Harp Amps?

[nastyoldtech]

I know nothing about the animal. I have a friend who would like me to modify one of his amps to use for harp.

Do I set it up like a mic preamp using a low to high Z transformer or is there something that harp players want from a standard guitar amp? Seems like there would be an impedance mismatch and ending up with a lot of feedback? What say you?

Any ideas by way of past builds, tips, or existing harp amp schematics? Bring me to your leader! 

[Boots Deville]

Here's a thread you might find useful:

http://www.el34world.com/Forum/index.php?topic=7332

...and check out the Ted Weber kits, the 5F2H kit was designed for harp.

[Fresh_Start]

It's not very active, but there's a Harp Amp Forum over on AMPAGE.  Bruce Collins of Mission Amps was working on a harp amp design not too long ago.

The "Normal" channel HERE was designed for harp when the "Bright" switch is off (bigger cathode bypass caps and only partial bypass of 2nd gain stage).  

You need more mids than a guitar amp, and feedback is a big concern.  That's the reason for lower gain.

As noted in the thread linked to above, lower preamp plate voltages help a lot.  A good starting point would be 12AX7 plate voltages of 150-180 or so.  Power rail node had 260-300 on it for this amp.  Even lower might be better for that greasy harp tone.

You also need to know what kind of mic the guy is using.  Some mics are fine with the typical 1 meg input impedance (hope that's the right term) of a guitar amp, but other mics may want to "see" as little as 100K.  That's why the input jacks are wired strangely.  The Lo jack has a 50% signal cut PLUS only 100K going to ground.

Also, harp amps tend to stay away from long-tailed phase inverters.  Concertina & cathodyne seem to be more popular, maybe because they're seen as being more Lo-Fi?

Oh, I left out couplig caps - bigger than guitar.

I'm no expert on this subject - just did a lot of research for this one build.  The harp-playing son of the owner really likes it though.

Cheers,

Chip

[tommytornado]

What amp/circuit?

[bigdaddy]

I read a lot about the differences with harp amps as opposed to guitar amps. The only thing similar is that they have tubes....

Seriously it's all about the gain. Gain causes feedback and that is NOT what you want in a harp amp. You want a BIG OT and a lot of power with little gain but have the amp still breakup and get some compression/sag from the power section. Not easy to get. I have also read a lot of harp amps have variables in places you would not normally use on guitar amps but some builders do. Like changing preamp cathode resistor/cap combos with a switch. Variable impedances on the input for different microphones. I forgot a lot of things because I never got around to building one. My harp playing sucks and my fingers are too short to hold the mic to the harp, although they have things you put the harp into that mounts a mic. But it is all about feedback and it's a midrange type of instrument, you want to get a saxophone sound from it, ballsy and brassy.

A lot too depends on what sound is he after, the earlier Chicago sound of Little Walter or Big Walter shakey Horton or the later tighter sound of a James Cotton, Junior Parker and others or something different and more modern or even rock sounding.

[nastyoldtech]

Thank you for all the help guys!

It sounds like it would be best to build them something from scratch like a small SE amp with a VVR. Strange, I've been reading that some prefer big combos like Super Reverbs.

 

[phsyconoodler]

The real pro players don't seem to do any of that.They can plug into any amp and get a killer tone.The mic is the issue.
  There is also Jukelemon's 'harp preamp' that lets you plug into a super or twin and get almost zero feedback.It can be done with a tube or a couple of diodes.
  I am not quite sure how it works but he repoarts great success with it.
  I haven't seen him on the forum lately,but maybe he'll chime in with his schematic.
 

[Fresh_Start]

Thank you for all the help guys!

It sounds like it would be best to build them something from scratch like a small SE amp with a VVR. Strange, I've been reading that some prefer big combos like Super Reverbs.

 

My build was based on a Super Reverb because that's what the harp-playing son had been using for years.  He does like this variation's Normal channel better. 

In a band situation, a small SE amp won't cut it for harp unless it's mic'd.  The harp player needs some volume without feedback.  Depending on what amp your friend has to start with, you may be able to make him really happy without open-heart surgery.  Is it a PCB amp?

Cheers,

Chip

[Baguette]

The real pro players don't seem to do any of that.They can plug into any amp and get a killer tone.The mic is the issue.

Right on. Mic + technique.
To tune the amp for harp I do:
- lower the preamp tube voltage
- open up the bandwidth (.1uF coupling caps at least) and cut the treble down a bit (no bright caps)
- lower the gain in the preamp (like unbypass the cathodes etc...)

[bigdaddy]

Many will also go with lower gain tubes like the 12AY7 and 12AT7 in place of a 12AX7. Some like to use 6SL7 and 6SN7 tubes instead.

It's all about getting volume without feedback. True, great mic technique and being a great harp player comes into it but some people have smaller hands and that hurts your technique, so it must be made up by the equipment. I have seen many great harp players go crazy with their amps because in a certain room no matter what they did it got feedback.

[bluesbear]

My experience with our harp player is you need to get the volume from the amp. Thr real feedback culprit is the PA monitor.
Dave

[chuggy]

What are the circumstances of use?  Duo, trio, with drums, horns, keyboard?  Micing a small amp for use in a loud band is challenging.  What is the skill level of the harmonica player?

Jim R