Vacuum Tube Harp Amps!

[Bruce / Mission Amps]

I've tried all kinds of things too. ha ha

One of my favorite harp tones came from a 25 watt amp I built for Kenny Dore (Maria Muldaur's group) where I used a 12AU7 for the first preamp tube with 27K plate load resistors biased solid class A but at an AMAZINGLY low plate voltage of .... 30-35vdc!
This then drove the other half of the 12AU7 at higher plate voltage, into a loose version of a Baxandal tone stack, recovered by a normally voltaged 12AY7, which then drove the other half of the 12AY7 as driver.... breathe... and then onto a pair of cathode biased... EL34s... yup EL34s.

[clyde1]

Howz about a pentode front end, like the 5687 in the Gibson Maestro, which is
a smoking harp amp? Ever used any of these Bruce? Also the 6J7 from a Masco PA. I play with an amazing harp player and he likes this stuff. He uses Astatic and Shure mics, the old ones.

[MWJB]

No real reason not to do it but if it is a 2 channel design, use a 9 pin preamp as well, as a back up. Some mics & players may have differing preamp gain needs and the flexibility of having plug in/plug out interchangeability of 12AX/12AY/12AU can be a big benefit.

[steel1953]

Are any of these mods good for a brown Concert?

I've got 2, and I'd like to set one up for harp.

[MWJB]

1st stop for a Brown Concert is to make sure that your power supply is up to scratch, ensure that both main and screen filter supplies are rated for 700v minimum (B+ is usually just a few volts off 500v, which is good for power & low end). I usually use 2x100uf/350v in series for the mains (each cap bypassed with a 220K 2W resistor) and the same for the screen supply (though pick up some 47uf/350v or 450v too as 100uf's can be a pain to cram in there - size of cap is critical, you won' be able to use Spragues at the screens). You can use 20uf/500v everywhere else. Whilst you're in the doghouse ensure that your preamp dropping resistor is 56K, you can go up to 82K (ideally at 3W) so long as you remember not to drown the amp in bass (about "6" tends to be plenty). I'd keep voltages at V1, 1 & 6 at 160v-ish.

Fit Sovtek 5881WXT and bias to 7.5 to 15mA.

Sub preamp and PI coupling caps out to .1uf.

Sub Normal channel cathode bypass caps to 100uf/25v, you can also try 220uf at V1, 3.

Change 1 meg Normal channel input load resistor to 5.6Meg.

And that's about as much as they need. They're great stock but have limited low end, especially if you use tenor tuned harps and a 64 chromatic. These mods (apart from the power supply, which is "non-negotiable") just fill out the low end and brown out the tone a little.

I always like 12AX7s in the preamp but play with how many - lots of harp players just use the Normal channel 12AX7 and one in the PI. I prefer a full compliment but wind up the Vibrato channel controls to tighten up the tone if necessary (Vol full up, Bass full up, Treb up until you get the cut you want) even if you get a bit more noise.

[Bruce / Mission Amps]

The brown Concert is one of my fav amps to use for a 'loud' harp amp also.
They sound great with a mix of the C10N and P10R but I still prefer a NOS 12AY7 in the preamp.

[bob p]

Have any of you seen the multitude of You Tube videos that demonstrate classic harp amps?

One fellow in Europe started the series, which features him playing harp through his collection of vintage amps. The videos are short, and are of good production quality, with rolling titles, etc. In response to his videos, other people have made their own. Its a really cool library that's worth looking into.

[bob p]

linkage:

Harmonica Amps Vol. 1 by Harpsucker

His other videos

[Bruce / Mission Amps]

Do a youtube search for my buddy Ronnie Shillist.
He has a new "learning blues harp" DVD out this month.

http://www.youtube.com/watch?v=bArT1imliRk

[tbryanh]

I've tried all kinds of things too. ha ha

One of my favorite harp tones came from a 25 watt amp I built for Kenny Dore (Maria Muldaur's group) where I used a 12AU7 for the first preamp tube with 27K plate load resistors biased solid class A but at an AMAZINGLY low plate voltage of .... 30-35vdc!
This then drove the other half of the 12AU7 at higher plate voltage, into a loose version of a Baxandal tone stack, recovered by a normally voltaged 12AY7, which then drove the other half of the 12AY7 as driver.... breathe... and then onto a pair of cathode biased... EL34s... yup EL34s.

I was wondering why you chose 30-35vdc for the plate voltage? What special effect did it have? Was the headroom of the first stage affected by the lower voltage?

[MWJB]

12AU7 are well capable of running low voltages with reasonable fidelity, (especially with Bruce's 27K plate resistor & with the right mic) compared to say 12AX7. For instance if you run a circuit that has a 12AX7 running at 130vdc (100K plate resistor), then you replace that tube with a 12AY7 you will get around 90v on the 12AY7 plates. Sub that 12AY7 for a 12AU7 and the 12AU7 may run at a little less than half the voltage of the 12AY7. The AU won't compress & grind up as much as the other choices as long as you have a mic with a strong enough output to stop it sounding dull/flat.

I sometimes run a 12DW7 in V1 on a 5F6A style amp to give a little more flexibility with the amp when using different mics (100pf bright cap removed), the low mu half runs at around 40-ish vdc and still has good headroom & fidelity with CM & CR mics, some hi output Hi-z dynamics...but you would need a REALLY hot crystal to make this work.

[Bruce / Mission Amps]

...

I sometimes run a 12DW7 in V1 on a 5F6A style amp to give a little more flexibility with the amp when using different mics (100pf bright cap removed), the low mu half runs at around 40-ish vdc and still has good headroom & fidelity with CM & CR mics, some hi output Hi-z dynamics...but you would need a REALLY hot crystal to make this work.

I've also used the 12DW7 with a hot mic and it works very well if you also use a split load "anode resistor pair".
On the AU7 triode, a 56K to 68K on the B+ side and a lower value at the actual plate side, like 56K to as low as 27K and then couple the audio out between the two split load resistors with something big like 100nF to 220nF.
Sock a bunch of current into the AU7 triode with a smaller value cathode biasing resistor (I recently used a 470 ohm 1 watter and a and big bypass cap... 100uF to 220uF or so).

[slidincharlie (Carlo P)]

Do a youtube search for my buddy Ronnie Shillist.
He has a new "learning blues harp" DVD out this month.

Bruce,
how do I contact Ronnie Shellist to ask about his DVD's?
You can PM or email me.

Thanks

[Bruce / Mission Amps]

Bruce,
how do I contact Ronnie Shellist to ask about his DVD's?
You can PM or email me.

Thanks

Carlo I think you can just get to him through here:
http://www.youtube.com/user/RonnieShellist

or,

harmonicainstruction@hotmail.com

got-harmonica@comcast.net

RonnieShellist@comcast.net



but if not, I'll give him your email address if you want.

[Marc]

Ha! Thomas is one cool guy! I sold him the Kay 503 off ebay this spring.
It was dead and ugly.
I think he did a great Dr. frankenstein thing and made it live again!

Marc