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previous: Randy Jamz OK, this is going to be Part 1 of a... -- 9/2/2000 12:50 AM

Re: "Crossover notch" biasing - my final answer! (Long-winded ramble: Part 1)

9/2/2000 12:35 PM
nuke	Re: "Crossover notch" biasing - my final answer! (Long-winded ramble: Part 1) Randy, I'm not sure what your point is in all of this, but what Aiken says is the truth and there really are no two ways about it. As far as the max ratings of tubes go, what you say is sort of true. Like any electronic component, one can choose to operate it beyond a rating, with the understanding that there may be consequences such as reduced lifetime. In the case of the 6V6, it actually carries some very (amazingly) high voltage ratings in some kinds of service. Used as a vertical deflection amplifier, it can withstand on the order of a 1kv in short pulses. Also it is tube with an awful lot of history. When the first examples of 6V6 were produced, state of the art tube manufacturing was not fully developed. Improved materials and processing technology meant later examples during the tube production golden era were far more robust than the original design center values. Fender et-al were tasked with creating a product with a certain amount of output power and beating the competition. Given the intermittant usage a guitar amp gets, pushing the ratings a bit is an acceptable tradeoff. As far as references to osciloscopes from 1958 texts, I think it would be a challenge to find any working scope made in the last twenty years that had anode voltages as low as 2Kv. Not in a world where $400 scores a brand new dual trace 20Mhz scope. As Randall Aiken says, the bias notch method is flawed, perhaps dangerously so, particularly without corresponding measurement of current and plate voltages. The notch/scope method seems to have been invented as a means to set some sort of safe operation without actually dissambling the amp in order to make measurements. On the other hand, setting the bias in a circuit of known design by setting the idle current with respect to plate voltage will always result in safe and reasonable operating points. From a design standpoint, one can specify a current level for correct operation by analysis of the operating curves and loadline of the output devices, load and operating voltages.

9/2/2000 12:35 PM

nuke

Re: "Crossover notch" biasing - my final answer! (Long-winded ramble: Part 1)

Randy, I'm not sure what your point is in all of this, but what Aiken says is the truth and there really are no two ways about it.

As far as the max ratings of tubes go, what you say is sort of true. Like any electronic component, one can choose to operate it beyond a rating, with the understanding that there may be consequences such as reduced lifetime.

In the case of the 6V6, it actually carries some very (amazingly) high voltage ratings in some kinds of service. Used as a vertical deflection amplifier, it can withstand on the order of a 1kv in short pulses.

Also it is tube with an awful lot of history. When the first examples of 6V6 were produced, state of the art tube manufacturing was not fully developed. Improved materials and processing technology meant later examples during the tube production golden era were far more robust than the original design center values.

Fender et-al were tasked with creating a product with a certain amount of output power and beating the competition. Given the intermittant usage a guitar amp gets, pushing the ratings a bit is an acceptable tradeoff.

As far as references to osciloscopes from 1958 texts, I think it would be a challenge to find any working scope made in the last twenty years that had anode voltages as low as 2Kv. Not in a world where $400 scores a brand new dual trace 20Mhz scope.

As Randall Aiken says, the bias notch method is flawed, perhaps dangerously so, particularly without corresponding measurement of current and plate voltages. The notch/scope method seems to have been invented as a means to set some sort of safe operation without actually dissambling the amp in order to make measurements.

On the other hand, setting the bias in a circuit of known design by setting the idle current with respect to plate voltage will always result in safe and reasonable operating points. From a design standpoint, one can specify a current level for correct operation by analysis of the operating curves and loadline of the output devices, load and operating voltages.