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In the Forum: Horn-Loaded Speakers
In the Thread: Macondo’s Midbass Project – the grown up time.

Post Subject: Properly-operating amplification for midbass channel.Posted by Romy the Cat on: 11/1/2010

This week I will be trying to address why the Milq midbass is clipping. This is very interesting one. Currently the Melquiades LF channel uses an output transformer 700R to 5R or 11.83 to 1. I thought before that I have much lower ratio, like 5:1 but I was mistaken. So, my 6C33C driven with 200V at 200mA is loaded with over 2100R, considering that it drives 15R driver. This is insultingly-idle operation mode for 6C33C; the tube is not loaded at all. The channel sound fine but at the max volume (when it VERY laud in the room) it clips very aggressively. Adding current to the output stage and ruing the 6C33C at 50-60W plate dissipation (250-300mA) completely eliminate clipping but the interesting question would be by. With such a idle loading of the output tube the clipping will be by deficiency of voltage. So, why adding current (and consequential drop voltage – Melquiades is not regulated) helps against clipping? I need to find the answer.

I need to find the answer as the clipping might not be where I think it is. I did not measure the amp driving my midbass channel but I will not. The clipping might be in the driver stage screen or anode or in output stage screen or anode. I need to find where it is and what clips first – current or voltage and then to reset the operation point or output tube to assure that I get max power from it. I know that I but a LOT of power and gain in output transformer. Any 6C33C amps that you see out there load this tube to 600R-800R but I load it to over 2100R. If I use this loading and run full range amps then it would be absolutely unlistenable, with all harmonics eaten and upper range brutally aggressive and even deformed. Here is however where the DSET advantage come – the channel has no upper range and in my view the super idle loading of the midbass horn is desirable. I am not sure the degree however, I might went a bit too far (as I tend to do) and loading of let say 1500R would be more appropriate. This would give some midbass “gloom” but I need to examine if it is what I want as it might change a few other things (like crossover order on ULF, like crossover order and cut off point on upperbass and a few other parameters). I have 700R to 8R transformer or 9.35 to 1 and I might engage it. It will load the tube a bit harder and will give me a few dB of gain and power. I might as well to get another 9:1 transformer with a fast core. With this upperbass channel it is not “open bottom” channel anymore and I do not need a huge inductance on it, in fact the channel has 20Hz high pass filter.

So, I need to find what the channel clips at 40W on 6C33C plate and how can I reset to assure the optimal operation of the Milq bass channel. I think now I use no more that 1-2W from 6C33C, with the same effect I could drive my midbass from type 45/2A3 tubes. I will be posting the findings…

The Cat

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