Kofi Annan in: "Kofi Makes a 300b DRD... and You Get to Help!" - Page 14

Kofi Annan · 14th October 2007, 04:14 PM

OK-- so I installed the Lundahl common-mode chokes and had a listen. The sound was very different-- softer with not as much gain. So, I figure it's a voltage problem, right?

I had to readjust the trimpot to lower the d3A anode / 300b grid voltage to about 177V (it was about 205V after the choke install) and things seemed about normal, but now the B+ is too high-- the 300b is seeing 375V instead of 350V, which I'm concerned is too high.

Also, the 1K cathode resistor (which measures .995K) is dropping 68.5V, which means it's passing almost 69mA of current.

I'm still taking some measurements, but it seems that the additional B+ is causing some problems. I may need to add a resistor in series with the B+ to drop some voltage, right?

Any suggestions?

Kofi

Kofi Annan · 14th October 2007, 04:26 PM

Oh, and the B+ is now at 660V. It's pretty surprising to me that changing the choke would do this.

Still investigating...

Kofi

Sheldon · 14th October 2007, 04:52 PM

At the risk of sounding like a broken record, it would be most instructive to show us your schematic with all the actual node voltages indicated. Before and after would be even better. Here's an example:

http://www.diyaudio.com/forums/showt...65#post1322965

Meantime, the voltage changes you see are not necessarily alarming. The new choke might have much lower DCR, but without the whole picture?

Sheldon

Kofi Annan · 14th October 2007, 05:15 PM

Here you go. The bolded figures that don't have a "V" after them are my measured voltages.

Still puzzled as to why it looks like I'm running at 69mA, though.

Anything jump out at you as particularly fishy here?

Thanks for your help.

Kofi

Sheldon · 14th October 2007, 06:22 PM

Looks fine to me Kofi. The bias current is as you calculated. I don't know what the dissapation limit you want on the output tube, but based on a cursory check, it seems you are well within the ratings. But double check that.

If you want to lower the current, you have several options. Easiest is to adjust the voltage at the grid with your lower pot. Or you can increase the other resistor values in series with the 300b cathode. I'd try the simple adjustment first. Then look at the curves for the input tube to make sure you are still in a nice linear region. And listen.

These kind of designs are a bit tricky because all the voltages are interdependent. But as long as you are in safe dissapation areas, play at will.

If you want to reduce ps voltage, I wouldn't use series resistance . A much better approach would be to reduce the size of the power supply input cap.

Sheldon

gingertube · 15th October 2007, 06:15 AM

Kofi,
Its a tube amp - that means +/- 20% on voltages - those voltage readings are fine.

Quick Calcs:
265-197 V across that 1K in the cathode.
That means 68mA and therefore also -68V bias from grid to cathode

644 - 265 = 379 V Anode to Cathode.
At 68mA that gives 25.8W Anode dissipation - a good conservative place to run the 300B.

Extrapolating off the 300B data sheet that operating point should be giving you about 8 watts of audio out.

Unless you are somehow unhappy with the sound I'd say its finished - sit back and enjoy.

Cheers,
Ian

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Kofi Annan in: "Cascodin' with Steve Bench's RIAA!"	Kofi Annan	Tubes / Valves	261	10th June 2011 05:48 PM
Kofi Annan in: "Kofi's Living Room"	Kofi Annan	Full Range	5	25th May 2008 05:19 PM
Kofi Annan in: "300beee!" or "Kofi Gets a Toy"	Kofi Annan	Tubes / Valves	125	25th August 2006 03:39 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

14th October 2007, 04:14 PM	#131
Kofi Annan diyAudio Member Join Date: Jan 2004 Location: US	OK-- so I installed the Lundahl common-mode chokes and had a listen. The sound was very different-- softer with not as much gain. So, I figure it's a voltage problem, right? I had to readjust the trimpot to lower the d3A anode / 300b grid voltage to about 177V (it was about 205V after the choke install) and things seemed about normal, but now the B+ is too high-- the 300b is seeing 375V instead of 350V, which I'm concerned is too high. Also, the 1K cathode resistor (which measures .995K) is dropping 68.5V, which means it's passing almost 69mA of current. I'm still taking some measurements, but it seems that the additional B+ is causing some problems. I may need to add a resistor in series with the B+ to drop some voltage, right? Any suggestions? Kofi

14th October 2007, 04:26 PM	#132
Kofi Annan diyAudio Member Join Date: Jan 2004 Location: US	Oh, and the B+ is now at 660V. It's pretty surprising to me that changing the choke would do this. Still investigating... Kofi

14th October 2007, 04:52 PM	#133
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	At the risk of sounding like a broken record, it would be most instructive to show us your schematic with all the actual node voltages indicated. Before and after would be even better. Here's an example: http://www.diyaudio.com/forums/showt...65#post1322965 Meantime, the voltage changes you see are not necessarily alarming. The new choke might have much lower DCR, but without the whole picture? Sheldon

14th October 2007, 06:22 PM	#135
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	Looks fine to me Kofi. The bias current is as you calculated. I don't know what the dissapation limit you want on the output tube, but based on a cursory check, it seems you are well within the ratings. But double check that. If you want to lower the current, you have several options. Easiest is to adjust the voltage at the grid with your lower pot. Or you can increase the other resistor values in series with the 300b cathode. I'd try the simple adjustment first. Then look at the curves for the input tube to make sure you are still in a nice linear region. And listen. These kind of designs are a bit tricky because all the voltages are interdependent. But as long as you are in safe dissapation areas, play at will. If you want to reduce ps voltage, I wouldn't use series resistance . A much better approach would be to reduce the size of the power supply input cap. Sheldon

15th October 2007, 06:15 AM	#136
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	Kofi, Its a tube amp - that means +/- 20% on voltages - those voltage readings are fine. Quick Calcs: 265-197 V across that 1K in the cathode. That means 68mA and therefore also -68V bias from grid to cathode 644 - 265 = 379 V Anode to Cathode. At 68mA that gives 25.8W Anode dissipation - a good conservative place to run the 300B. Extrapolating off the 300B data sheet that operating point should be giving you about 8 watts of audio out. Unless you are somehow unhappy with the sound I'd say its finished - sit back and enjoy. Cheers, Ian

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)