3dzapper Posted December 22, 2004 Author Share Posted December 22, 2004 "At least the hum problem is from your furnace, it did give you a excuse to improve on the 300B amp." It did that Mike! I can't believe the transformation. The indirect heated cathodes on the 6BQ5 makes them easier to quiet than the direct heated cathodes like the 45, 2A3 and 300Bs. I'm keeping eyes open for a more suitable input capacitor. By the way you were right about the drivers. They have a seperate filter cap after the dropping resistor. I couldn't see any ripple on the scope from them. Rick Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 22, 2004 Share Posted December 22, 2004 "By the way you were right about the drivers. They have a seperate filter cap after the dropping resistor. I couldn't see any ripple on the scope from them." I bought the 300B amp in parts from a good local audio friend. He designed and breadboarded the circuit and had it all tweaked in, but never built the amp. His schematic indicated driver de-coupling "if necessary" Trust me, it was "necessary". After a scolding, I scored some some filter caps. There was some confusion on the resistor for the plate load on the driver tube. A 1/2 watt 1K resitor ended up where a 2 watt 10K should have been. Those little Holco's go up fast! A pair of 2 watt carbon comps and all was well. I've done driver de-coupling with the little 6BQ5 single-ended triode strapped no feedback screamer I built, on each side of 6EU7. It's probably why it's not more noisy than it is. My 300B amp needs some hum chased out, I swapped to a higher votlage power transformer to get the tubes to 300B spec, and the hum got a tad bit louder. Drawing up a schematic for these Lowery 6550 PP amps is kinda the priority now. It would be easy, but they have a turret board with parts on both sides. I got the PS part! Quote Link to comment Share on other sites More sharing options...
3dzapper Posted December 22, 2004 Author Share Posted December 22, 2004 Did Lowrey use Dynaco transformers? I know that some organ builders did. Rick Quote Link to comment Share on other sites More sharing options...
mike stehr Posted December 22, 2004 Share Posted December 22, 2004 All I know is they are around 3500 ohms primary impedance, and can get to 20 kHz. They should get a fair bit of power at 30 Hz. Lowery info on the net is pretty limited at best. Quote Link to comment Share on other sites More sharing options...
3dzapper Posted January 7, 2005 Author Share Posted January 7, 2005 Last night I brought the residual noise down to under 1mV, VOM flashing 0-1 on the left channel 0 on the right by rectifing the 300B filaments. I just used a 1500uF capacitor across the DC this time instead of a CRC filter. As far as I can see most of the residual noise is very high up the frequency spectrum originating in the driver stage and is not audible. This being a zero feedback design. I'm wondering if it would work to put a small value ceramic disk across the plate of the first stage to shunt this to ground? The effect is probably caused by the resistive coupling from the first stage to the driver output grid and the inate capacictance/inductance of the resistor. The input stage design is the one known as the "J.E. Labs 300B". The schematic can be seen here: http://indigo.ie/~walton/300bschematic.html Rick Quote Link to comment Share on other sites More sharing options...
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