Suggesting A Donation For Birthday Gift

I am throwing my husband a 50th birthday party. Over 100 guests are coming and some are asking about a gift. I was wondering if it would be inappropriate to let them know that a fund could be taken up to purchase him a new amplifier for his guitar. Most of the guests are musicians and have been friends for 25 + years. Many have not seen each other in a long time and they are all very excited about the party. Is it tacky to suggest a donation? Please let me know very soon. I need to send out invites in a few days. Thank you. Sue