Hello everyone,
I am working on an amp (schematics attached). I have a pair of SAC Silk grid chokes that I would like to use for 300b. Silk states that those chokes are "7000 H at 12Hz".
What would be the considerations for replacing 210k grid resistor with aforementioned chokes? My concern is corner frequency and phase shift given coupling and bypass capacitor values. I wonder if those with access to LT_Spice could give me some pointers to proper values.
Your help is appreciated.
Thanks
I am working on an amp (schematics attached). I have a pair of SAC Silk grid chokes that I would like to use for 300b. Silk states that those chokes are "7000 H at 12Hz".
What would be the considerations for replacing 210k grid resistor with aforementioned chokes? My concern is corner frequency and phase shift given coupling and bypass capacitor values. I wonder if those with access to LT_Spice could give me some pointers to proper values.
Your help is appreciated.
Thanks
Attachments
Here's the problem I am experiencing:
When 300b grid resistor is replaced with Silk choke I get audible bass roll off, not a lot but enough to know that it's there. I tried swapping 0.1uF coupling caps on the last stage with 0.47uf (had them on hand) and the result was audible distortion. Apparently the values of components need to be adjusted. I need some help.
When 300b grid resistor is replaced with Silk choke I get audible bass roll off, not a lot but enough to know that it's there. I tried swapping 0.1uF coupling caps on the last stage with 0.47uf (had them on hand) and the result was audible distortion. Apparently the values of components need to be adjusted. I need some help.
Your enemy here is the high inductance combined with low DCR- inevitably, you'll have a bad subsonic peak. A 10 cent resistor will greatly outperform the choke. You can get better results with something like a Lundahl, which is around 5k resistance and 500H. But still inferior to a resistor...
I would suggest leaving the 210K resistor in there and putting the grid choke in parallel . You will need a bigger coupling cap than 0.1uF . I have tried these Silk Supermalloy chokes in a few circuits and have never been 100% happy .
There is a spice simulation on the intactaudio.com website which is useful for modelling , although I have had differing measurements in-circuit
316A
Try modelling with a larger cap and lower shunt resistor values . I found that with these grid chokes , even with the extra capacitance of the choke , the top end really opened up . I never got the bass right no matter what cap/resistor combination . Always ended up with false-sounding bass , even when measurements indicated only a very small peak or none at all . So now my very expensive Silks sit on a shelf . One thing I did not do was to try with one bobbin's turns rather than both in series (these chokes are centre-tapped) .
316A
Try a coupling capacitor of at least 1uF, note that there is no reason why you can't also insert a resistor of 1K - 20K in series with the choke, this will substantially reduce the Q while effectively maintaining the choke's high load impedance. (Shunt resistors don't)
FWIW, the only DHT that really requires IT or grid choke is the 50/250 and in that case the total grid circuit DCR must be no more than 10K or runaway is potentially possible.
I've used grid chokes successfully in a number of designs for others who wanted them.
FWIW, the only DHT that really requires IT or grid choke is the 50/250 and in that case the total grid circuit DCR must be no more than 10K or runaway is potentially possible.
I've used grid chokes successfully in a number of designs for others who wanted them.
Hi Kevin,
Thank you for your post.
I wonder now if the benefits of the grid choke are that much greater than a simple grid leak resistor. I am growing some serious muscle from moving the amp from the listening room to the bench and back, and my wife wonders if I ever go to sleep at night... I love the mids and highs that the choke opens in the amp, but I am unable to achieve the level and clarity of the bass that grid resistor seems to provide.
I would like to hear from those that successfully used these chokes in their 300b and see their schematics.
Thank you for your post.
I wonder now if the benefits of the grid choke are that much greater than a simple grid leak resistor. I am growing some serious muscle from moving the amp from the listening room to the bench and back, and my wife wonders if I ever go to sleep at night... I love the mids and highs that the choke opens in the amp, but I am unable to achieve the level and clarity of the bass that grid resistor seems to provide.
I would like to hear from those that successfully used these chokes in their 300b and see their schematics.
I have read a couple of threads now on grid chokes...I understand that some people rcommend series restors, some parallel resistors etcetc.
Besides that, could you give me a view if they make sense in a fixed bias setup at all ? So, when the negative voltage is applied through the choke to the grid ? Are the theoretical advantages like grid current goes into ground in reliable way etc still true ? I guess a power triode would be happy about the resistance of the 1k2 of the silk choke instead the usual 100k, right ?
Besides that, could you give me a view if they make sense in a fixed bias setup at all ? So, when the negative voltage is applied through the choke to the grid ? Are the theoretical advantages like grid current goes into ground in reliable way etc still true ? I guess a power triode would be happy about the resistance of the 1k2 of the silk choke instead the usual 100k, right ?
Plate and grid chokes could make some sense in the past for those people who didn't like the best solution: DC-coupled cathode/source follower. They only had interstage as an option. At that time high quality interstage transformers were difficult to source and quite more expensive.
This is not the case anymore.
The only case where a grid choke makes sense today is DC coupling. If this is employed (so that choke CAN withstand some DC current and serve also as the plate load for the voltage amplfier) it might work ok. However higher voltage supply is needed....more expensive capacitors, more insulation......
LC coupling doesn't make sense anymore because of a worse performance respect to a good interstage basically at the same cost! Since you have the coupling capacitor you will get blocking as soon as grid current flows. You can mitigate this effect inserting grid resistor and optimizing the recovery from overload but never get rid of it completely.
If you look at the cost of those Silk grid chokes and add 2 quality coupling capacitors you might spend more than a pair of their 511 interstage which has very good performance and doesn't have all this troubles.....
People often do not realize that LC coupling is a resonant network and think that having very low resonance frequency and flattening frequency response is enough. Not at all. Even if FR looks flat the load is still REACTIVE (look at phase) for the driver and when swinging several volts it will have more distortion that a less reactive or non-reactive load at low frequency.
But they never measure distortion at low frequency (below 100Hz). The only thing that could "save" them is that the low frequency performance of many output SE transformers is so bad that it might not make a big difference. In reality such a circumstance is just bad!
45, thanks a lot....this is in a very condensed way a lot of great advise...I can easily image that you tried everything and measured/listened to it. Can you talk a bit on how the sound of your system changed with trying these options ?
I am on my own learning curve and have time to enjoy this path. I do have a couple of silk grod chokes laying around, which I want to try in my El-34-pp-Ul (http://www.triodedick.com/monobill_2/monobill_2_schema_versterker.GIF).
I do have as well some nice IT from Monoloth Magnetics waiting for their application...but I want to do it step by step to have the learning experience what does what on sound quality...
Your comments on DC-coupling are as well highly appreciated, it took me a lot of time and $$$ to end up with Duelund CAST as my coupling caps, which do a good job, but no cap is better...just have not found yet an EL34-pp-schematic how to apply it...and I am not a SE-guy. If you can help me to get into the subject, highly appreciated...(pm?)
So back to the topic: How would you apply the silk grid chokes in the schematic above? Just exchange the 39k resistor ? Or parallel to it ? Or silk-choke and 8k2 resistor in series replacing the 39k resistor ?
I am on my own learning curve and have time to enjoy this path. I do have a couple of silk grod chokes laying around, which I want to try in my El-34-pp-Ul (http://www.triodedick.com/monobill_2/monobill_2_schema_versterker.GIF).
I do have as well some nice IT from Monoloth Magnetics waiting for their application...but I want to do it step by step to have the learning experience what does what on sound quality...
Your comments on DC-coupling are as well highly appreciated, it took me a lot of time and $$$ to end up with Duelund CAST as my coupling caps, which do a good job, but no cap is better...just have not found yet an EL34-pp-schematic how to apply it...and I am not a SE-guy. If you can help me to get into the subject, highly appreciated...(pm?)
So back to the topic: How would you apply the silk grid chokes in the schematic above? Just exchange the 39k resistor ? Or parallel to it ? Or silk-choke and 8k2 resistor in series replacing the 39k resistor ?
Last edited:
Hey Blitz, that was a long time ago. It was a 300B SE with 46 triode driver. The difference between LC coupling and Tango NC-20 was like night and day.
If you want to employ those chokes in that schematics I think you need 4 for the stereo amp as I remember these are not symmetric double-coils (which you could also use to balance the static induction due to DC current in a DC coupled amp).
If you have to buy another pair just buy a pair of their SE-to-PP insterstage model I-511SD if you want to do a SE driver or the I-511P if PP-to-PP.
For the SE-to-PP transformer you can use the 6N6P or, even better, a triode wired 6E5P which is very linear and is ideal for those specs (25-30 mA DC and 55H). For the other IT you have plenty of choices in addition to 6N6P.....
If you want to employ those chokes in that schematics I think you need 4 for the stereo amp as I remember these are not symmetric double-coils (which you could also use to balance the static induction due to DC current in a DC coupled amp).
If you have to buy another pair just buy a pair of their SE-to-PP insterstage model I-511SD if you want to do a SE driver or the I-511P if PP-to-PP.
For the SE-to-PP transformer you can use the 6N6P or, even better, a triode wired 6E5P which is very linear and is ideal for those specs (25-30 mA DC and 55H). For the other IT you have plenty of choices in addition to 6N6P.....
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.