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AMI DD amp from a jukebox
AMI DD amp from a jukebox
AMI DD amp from a jukebox
AMI DD amp from a jukebox
Moviola URS schemtatic

News[edit | edit source]

The Sherwood had to be shut down because of a number of noise problems, causing research to move forward to try to exmplain a) how it works and b) how it can be fixed. The research easily explains one of the project amps, a Moviola "Squawk Box," but the Sherwood is to complex. Most recent discussion is here.

The project is underway with the addition of a few significant text contributions and a search for materials are being ordered, and a senior local technologist is offering help. The technologist, a retired teacher with a PhD, confirmed that tube sound is better; he said that he has seen the interferance added by solid-state as "squiggles" along the curves on an oscilloscope, where tubes had shown clear curves. He still seems interested in project-based education, and there would be a significant contribution if we could repeat and document this experiment for this "class."

This is a DIY audio course for those who are interested in restoring vacuum tube, or valve, systems, specifically amplifiers. It is project-oriented, and the simplicity of vacuum tube amplifiers, and other devices, makes the topic idea for electrical engineering novices.

You might have seen AudoFOS. Clearly I am frustrated by the misinformation that is possibly 99% of tube information. So, I was excited to find these explanations (Michael S. McCorquodale) for ideal amplifier design parts (cap, resis, etc); they are part of the Berkeley SPICE emutulation program, which seems like the best reseach direction because it has to make sense--unlike AudioFOS, which is, apparently, self-appointed audio misinformation for marketing.

HiFi theory[edit | edit source]

  1. Figuring it out
  2. Glossary
  3. Spice

Most recent strategy[edit | edit source]

Initially the idea was to build up a glossary describing all the parts (tube, resis, cap, choke, etc) and then assemble them based on what they do. Now it is obvious that "parts" are circuit components, or ideal amps, that presumably can be linked in series linking mic to speaker. This covers one project, the Moviola, which is serial enough to actually give me this idea as soon as I looked at the schematic. The issues surrounding the signal path (frequency, inductance, etc) in apparent relation to current path (NST to PST) create needs that complicate this end-to-end idea such as the push-pull design that splits the signal path; and feedback has not been grokked yet. Also yet to be grokked is ultralinear, which connects the transformer (TOA) internally to the circuit which is said also clarify the sound.

Example of ArtSci[edit | edit source]

This is appropriate for electrical engineering beginners, because tube devices have fewer components and therefore seem simpler than solid state counterparts in terms of diagram complication. But, as it happens, tube systems require a great deal of theoretical imagination, as their use depends on physics knoweldge, and, as it happens, whole systems theory. What is perhaps most beneficial is that tubes cross the two aspects of intellect: art and science. Historically they represent the art of science, and as musical recording equipment they are the science of art. Comprehending, and implementing them is an art, but it is not subjective as many suggest such as with "pscyhoacoustics." Both aspects are purely objective and touch on physical and artistic reality.

  • What are the different components of an amp?
  • How do they connect together?
  • What do the little things such as the round orange things or the colored block do?

Then a block-model can be created that in turn can be developed into constructed knowledge that can lead to innovation, such as the development of new systems (perhaps built from cast-off equipment), and improvements to create technical inertia.

First project[edit | edit source]

A Moviola "Squawk Box"

This amp (actually there are two), have easily-obtained tubes (which are here), and is also very easily explained theoretically from the material being developed here.

Second project[edit | edit source]

Illustrated is an AMI jukebox amp from the 40s that uses generic 6L6 tubes, but has hanging wires. The schematic is here.

I need to know what kind of testing I should do before restoring insulation and crimping the loose metal in the mounts. There are two loose wires coming from the speaker-side transformer (that hang in the breeze w/o insulation), and I cannot imagine what they do as they don't seem to be on the schematic. I am very hopeful for this amp, as it should have a great sound if/when it gets up and running again.

Third project[edit | edit source]

This will be the revival of a Sherwood 8000 amp, that was one of the last amps built prior to the switch to (much simpler) transistor technology.

Goals for the class[edit | edit source]

This amp:

  1. Identify high-voltage hazards
  2. Determine how to test the system prior to startup
  3. Fix insulation
  4. Identify loose wires
  5. Add volume control
  6. Find appropriate speaker
  7. Power-up

Getting started[edit | edit source]

Marcus provided material for testing the system:

Here is a link to an assembled "dim bulb" tester.

Dave, or dcgillespie, states that this is not so much a HiFi as a PA, or public address, amplifier. The difference, as he says below, is that HiFis limit base to the output, implying that PAs have high bass response. In my opinion, there is no shame in critical base response, as bass is a key component of contemporary music.