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 
Most recent strategy 
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.
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 
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 
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 
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 
- Identify high-voltage hazards
- Determine how to test the system prior to startup
- Fix insulation
- Identify loose wires
- Add volume control
- Find appropriate speaker
Getting started 
Marcus provided material for testing the system:
|“||There are a few things I do when first testing out an amplifier which has not been used for some time.
I check the schematic, as you have done and normally put the wiring back to the standard unless I understand what changes have been made and possibly why. Many valve amplifiers have had a number of modifications before we end up with them many decades later.
I always use a variac to slowly increase the voltage into the amplifier, usually over about five minutes. This prevents blowing up things because the capacitors are not operating correctly. By slowly bringing up the voltage electrolytic capacitors are reformed and will normally operate properly as long as they're not damaged. If you do not have a variac, you can wire a light bulb holder in series with one side of the mains and start off with a 10 W lightbulb in the socket. The light should glow brightly and possibly start dimming a little after a short period of time. Once it is dimmed you can slowly increase the wattage of lightbulbs up to 100 or 200 W. Each lightbulb should slowly dim as the amplifier settles down and then you can go onto the next one. As long as the lightbulbs are not glowing brightly then the amplifier is not drawing too much power and could safely be connected directly to the mains. If by the time you get to somewhere around 50 W, if the light bulb is glowing full brightness then you've got a problem need to figure out why the mains input of the amp is behaving a bit like a short circuit.
I would always fit all of the tubes required into the amplifier because you can't really tell if things are working without everything in place.
Most pieces of electronics over 15 years old, especially if they've been left not operating for some time, require new electrolytic capacitors. I normally replace all of the electrolytics in any old piece of electronics, including semiconductor electronics. Sometimes just doing this sorts out whatever problems and noise that the item may have
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.
|“||Determining what kind of effort and resources to put into a piece of equipment starts from accurately defining what it is. This amplifier was designed to be a "PA" type of record player amplifier. That is hardly to diminish what it is, as no doubt it did/does it's intended job quite well. But it is also to say that it was not particularly designed to be a high fidelity piece of equipment either, as that term has been established to mean.
As designed, this unit has very minimal tone control facilities, was designed for use with an economic crystal type pickup, has remote volume control capabilities, and has distortion, frequency response, and power bandwidth capabilities and a damping factor in keeping with PA type equipment.
In looking at it from a modification standpoint, you can look at two basic building blocks of the design: The AF amp/tone control section (1st 6SN7), and the power amp section (2nd 6SN7 and 6L6s).
You can certainly place a simple volume control at the input to control high level inputs into the unit. It's value is best determined by what source will be driving it. Use a 100K control for SS sources, and 500K for VT sources. However, a much better approach would be to bypass the AF amp/tone control section all together, use a good outboard mono preamp, and inject it's signal straight into the power amplifier section of this unit. This would bypass all the existing tone switches and crystal pickup equalization built into the unit now, allow for much more versatile use from a more flexible and capable preamplifier, and allow the unit to basically operate as a mono block power amplifier.
In doing that job, one of the biggest differences (i.e. most noticeable differences) between PA type power amplifiers and high fidelity power amplifiers, is that PA amplifiers offer very little electrical damping to the speaker. Therefore, PA type amplifiers often tend to have a larger bass sound than high fidelity amplifiers do. It may be enjoyable, but it is not accurate. However, used with a more flexible preamplifier, this could be addressed to some degree with the bass and/or loudness controls.
Finally, one of the greatest things about this unit is it's size, design, and construction. There's lots of room to work with, it's a straight forward, well proven circuit format, and is a great piece of equipment for you to learn and cut your teeth on. Basically, the sky's the limit as to how much you might want to modify it.