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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: Re: Bogen / NTS-1125 look like a 50-60 Watter... posted by m8o on March 13, 2007 at 21:52:05:
No trouble at all...while I have the time...
Since these Bogens are being used, no Variac slow start-up is necessary. However, using a Variac is often a good thing. Your wall socket has about 121 Volts AC. Late nights, it might even yield 124V AC. Much of the gear we vintage nuts, I mean aficionados, use, was designed for 110-117VAC. I use gear from the 1920s and 1930s, often. I always use a Variac as a sort of isolation transformer, for the gear designed for 110-117VAC. Without the Variac 121VAC would overstress many parts.
When older gear has been stored for long periods, over 9 months, a slow start-up with a Variac is worthwhile. Items stored for years, must get slow increases of AC input voltages, or they will certainly blow parts. Let me describe how it can be done for the older, stored for a long time, amp variety.
First, you must have a load on the amp, that is a speaker, or heavy duty 8-16 Ohm resistor, connected to the proper output taps. Set the Variac to 50VAC. Remove the output tubes before turning on. Turn on the Variac with turned on amp connected, yes, without the output tubes. Over long time periods, hours, you slowly increase the Variac AC voltage, a few volts at a time.
The reason I remove the output tubes, is very practical, learned from many years of fooling around with tube gear. The output tubes draw the nmost current. They are the likliest, to ruin the power supply caps, the fastest. Without the output tubes, we can reform many of the original power cap cans, very easily, as proper voltages are reached.
So, we can monitor the AC leakage on the power supply caps. Use a meter with the neg. lead secured to chassis ground, and use only ONE HAND to move the positive meter lead, carefully. As the leakage gets very low, less than .3 Volts AC, we can slowly increase the AC Voltage from the Variac. If an output stage electrolytic gets hot, back off on the Variac, and wait, hoping it will cool and possibly reform. When cool, turn up the Variac, a little at a time. Check the AC leakages as you go. This can take one hour to many hours, even more than a day, depending on the encountered problems.
If the AC leakages stay low, keep increasing AC, SLOWLY. Amps that are good, will reform quickly. Monitor the DC voltages at the power supply caps, also. As we increase the AC in, the DC comes up quickly.
You can get rated DC Voltages on the power caps, long before 110VAC !
That is important. As we reach rated DC voltages, back off on the Variac, back to about 60V AC. Then, insert the pairs of output tubes, and start the process over again, monitoring AC leakages.
As the AC leakages get low, step up the AC SLOWLY. Keep on keeping on, and soon, or later, the amps can work with 110-120VAC.
Bias is not all that tricky. First, try to realize that bias is really measured from the cathode to the grid. While fixed bias amps can measure negative bias from ground, any tube's bias is really the difference between its cathode voltage and its grid voltage. With tube manual or diagram at your side, you can see which socket pin is grid and which is cathode and which is plate, screen, etc.
Without a schematic, tube manuals can give insight to certain bias voltage points at certain plate voltages. Decreasing bias, that is less negative, decreases plate voltage, increasing current, closer to Class A or AB1. Increasing bias, increasing negative bias value, also increases plate voltage, but decreases idling current, closer to Class AB2. Don't sweat the details about Class here. Bias must be measured with no signal into the amp. Remember to have the amp connected to a load. Tube manuals are a guide. My amps often have different bias values than the manuals, but I like to run my triodes hard, in Class A or AB1, often written as A Prime in the 1930s. By the way, books from the '20s and '30s are much more informative.
So, in the case of the brutish Bogens, let's say their bias is about -30V, with 600 Plus Volts on the plate, 300V on the screens. Those 807s will run very hot, even yielding red spots on the plates. Raising the negative bias value to -35 or possibly -40, will reduce the idling current, lessen the red, add stability and life to your amp. If there is no adjustable pot/knob, you might have to change reistances, if needed. There are resistors, in the bias circuit, that are in series with the bias voltage, while others are in parallel, connected toward ground, the bleeder resistors. Juggling these resistor values is what changes bias.
If this is your first amp setup or "repair," DO NOT change anything on the Bogens, until you understand what you are doing. There are very dangerous voltages under that amp ! It would be safer, and better for you, to use a much smaller amp in your first experiments.
Absorb this all, and gather some more questions. Read and Reread.
The more you read, the more sinks in the brain. Some of the theory needs to simply be remembered, rather than understood. The best theory books are the early radio books and magazines, before WWII. I learned the most from 1920s and 1930s texts.
Gotta Go...Later...
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Follow Ups
- Variac Slow Start-Up; Bias & More... - Interstage Tranny 09:21:43 03/14/07 (2)
- Re: Variac Slow Start-Up; Bias & More... - m8o 21:41:36 03/14/07 (1)
- Understanding & Patience Pay Off... - Interstage Tranny 05:46:20 03/15/07 (0)
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