886 ballast

[RetiredRich]

I am working on a Victor Electrola 12-15 for a friend. Problem is the 886 is only dropping 20 volts which makes everything on the chassis too hot. It gets warm but if I had just put in all the tubes and not brought it up slow on a variac it would have popped all the tubes. I have found by using reststors to drop the proper voltage that the amp seems ok. Even ran a tone through it. I believe a good 886 will fix the problem but don't know where to find one. I am really confused as I read somewhere the 886 is supposed to be used in 25 cycle supplies. The 887 tube is used for 60 cycles. But all the name plates on this machine says 60 cycle supply and the tube is an 886. Any help appreciated

Rich WB4VFN

[gregbogantz]

The 886 is a so-called "ballast" tube. It is basically a resistance wire operated in a vacuum bulb somewhat into incandescence. This operating point is supposed to allow the ballast tube to function as a (poor) form of voltage regulator. As the input voltage to the circuit increases, the bulb passes more current which causes it to glow brighter into incandescence. This causes the resistance of the bulb to increase, thus limiting the current somewhat. This results in a form of voltage regulation to occur at the primary of the transformer, but the regulation action is pretty poor. The Victor 12-15 was fitted with two quite different amplifiers. The one you have sounds like the early one with the #10 output tube. The later amp used a #50 output tube and did away with the ballast tube. Ballasts were a dumb idea, made LOTS of heat, and Victor used them for only a short while.

Since the ballast tube starts out with a low resistance when cold, it will cause a voltage surge to occur in the power transformer secondaries when the unit is first turned on. The 199 tube filament runs on DC off a voltage divider from the main B+ supply. So even with the surge in the turnon voltages, the 199 filament won't see that immediate surge until the rectifier tubes warm up. By which time the ballast should have also heated up and reduced the transformer primary voltage. Still, it's a flaky arrangement even when it's working as designed. For example, unplugging any substantial load from the circuit such as the rectifier tubes will cause the power transformer to draw less current which will decrease the drop across the ballast which will raise the transformer secondary voltages. Be careful that the high voltage winding does not get too high under these conditions which may arc and break down the transformer insulation. Also make sure that the long chain B+ voltage divider is all working. An open resistor in this chain will result in VERY high B+ voltage which can arc and break down the filter capacitors. A shorted resistor in this chain can result in too much voltage across the 199 filament.

But your main problem is that these early amps designed for use with the ballast tube are designed to have only about 80 volts or so on their transformer primary windings. You need to provide some means of getting that primary voltage if you don't use the ballast tube. The #886 is the tube specified in the Victor service data for this amplifier, but it is pretty hard to find. You can pay a lot of money for an 886 or go to plans B or C. Which would be to either use an autotransformer ahead of the power transformer to reduce the input voltage, or you can replace the ballast tube with some high power wirewound resistors. Many of the restored early RCA amps that I've seen have used the resistors. This works perfectly well with modern AC mains power which is pretty constant voltage. But using an autotransformer (or a voltage-bucking transformer with its secondary wired in series with the power transformer) will run MUCH cooler than the resistors and be an even better solution.

[RetiredRich]

Appreciate the info. I played all day with high wattage resistors and got the input voltage dropped to the right value. Already replaced the filter caps and the amp now works good. Even hooked up the tone arm and got input from the needle. Now to order some high wattage resistors. I usually do old radios but this has been a fun challenge. Again thanks for the input. Rich WB4VFN

[gregbogantz]

Rich, glad to hear you're having success with the 12-15 amp. Since you say you have mostly worked on radios, you may not be familiar with rebuilding the horseshoe magnetic pickup on these old Victors. Long story short - the rubber suspension parts and bushings around the needle bar get hard with old age. If you do not replace these parts you will grind up your records FAST because the needle assembly on the pickup is much too stiff. There have been several threads on this forum about rebuilding these pickups. Check them out.

[Guest]

You are right I did not know about rebuilding the pick up and appreciate your heads up. I will look into it as I want to make this old thing work and last good. Thanks Rich