What makes an amp the best Harp Amp?

[Bruce / Mission Amps]

Just use the 6AB4 as your first preamp tube... for a harp amp you DO NOT want a high gain first preamp stage anyhow.... with a gain factor of 60, maybe the 6AB4 is perfect anyway... never used one.
You could rewire the socket and swap it out with a very hot pentode... like a 6SJ7...?

Anyhow, I'd try to build this with what you have.
Your input jack(s) into the 6AB4, output to a 1M audio taper volume pot with a single knob 500K tone pot with a grounded .005uF treble cut cap (like the Masco ME18), output of that right into to either a paraphase driver or a long tail PI driver.
You could use a cathodyne phase inverter with the first half of the 12AX7 being the driver.
Regardless use a phase inverter stage with some gain and then right into two cathode biased 6V6s, wired up in push pull.

[RitzRed]

what does it mean wired in push pull or better yet how is it done?

[Jazz P Bass]

Good read.
What Do the Terms Push-Pull and Single-Ended Mean?

[J5&10's]

I know I'm coming in way late here ...but I would suggest vintage tone and simplicity. All octal tubes for sure.

I own two Gibson EH-125 dometop/suitcase amps from 1941-42, but they are slightly different schematics.
One has a six tube complement: 2 X 6SQ7, 6J5 for phase inverter, 2 X 6V6 power tubes, 5Y3 rectifier.
To make more harp friendly, I sub a 6SR7 for one of the 6SQ7's. Favorite is a GE 6SR7GT. Various metal and GT 6J5's work fine. Use one Ken-Rad 6V6G and pair with a CBS-Hytron JAN 6V6GT/G greyglass, but many others work well: RCA, Sylvania, Raytheon 6V6GTY's; Raytheon, Tung-Sol, National Union 6V6GT/G blackglass, GE and RCA clear 6V6GTA's.
Best rectifier... Sylvania 5Y3WGTA, but Tung-Sol JAN 5Y3GT/G VT-197A is also quite good.
Original 12" field coil speaker. One knob: volume for Mic channel. Sounds like a mini Bassman with all the honk and no feedback.
The second EH-125 is the same layout except for pre-amp stage, where it uses a single 6SJ7 pentode. I have swapped a
6SK7 for less gain, earlier distortion to voice more harp friendly. Rest of tube layout similar to above.

Why no harp amp builders haven't gone with these early Gibson models that are relatively simple is curious; since so many of the legends used amps of this style or similar. I know Big Walter Horton, Junior Wells, and Howlin' Wolf used them. Hubie Sumlin was my source, along with Dave Myers.
Hubie actually discovered the overdriven guitar sound by accident, when he plugged his guitar cable mistakenly into the microphone input instead of one of the instrument jacks one time during a recording session with Wolf. Everyone went WOW ! The amp was a Gibson EH-125, and all the cats wanted one, but Gibson had stopped making them at least a dozen years before ( mainly due to WWII ).

I do gig with both of these amps, but am real careful because they are museum pieces now. I use a Variac to keep the volts 110-115 to protect components.

My other main gigging amp is a VHT Special-6, but there are plenty other Class A, SE amps on the market now.

[km6xz]

You know of course that most of this is hogwash. The tubes mentioned are all over 50 years old and not likely to be of the same characteristics as they were when they left the factory so attributing sound qualities to devices that have no inherent sound characteristic is either misplaced cause and effect or pure intentional BS.
The perfect harp amp is whatever the best more enjoyable-to-listen-to player is plugged into at the time. I favorite and long time friend was Horton Buffalo and he sounded GREAT whether plugged into a Bassman or a Roland JC120 and everything in between. One of his most interesting performances, the small club only had a Polytone and no one could have imagined a better sound. Masters do not stress over gear, unless it goes in their mouths, just as great guitar stylists are much less concerned over amps than amateurs are, the latter seem to think the 100,000 hours of practice can be overcome with buying some gadgets or hardware.

[Randall]

I have not read any of the responses to this thread, I went straight to the bottom here. All I want to add is I have been told by a guy who I know who has played with many of the greats and sounds likes a walking talking 78 blues record... cheap small speakers are were it's at for blues harp. 8" mostly. Mic it and play. That is all.

[harpslinger]

Im new to this forum and a novice when it comes to working inside an amp but Ive been playing harp for over 40 yrs.
By the way. What an awesome forum. There seems to be a lot of chatter goin on here and all really helpful stuff by knowledgeable folks.
My first amp was a fender super reverb then later a Princeton reverb but I now prefer Gibsons and now even more Valco made amps.
The best sound Ive ever gotten is from my 53 National tremotone. Im sure you've seen this but if not check this out Blues Harp Amps 11: 1954 Valco National Tremo-tone - YouTube
I know that clip is a Little Walter style but turn it up and lose the juke thing (no disrespect) and you get a very different sound.
I have not done anything to this amp yet but it is a love hate thing. I have to get it turned up to get the juice out of it but when I do it sounds like nothing I have ever heard. The downside is that it is not only a crunchy tone generater, it is also a feedback generater. But that is something that Im gonna work on.
I think ideally a separate head that can be reached on stage so I don't have to put a volume pot on my Black CR mic. The volume pot takes away the gritty edge from my mic. Then a custom made wedge monitor that has a seperate volume control so I can get a nice level to hear myself. And thirdly a separate cab possibly with a closed back that can be positioned somewhere else in the room to get a louder sound to the audience without being picked up again by my mic. I figure even a closed back is an improvement over micing thru the PA because you stay more in control of the sound being heard by the audience. the PA is usually set up by someone else and for someone else. And it is closed back too.
That's my two cents and maybe that's all its worth but what the heck its free.