Bedrock Lead 50C Guitar Amp Checkout


Pete from Pasadena is the proud owner of this like-new Bedrock amp.  Except, it doesn’t play anymore.  As he makes his living doing guitar setups and repairs for a major national chain, he lamented that he didn’t have the desire to reach into high voltage and die.  But he really missed playing thru this amp, which would have been considered a boutique amplifier nowadays.  Bedrock Amps was a once-fine amplifier builder, sued out of existence by greedy capitalists in the day before the Internet, so there are no manuals for this particular model available online.  No Problem, I told him, and lugged the amp back to the shop.



I inverted the amplifier and disconnected the loudspeaker.  To remove the chassis from the cabinet, I had to remove one tube and shield.  Tight fit!



The two large tubes are the EL34 pair.  It appears that the envelope of the tube on the left had been compromised somehow, allowing air and moisture into the tube.  Note that the silver ‘getter’ was ‘gotten’ and turned white.



Sure enough, the envelope was cracked near the base.


This is not an unusual failure mode for vacuum tubes.


The thermal coefficient of expansion of steel, widely used throughout the internal structure of the tube, is about seven times that of glass.  Where the steel and glass meet is right at the tube base.

Worse, with the chassis inverted, the rising heat would raise the temperature of the base of the tube higher than the rest of the envelope, further exacerbating this type of failure.

From time to time, I will see an amplifier chassis with holes drilled around the tube sockets, in an attempt to mitigate this problem.  But we cannot escape the laws of physics forever!



Interestingly, the glass envelopes of these 12AX7s were slightly larger than the JAN-compliant tube pin straighteners in my tube tester.  Not a big deal, because I can unscrew the pin straighteners from the aluminum plate and use the back side of the tube pin straightener to clean up the pins, without interference from the aluminum walls on the straighteners that you see here.  All the tubes in this amp were GrooveTubes, with the familiar clear plastic labels.  But the thickness of the label was not the source of the interference…  the glass envelope is slightly larger than American-made tubes of old.



All of the preamp tubes were in excellent shape, so they were wiped off and set aside for now.



Flipping the chassis over reveals a neat hybrid of point-to-point wiring with an efficient circuit board layout.  All of the components operate WELL within their ratings.  Whoever did the design and construction of this amplifier (reminds me of a Duncan amp…) had the head of an engineer and the heart of an artist.  Here, I’m checking the ESR of all the caps with a Blue ESR Meter from AnaTek.



The only sign of any deterioration I could find was this thin layer of mildew which appeared on the red insulated wire.  And only the red insulation.  You have probably seen this same light-colored dust on Craftsman screw driver handles that have been stored for a long time.  The mildew is of no consequence here, but I cleaned it off anyway.



Time for a new set of final amp tubes.  These are exactly the same type and rating as what was removed.



The white gloves keep finger oil away from the glass.  Yeah, I know that the plastic label on GrooveTubes obviates the need for Clean Room procedures, but I’m old school;  Old habits die hard.



The tube base locks were bent back upwards to grab the tube bases.  As you might guess, this is mildly important on amp chassis such as this one, which live their lives inverted.



Yeah, no matter how many amps I work on, “First Light” always makes the hair on my arms stand up.



These are the 100W eight ohm resistive load blocks that I use to check amplifier performance.  The basic concept of bias checking with an oscilloscope, and performance verification under resistive load is described at various places on the Internet.  Here you see an oscilloscope probe tied right across the load.  Another lead was added to at the same point in the circuit for the THD meter on the bench.



A 400Hz sinewave was applied to the ‘clean’ channel of the amp.  After everything was adjusted, output was gradually increased until the THD meter began to show distortion.  This screenshot shows clean crossover between each output and just a hint of flat-topping (probably the origin of the detected distortion.)  You are looking at 78 watts RMS at 0.1% THD from an amp rated at 50 watts.  As I said, all the components of this amp are conservatively rated and operated well within their parameters, and this is proof.



When I next saw Pete, his grin reached from ear to ear.  His band held a successful audition and he couldn’t have been happier with his band and his amp.  He promises me video…  Stay Tuned!

On Performance

“All the world’s a stage, And all the men and women merely players” wrote William Shakespeare, in his play As You Like It.

Hmmm…  Not much of a stretch to visualize a guitarist up on the the stage.  We’ve seen thousands of pictures and videos (no exaggeration here) of guitarists in performance.  However, even with your guitar in your lap in the most secluded closet recording studio, or practicing alone in the darkened living room, you the musician are exposing yourself in ways that would make the non-musician feel squeamish.  But being up on the stage…  Woah!  Does that terrify you? Are you terrified that someone in your own house would hear you play? Public performance is not why you play guitar, is it?

Let me speak to those of you who haven’t performed in public.  You practice your drills and repertoire diligently, yet you play in private. Why? To what end? “Where is your head at?”

Let us delve into the unseen world of the human mind. Allow me to reduce the workings of the human mind to a more-manageable level by introducing three terms from a field of psychological study called Transactional Analysis:

(1) The Child corresponds to the “id.” The child in you and me offers our most basic, instinctual drive.
(2) The Parent corresponds to the “super-ego.” The parent in our mind tells us what we ‘ought’ to do.
(3) The Adult corresponds to the “ego.” The voice of reality in our mind attempts to reconcile the conflict between the child and the parent.

This is a handy tool to analyze many of the conflicts we endure, and to posit a solution. For instance, the Parent says we don’t practice enough because the Adult says that we must seek perfection. The Child wants to have another cookie and go to sleep.

Or, my parent says that I need to work on this Blog, and my Child says to play another game of Internet Spades. My Adult is particularly weak, so, more often than not, the Child wins out.

Here’s another one. The Parent is fearful of humiliation if we perform our art in front of others, and so the Child is happy to take flight to escape the fear. Thus we run away from the possibility that we perform in public.


What is an Adult to do?

Let me offer a glimpse of something that may be of value to the Adult in resolving the conflict between the Child and the Parent, to the end of moving out into public performance.

Dave Goggin was VP of Data Processing at Gulf Oil Corporation in Houston, Texas up until Chevron purchased the company back in the 1980s. I was fortunate enough to know Dave socially through our joint involvement in square dancing and clogging.

Dave had many roles to play in the day-to-day operations of the Data Processing division. At the time, Dave’s division printed 2.2 million credit card statements a month. He had over two hundred employees to manage. He wore many hats. To keep it all straight, he shared with me the fact that each morning, as he sat on the pot, he read through his personal planning calendar to see what needed to be done at work that day. He said that he would check on a department first thing when he got there, call a field office at nine AM, pick a fight with an employee at 10am so he could fire him, speak at a board room luncheon at 11:30 am regarding changes in billing information in two Western states, go back to his office at 1 pm and close the door for a nap… you get the picture.

He really got my attention about that ‘task’ on his calendar of starting a fight. Other than knowing that he was a Marine radio operator and prisoner of war in the Korean conflict, I never perceived a single mean fiber in his body. Sure enough, Dave told me that work was a lot like acting. He had different roles to play, and he was a ‘real good’ actor. He would fulfill the role of supervisor at 8 am, play the part of a friendly voice on the phone at 9 am, act like a tough guy at 10 am, become a news anchor at 11:30 am, and so on. Driving home at night, he practiced his acting skills while fulfilling the role of courteous Houston driver, which, he intimated, was particularly difficult for him on those days of the week that end in -day.

Dave told me that “All the world’s a stage, And all the men and women merely players.” You act. Even when you don’t want to. Very Adult. …and it seems as if I’ve heard that quote before…

So, the Adult may use the ‘thespian arts’ to reconcile the Child and the Parent. The Adult may now motivate us to behave in such a manner to change us for the better, to cause us to take actions that will improve us, and the world, for the better.

So you have art within you. You are made in your Creator’s Image, so, obviously, a portion of that Image is to be creative.  You make music, dontcha?

You must ‘do’ something to release that art, to give it an existence in the real world. Just as a sculptor sees a statue trapped inside the block of stone, or the wood carver sees the finished carving inside the block of wood, you have a song, a riff, a melody, a chord progression, a lyric that MUST be released. You are powerless to do otherwise. In the current context, you pick up a guitar and create melody, rhythm, lyrics, and/or accompany someone else going the same musical direction as you.

Now you have created the art. What are you going to do with it?  The Adult creates a New Guitarist Person.

My premise in this blog post that you must perform your melody, rhythm, and lyrics for someone else to see. To do otherwise is to hide your art under a basket. Your creation is, by definition, a candle, a point of light, a piece of love. It is something good and of value. This is a dark world, and can use all the light, all the candles, all the love, we can muster.

Further, the act of sharing, of giving, changes you. I don’t mean that your bank account gets larger because you are booked for a hundred dates around the world in the coming year, although it would be cool to find out what that feels like, but rather, your soul is nourished by the act of giving away your art, your creation.

Don’t ask me how this works. The old saying goes like this: in the baseball game of life, you can’t really play ball wearing two catchers mitts. There is a time to catch the ball, and there is a time to throw the ball to someone else, for their benefit and for the good of your team. This, herein is the benefit of performance. Giving has a bigger spiritual effect on the giver than on the audience.                                                                                                                                       2013-03-03_14-13-48_680

To get over the threshold from introversion to giving, the Adult must construct a new theatrical character, and give that theatrical character a purpose, dialog, musical chops, and the opportunity to hone thespian skills. Plainly stated, you are the men and women on the stage. You are the playwright. You are creating not just music, but the character to play that music.  As the creator, you arrange rehearsals, costuming, find a stage, props, lighting, equipment, and promotional opportunities that will allow this new theatrical character.

Haven’t you dressed up for Halloween?  Or participated in the annual “Talk Like A Pirate Day?”

What I’m talking about is creating a new character.  This character plays guitar.  This character has a name (not your real name) and a costume.   You are now “Doing Business As” someone else.  Ever heard of Slash?  The Edge?  Zakk Wilde?  Piggy D?  Their mama didn’t give them their names.

That new person is who goes out in public.  That new person gets the stage fright, not you.

Yeah, that works.  Get out there and let’s see what you’ve got, New Guitarist Person!