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|25th August 2013, 11:47 AM||#803|
Join Date: Jan 2004
A great suggestion was sent to me by Lauscher to use 6A3's or equivalent in filament bias with the coleman regs for the output tubes.
I've been considering this and here are the pro's and con's:
Gets rid of the big bypass cap.
Less heatsinking required for coleman regs
NOS tubes are much cheaper and easier to come by
The Ra raises over 1k ohms which saps power and power delivery.
Less flexibility with output transformer due to the higher Ra.
Higher Filament voltage may mean more noise, of course with a DHT headphone amp noise is the greatest challenge.
I think I am sticking with 2A3 for now, almost think PSE 4p1L's would be a better choice if you must have filament bias than the 6A3. But for ultimate power PSE 6A3's in filament bias may be the ticket over a 300B.
|25th August 2013, 06:15 PM||#804|
Join Date: Oct 2007
to get a good sound from a DHT tube with DC filament, you have to decouple the music signal from the DC supply.
- conventional CLC supply and then a 0,2H to 0,5H choke before the filament. A CRC supply will generate a lot of heat with 1A - 2,5A.
- the reg. from Rod Coleman are no usual voltage or curent regulators. They sound so good because they separate the signal from the DC supply.
On my 300B amp there is no noise on my 101db speakers. Headphones are much more sensitive. I suggest send a pm to R. Coleman. He is verry helpfull and has a lot of theoretic and practical experience. His kits are not expensive. You can not use them with a simple 10,000µF cap in front. With a CLC you get them silent.
The input voltage must be 6V-7V higher than ouput voltage. It dissapates more heat if you use 2,5A instead of 1A.
The Ra does generaly rise when you use filament bias, because you remove the cathode capacitor.
Ra total is=Ra+(mu+1)xRcathode
Your OPT has 5KOhm, so 1K Ra is no problem. Usual formula for OPT is 3 to 4 times the Ra. For the Hifiman HE-6 you need a lot of damping. PSE with 4P1L is one solution. Plus you have a lot less heat dissipation in filament bias than with 6A3/6B4G/6C4C.
If you want low Ra, then use 6C33.
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