Tube DIY Asylum

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If you are looking for better bass response...

Posted by Ivan303 on June 21, 2021 at 10:04:43:
then DON'T change the value of the grid to ground resister very much as that will not change the frequency passed to the output tubes as much.

That said, the existing values calculate out to pas about 20Hz (see link below) which is likely lower than the output transformer would be able to do too much with.

Are you using original iron?

The HF81 used GREAT iron, and if I were to do what you are doing I'd look for a trashed HF-81 with original, and still working, iron as that was the secret to the HF-81's great sound.

That and the EL-84 being a great sounding tube.

HF-81 iron will also work using the 6V6 tube as well.