Hey All,
I installed the grid chokes and connected them between the coupling cap and the grid stopper. The grid stopper and the tube and choke created a voltage divider which sapped most of the bias voltage. I can either add resistance between the choke and ground or connect the choke directly to the grid pin on the tube socket. Which is better?
I installed the grid chokes and connected them between the coupling cap and the grid stopper. The grid stopper and the tube and choke created a voltage divider which sapped most of the bias voltage. I can either add resistance between the choke and ground or connect the choke directly to the grid pin on the tube socket. Which is better?
I know this may sound obvious, but from looking at the schematic...
Do you have the grid chokes connected from output tubes grids to *ground* or from output tube grids to the negative bias supply?
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They don't go between the coupling cap and the grid stopper. They replace the bias resistor and attach at the junction of the coupling cap and grid stopper.
They are in place of the grid resistor, one end between the coupling cap and the grid stopper the other end to PSU ground.
Is your output stage using fixed bias, or cathode bias?
I thought you'd posted that it's using fixed bias. If that's the case, then the grid chokes need to be connected to the negative bias supply instead of ground. Excuse me if I misread that. It's difficult to keep all the facts straight by looking through a long thread.
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Yes its fixed bias. They should be connected to the ground, (positive) side of the bias power supply?
It should go to the output of the bias adjustment pot (assuming that you have one like nearly all amps).
Are you using the choke to solve a problem here? What is the purpose for it?
My understanding is that a choke should be used when the max grid bias resistance is really low (like on some of the very old tubes). That way you can give it a low DC resistance to the bias voltage while giving it a higher AC impedance so you don't load down the previous stage and generate a bunch of distortion.
More modern tubes didn't need it because they had lower grid current and so didn't require low value bias resistances to keep from running away.
Are you using the choke to solve a problem here? What is the purpose for it?
My understanding is that a choke should be used when the max grid bias resistance is really low (like on some of the very old tubes). That way you can give it a low DC resistance to the bias voltage while giving it a higher AC impedance so you don't load down the previous stage and generate a bunch of distortion.
More modern tubes didn't need it because they had lower grid current and so didn't require low value bias resistances to keep from running away.
Sorry to be a pain, but I don't see a ground path there? Also in AJT's example the bias pots are meant to balance the bias voltage between the two tubes. Would I have to adopt this bias scheme to make it work?
Man that 813 tube has some pretty amazing specs for AB1. Anyone ever heard one?
Man that 813 tube has some pretty amazing specs for AB1. Anyone ever heard one?
Path to ground is through the -ve bias supply. Check some fixed bias schematics to familiarise yourself. In your situation all you want to do is replace the grid to -ve bias resistor with the gridchoke. The rest of the bias supply remains.
AJT's circuit above adds a resistor in parallel to the choke, ? as a safety measure if the choke goes open circuit or maybe if its reactance exceeds the tubes max grid resistance.
AJT's circuit above adds a resistor in parallel to the choke, ? as a safety measure if the choke goes open circuit or maybe if its reactance exceeds the tubes max grid resistance.
The output tube's plate voltage is positive, its grid voltage in fixed bias is negative, and its cathode is at ground potential (0V).
The path to signal ground is through the cathode of the output tube.
The negative bias supply's path to DC ground is through the voltage divider that adjusts the bias voltage. (The negative supply's reservoir and smoothing capacitors also go to ground, of course.)
Take a look at the negative bias supply for the output 6550 tubes in a Dynaco Mark III amp.
In this circuit, all you'd be doing is replacing the two grid leak resistors marked 100K* with chokes. You should be able to translate this information to the circuit you're working on.
That's optional.
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..a very bad one.example....
WHY? Imagine this:
lundahl LL1670 540H = @1kHz 3,4M (DCR 5k4)
Therefore a paralel resistor will destroy any advantage of grid choke.
Fail nr2: other end of choke must be AC grounded- add cap to the ground
CT g choke has advantage for PP, more signal symmetry.
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Again, what problem is trying to be solved with the choke? The amp seems perfectly capable of driving a grid resistor with low distortion.
Or maybe this is just a case of you getting a new hammer and getting a feel for how it swings? Nothing wrong with that of course, but letting us know that is helpful. We can then help you line up a nail to test it out and hopefully help you avoid hitting your fingers.
Or maybe this is just a case of you getting a new hammer and getting a feel for how it swings? Nothing wrong with that of course, but letting us know that is helpful. We can then help you line up a nail to test it out and hopefully help you avoid hitting your fingers.
damping the choke......the choke itself has much much lower dc resistance compared to the 180k resistor...
the 813 is not very high in transconductance compared to say a 6c33 so i myself wonder why the choke is there....
who knows what goes on in the mind of designers?
if i were doing these choke, i will do it with a dual bay bobbin and 4 sections.....
IMHO 540H as grid choke is on the weak side (this Lundahl has about 68k at 20 Hz).
I have 640H (3k7, 44pF) custom made -amorphous- choke, but it has limited usability.
I have 640H (3k7, 44pF) custom made -amorphous- choke, but it has limited usability.
plate+grid resistors lowers gain.
I agree Gchoke is money "waste" if driver has fet follower
But aren't we talking about fitting a grid choke on the Uniamp? It has a 33k plate resistor on the driver, driving a 220k grid resistor. I don't think there will be much improvement in gain, and there will be potential pitfalls.
Personally, I'd save a grid choke for a power tube that requires a low value grid resistor, unless mr2racer is just doing it for the practice.
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IIRC We are talking about AD1s in the output stage and these like relatively low dcr in the grid circuit.
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