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Too much gain: please explain some things

Posted by Frihed89 (A) on January 18, 2007 at 00:45:06

First, some experience:

I have an OTL headamp/preamp. It has 3 tube positions: 1 input and 2 outputs. The manufacturer has designed the amp and tube adaptors so that this amp can be run with a number of different tubes in each of the two positions.

So, I put a 5751 in the input position and 2 5687s and it sounds really nice. But I change the output tubes to 6SN7 VT-231s and there is a crackling noise when i turn the volume up. Conclusion: the gain on the 5751 is too high. I change the input tubes to lower gain tubes, such as 6414, 5965, 7062....no noise.

Second, some questions:

1. When someone says my 5751 has too much gain, what part of the "too much" is due to the circuit and what part is due to the interaction between the input and output tubes?

2. Specifically what deifferences between the 5687 and 6SN7 VT-231 are causing the 5751 to have too much gain?

3. Would this problem be solved by switching to a higher voltage version of the 6SN7 (or like the 6BL7GTA) and running it at higher voltage (I can do that with either 300V or 450V choices for the output tubes)?

Thanks.

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