Friday, March 29, 2013

1949 Gibson BR-9

These were originally packaged and sold with a Gibson lap guitar. Very cool; the amp has two 6V6 power tubes and it uses an interstage transformer (not a phase inverter tube) to drive them in push-pull. It makes maybe 10 watts. 

The cab is a little trapezoidal thing, with a 10-inch field coil speaker. One knob for volume, no tone control. It is similar to the 1947 Gibson amp I once had. It has a nice old school tone.

Wednesday, March 13, 2013

New Small amp from Mission Amps

This is the new small amp from Bruce at Mission Amps, sitting on the bench in his shop. The amp is now down in my amp room. Bruce asked me to thrash it out for a couple weeks.

A few design decisions have been made:

-It will come in a tweed Harvard cab, as you see in the picture.
-The speaker will be the 10-inch Eminence Lil Buddy.
-The power tubes will be two 6V6,
-The preamp tube will be a 12AY7.
-It will have a line out.
-It will sell for under $900.

It makes an honest 15 watts and sounds great. It weighs in at 24 pounds.  I'll put up some videos over the next few days to show how it sounds. I also plan to take it to Doc Brown's Jam at Ziggies in Denver on Sunday to let other players try it and get their opinions.

One thing the amp does not have is a name. Bruce wants to hear your suggestions. The amp is cathode biased for some mellow crunch and warm sag tone. You got a name for that? Let me know and I'll pass it along.

Tuesday, March 12, 2013

Watts Revisited

Somebody asked me in another forum why I use the term “watts at clipping” to describe the power an amp makes.  I thought I’d devote a new blog post to this topic.

It may surprise you to know there is no hard and fast formula that amp makes use to calculate exactly how many watts their harp amps make.  In 2009 I researched an article I was writing on this topic by contacting all the big harp amp makers at the time to find out how they arrived at their wattage claims.  Some of these amp makers were quite annoyed that I even asked the question.

I believe we need a standardized method for calculating amp power so the numbers are meaningful and consumers can make rational, informed decisions.  Wattage numbers in amps are like horsepower figures in cars.  There is a powerful incentive to fudge the numbers upwards – higher than your competitors – because many buyers are greatly influenced by it.

You've probably seen wattage claims for boom boxes or car audio or computer speakers that seem, um, unlikely.  They sometimes claim hundreds of watts when the truth is a small fraction of that.  Harp amp makers have been more restrained in their numbers, but still there is no real standardization.

Those of you who like vintage hifi gear will recognize this phrase:  110 watts RMS per channel @ 8 ohms from 20 to 20K Hz with .1% THD.  I found that harp amp makers borrow parts of that formula to arrive at their wattage spec.

This is what they do:  They drive the amp with a test tone – a sine wave – and measure the AC output at the speaker tap by watching the waveform on a scope.  You may have opinions about what the proper method should be, but that is what they actually do.

One problem is that the frequency and amplitude of the test tone is not uniform (and certainly not announced by the amp maker) and it can make a big difference.  Another problem is the amount of deformation they tolerate in the wave form at the point they claim as their wattage number.  In other words (as in the hifi formula), how much distortion is included in the number?

This is where the question arose in the other forum:  What the heck is wrong with distortion?  We love distortion in our tone, right?

Indeed we do, but for the sake of arriving at a meaningful wattage number that allows us to make real comparisons we have to stipulate the level of distortion in the test.  The easiest and best way to do that is to measure the amp’s power at the point that the sine wave begins to clip.  That is a very good indication of the amp’s true strength.

Can the amp make more power beyond the point of clipping?  Sure.  But depending on the design of the amp the distortion (the deformation of the sine wave) can start to sound unappealing pretty quickly.  Amp makers could just crank everything to the max and report that number but it would be unrealistic and meaningless for musicians, and there are lots of ways to juice the max number.  We want to know:  How much clean power can the amp make?  How does it compare with other amps?

Here is what I am suggesting as a standard for harp amp makers:  Drive the amp with 150mvac@130Hz and measure power as peak clean voltage into the appropriate true non-reactive load.

Is it a perfect formula?  Probably not, but it is not meant to be.   We need to insist that amp makers use a standard, uniform and verifiable method of calculating their wattage claims, and this method is a good place to start that conversation.

Monday, March 11, 2013

Blues Harp Solos

Here are three short video clips of blues harp solos from the Mile High Blues Society jam at Ziggies in Denver, Mar 10, 2013. The players are Ronnie Shellist, Gregg MacKenzie, and Nic Clark,

I brought two amps to the jam: Bassman and Mission 32-20. Just for fun, can you guess which player is playing which amp? No pedals or effects (not even delay) on either amp.

Ronnie Shellist at the MHBS Blues Harp Workshop

Here is a a snippet of Ronnie Shellist at the MHBS Blues Harp Workshop yesterday, discussing how he likes to mix amp tone with the vocal PA mic.

Ronnie is playing through a '57 Supro he found at a thrift shop for $79. The guitarist on stage with him is Matt Hendricks, a very fine blues player and singer who recently moved from Chicago to Denver.