Discussion in 'General Technical Questions and Answers' started by K7URX, Dec 14, 2019.
Can a grid resistor that's gone up in value cause lower output power in a grounded grid amp?
Of course it can.
Best thing to do with GG amps is ground the grids directly, then there aren't any resistors in the path.
I thought so but wasnt sure. Mine are out of spec. Thank you.
Is it as simple as removing the resistors and wiring directly to the chassis? What about the capacitors that are on that same pin?
That depends on the specific amp but generally speaking no. Unless you happen to have an amp that is set up for appropriate idle mode current with zero bias you'll need to change the bias circuitry when you hard ground the grids. That usually also means changing the grid current metering circuitry as well. None of those things are difficult or expensive but in most amps when you hard ground the grids you have to change a bit of supporting circuitry. There are exceptions in amps that don't introduce negative bias voltage to the grids in their stock configuration but it all depends on the specific amp and how it's currently configured.
What make and model amp are you thinking of changing to hard grounded grids?
Thanks for the reply. The amp is a sb-200 with the usual harbach upgrades.
I converted my SB-200 to direct grounded grids. It's not too hard, the basic steps are:
- Remove the resistors and capacitors currently connected to the grid pins of the 572B sockets
- Bond those pins to ground through a very short wide copper strap straight to the chassis right next to the sockets
- Unscrew the filament transformer secondary center tap that's currently bolted straight to the chassis
- Introduce a bias voltage of around +2 to +3 VDC during active transmit mode operation to that filament transformer center tap, this can be done several ways but the simplest is probably a string of diodes each forward biased to achieve the desired no signal plate current with the amp keyed (~90 mA)
- The current cutoff bias supply is a negative DC supply (approximately -100V), you'll want to turn around the diode and capacitor on this supply so it becomes a +100V suppy or thereabouts (specific value not critical just high enough to make sure the tubes are in cutoff when the amp is not keyed)
- That cutoff bias supply runs through the T/R relay coil to the amp keying line that goes to your Harbach soft key board which. If you used the Harbach SB-200 soft key kit you'll likely have to change that to something that can key a positive, not negative voltage. I'd have to double check but I believe the Harbach SB-200 soft key board is designed to key a negative cutoff supply, not a positive supply.
- The grid current shunt resistor will also have to be moved because of the new positive bias scheme
There are various ways to do the steps above but here's a good schematic from W0ANM's blog using string of diodes biasing and a universal keying module: http://w0anm.com/blog/wp-content/uploads/2011/07/sb-200_amp_color.jpg
R7 is the grid meter shunt and R15 is the grid meter multiplier in this new topology
There are some other mods shown on this schematic such as adding an extra coupling capacitor and swamping resistors in the cathode drive. Also there's an added glitch resistor in the HV line which is a good idea that can help protect the metering circuit in the event of a grid to anode short in a tube as well as meter protection diodes. There's other approaches to adding safety like a single diode on the B- line like Ameritron uses. I personally would just add a pulse rated glitch resistor in the B+ line but wouldn't recommend the HV fuse but there are many ways to skin this cat.
It's not as daunting as it might look but the SB-200 takes a bit more than just some copper straps if you want to hard ground the grids.
That amp is an example where cutoff is supplied to the grids as -120 volts in receive.
Setting it up to directly ground the grids isn’t difficult but does require quite a bit of topology reconfiguring. Things will not go well if the grids are grounded without taking care of several other items.
Edit: What K7TRF said!
Excellent. Thank you for that very informative post.