valve amplifier


1968 Telecaster with Marshal MA100C.

The amp belongs to my cousin, who actually got it in a private deal on Craigslist for a PRS SE guitar.  The Reverb doesn’t work so he asked me to take a look at it. I’m no tech, but apparently this is a common issue with this model…one of the wires in the reverb tank gets disconnected.  When I opened the tank up, sure enough, that was the issue.  Now I just have to figure out how to fix it.  the wires are totally insulated, so I am not sure if I can just take the insulation off and solder it…or if the insulation is required (once I remove it it ain’t going back on!)

Digital is too deterministic. At the purely electronic level, there are very few molecules involved, and their behaviour is amplified. The closer you get to ‘real’ instruments - including physical devices such as tapeheads, tape, loudspeaker cones, old echo units, analogue synths - the more molecules are involved, and the closer you get to a 'probabilistic’ condition. This is an argument for strapping a lot of old junk on to the end of your digital signal path - valves, amplifiers, weird speakers, distortion units, old compressors, EQs, etc. - in the hope that you reintroduce some of the sonic complexity of 'real’ instruments. There’s nothing wrong with the pristine formica surfaces of digital: it’s just that one would like to be able to use other textures as well.
—  Brian Eno, 1 December 1995

My 1968 Gibson SG Special,with my cousin’s Marshall MA100C.  He lent this amp to me to see if I could fix it…the reverb is not working.  Apparently this is/was a common issue with these amps.  I did not have the heart to tell him that… 

1) I am not nearly as “handy” as he thinks I am, and 

2) There is a high probability I will go in there looking for the reverb issue and end up irreversibly f*cking up something else completely and irreversibly! 

5 BIG questions -- buying new tube amp

External image

1. Should I pick Single End (SE) or Push-Pull (PP). 

2. Is Class A PP any better than Class AB or Class B?

3. Should I use Ultra-Linear or Triode mode

4. Negative Feedback – it is OK?

5. Is chinese amp any good?

1. SE or even Singe End Triode (SET) or PP? Less is more. SE (or SET) has less electronic components on the signal path, so SET has less influence on signal, has less sound deterioration. At the extreme, only one resistor, one tube and one transformator will do.

Push-Pull has 2 tubes, working in opposite directions, doubling the output power. More bass, more volume, less detail and imaging.

So, is it quality or quantity what count for You?

2. Ok, it gets more complex. Push-Pull can work in class A (both tubes in constant action all 360 degrees), in B (only 1 tube in full action in 180 degrees, only when it’s phase) or mixed – AB. In short, its pretty much the same. A gives less power, but cas cleaner sound, AB is a compromise between distortion and power. Class B is so distorted, that is not generally used in audio systems.

3. Get decent high-efficient speakers and Triode mode. Triode versus Ultra-Linear is again fighting for additional power. You lose power, going to Triode way, but gaining quality. Use high-efficient speakers and You will not lose in listening sound levels.

4. As little Negative feedback (NFB) as possible. NFB is to cure other issues in circuitry. Good SET does not need any NFB and sounds brighter than one with NFB. Negative Feedback is for gaining good characteristics on the paper, not for human ears. 

5. Give a try. Your ears will tell. I got one and like it! If You don’t like it, sell it. Anyway, they are 5-10 times cheaper than audiophile European or USA made units.


Every technique to improve efficiency and power in tube amps, causes added distortion. My advice is to purchase high efficient back loaded horn type speakers. I recommend full-range 1-speaker crossover network -less setups. These speakers allow to use simple single end triodes with very few components on signal path. Crossover-less full-range speakers are easy loads for amp and final result will be precise imaging and uniquely airy feeling at regular listening volumes.


Guitar shopping at Cosmo Music just north of Toronto, Canada:

  1. Custome Shop ES-355.  Note: no Varitone.  Obviously not trying to be a reissue!
  2. Suhr and Two Rock amps + Gretsch guitars
  3. Fabric-covered Ritter guitar.
  4. Another odd yet interesting Ritter instrument.
  5. The 355 again.  Not the priciest guitar in the room at $4k ($CAD) either!