Since I had gotten used to the SA100 and it's external test points (including the two I added), I thought it would be an excellent enhancement for the SA300. But once I looked at the circuit and the amp's real estate, I realized it wouldn't work the same way as with the SA100 and wasn't feasible with the SA300.
When I talked about drilling holes for access, I was talking about through the removable side panel to the bias and hum controls. If I were to split the bias circuit into left/right and wanted access without removing the side panel, I would drill 2 holes and cover them with metal plugs. Those same metal plugs that cover the AC/DC balance controls are available in chrome finish at Lowes, they would only need to be painted to match the amp. Done in this manner, I think the amp wouldn't look so bad and I don't think anyone would even take notice, much less complain. However, given that the side panel is easily removed (by design I suspect), drilling is not even necessary to access a single or dual bias controls. Splitting the single bias control into two would remove the need for a closely matched quad of output tubes, which makes perfect sense to me, but isn't even an absolute necessity. And honestly, how often do we check bias? That's when I came to realize that fabricating individual bias test sockets (similar to those sold on ebay for $20) for each output tube would actually be the simplest and most accurate way to go with the least overall effort. I have one VTVM, two VOM's and six DMMs (including 4 Flukes) so anytime I want, I could get a complete picture of the amp at a single glance. Using this DIY approach, I can custom construct test sockets to read any parameter with a minimum amount of effort. I can build them to read voltage and currents for the plates, screens and cathodes simultaneously. After much thought, it's the only scenario for which I cannot find a negative aspect. It's the ultimate tube tester because it tests each tube in use at real working voltages. I'm the first to plead guilty when it comes to over-engineering, but when you get down to it, individual bias test sockets for each tube makes the most sense to me because they can can be used on any amp.
I liked Dave's solution employing the (rarely used) terminals on the SA100 speaker strips, but I thought replacing the 3 terminal stock test points with a 5 terminal strip was even better in that it was more intuitive and obvious. I was, as he suggested in another post, doing what Fisher probably should have done in the first place. If you look at some of my pictures and compare them to a stock SA100, a five terminal strip fits and looks like it was part of the original design. The biggest problem was finding a five terminal strip, which I did, including a few spares. Installation was a snap, no major surgery. My governing principle is to make any project look as though I was never even in there and why I wanted to see an image of a factory condition SA300B amp.
With my SA100 now running stable, I hardly ever find the need to check bias, only when I swap tubes or when my ears seem to play tricks on me. I like to switch tubes around every once and a while because they sound different, so the bias gets checked often enough anyway. I have sets by Amperex, Sylvania Black plates and GE-7189-A, and they all sound different. Depending on my mood or the kind of music I'm into, I'll swap tubes to get the best sound for my mood.
The first amp I rebuilt was the Dynaco ST70 using the VTA driver board. The VTA board has 4 adjustable pots and provisions for external test points for the four individual EL34's via the front preamp octal sockets. An empty octal plug, test wire, banana plugs and two DMM's-I had a simple and very safe way to plug in, observe and adjust the bias and balance of each channel without ever having place a test probe near a high voltage source, inadvertently slipping the probe and causing damage. (Read "Screwed the Pooch" on this forum for an example).
Audion:
What Dave noted about the SA100 riveted ground terminals does not apply to the SA300. As I recall, the chassis of the SA100's have a high copper content, and riveting the dissimilar metal of a grounding lug creates a recipe for a galvanic reaction between two dissimilar metals resulting in corrosion and resistance between the two dissimilar metals. The Chassis of the SA300 isn't the same, and I haven't noted any corrosion.
You changed the couplers (Jensens and Orange Drops) but what about the electrolytics used for bypass? Are any of them near a source of heat? Like the orange drop next to the fat resistor by the AC/DC controls?
Any chance you simply had a tube go bad? It happened on my Dynaco ST70 recently after sitting idle for a few months. Red plate out-a-nowhere, and the tube was gone. Fortunately, that amp doesn't require closely matched quads as each socket is adjustable. Swapped in another tube, adjusted bias and good as new. Maybe it was just that tubes time. Part of God's plan.
5. On the SA-300, there is a ground terminal and next to that a common terminal. On mine, I used the common terminal. The only thing separating the common and ground is a 0.47 ohm 5 watt resistor on each channel. That's an oddity. It's possible that the SA-300-B may have speaker common terminals that are not at ground potential to support the use of a summed center channel output. Dave has mentioned on the Fisher 400, X-1000, and X-101-C restoration threads that the speaker common not being at ground potential is an issue when testing.
03-28-2014 02:57 PM
It's my understanding that you're supposed to clamp the negatve probe to the chassis for ground when testing voltages (at least that's what I do).
I could be wrong here, but on the SA100 I thought the "x" terminal served as the ground terminal for speakers needing more damping factor and one speaker connected from Channel A common to Channel B common was an amplified mono amp (when the "stereo/mono" switch was switched to mono). I never personally verified this so I am only speculating.
And why the inrush limiters? I didn't think they were necessary with a GZ34 rectifier because it brings the B+ up slowly? Did you do it to reduce input voltage? Do they get hot affecting the nearby hum adjust pots or the rectifier tubes?
To all:
Regarding the Selenium Rectifier Stack...get it the hell out of there! When they blow they release a toxic gas. I didn't even want it around if it got heated by the nearby diode.
Selenium dioxide is formed when selenium is heated in air. Direct exposure to selenium dioxide is, therefore, primarily an occupational hazard and not likely to be a risk at hazardous waste sites. Selenium dioxide forms selenious acid on contact with water, including perspiration, and can cause severe irritation. Acute inhalation of large quantities of selenium dioxide powder can produce pulmonary edema as a result of the local irritant effect on alveoli (Glover 1970). Bronchial spasms, symptoms of asphyxiation, and
persistent bronchitis have been noted in workers briefly exposed to high concentrations of selenium dioxide (Wilson 1962). Kinnigkeit (1962) reported that selenium dioxide concentrations of 0.007–0.05mg selenium/m3
in a selenium rectifier plant producedslight tracheobronchitis in 9 of 62 exposed workers
Selenium rectifiers
Selenium dioxide is the major compound produced when a selenium rectifier is overheated. It can cause severe burns to the mucous membranes and severe respiratory tract, skin, and eye irritation. It can also promote allergic reactions with the skin. Fortunately it is not considered a carcinogen.
Another fortunate thing is the awful smell which gives it away. If you smell something really rotten, like decaying onions and garlic, coming from your equipment, it is best to leave the area immediately, opening some windows on the way out. Allow the selenium dioxide vapours to dissipate for several days before you go back - you will not want to go back very soon anyway!
