My unique new pedal steel amp

John Polstra · From: Lopez Island, WA, USA

Hi everybody,

I just have to show off the new pedal steel amp that I built for myself. I just finished it yesterday.

Except for the solid state reverb module, the amp is all tube. It uses a pair of 6550 power tubes, and puts out a totally clean 100W RMS. I've been refining the design since the beginning of the year, and I'm really happy with the sound. It's voiced similar to a Session 400. I'm using the amp with an Eminence Commonwealth 15 in an open-back cabinet.

I wanted the amp to be a head rather than a combo, so there wasn't room for a spring reverb. I'm pleasantly surprised by the sound of the Belton solid state reverb, though. Those things have come a long way.

In case you're interested, I put up a bunch of photos documenting the entire build process on a web page.

Click here for more photos

John

Greg Cutshaw · From: Corry, PA, USA

John, that's pretty neat! Thanks for posting this with all the pics. I've had good luck just porting the Session 400 Mid/Shift circuit topology to preamp boxes. It's a great design.

Greg

Kevin Hatton · From: Buffalo, N.Y.

Beautiful job John. Bravo. I also like the Commonwealth.

Jerry Overstreet · From: Louisville Ky

Nice looking Sierra and speaker cab too, John. I expect you could sell some of those amps if you build them for resale.

John Polstra · From: Lopez Island, WA, USA

Thanks for the comments, guys!

Greg, I agree about the mid/shift setup in the Session 400. Have you ever gotten to compare that amp against the Fender Steel King? I've never heard a Steel King, but I studied its midrange circuit pretty intensely. From a technical point of view, that circuit is "perfect", in the sense that the Mid and Shift controls are completely independent of each other. Adjusting the Shift control just moves the midrange cut up or down the frequency scale, without changing the shape or the depth of the cut. Likewise, adjusting the Mid control doesn't affect the cut frequency at all.

In contrast, the Session 400 controls are very interactive (or "organic", if you prefer the euphemism). But the results they produce are musically wonderful. I'd love to compare the two amps side by side.

Kevin, I chose the Commonwealth 15 based on your recommendation here back in September, 2009. I've been very satisfied with it.

Jerry, I've sold a couple of my amps to friends in the past, and I have a small business set up for selling and repairing amps. I make a little money doing repairs (it's a very small market here in the San Juan Islands), but I don't see any way that amp building could become a viable business for me. There's just too much labor involved. Even at top-dollar boutique amp prices, I doubt I'd earn even minimum wage doing that. So these days, I just build amps that I want for myself, selling one occasionally to buy parts (and to make space) for the next amp.

John

Greg Cutshaw · From: Corry, PA, USA

John , years ago I modeled the Session 400 circuit in IBM's ASTAP, their version of SPICE, and it behaves just as you described.

I play through a Steel King every day but never studied the circuitry. Both circuits work well to give me the mid range cut I need and I seem to end up setting both of them to the same mid range cut sound, allowing for the huge difference in the Black Widow and Commonwealth speakers' frequency response.

Greg

Brad Sarno · From: St. Louis, MO USA

John,

that's both beautiful and impressive.

Brad

John Billings · From: Ohio, USA

Nice transformers! What are they?

Ken Fox · From: Nashville GA USA

Beautiful job! I have been thinking about build a 2 or even 4 6550 amp with the same Belton reverb module as a project. Maybe in the near future, right now too many other irons in the fire.

I love the Sierra Steel too, I used to have the same model, wish I had never sold it.

John Polstra · From: Lopez Island, WA, USA

Greg, it sounds like I asked the right guy about the Steel King! I'm glad to hear that their approach works well, too. I simulated both the Session 400 and parts of the Steel King, using LTSpice. One interesting thing about the Steel King midrange circuit is that, standing alone, it can boost and cut:

But there's other tone shaping elsewhere in the amp that must get rid of most of the boost. (I didn't simulate that part.)

John

John Polstra · From: Lopez Island, WA, USA

John B., the output transformer is a Hammond 1650R.

The power transformer is from an old Tektronix 545A oscilloscope. That scope had 104 vacuum tubes in it, and the transformer is a beast. It has 5 high voltage windings ranging from 115V to 204V. They can be connected in series to get just about any voltage you want. It also has 7 separate 6.3V filament windings.

John

John Billings · From: Ohio, USA

Thanks JP! I knew it was something special. A lot of people don't realize the importance of transformers. I traded for a Delta Blues 15, to have something to play Baritone through. I didn't cut it. I looked at the trannies, and they were pitifully small. Put in a honkin' big old Hammond PT, and a Dr. Z OT. Made all the difference in the world!
JB

John Polstra · From: Lopez Island, WA, USA

Brad and Ken, thanks very much!

Ken, the 6550s are nice because they use less chassis space than, say, a quad of 6L6s. On the other hand, they need a more complicated power supply. And they generate a lot of heat concentrated in a small area. In my amp, counting the filaments, each tube is idling at around 40W. It's like having a pair of soldering irons in there.

That Sierra is my first and only pedal steel, and I do like it. I got a chance to play a few other brands recently, and experienced no buyer's remorse whatsoever.

John

Dave LaSalle · From: Ontario, Canada

That wire job looks cleaner then my Web or Steelking.Good job!
_________________
Politics:Just vote .79' Sho-Bud "Super Pro"Webb,Fender Steel King,

Marc Jenkins · From: Victoria, British Columbia, Canada

Nice work!

Olli Haavisto · From: Jarvenpaa,Finland

I`m picking up a Dynaco monobock power amp tomorrow, w. two 6550s.
Any other similarities to your design ?

Great work, for sure !!
_________________
Olli Haavisto
Finland

Greg Cutshaw · From: Corry, PA, USA

John, there are often differences in phase response in different amps as a function of frequency in addition to the usual plotted amplitude frequency response. Radical adjustments to mid shift circuit controls can also inject large phase differences in the signals at the mid frequency points which also determine the characteristic sound that we hear. An obvious proof of this is using a 7 band EQ to simulate a dobro sound with a pedal steel. The radical phase shifts introduced by staggering the bands does more than just affect the typical tone. It alters the basic sound of the instrument! I also had access to an HP network analyzer back then. Such a tool greatly speeds up circuit analysis! Long live tubes!

Greg

Rich Santucci · From: Perkasie Pennsylvania USA

That looks nice and I'm sure it sounds great!

I see that you are using three separate solid state bridge rectifiers in the power supply. How are they split up?

John Polstra · From: Lopez Island, WA, USA

Greg, that's a good point about the phase shift. Surprisingly, the Steel King's mid circuit has phase shift characteristics very similar to the bridged-T, which is essentially what's used in the Session 400. Here's the Steel King:

And here's the bridged-T:

The circuits couldn't be much more different. The Steel King uses two op amps, one of which simulates a negative valued resistor (!).

John

John Polstra · From: Lopez Island, WA, USA

Rich, the amp has two high voltage supplies of about 300V each, using separate transformer windings and separate bridge rectifiers. They are connected in series after rectification, to give me 300V for the screens and 600V for the plates of the 6550s.

The third rectifier is for powering the Belton reverb module. It's driven by a spare filament winding on the power transformer.

Finally, there's a bias supply driven off yet another winding. It just uses a half-wave rectifier made from a single diode.

John

John Polstra · From: Lopez Island, WA, USA

Olli, that Dynaco amp has what's called an ultralinear output stage. It's no doubt more "hi-fi" than my steel amp. I bet it'll sound good.

John

Jeff Heard · From: Lopez Island, Washington, USA

I confess total ignorance as far as the technical aspects of John's amp, but I can tell you, my steel sounds better through it than any other amp I've used!
Nice tight bass, smooth highs, and warm tube sound.
I think it's a masterpiece.

Tommy R. Butler · From: Nashville, Tennessee

John Polstra · From: Lopez Island, WA, USA

I made a couple of recordings with this amp several months ago, but haven't shared them up until now because I'm not what you'd call an advanced pedal steel player. But I decided I'll probably never feel like I'm "good enough", so I might as well just go ahead and post the clips.

The "Country Road" Steel Guitar Amp

The first bit is the steel solo Greg Leisz plays on "That's the Way Love Goes" on Buddy Miller's "Majestic Silver Strings" album. The second clip is, of course, the classic steel break from "Because of the Wind".

John

Per Berner · From: Skovde, Sweden

Very neat! Smile

Forum Index > Electronics	Goto page 1, 2 Next	Next oldest topic :: Next newest topic