Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on September 29, 2010, 05:51:38 pm
-
What are the advantages?
The blackface Princeton Reverb AA1164 circuit does not have either power tube grid stoppers or screen grid resistors.
The blackface Deluxe Reverb AB763 has 1.5K grid stoppers on its 6V6 power tubes and 470 ohm, 1-watt screen resistors. Fender must have included them in his more expensive amps for a reason. What is the purpose of each of those components?
Thanks for your thoughts,
Chip
-
Grid stoppers mainly act as RF filters combined with the tube input capacitance (small for pentodes, high for most triodes), cutting out high frequency noise and curbing oscillations. They can also limit grid current when the tube is overdriven - less blocking disto, nicer sound.
Screen Rs limit current, thus protecting the tubes.
-
Stoppers are -sometimes- needed, more so with "highly strung" amps.
You think it is an Audio Amplifier but the tube is happy to sing past 10MHz. Our coupling networks and transformers tend to roll-off past 10KHz-50KHz. But wire length, OT windings, and stray capacitance sometimes form MHz tuned circuits.
Do you need them? Hard to know in advance.
Sometimes hard to diagnose.
Sometimes annoying to retrofit unplanned stoppers.
With low plate voltages and low loads, screens may be fed direct from a power node. At higher plate voltages and with mismatched loads, screen current may jump from 'small' to LARGE. This is dead-waste, and also large heat in the relatively delicate screen. And screen current calculations are very-very difficult if you know the exact parameters; impossible with real-world loads and 3rd-world tubes.
> Benefits of... stoppers
Invert the question. What is the DIS-advantage? 12 to 48 cents for resistor, mounting, labor.