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Tag Archives: Pat kauffman

  • Modifying a Fender Bandmaster Reverb

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    What does the pedal steel player have in common with the armies of tube disciples, weekend warriors, and every other six string mother post 1958 Link Wray? Nothing. Guitar Amp doctrine presumes everybody wants to distort. Granted, 50 million Elvis fans can't be wrong, but players often wanting clean tone are left out of the discussion. Many choices and advice for those who are searching for clean sound can be confusing. Technical advice coming out of the HiFi world or well-meaning advice from endless forum jockeys debating how Carol Kaye got that bass sound from a super reverb, can be exhausting. From what I have found as the amp tech here at Main Drag Music, a good guitar amp makes a terrible HiFi amp and a good HiFi amp makes rather lifeless guitar amp.


    Steel players usually desire a tight amp with plenty of clean headroom without it being stiff. Not just steel players though. I am constantly in conversations with songwriters, jazz, and country players wanting to hear that weirdo chord they came up with resolve to a Gm7, without it being muddy. A correctly working tube amp will be able to dial in a clean sound on low and intermediate level volumes. Once you open the gates and push the amp into clipping, then it will distort, compress, and yes, sing.  More on that in my next blog on harp amps and pentodes. Seriously!


    Recently, I had a fun project in the shop. A Fender Bandmaster Reverb head had been fitted into a custom 1x15 cabinet. The player wanted it cleaner and tighter through the wide tonal range of the instrument. I started with choosing a better output transformer than the stock one, and switched over to solid state rectification. Solid state rectifiers reduce sag and increase voltage. Yes, voltage sag can bring the soul in a tube amp Chester Burnett style, but today's alchemy is not that simple. Having more voltage and a faster power supply is the foundation we wanted for the output section. The output transformer I used was a Mercury Magnetics Beauty, which gave us a low end strength and tonal balance. This transformer gives the Bandmaster no restrictions on the back end and lets us actually "hear" the subtle and not so sutle "sound" of the tubes. A good output transformer breathes effortlessly with the power tubes and the speakers. It magnetically couples the two with invisible lines of flux casting out some harmonically rich AC voltage to the speakers. Transformers to me are undoubtedly magical, like AM radio, Springsteen's Nebraska, or driving all night in the Jersey Pine Barrens in a 1968 Cutlass, alone.


    Next, I wanted to clean up the preamp. I removed the first preamp tube, which was a 12AX7 for the reverb channel, and swapped it with a lower gain 12AY7. In most guitar amplifiers the preamp section is designed around the notable dual triode, the 12AX7 (ECC83). A dual triode is a type of vacuum tube with two separate gain stages in one tube. They share their glass envelope and heater but you have two independent stages of gain, EQ, vibe, taste, etc. The amplification factor of a 12AX7 is 100. When you plug into the input jack of your amp, a weak signal gets immediately amplified by the first tube in the circuit. From there it moves to the tone controls and then the volume control, which is only an attenuator.  Then the signal goes right back into the same tube, then onward through the rest of the preamp.  This first tube in the circuit is crucial to your tone and how you distort. Other tubes with less gain can be substituted in the first preamp stage without modifications to achieve cleaner tone. A lower gain tube in the first position will be softer and less likely to clip. Once you have distortion in the beginning of that chain, that distortion will remain throughout. A different tube type will still give you plenty of volume and help keep it clean.



    Here is a video of the finished modded Bandmaster and a list of different preamp tubes with their amplification factor. These are all substitutions that you can experiment with, for a 12AX7. I also advise against using a 12AT7 which many people do, as they tend to be very microphonic in the first stage. Also if using a 12AY7, get a new old stock one. The vintage 12AY7 will actually work (unlike the new ones) and summon some serious spirits, just like the croon of the pedal steel itself.


    Tube 12AX7, amplification factor 100

    Tube 5751, amplification factor 70

    Tube 12AT7, amplification factor 60

    Tube 12AY7, amplification factor 45

    Tube 12AV7, amplification factor 40

    Tube 12AU7, amplification factor 18


  • A Day In The Amp Shop

    A Day In The Amp Shop

    A day in the amp shop for Pat Kauffman doesn't always involve Twins, JCM 800s or speaker jobs. On Monday, Feb. 22, I came in specifically to repair 2 radios for 2 fellow co-workers.

    First project was Joe's family's 1948 Philco model 48-250 which was bought new, in Brooklyn. The legend of the Philco starts with Joe's grandfather buying it for his daughter (Joe's mother)and she had it in her bedroom where she grew up. Joe's parents had it in their loft in Manhattan through the 60s and 70s. In the early 80s, when it stopped working, they took it to a local TV & radio repair shop. The technician said that "tubes are on the way out and solid state is the way to go". So he modified the radio with a solid state diode to replace the 35Y4 tube that they thought they couldn't get anymore. Since the 80s, Joe's family continued to use the radio but as time when on, the stations started to fade. A 60-cycle hum started to appear.

    First, I removed the modification of rectifier diode and power resistor and installed a NOS 35Y4 tube, which we had in stock. Then, I replaced all capacitors in the radio, cleaned & lubed all pots, all controls, tube sockets and put a fresh coat of a Main Drag Secret Bakelite-Lovin' rejuvinator.  Even I was impressed by the quality of reception, clarity and volume.

    Shine and tune, Joe.

    Second repair was a recently acquired 1959 Grundig. Tom bought this at  a local junk shop but unfortunately to our surprise, about half of the circuitry inside has been ripped out violently by previously frustrated technicians. After feeling stuck in a conundrum of a totally badass radio and a possible high repair bill/ frustration, it was probably not worth the time. I then struck upon the idea of just making it a line level audio amplifier, or to put it another way, Tom's new ipod player.

    I built a 6-watt audio amplifier with a single EL84 and single 12AU7 tubes. Using the existing transformers and speaker, I wired the circuit on the cuff, even designing and using the old tone controls. After digging through the entire shop for the correct lightbulb for the faceplate, I finally salvaged them from Tom's other German radio under my bench, which has a habit of smoking during use. Maybe I could talk Tom into turning his radio into a waffle iron.

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