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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps
Post Subject: 2A3 with slightly lover filament voltage?Posted by Romy the Cat on: 2/23/2009
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 floobydust wrote:
Question: the 0.75 ohm resistors in the filament lines on the 2A3. Is it your intent to drive them from a 2.5VAC transformer or a 5.0VAC transformer. Voltage drop considering the 2.5amp filament requirement would result in insufficient voltage to light it up.
Nope. The 0.75 ohm resistors in the filament lines on the 2A3 will be driver from 6.3V transformer. What I would like to have ultimately would be to have the SAME filaments droppers for 1A/4V on YO186 and 2.5A/2.5V on 2A3. The 1R and .75R are VERY close and it will be a good idea to combine them.  Would it be a good idea to use 2A3 with slightly lover filament voltage? 
 floobydust wrote:
Comment: On the balancing scheme. Using a 100-ohm balance pot is quite common. Effectively this results in a 25-ohm resistance in series with the bypass cap. I also use a 100-ohm balance pot but pad it with a pair of 12-ohm resistors (end to wiper) which widens the adjustment range and drops the effective resistance down to ~4.8 ohms.
Yes, I know. I will always have a chance to add/remove resistors in order to make the “shaper” selection. From a different perceive with 25-turm it might be “precise enough” even from 100R.
 floobydust wrote:
Secondly, you might want to try a split balance approach. This has worked well for me with the 45 and 2A3 (dual-section types). I use a center-tapped filament transformer and connect the cathode resistor from the center-tap to ground. The balance pot has the padding resistors and the bypass cap only from the wiper to ground, i.e., you have a fixed DC balance and an adjustable AC balance.

Ok, thanks.

The Cat

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