SR45 - nice upgrade - MQ grid chokes

[Doc B.]

We have been doing some organizing around here lately, and in the process I came across a pair of Mikey's BCP-16 GC NI nickel grid chokes, mounted to an old prototype chassis. I wanted to use them in my system somewhere and the only possible location was in my pair of SR45 tweeter amps, since the mids and woofers use 4 amps per channel. During some discussion it was suggested that there might not be much improvement had by replacing the grid resistor in the tweeter amp with the grid choke because most of the comments we see about grid chokes talk about bass improvement. My experience with grid chokes has been that there is an overall improvement in dynamic punch and liveliness, so I forged ahead and asked Shawn to install the chokes along with upgrading the existing Solen coupling cap to a Russian Teflon cap.

I guessed right. The system treble sounded good before. But the clarity, dynamic snap, and the levels at which we can now play the system without strain in the treble are greatly increased. This is a very worthwhile upgrade. The two holes that sit between the tube sockets normally mount some cathode resistors on a Paramount kit, which uses this same chassis. Some work with a rat tail file will lengthen the holes to fit the choke mounting holes and the BPC-16 will just squeeze in between the T strip next to the 45 socket and the SRC4S board on over the 6CM7 socket. One might be concerned about the possibility of hum pickup. In my application the output of the amp is cutoff by the use of a very small (.016uF) parafeed cap, which works in conjunction with an active crossover to give a steep slope on the RAAL ribbons. So if I was picking up hum, it wasn't getting out to the tweeters, and I can't say for sure if you would experience hum or not if you try this on a full range setup.

I should mention that I have also removed the cathode bypass resistor on the 45 since I am not looking for great bass or gobs of gain, and that we also upgraded the 1.3K sandcast cathode resistor to a pair of 680 ohm 6.5W Dale wirewounds.

[2wo]

I agree, I  recently added grid chokes to my full range SR-45, more snap throughout the whole range. I am using a Russian  K40, PIO coupling cap.
  I removed the cathode bypass cap and did not see any loss of gain. With the output transformer returned to the cathode, I don’t see why it is needed at all…John     

[Paul Joppa]

...
  I removed the cathode bypass cap and did not see any loss of gain. With the output transformer returned to the cathode, I don’t see why it is needed at all…John     

I experimented with this when developing the Paramount 300B amp. The cathode bypass capacitor on the output tube does improve the deep bass response. I think this is because neither the plate choke nor the parafeed capacitor are infinitely large. We're talking 20-30Hz so it may not be easily audible unless you have speakers - and a room - that have good response that low. But the phase shifts could still have a subtle effect.

[2wo]

Thanks Paul,  That makes sense, no way do my speaks do 20-30 Hz. Even "hot swapping "a cap on one channel made almost no difference. I will keep it in mind if I try to run a truly full range speaker…John 

[tsingle999]

Thats exciting Doc! I tried mounting a pair in my Paramounts unsuccessfully. My SR45 sounds so good I am afraid to change anything! lol. I will try the grid chokes in there though.

[Grainger49]

Bwa, Ha, Ha, Ha! (I had to spell it out)  I just went to MQ's site and looked at the BCP-16 specs.  Tip O' the hat to Mikey!  

http://www.magnequest.com/products.htm

Doc, do you think there would be an improvement on my old Paramour 1s?

I accept that it is your guess but I trust you.

[Frank Mena]

                                   > H              DC resistance (Ohms)
BCP-16 GC M6                >1,000          3,850
BCP-16 GC NI NICKEL       >2,000          3,850
BCP-16 GC HN HI NICKEL  >3,000          3,850


An earlier post from Paul Joppa on  determining which Grid Choke to use

Posted by Paul Joppa on October 28, 2000 at 08:51:07
In Reply to: Parameters for grid chokes-for Gordon or anyone. posted by Adrian on October 27, 2000 at 18:38:38:


There's not much information out there because there's not much experience yet, at least within this audio community. There is no way I know of to get a 100% reliable value, and guarantee you won't want to try another value later.
Basically, the grid choke has to present a high impedance to the previous stage. It replaces the grid leak resistor. As a rough estimate, to get adequate bass you want at least 4 henries for each 1000 ohms of the resistor it replaces. So for example if it replaces a 100k resistor, you want at least 400 henries.

To keep this reasonable, you should first figure out the smallest grid leak resistor that you can use - otherwise the grid choke becomes bigger than necessary. That depends on the driving circuit. As a very rough estimate, if the driver is a triode, take its plate voltage divided by its plate current. For example, a 6SN7 operating at 250v and 8mA has a ratio of 250/0.008 or 31k ohms. The minimum grid choke on the driven stage would be 125 henries. This is a pretty crude approximation, but at least it's easy to calculate.

A higher inductance will give better bass, but it might have a higher capacitance which will limit the treble. So you don't want a choke much bigger than the minimum needed for decent bass. I've not seen a grid choke with a capacitance specification, so that's not as useful an observation as you might hope. I suppose that if the capacitance is smaller than the Miller capacitance of the driven stage, it will probably not have too bad an effect, but again it is dependent on the specific circuit.

[Grainger49]

Frank,

Thanks!  

I don't want to skid this thread sideways I will start a post on the Legacy Products section but will include a link to this post.  

I'm just trying not to muck up the system Doc et al are using.

The thread is here:

http://www.bottlehead.com/smf/index.php/topic,132.0.html