I don't play harmonica but an acquaintance who does ran these two web pages by me...
Keith's Amp Picks
For those of you that are still confused or need to be hit over the head, here are my suggestions on what amps to consider.
Every Bluesman needs a small practice type amp that is light weight and still sounds good.
Any amp with a 5Y3, 6V6, and a 12AX7 tubes with an 8 or 10-inch speaker.
Fender Champ - Tweed is best because it sounds raunchy, but all before about 1970 sound good.
Gibson Skylark - The white tolex kind from the 50's are my favorite, the later ones are not as loud or nasty sounding. I got mine for $75 and will never sell it.
Premier 50 with the 12AX7 tube about 1960 - This is my favorite amp.
Small Valco amps from the late 50's and early 60's - This includes Harmony, Silvertone, Supro, Danelectro and National amps. I have a Valco National that has the exact same circuitry as a tweed champ.
Medium Power Amps
These are Fender Tweed Deluxe style amps from the late 50's and early 60's. They have usually two 6V6 tubes and a 5Y3 rectifier. They have 2 or three 12AX7 tubes and a good one will have reverb. These can cut through the guitar players at a jam, but may have to be miced. They have a 12-inch speaker or 2 10's.
The best choices are:
Fender Tweed Deluxe - This is way out of my price range.
Fender Princeton - Not the Princeton reverb, but the smaller amps based on two 6V6 tubes. The silverface amps are even good for harp. These are too tame for many guitar players and can still be had for under $500. The silverface might go for under $300.
Premier Model 120. The ones from the early 60's are the ones I like best.
Gibson GA-40 from the mid 1950's - This is a killer amp.
If you have to cut it with guitar players, your best bet is to get something with 2 12-inch speakers or 4 10-inch speakers and 2 6L6 tubes.
1959 Fender Bassman. This is the holy grail of harp players. It had a 4x10 speaker configuration and minimalist circuitry. I found the one I played through to be too sensitive and got a lot of feedback, but everyone swears by them. This amp is the basis of most boutique harp amps.
Sears Silvertone 1483. I have been told this is the best alternative to a '59 Bassman that you can get and the heads go for about $75. The cabinets are very cool and you can slide the head into the back of the cabinet for transportation. For $150 you can get an amp combo that sounds even better than the bassman.
Premier Model 71. This is a buttery smooth amp that breaks up nicely and resists feedback. This is my favorite big amp.
Older Ampeg Jets and ReverberRockets. These are great harp amps and readily available at affordable prices.
Gibson GA90 - I am told this is one of Kim Wilson's favorites. It has 6 8-inch speakers. They are way too expensive for my wallet.
There's also an interesting ongoing 6L6 harp amp build project at:
Lone Wolf Harp Amps
Lone Wolf Blues Co.