{"id":832,"date":"2012-12-09T21:46:59","date_gmt":"2012-12-09T21:46:59","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?page_id=832"},"modified":"2017-07-08T07:51:28","modified_gmt":"2017-07-08T06:51:28","slug":"01a-dht-pre-amp","status":"publish","type":"page","link":"https:\/\/www.bartola.co.uk\/valves\/dht-pre-amplifier\/01a-dht-pre-amp\/","title":{"rendered":"01a DHT pre-amp"},"content":{"rendered":"

After working for many months in refining the 26 DHT preamp, it was time to go back and revisit my 01a. I always liked this valve, it has a superb tone which probably is due to its thoriated tungsten filaments. Challenge is though, you need to find a good pair of CX301a as they tend to be microphonic. This is why many discarded them, I found them unique in tone and clarity.<\/p>\n

As I removed my Salas HT shunt regulator from the HT supply chassis, the raw power supply now comes up to 280V with no load. The preamp has a cascoded gyrator which has a good PSRR, so decided to test it as is, so only had to adjust the bias point by setting the right voltage on the gyrator. The anode current was set to 3mA for minimum distortion. Yes I know I’m running the CX301a at a higher anode voltage than permitted, but it works fine. Problem is my bias resistor, so the filament bias can be optimised. Ideally the resistor should be around 15 ohms, but I had the 51 ohms around which worked fined with the Rod Coleman regulators I had as well and allowed for some filament starvation (circa 245mA). The bias voltage is then 12.4V which is not ideal for maximum swing at minimum distortion. I will change it in future. THD at 5V rms output was only 0.04% and at 1 Vrms just 0.01%:<\/p>\n

\"CX301a<\/p>\n

Distortion plot:<\/p>\n

\"01a<\/a><\/p>\n

It may be referred as the poor man’s DHT preamp. However, getting hold of a good pair of CX301a may be not that easy. I’m currently using Sylvania CX-301a, which are really nice.<\/p>\n

My thoughts:<\/p>\n