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I've noticed the driver stage in this amplifier is susceptible to the side effects of secondary emissions. That is, it keys at one level and after you modulate for a few seconds, the power can rise by as much as 200 watts. Lesser quality or weaker tubes in the driver stage can cause this amp to behave like this because the screen grid already does not have a low enough resistance to ground as Nomad mentioned. This amp just uses a resistor but it works fine when used with great tubes. The use of a shunt regulator reduces these issues and often allows the tubes to be used much longer before instability appears.

I'm not saying that is what's wrong with this particular amp, but if the problem persists, you might consider temporarily swapping the driver tubes for a pair of finals and see if you notice any improvement. 6LF6 driver tubes seem less prone to this and some 300A's came with them in the driver. If they aren't all matched, be careful this swap doesn't cause some to turn red when keyed a while. Everything Nomad said needs to be done first. Those white caps are almost never good anymore. If the PC board screws are still tight, it's only because someone re-torqued them within the last decade. Note that these screws have nuts on the top and bottom of the chassis. The screw head must make a tight connection to the PC board ground before the nuts lock the screw up.

Years ago it was a fair argument to favor the 572B or 3-500Z over sweep tubes just based on tube life alone. Since Cetron and Eimac stopped making these and they are all made in China now, things aren't the same. The old Cetrons in my FL-2100 lasted 20 years when the new Talors lost full output within a year. Four RCA 6LQ6 made in 1968 are still running strong in an old Dentron. One Eimac 4-400A made in 1963 peaks 1500 watts with a 375 watt carrier in a T-368. Point is if you need glass that will last, buy NOS.

<blockquote data-quote="Shockwave" data-source="post: 658760" data-attributes="member: 10882">I&#039;ve noticed the driver stage in this amplifier is susceptible to the side effects of secondary emissions. That is, it keys at one level and after you modulate for a few seconds, the power can rise by as much as 200 watts. Lesser quality or weaker tubes in the driver stage can cause this amp to behave like this because the screen grid already does not have a low enough resistance to ground as Nomad mentioned. This amp just uses a resistor but it works fine when used with great tubes. The use of a shunt regulator reduces these issues and often allows the tubes to be used much longer before instability appears. I&#039;m not saying that is what&#039;s wrong with this particular amp, but if the problem persists, you might consider temporarily swapping the driver tubes for a pair of finals and see if you notice any improvement. 6LF6 driver tubes seem less prone to this and some 300A&#039;s came with them in the driver. If they aren&#039;t all matched, be careful this swap doesn&#039;t cause some to turn red when keyed a while. Everything Nomad said needs to be done first. Those white caps are almost never good anymore. If the PC board screws are still tight, it&#039;s only because someone re-torqued them within the last decade. Note that these screws have nuts on the top and bottom of the chassis. The screw head must make a tight connection to the PC board ground before the nuts lock the screw up.Years ago it was a fair argument to favor the 572B or 3-500Z over sweep tubes just based on tube life alone. Since Cetron and Eimac stopped making these and they are all made in China now, things aren&#039;t the same. The old Cetrons in my FL-2100 lasted 20 years when the new Talors lost full output within a year. Four RCA 6LQ6 made in 1968 are still running strong in an old Dentron. One Eimac 4-400A made in 1963 peaks 1500 watts with a 375 watt carrier in a T-368. Point is if you need glass that will last, buy NOS.</blockquote>

[QUOTE="Shockwave, post: 658760, member: 10882"] I've noticed the driver stage in this amplifier is susceptible to the side effects of secondary emissions. That is, it keys at one level and after you modulate for a few seconds, the power can rise by as much as 200 watts. Lesser quality or weaker tubes in the driver stage can cause this amp to behave like this because the screen grid already does not have a low enough resistance to ground as Nomad mentioned. This amp just uses a resistor but it works fine when used with great tubes. The use of a shunt regulator reduces these issues and often allows the tubes to be used much longer before instability appears. I'm not saying that is what's wrong with this particular amp, but if the problem persists, you might consider temporarily swapping the driver tubes for a pair of finals and see if you notice any improvement. 6LF6 driver tubes seem less prone to this and some 300A's came with them in the driver. If they aren't all matched, be careful this swap doesn't cause some to turn red when keyed a while. Everything Nomad said needs to be done first. Those white caps are almost never good anymore. If the PC board screws are still tight, it's only because someone re-torqued them within the last decade. Note that these screws have nuts on the top and bottom of the chassis. The screw head must make a tight connection to the PC board ground before the nuts lock the screw up. Years ago it was a fair argument to favor the 572B or 3-500Z over sweep tubes just based on tube life alone. Since Cetron and Eimac stopped making these and they are all made in China now, things aren't the same. The old Cetrons in my FL-2100 lasted 20 years when the new Talors lost full output within a year. Four RCA 6LQ6 made in 1968 are still running strong in an old Dentron. One Eimac 4-400A made in 1963 peaks 1500 watts with a 375 watt carrier in a T-368. Point is if you need glass that will last, buy NOS. [/QUOTE]

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