Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Topic started by: Grainger49 on November 03, 2009, 03:24:46 AM

Title: Grid Choke For Parmaour 1?
Post by: Grainger49 on November 03, 2009, 03:24:46 AM: I asked in another thread about grid chokes on my beloved Paramour 1s. Frank Mena found an old PJ post and it is included in response #6 on this page:

http://www.bottlehead.com/smf/index.php/topic,127.0.html

Restating my question would there be any benefit of grid chokes on my old Paramour 1s?

Edit: I have one of the final run Paramour 1s with upgraded MQ iron and Obbligato 10uF Film/Oil Parafeed caps. I also upgraded every cap in the amp. The cathode bypass cap is bypassed with a PETP KK. My interstage is a KK Teflon 0.1uF. My power supply caps are PP film.

The thread above is about the SR45 amp. I am unfamiliar with the circuit. Could the capacitor coupled, 76 driver 2A3 amp benefit from this circuit improvement?

Edit: This part of the thread assumes the wrong resistor is the grid load resistor:

~~The grid stopper resistor is 1/2k, or 499 ohms. Seems that the formula would yield 2 Henries. I'll calculate the 76 driver impedance and see what that gives.~~

More later, less sooner.
Title: Re: Grid Choke For Parmaour 1?
Post by: bernieclub on November 04, 2009, 03:47:01 AM: Hi Grainger
I'd like to know also...I'm almost ready to buy the chokes to see for myself. The resister it replaces is the 270K grid lead resister. I also remember some posts from the old archives that recommend substantially increasing the size of the coupling cap. More reading to do!
Bernie
Title: Re: Grid Choke For Parmaour 1?
Post by: Grainger49 on November 04, 2009, 05:06:22 AM: Bernie,

Welcome to the new board. I see it is your first post.
(I hope that isn't too aggravating because I had to try it.)

Ok, I get it now, it is the resistor from the grid to ground/common. I should have understood from the load part of "grid load resistor."

Hopefully Doc or PJ will chime in and tell us. I love my Paramours but the tiny chassis is pretty full. I guess I need to take a temp job and afford the new Stereomours when they come out.
Title: Re: Grid Choke For Parmaour 1?
Post by: Doc B. on November 04, 2009, 05:29:30 AM: Hi Grainger,

Yes you should derive the same benefit from installing a grid choke in a Paramour I as any other cap coupled amp with a grid load resistor. I don't have a specific value to suggest for the coupling cap. Perhaps PJ does. I would say try some larger values and see if you note any improvement with bigger caps. If the Paramour I is using a stock Speco transformer that will tend to be the limiting factor in bass response, and increasing the cap value may not be that critical. If you have upgraded to MQ Paramour iron with it's much better bass response then it could be worthwhile to experiment with larger coupling cap values.

The M6 grid chokes are pretty inexpensive and even those should create a very positive improvement in the dynamics of the amp.
Title: Re: Grid Choke For Parmaour 1?
Post by: Paul Joppa on November 04, 2009, 05:51:31 AM: Quote from: Grainger49 on November 03, 2009, 03:24:46 AM
... would there be any benefit of grid chokes on my old Paramour 1s?
Doc B's answer is good. I posted at the same time but it seems lost - I probably neglected to hit the "Post" button ...

I have no experience, and nowhere near enough data, to suggest a coupling capacitor value.

The main risk is hum pickup. Grid chokes pick up magnetic fields and inject the resulting hum voltage directly into the grid, and few if any of them have good magnetic shielding. Doc's successful experiment was in a tweeter amp with 60dB attenuation at 60Hz between amp and tweeter, so it says little about this issue. Be prepared to experiment with location and orientation, and possibly get some molyperm tape or other magnetic shielding material to play with. The hum pot might be a useful tool to cancel 60Hz hum as well. And post your results! There is a dearth of useful direct experience on this application.
Title: Re: Grid Choke For Parmaour 1?
Post by: Doc B. on November 04, 2009, 05:57:17 AM: Quote
Grid chokes pick up magnetic fields and inject the resulting hum voltage directly into the grid, and few if any of them have good magnetic shielding.

That might be another good reason to try M6 rather than one of the higher perm core materials.
Title: Re: Grid Choke For Parmaour 1?
Post by: Grainger49 on November 04, 2009, 06:09:38 AM: Doc and Paul, I edited my post to include my interstage coupling cap and the output upgrade and Parafeed cap (MQ iron and Obbligato 10uF Film/Oil Parafeed cap). I should have included that to start with.

I have one of the final run of Paramours. When you mention the Speco transformer, that is the output, right? If so, I called you a few years back (after the Paramour II was current) and got your recommendation for an upgrade. This upgrade was somewhat powered by Bourbon but a great improvement regardless. I bought new, and beautiful brass end belled, transformers from MagneQuest, plate chokes too. I'm looking for the numbers but right now it is eluding me.

Edit: The output is TFA-2004 and the choke is the BCP-15, I found the model numbers. I have the output wired per VoltSeconds suggestion to return it to the top of the cathode bypass cap rather than common/ground.
Title: Grid Choke For Paraglow II?
Post by: Len on November 04, 2009, 06:53:20 AM: Quote from: Doc B. on November 04, 2009, 05:29:30 AM
Yes you should derive the same benefit from installing a grid choke in a Paramour I as any other cap coupled amp with a grid load resistor.

270K sounds like the grid of the first tube (in my case 6n1p). Would the improvement be as noticeable in shunt regged direct coupled amps (Paraglow II and Excite)?

Now that the Quickie low pass filter has made such a dramatic improvement in my system, I am looking for ways to improve the bass even more, especially in the PGII.

My PGII is using MQ M4 iron, Excite uses nickel.

PS: Holy cow, I just saw the post about the PJCCS. I'll ask a similar question there!
Title: Re: Grid Choke For Parmaour 1?
Post by: Paul Joppa on November 04, 2009, 11:42:22 AM: Quote from: Grainger49 on November 04, 2009, 06:09:38 AM
...
Edit: I am pretty sure the output is TFA-204 and the choke is the BCP-16. I have the output wired per VoltSeconds suggestion to return it to the top of the cathode bypass cap rather than common.
Probably a TFA-2004, possibly a Junior (Senior has a 1.75" thick lamination stack; Jr is 1.25"). And probably a BCP-15. You probably got those before we and Mike defined the official upgrades, which are variations based on those two designs (and are among Mikes best work, IHMO).
Title: Re: Grid Choke For Paraglow II?
Post by: Paul Joppa on November 04, 2009, 11:47:08 AM: Quote from: Len on November 04, 2009, 06:53:20 AM
... Would the improvement be as noticeable in shunt regged direct coupled amps (Paraglow II and Excite)?
The advantage of a plate choke, at least in theory, is that it increases the impedance at teh grid of the output tube. The direct coupled amps have no grid resistor, hence that impedance is already infinite and would be hard to increase... :^)

I have not heard of enough examples to have any clear idea about using a grid choke for small signals such as the driver grid. The extra gain after that point would make the magnetic induced hum an even larger problem though.
Title: Re: Grid Choke For Paraglow II?
Post by: Len on November 04, 2009, 11:56:21 AM: Quote from: Paul Joppa on November 04, 2009, 11:47:08 AM
Quote from: Len on November 04, 2009, 06:53:20 AM
... Would the improvement be as noticeable in shunt regged direct coupled amps (Paraglow II and Excite)?
The advantage of a plate choke, at least in theory, is that it increases the impedance at teh grid of the output tube. The direct coupled amps have no grid resistor, hence that impedance is already infinite and would be hard to increase... :^)

I have not heard of enough examples to have any clear idea about using a grid choke for small signals such as the driver grid. The extra gain after that point would make the magnetic induced hum an even larger problem though.

Thanks.
Title: Re: Grid Choke For Parmaour 1?
Post by: Grainger49 on November 04, 2009, 12:51:08 PM: Quote from: Paul Joppa on November 04, 2009, 11:42:22 AM
Probably a TFA-2004, possibly a Junior (Senior has a 1.75" thick lamination stack; Jr is 1.25"). And probably a BCP-15. You probably got those before we and Mike defined the official upgrades, which are variations based on those two designs (and are among Mikes best work, IHMO).

Paul, you are certainly right on both counts. I had thought the number was 2004 ~~but found a 204 on Mikey's web site~~. I had BCP-15 in the post and somehow decided it was a 16.

Edit: I found out that the output transformers are TFA-2004 (Sr.)

Yes, I bought just before the debut of the Paramour II and the upgrade iron that came a little later.
Title: Re: Grid Choke For Parmaour 1?
Post by: Grainger49 on February 09, 2010, 04:48:16 AM: Don't ask me why, but I decided to check this today. The transformers are large. The lamination stack is 1.75" thick. I bought what Mike had on deck to make soon. They are TFA-2004 Seniors. I guess I'm not stressing them.