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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: The problem is your Tube Amp not Tube Amps in General (nt) posted by Cut-Throat on June 2, 2002 at 08:51:38:
Moldog,You say that the power tubes went ballistic, once, and took out your bias resistors.
Ever thought that it may be the other way around. I've seen a lot of unstable amps trace back to unstable bias power supply for the idle bias circuit.
I'd check the diodes, electolytic caps, resistors of your idle bias circuitry. If the schematic say that the negative voltage of the idle bias tap should be -40 volts (say)---it better be a steady -40 volts at idle.
Should it vary up-&-down by 5 volts, with several readings at idle---you are, in essence, changing the idle bias point of the power tubes. This is playing with fire, especially with power tubes that are capable of drawing a lot of current.
Many of these amps have a screen resistor (usually 1-2 watt rating) that limit the current draw of the power tubes. These should not be allowed to be changed to higher wattage rating resistors (like 5 watters). Or it defeats the purpose of this resistor.
Finally, dressing on the power tubes and even the gain stage tubes is critical for a stable amp. If an oscillation is started in the gain section---it will be amplified and carried to the power tubes. The power tubes will try hard to reproduce these oscillations----leading to a parasitic condition.
More current is required to reproduce the oscillation. This taxes the power transformer---and causes an increase in the oscillation peaks. So, the power tubes suck more current to reproduce this increase in oscillation. A deadly vicious cycles. Like a dog chasing its tail.
What is worse---these oscialltions often have a fundamental frequency well above the audbile range. Like around 25 kHz. So you can't even hear the havoc wreaked on your amp. All you see is overheated parts and often a dramatic drop in volume of music, at the same volume settings. And eventual failure.
I've found that parasitic oscillations and failed bias circuits are two of the biggest causes of power section failures, in fixed bias amps.
Neat soldering and shortest leads are the key. Resistors butted up against the socket lugs. Caps kept as far from heat sources (heat is cap's worse enemy).
And I strongly consider that you replace all the bias power supply diodes (watch the polarity), electrolytic caps, and resistors (go to 1% Rikens or equivalent). Form those caps. And test the bias voltage. Should be steady as a rock and near the schematic's value (+/- 2 volts max).
Check the dressing around those gain stage tubes and phase inverter---look for poor or cold solder points. Excess leads. Kinked in leads. And NEVER wrap a bunch of wires together. You may be inadvertently causing an EMF coupling which could lead to instability.
Change all the screen resistor with proper wattage rated resistors. 5% carbon comp are fine---since this resistor is more of fuse than a contributor to the tone. Remember, body of resistor butted right up against the lug.
If you don't do this work, try to make sure your amp tech follows these suggestions---most of the time when these things are done, the infamous "tube eating amp" becomes a long lasting stable unit.
Good luck.
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Follow Ups
- Not tube took out bias supply, but maybe vice-versa. - steven oda 09:54:42 06/02/02 (0)
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