It is well known that all followers are prone to oscillation, especially with a capacitive output load. The usual solution is an output stopper, to isolate the output from the capacitance. Inductance does no harm.SemperFi said:
Yes, put it at the pin. But not for the reason you say.
When following a rule made up by wiser men it helps to know why they invented the rule; then you will know when you don't need to follow the rule. CC resistors are good at coping with temporary overloads, so they may be useful in a guitar amp output stage suffering from heavy overdrive. This reason does not apply to hi-fi, so there is no need to use CC stoppers elsewhere.
Guess what happens when u overdrive a power tube... ever heard of grid current?
Sometimes it may need to carry the current but one needs to wonder where that current is coming from in the first place. In a hifi application the only time grid current flows deliberately is in A2 or AB2 operation, and that's relatively uncommon in modern tube hifi amps. In most cases it will likely be short term overdrive or a fault in the driver or output tube itself - the stopper is not going to provide any protection in such events.
In many power tubes an open circuit in the control grid circuit will result in the destruction of the tube.
In many power tubes an open circuit in the control grid circuit will result in the destruction of the tube.
I've definitely had metal films open when used as cathode sense resistors, but only when something catastrophic happens. Usually this occurs when I get too lazy to burn in some NOS WWII power tubes and one of them arcs.
I don't think there's a lot of debate about whether metal film resistors are good for pulse duty- they're not. About the only thing that handles pulse currents well is carbon comp / carborundum.
A lot of people don't necessarily realize it, but wirewound resistors are quite poor with pulsed power as well. I've had quite a few of them go open on me. The "Non-inductive" type tend to be more fragile when it comes to pulse currents.
I once had the naive idea that musicians are artists, and being original is every artists goal. So offering guitar amps that were NOT clones (as every other amp builder seems to do) was a good idea. So I built guitar amps with 6AS7 triodes on the output, and trioded 5881s and drove 6V& in class AB2, etc.
I cannot remember which blew the MF gridstopper, but rest assured some of the amps had plenty of grid current on overdrive.
Btw I learned the hard way that guitarists even though make the greatest sound known to man, all want to look and sound the same on stage, so very little market for any amp that sticks out as different...
I once used those Caddock MP930 TO220 non-inductive power resistors as source resistors on a MOSFET amp. They opened up on me once! I talked to a Caddock rep at Electronica Fair in Munich and sure enough he admitted they are not very pulse tolerant.
Picking resistors for pulse/overload is not trivial.
In most applications it does not make a difference. The TSE amps I designed use a 5842 driver tube with a CCS load for high gain. That gain extends well into the VHF RF region. I had an early prototype oscillate so hard that it whacked TV reception in the same room yet sounded normal.
Since then, I routinely "sniff" my audio amps with an RF spectrum analyzer, preamp and broadband antenna while messing with input cables and running my fingers around the glass on all the tubes and around all non conducting parts. The 5842 in the TSE and TSE-II boards can be made to oscillate sporadically in the 50 to 150 MHz region by doing this with a 1K to 10K metal film stopper resistor. The same amp is benign with a 4.7 K CC resistor OR the metal film with a ferrite bead over each lead.
For this reason I recommend a CC stopper resistor. The amp is dead quiet with headphones connected directly to the speaker leads regardless of resistor type. A gentle thump can be heard when oscillation is provoked with the film resistor. There may be some increase in the background noise with the oscillation, but if so it is below the threshold of my scope or 67 year old ears.
I have blown exactly one grid stopper resistor with overdrive. It happened when pushing a pair of vintage 6L6GA's beyond 110 watts per pair in AB2 with mosfet followers driving the grids. The cathode resistor was destroyed as well due to a tube arc. The tubes survived and still work today. These were NOT typical conditions even for the most ardent metal shredding guitar amps. If someone blew a screen resistor with extreme overdrive there might have been a non fatal tube arc. There would still need to be a path for a LOT of grid current to flow which is not present in the typical guitar amp.
I also routinely stress test all of my HiFi amp designs by making them eat my lousy guitar playing at full crank for several minutes. I connect my old ADA MP-1 guitar preamp to the inputs, and dial up a preset that I named Jimi. It was here that I discovered that I could blow a 1 watt metal film screen stopper on an SE EL34 in UL mode at full crank. The stopper only blew on EL34's, 6L6 types and KT88's did not blow screen stoppers. A 2 watt resistor is recommended on all SSE builds ever since board #1 for this reason.
Tubelab, I'm no electronics expert but I am exploring this subject. This is what I'm reading and being told. Carbon comp resistors have the most Johnson noise, so why use them if not needed? It goes from people seeming to think that the grid stopper is or might be the most important resistor for sound all the way to it makes or likely makes little to no difference on sound. Inductance of this resistor is said to be the big factor. I'm getting figures that metal film resistors can go from less than 2nH all the way up to 700 microHenrys. I'm told that even 1 microHenry doesn't effect the sound of an amp. It seems quite clear that there is or can be a huge difference in the inductance of different metal film resistors. Likely enough, IMO, to reasonably speculate that there is a MF resistor that would prevent the high frequency oscillation that you discussed. It is very hard to get inductance figures of most metal films. The inductance on metal foil numbers I see published are vanishing small. If expense wasn't an issue they would seem to be a great choice unless there is some other problem I wouldn't know about. Super low Johnson noise, super duper low inductance. I get one report that carbon comp are on the order of 20-30nH. To sum up for now, it sure seems likely that an appropriate metal film exists to satisfy the most demanding requirement for a grid stopper and I see no reason not to use a metal foil. I would like to see inductance numbers of some of the famous metal films like Dale...
The thermal noise (Johnson or Nyquist) is exact the same for any resistor tupe. It is only a factor of the resistance value. The excess (current) noise is very different from different resistor types, CC being very noisy. But if there is no current thru the resistor, there is no excess noise. So for grid stop the good old CC is concidered best. It is the least inductve of any type which is probably why they work so well, in addition the their great pulse tolerance.
SF, it seems that you are correct about the Johnson noise. I either read that the CC had the worst Johnson noise or maybe I conflated Johnson noise with current noise. I don't get how they can all be the same but I guess I'll have to live with it. Many think that only in very rare instances if at all will the inductance of metal films have an effect on audio frequencies. If there is an oscillation at frequencies much higher than audio I don't know if that would effect the sound or the proper operation of the amp. I assume that people in the know who think it's fine to use MF as stoppers think it is not a sonic of operational issue.
If the stopper fuses before the grid, it protects the grid...
Only if you don't have anode over-current protection, such as a fuse (OK, a HV fuse may not be the cheapest type, but its not rocket science to see the advantage over using the tube as its own fuse!).
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