Discussion in 'Amateur Radio Amplifiers' started by K5VV, May 28, 2016.
I think the output tank circuit and its efficiency has something to do with it as well.
The point being put across is the difference in design of the two amplifiers.
The tube plate load resistance is different at lower plate voltage higher current operation requiring a different conversion transform to output nominal 50 ohms.
What are the differences in tank current, Q , parts position, input drive configuration etc.
The drive input resistance, if lower requires more drive power just from that fact alone.
Old amplifiers in their day were driven by tube radios with spare power to burn so it made little difference.
Pretty silly to jumper switch... Nothing wrong with using lower for lower output levels. Higher voltage means higher zero signal dissipation which means amp would run hotter at say 700 out in SSB position than in CW position at same output. Higher is not always blindly better as suggested above.
The idea is to repurpose the switch and not just jumper it. By repurposing the switch you use it to add more operating bias which lowers the ZSAC you talk about. All newer amps use full plate voltage on the tubes and do not switch from high to low. That is a passe' method.
Just in case somebody doesn't know it, changing the plate voltage on a triode tube, also changes ZSAC without the use of another potential failure item like a zener diode which is considered by real engineers to be a poor choice to bias a triode power grid tube. So the low/high power switch already changes the "effective" ZSAC of the tube without changing any circuits. And the change in idle current is already correct for SSB vs CW use.
Also, most hams who like to play around with zener voltages to bias tubes don't have a clue what they are doing to the operating point of the amps linear transfer curve. People should not change the design of an amplifier when they don't know how to design an amplifier in the first place.
The statement "passe method" is really meaningless. It has no legitimate engineering significance.
One other issue nobody seems to want to talk about is whether the typical SB220 user wants to hold down the key, maybe a little too long with 3kv on the plate instead of 2kv on the plate which was the original design. The power transformer in a SB220 is a 450 VA transformer. It feeds a cap input doubler which forces the transformer secondary and the diodes to pass huge peak rectifier and secondary currents. Are those "passe" concerns?
Still a bad idea. Pretty silly to disable slower power mode that not only reduces idle dissipation on tubes but also reduces heat and stress on equalizing resistors. I would not disable this feature. Plus lower voltage is good if you may have a gassy tube. The higher the voltage the more likely it will arc over.
Can the tubes handle 100% duty cycle in SSB mode while tuning? If the anodes do not get hotter than 500 watts on each anode I see no problem. To gauge what anode color corresponds to 500 watts key down dissipation the amp can be run in CW mode at a power such that DC-RF power equals 1000 watts. In any event the amp can be tuned by sending DOTs with a keyer for one-half power.
In an SB-220 Yes, I wouldn't keep the amplifier key down for very long though considering that the stock plate transformers are over thirty-something years old.
In some amps, the issue isn't the tubes as much as the whole amplifier including the power supply and cooling system.
50 year-old Henry 2K amps could run "key down" at full power for hours and hours and hours...but the power supply was rated for it and the cooling systems were very effective. An SB-220 was designed for affordability and ease of construction, and can't do that.
But it was a great deal, and I hear them on the air almost every day.