How to connect Gridchoke?

[papparazzi]

Hi

I just got 1000H gridchoke and I intend use it in my 300b Waltons/Jelab Circuit with single 6sn7 tube.
I allready connected Choke parallel with old 220k grid res to ground but that doesnt seem to be right? Output bias increased from -66 to near 100 vdc . Choke seems to have 24k DC res.
How to use it?? Thanks for help.


Papparazzi

[lt cdr data]

take the resistor out, and the choke stays in place.

[papparazzi]

take the resistor out, and the choke stays in place

220k res should not mean anyting parallel with choke???? Or?

papparazzi

[papparazzi]

Removing 220k res didnt make any difference Connected from 300b
grid to ground lift up current near 100 ma per tube across cathode resistor.
I didnt find any such kind a circuit using Google where gridchoke is installed?????

papparazzi

[kevinkr]

Hi,
I suspect your 300B tubes are perhaps a bit gassy and with a 220K grid resistor the grid current was generating some additional contact potential bias.

Some grid current in 300B is not unusual, but I think more than ~5 - 10 uA should be a cause for concern. Whose 300B are you using? I only ask because in the past I have encountered unusually high grid currents in both Shuguang and Valve Arts 300B tubes operated at 400V.

IMO 100K should be the upper limit, particularly if fixed bias is used.

The DCR of the grid choke is much lower. To test this hypothesis, put a 200K resistor in series with the choke, if the cathode current then drops back to about where it was the hypothesis is confirmed.

The solution is simple, just increase the value of the cathode bias resistor until the operating current is the same. No big deal.

The grid choke is commonly used here and I' m surprised you have been unable to find schematics showing this.

When you use a grid choke you also end up with a subsonic high q resonance that can be most easily fixed (if you don't like the way it sounds ) by increasing the value of the coupling capacitor. There are ways of reducing the Q of the resonance, but entail adding varying amounts of R in series/parallel with the choke. Incidentally adding parallel R sort of defeats the point of a grid choke in most cases.

Edits: I fixed a lot of typos, apparently I can't think and type at the same time..

[papparazzi]

Some grid current in 300B is not unusual, but I think more than ~5 - 10 uA should be a cause for concern. Whose 300B are you using? I only ask because in the past I have encountered unusually high grid currents in both Shuguang and Valve Arts 300B tubes operated at 400V.

Thanks Kevin

Im running TJ mess tubes near 400vdc across 1k autobias ,voltage is normal 66 vdc Now adding Choke from grid to ground (24k dc res)
makes bias -100 vdc across Cathode resistor

I havent find any circuit for help this.

Papparazzi

[kevinkr]

Hi Papparazzi,
Hmm, I have heard a lot of positive press on the TJ, but it does sound like there is really a lot of grid current with these particular tubes or there is something else going on. (Like a faulty meter - albeit unlikely as that is or HF/VHF oscillations.)

I would insert the original 220K resistors in series with the chokes and remeasure the the voltage across the bias resistors, I suspect it will be right around 64 - 66V.

If you have a dvm with a sufficiently high input resistance (say 10M) you might be able to measure the voltage at the grid. I have never encountered a grid current induced voltage drop of more than a couple of volts with any half way good 300B.. YMMV though..

Your current circuit implementation if you have the choke connected where the old 220K resistor was is correct. You do have a 1K grid stopper resistor in series right at the grid of the 300B don't you?
(I'm thinking HF/VHF oscillation here, which you probably cannot measure with the instruments at hand.) Try adding stoppers if not already present and then remeasure the cathode bias voltage.

If all this checks out ok, and bias is still too high an acceptable solution for the TJ anyway would be to change the 1K cathode resistor to 1.5K, this should put things back exactly where they were.

[papparazzi]

Your current circuit implementation if you have the choke connected where the old 220K resistor was is correct. You do have a 1K grid stopper resistor in series right at the grid of the 300B don't you?

I have 1k grid stopper .Using Grid choke to to ground also drops B+ voltage about 35 volts according to PSU impedance .I might solve the problem increasing 1k to 1,5k ,but does these tubes anymore work correctly ,what is the real current drawn through Cathode? Is the only way to measure current through primary DC res.


papparazzi

[arnoldc]

Originally posted by papparazzi

I havent find any circuit for help this.

Papparazzi

Maybe this will help - http://www10.brinkster.com/azcruz/My5010schema.html

For what ever it's worth, I've tried the TJ globes on this circuit.

[papparazzi]

Maybe this will help - http://www10.brinkster.com/azcruz/My5010schema.html

Thanks ArnoldC

Are you using both resistor parallel (1320 0hms) in your 300b configuration,or do you have same kind of experience as me about
current change using GridChoke
Your choke has 5 times Inductance and isuppose also DCR than mine

Papparazzi