011, 07:43 PM   #26
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So--Probably overkill to use one in a Line-Stage too you reckon...?
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Old 27th August 2011, 07:45 PM   #27
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Quote:
Originally Posted by Alastair E View Post
So--Probably overkill to use one in a Line-Stage too you reckon...?
I did not try that, but in output stage they did not ring.
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Old 28th August 2011, 01:35 PM   #28
hihopes is offline hihopes  South Africa
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I think that in a power-amp, the step-down ratio of the output transformer probably lowers the level of the ringing considerably. (At leaast I hope so! as I intend to build one soon) As a preamp, they ring if any loud noise is made in the room, although admittedly, I have done nothing in the form of damping for them, as this pre is still an experiment.
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Old 31st August 2011, 08:13 AM   #29
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The idea of putting leds in the cathode is to give it constant voltage. But leds have rather high dynamic impedance. I did some measurements to determine the Rdc of leds by changing the current through them and measure the voltage difference and found the following values: Red 5 Ohm, Green 15 Ohm, Yellow 18 Ohm and Blue 37 Ohm. When you put a string in the cathode these Rdc values add.

Mogliaa:
In my ears bias with leds alone sound only so so.
Try to bypass the leds in your schematic in post #21 with good quality foil types and listen to the differences with/without. I am sure it will put a smile on your face!
My recipe is to start with something big and bypass it with several capacitors from large to very small, for example: 10u mkt or pp, bypassed with 2.2u pp, then 100n polystyrene, 10n polystyrene, 3n Russian Teflon, 560p Russian Teflon. Or whatever types you prefer or have on hand! Even the smallest values in the pF region give an easy to hear improvement, so don't forget these.

Peter

Last edited by pietjers; 31st August 2011 at 08:24 AM.
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Old 31st August 2011, 12:04 PM   #30
hihopes is offline hihopes  South Africa
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My LED's are bypassed with new OSCON organic caps. I tried quite a few different types here and these give best balance and clarity in my system with my current operating point. They are ULTRA-low ESR. No small bypass caps needed.
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