Posted by PakProtector [ 205.188.116.67 ] on October 08, 2005 at 17:48:47:
Hey-Hey!!!,
I got the 1:1 linestage up and running. AC to the 5687's heater no less. While waiting for Mouser to deliver parts for the DC supply( 60,000 uF of Low-Z caps, Schottky diodes and somemisc bits for the rest ), I hooked it up.
The buffer is a hybrid cascode with IRF820 MOSFET's and a section of 5687 running at 15 mA. This faux pentode has some seriously flat "plate" lines, so I went with a single stage of B+ filtering.
Output is through a 1:1 80% Nickel TX by Intact Audio. .004" gap for 15Hy of primary L.
I ran a $2.99 RadioShack Alps pot for the volume control. It sounds quite good. The low gain removes the principle complaint with those cheap pots: tracking mismatch.
Compared to the previous active design( with gain ), this one is a bit cleaner. Slightly less euphonic perhaps. The active loaded 12B4 is not very colourful tube, and that's part of the reason I like it so much. The buffer circuit is a step cleaner. How much from the cascode buffer circuit, how much from the high perm, bifilar output TX's?
I have got to say I am pleased with it. If not for the Nickel outputs, this would be quite inexpensive. No DC filament requirement, single stage L-C filter for B+. It is quiet...on AC no less. That one was a very pleasant suprise. The 6754 allows a full warm up of the 5687 before bringing on the B+ slowly enough to make a damper diode jealous.
cheers,
Douglas
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