Tom: Continuing on with this discussion.....I read most of the information on this link: http://www.w8ji.com/vhf_stability.htm
While it explains a great deal it will require 2 or more readings to digest the most important information.
Now, since most amplifiers use either 2 X 3-500Z or 2 X 572B tubes, give us a good design for anode parasitic suppressors, including number of coil turns and resistor values. Please.
I'm quite sure this information will be welcomed.
So far we have learned that Ohmite OX or XY resistors are ideal for parasitic suppressors. Please finish the design.
I cannot finish the design without making up a big tall tale.
Anyone telling people they have a universal suppressor design for a certain tube type, that works in all amps using that tube type optimally, is feeding people pure nonsense.
The suppressor is part of a SYSTEM that involves the anode system in the tube and all the leads and components from the anode to the chassis, AND back through the chassis to the cathode and grids of the tube.
The likely frequency of oscillation, if oscillation occurs, depends mostly on the control grid path characteristics in grounded grid triodes. To oscillate at VHF, the grid has to become divorced from the chassis at a frequency where the ***internal*** elements of the tube are still producing gain. So even grid path impedance, from inside the tube itself to the chassis, will play a role in determining if and when a parasitic suppressor is necessary and what the optimum impedance of the suppressor would be.
Anyone ever telling people they can sell a "slap in" design based on tube types is deluding everyone. Some amps, in particular using tubes with grid flanges like the 3CX800A7 and 8877, can be unconditionally stable with no suppressor at all.
As a matter of fact I'm working on an AL800H amp right now, and I pulled the suppressors totally and it remains 100% stable. When I built the very first AL800H amp prototypes, they were 100% stable with 3500 volts on the anode and NO suppressors at all. I put suppressors in because Hams give people some much crap about needing suppressors, it was easier long term to add unnecessary parts than to face questions about why something expected to be there is not there. So obviously I could sell anything as a suppressor for that amp and it would work (like I could an AL1500) because they are 100% stable, even if anode voltage is nearly doubled, under all tuning conditions. This is because the grids have a VERY short low-impedance connection to the chassis.
The same is not true for a 572B or an 811 tube, which have long skinny grid leads. Even with the grids directly grounded they can be unstable, as can be a 3-500Z and 3CX1200D7 or A7.
A 3CX1200Z7 is unconditionally stable because of the conical grid support and flange. Its sister tubes, the D7 and A7, are not.
As a general rule this is how it works:
1.) Longer thinner grid leads move the tendency for oscillation lower in frequency. This means the suppressor needs more INDUCTANCE.
2.) Longer thinner anode leads, and longer "thinner" paths through the chassis back to the tube, require more suppressor impedance to have an effect.
3.) Some resistance value will maximize damping. This is generally where the resistor dominates the impedance at the parallel resonant frequency of the grid (where the grid is divorced from ground). At that frequency, a higher anode to chassis path impedance would require a higher resistance resistor. A lower impedance path would require a lower resistance for optimum damping.
Heath, if the engineers were any good, went all through this when they did they did the SB200. Any other manufacturer, if they are any good at design, would do the same. They would have also checked stability at maybe a 30-100% overrun of plate voltage with the tube biased at various voltages on all bands with all control settings to check for unconditional stability.
It is unlikely some 5-minute Ham design would improve on something like that.
As a matter of fact if you do a stability test on an AL80B and substitute a certain west coast nichrome kit, you will find it becomes substantially LESS stable. In either case it is OK at normal voltages, but when run through the same test of over-voltage and control settings it will take off like a buzz saw with the nichrome kit. This is because the inductance of the nichrome kit is too low, so the resistor does not load the circuit enough at upper VHF where the tube oscillates.
The SB220 Heath requires even more inductance than the AL80B does, because of socketing and grid leads. So an optimum suppressor in the AL80B has half the inductance of one for the SB220. The same nichrome kit that has too little inductance for the AL80 amp is even further off in the SB220, and the tube types are the same. The only difference is in socketing and impedances of paths from the grid to chassis and anode to tube circuit in each of them.
If I told you I could give you one optimum design for a certain tube type, it would clearly show I did not really understand how amplifiers work.
My advice to anyone would be if the amp was properly engineered, duplicate what they had. If the resistor is bad, replace it. Carbon resistors age fast, especially when hot. Use an OX or OY metal composition of the same value and use the same size inductor as the original suppressor.