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Finally getting around to restoring some Fisher gear I pulled from a console a few years back - 100T Preamp Tuner and SA-16 amp. I just finished the 100T preamp / tuner. Replaced all the electrolytics and made some resistor changes in the power supply to adjust the voltages to compensate for the higher modern wall voltages. Using it with the DIY 27- 6A5G amp I recently reworked and it sounds fantastic.
The SA-16 amp had obviously developed some issues. Two of the output tubes (one in each channel) had lost its vacuum and the bulbs were cracked at the base so I had to pry them out of their sockets. Then I discovered that the two 220 ohm resistors that are used to create a center tap for the filament winding were burned up. Now I'm wondering what would have caused this. Perhaps a short in the tube's filaments? The fuse was intact, though. Any ideas?
I pulled all the tubes, disconnected the PT leads, and put 50v in via variac. Both secondaries seem to be OK - I measured 294v across the HT secondary and 2.9v across the filament winding.
Guess I'll break out the Dremel and remove the rivet that holds the charred terminal strip, replace it and install new resistors. I'll probably just power it up slowly and check voltages before I replace all the PS caps.
Most all the stuff I've restored previously hasn't been damaged - out of spec parts, sure, but nothing burned up. Anything else I should look out for?
Follow Ups:
Got the SA-16 up and running again. I'm very impressed with the sound even after doing minimal work on it.I had to mount a new terminal strip to replace the one that was damaged when the heater resistors burned up. Still not sure why that would have happened. I did figure out why one of the two output tubes died. One of the .022 uF ceramic coupling caps was leaky and the replacement tube started to red-plate. I was a little surprised because ceramics have a reputation for being somewhat indestructible. The coupler on the other dead tube's socket seems fine though, so that is still a mystery.
For now I just replaced the bad one with another ceramic because I wanted to hear the amp in stock condition. It sounds so good that I'm actually hesitant about replacing them. I do have some nice .033 uF Good-All PIOs so I'll use those if I decide to replace the ceramics.
Another surprise was that, even with the variac at 121.5v the voltages were very close to being spot on. I figured I would have to install higher value resistors in the power supply to adjust the voltages but it doesn't look to be a problem.
Next I'll be replacing all the electrolytics. The first cap will be a 35 uF 500v Panasonic film and I'll use 47 uF 450v Panasonic lytics for the others.
Edits: 09/26/16
I have a similar Fisher console (Futua I think) amplfier, looks like the SA-16, but 7591 outputs. The power supply in the amplifier also powered the preamp and tuner. Without the preamp and tuner plugged in, some of the B+ voltages were too high. The amplifier actually played like that, but there were some resistors that were getting hotter than they should, and I didn't catch on until I subsituted 5751's for the 12ax7's and no sound. I know it was an overkill but did a SPICE simulation and found the drivers were running off the charts. I corrected it with changing some of the power supply B+ resistors.
Whatever you have to do to that amp it is probably worth it. It's close cousin is a very good sounding amp.
Yeah when you remove the pre / tuner current and then on top of that connect to the higher modern wall voltage the voltages can get out of hand. Fortunately these components (100T pre / tuner and SA-16 amp) each have their own power transformer. What I did to the 100T was to increase the resistance in the power supply so that the voltages were correct with 121.5v on the primary.
I replaced the burned resistors on the amp and put in new AC suppression caps and started powering it up. Unfortunately, the two remaining 6BQ5s and the two 12AX7s - all Amperex - all tested bad so I had to put in some other tubes. Got up to 80v input before I went out to hear some live music. I'll continue the process over the weekend and see what happens. I want to make sure there's nothing seriously wrong before I go to the trouble of replacing the PS caps and going through the voltage adjustment process like I did with the 100T.
Charlie
The 220 ohm resistors are put there to create an artificial center tap for the filament supply.They are also there to shunt leakage current from the primary that can cause hum from being imbalanced and creating a magnetic field.
This would not cause your tubes to lose vacuum.The amps were probably sitting in an area with extreme temperature change or some physical jarring caused these tubes to crack and lose vacuum.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 09/23/16
I understand that the resistors are used to create a center tap and also fulfill the same role as a hum pot, though not adjustable. I didn't think they caused the two output tubes to lose vacuum, I was speculating that it might be the other way around - that the tubes lost vacuum and perhaps their heaters were shorted somehow causing the resistors to burn.
I suppose temperature could have been an issue regarding the tube losses. I recall when I pulled these that the amp was mounted in a fairly small, and completely enclosed, section of the cabinet which seemed odd to me.
I assume there was either excessive current or excessive voltage going through those resistors. I'd just like to figure out what caused it, if possible, so any related issue can be dealt with.
The other thing that occurred to me just now is that the so-called "death cap" (a .1uf 600v poly) went bad and was allowing too much current or voltage to the chassis. Would that affect those resistors or not?
The fuse is original - a 2A Slo-Blo. The output tubes (6BQ5 Amperex Bugle Boys) may have been replacements installed in the very distant past but if I had to guess I'd say the console was never serviced in any way during its lifetime.
I'd replace the needed parts (resistors and caps) and bring it up slowly on a variac while watching the current draw. If you start drawing excessive current before you get to 60-70VAC then there is some other issue.
As Mike noted, I highly doubt the vacuum loss on the tubes was due to an electrical issue, much more likely due to thermal or mechanical issues.
The 16 is a nice amp, as is the SA-100 Fisher. Both similar in sound to the Pilot SA-232. All 3 of these amps you can listen to for days without any ear fatigue...
Good luck in getter her going again.
I've seen this when someone substitutes the wrong fuse. Like using a 20-amp car fuse one in place of a 3-amp slow-blow.
Excessive initial current draw fries the artificial CT tap resistors.
Steve
I've often wondered at times if the fuse acts as an inrush current limiter.If you watch a fuse that is exposed that sits in the fuse clips,many times on startup you can see a movement inside the fuse wire or it gets slightly brighter and then settles back down.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 09/23/16
Why tube amps often use slow-blo fuses. Fast blows, at same current rating... pop. Esp, cold startups.I've worked on or read around a dozen amps with fired heater lines and/or pilot lights that keep popping. You check the fuse holder and someone had replaced the OEM one with a 20-amp car fuse. Or worse, a straight wire.
Edits: 09/23/16 09/23/16
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