Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.
Here are some photos. Maybe somebody can see something from these.
Rube Goldberg would appreciate unnecessarily using more parts to perform a simple task. Meanwhile, simple carbon composition parts (that are rated for the application) are still available. Hocus-pocus unneeded.
UPDATE! Replaced the two 200pf caps with two .01 caps after reading an old write up from W8JI on these QRZ forums. It acts like any other grounded grid amp I’ve ever messed with now.....even on 10 meters going into my 80 meter dipole/tuner combination. Directly grounded grids would probably be even better, I’m sure. I’m still going to rewind the suppressors with NOS resistors, but right now, I’m happy
If it works as you wish after the suppressors go in, it may not be worth the trouble to rewire the thing right away. If you use it for a while and then decide to experiment with it, you'll know how it changed... possibly a better learning experience.
Great news about the repair! That's one of the best feelings.
By no means do I have the technical capabilities to fully understand why this has helped my issue. I’ve tried to stick to the “original design” but nothing has worked. I just happened to run across a conversation on this forum where W8JI recommended that swapping out the two 200pf caps for .01 would be better. I’m at the point where I basically have nothing to lose so I threw in a couple .01 1kv ceramic caps. It now drives easier and it’s much easier to tune on every band. Now, when keyed with no rf applied, it seems 100% stable on every band but 10. But even 10 acts much better. The only signs of instability now come when I key with no rf applied and turn the plate cap which still shows some grid current being drawn.....only on 10. I’m hoping , now, this can be remedied with different suppressors/resistors. It does seem now as if it is light on loading capacitance now, so it’s still not 100% “fixed.” But, it behaves like a different amplifier and in a good way at the moment. It behaves more like every other grounded grid amp I’ve ever messed with. I’m gonna get the NOS suppressor resistors and play around with those and the inductors to see if I can get this thing to not draw grid current while turning the plate cap when keyed/no rf. BTW, I’ve found no other “voodoo” mods that haven’t aligned with the original schematic. Nor have I been able to find any other components that measure out of spec. And YES, I do feel much better about this amp now.
Glad you're moving in the right direction.
Given what you saw from increasing the value of the grid bypass caps it seems like your amp is a good candidate for actually hard grounding the grids if the instabilities persist even with new suppressors. Yeah, it's a few more steps but it's not all that hard. There are several web resources for the conversion or many of us here could sketch out a schematic and give you steps for strapping the grids to ground. You basically do a few things:
- Remove the existing components running to the grid terminals and the negative supply that's used to bias the grids and provide power for the T/R relay in the Heath design. You'll still use that supply but will turn the diode nd filter cap around and create a positive cutoff bias supply using the same transformer taps and use that new positive supply as the hot side of the T/R relay.
- Actually strap the grid pins on the tube sockets to chassis ground, ideally with short wide pieces of copper flashing for a low inductance grid path to ground.
- Lift the grounded lug that ties the filament transformer center tap to ground. That's where you'll introduce the positive DC cathode bias (which does the same thing as the negative bias applied to the grids now), that will either be cutoff bias from that positive power supply (the old negative supply with the diode and filter cap turned around) or the operating state bias in the 2.5 to 4 volt range.
- Lot's of ways to develop that operating bias such as zener diodes or small dedicated supplies but the easiest is probably a sting of diodes approach. You basically string together 4 to 6 silicon rectifier diodes in series with the anode end of the full chain tied to that filament transformer center tap that was previously grounded and the cathode end of the string tied to the side of the T/R relay coil that gets dragged to ground when the rig gets keyed (most likely via a soft keying circuit if you're using this amp with modern rigs). So basically when the amp is not keyed there's somewhere near 100V positive coming through the coil of the T/R relay from the old negative supply you repurposed as a positive supply and then the voltage drop across the string of diodes which is pretty meaningless in standby mode but it puts somewhere around +90 to +120 volts DC on the center tap of the filament transformer which creates the same result as a negative bias at the grids and the tubes are deep in cutoff.
- When you key the amp by dragging the bottom end of that relay coil to ground the positive supply triggers the T/R relay and now the positive bias applied to the filament transformer center tap and the cathodes of the tubes is just the voltage drop across the string of diodes. This is what sets your no signal idle plate current when the amp is keyed. So you add or subtract diodes in that string to get the desired no signal plate current per the manual or per linearity testing. When I did this mod to my SB-200 I used a terminal strip to hold all the diodes in series and then when testing for no signal idle mode current I jumpered around diodes to get the idle current I was after.
- The only other thing you have to change is the metering and that's done with a resistor from the B- line to ground acting as your meter shunt.
It sounds a lot harder than it is but yeah it'll take a day or so in the shop if you don't regularly dig into electronics and perform mods but it really isn't a very difficult mod to perform.
All that said, if the amp is stable after you add the new suppressors or you don't intend to work 10m then maybe you just run with what you've got unless it starts acting up again.
I’m gonna play around with the suppressors after I get some NOS resistors of a few different values. I know that it still wants to oscillate only on 10 meters. I truly don’t operate much at all, but when I do, it’s mainly on 40 meters. I’ll let someone else who actually needs an amplifier have this for a good deal after I’m satisfied that it works 100% and is stable on ALL bands. I realize that most hams will never use this on 10 meters, but if I can modify it where I feel comfortable letting someone else have it and learn at the same time, I’ve accomplished so something. I just prefer to “understand “ what I’m doing and why I’m doing it. Someone could sit here, hold my hand, and tell me every component and wire to change and all would be well, but if I learn nothing, I truly gain nothing. I thank everyone on this forum for their advice and guidance!
The way the current parasitic suppressors are made contributes to the problem. The leads from the resistors go over to the coils but that adds inductance in series with the resistor which is exactly what needs to be avoided. The coils should attach right were the leads come out of the resistor with the minimum resistor lead between coil and resistor attachment point. Here is a photo of better suppressor technique. Notice the coil attaches right were the leads comes out of the resistor. The more wire lead between resistor body and coil attachment, the worse the suppressor works.
I am gonna make them as the originals were when I get the older type resistors. The first ones i wound used two 100 ohm 2watt metal film resistors in parallel. They got hot pretty quick when testing on 10 so I redid them using the larger 5 watt resistors. I will take my time constructing the next set. If I remember correctly also, my LCR meter doesn’t accurately measure very small air wound inductors which doesn’t help me either.
Perhaps instead of random mods you read about on the internet....
You should consider returning the amplifier to the exact configuration heathkit sold it as.....
After all, they sold thousands that worked .