Hi Ray - Yep, I looked into that as well. The designers were biasing the 3Q4 at the proper voltage to minimize AF distortion. When properly biased the 3Q4 is rated at 7% distortion, not biasing it makes that worse. But as a practical matter I can't tell the difference at low audio levels after some experiments and data gathering. So I bias my sets at - 4.5 V as the tube and radio's designers intended. I use 3 series cells taken from a 9 volt battery. Easy, works, lasts "forever".

For my deeper dive into the GRC-9 RX bias battery effects take a look here, about midway down the article:

Have fun with your GRC-9 !

Tim

N6CC

On Mon, Nov 18, 2024 at 11:56 AM Ray Fantini via groups.io <RAFANTINI=salisbury.edu@groups.io> wrote:

Just started doing awork on a bunch of GRC-9 transmitter / receivers. Never spent much time on them before but getting an education now. The big question for today is the receivers bias battery, lot of stuff on line about this from its only there to improve audio response to it is limiting current in the audio output tube and absolutely necessary.

It only provides cutoff voltage to the audio output tube, nothing else. I have seen where people have built up battery packs and all sort of stuff but when I look at it am going to assume it draws almost no current at all in normal operation thru grid current. When the radio is turned off no current is drawn, unless something like C54 gets leaky So why use big cells (AA, AAA) for this and just stuff a lithium 3.6-volt clock backup battery in its place?

Would look a lot better than some clump of AA cells and can fit in the original holder. The other option is to just use the radio without the battery. Cannot hear any difference between the audio with or without the bias.

Another thought, if you don’t use the battery should you jumper C54 to ground to prevent any positive DC voltage from developing acrost the grid of V6, maybe a 1.0 K resistor to keep you from grounding the grid?

So, lets see what the prevailing opinion is on this?

Ray F/KA3EKH

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