Making a quad out of 2 matched pairs

[GainFreak]

Hi,

Let's assume a 100W amp and two matched pairs available. Also two bias adjustment trimpots for both sides of the OT. What is better:

1/ To combine 1 tube that draws more current with one that draws less current (for a given bias voltage) per side

or

2/ Use the two pairs without mixing tubes and adjust bias individually for each side?

[MWJB]

Option 2, assuming you want proper fidelity.

Option 1 might be worth a try if looking for a browner, less pristine tone. Just make sure that the hotter tube is not over recommended idle current.

[GainFreak]

Thanks for your reply.
What if no bias trimpot is available and bias voltage is fixed? Is it better then to mix the tubes to get approx the same idle current per side?

[Enzo]

For tone it is up to you. For hum reduction it is better to have the two sides balanced.

So if you have two pairs of tubes - AA and BB - then split each pair to the two sides. For example ABBA or BAAB. Even ABAB or BABA. What you wouldn;t want is AABB.

But really it is nowhere near as critical a thing as many people make it sound.