Amp de-bugging

jkeny · 10th February 2007, 07:34 PM

Hi all,
I recently changed the output stage of a Rogers Cadet III into a Baby Huey schematic seen below.

My problem is the voltages are not what are on the schematic i.e. the 180V at the anodes of the Input Diff amp is 305V in my case (with the tubes removed - I removed the tubes because about 30 sec after turn on I heard the beginnings of a high ptched whine coming from the circuit & I was afraid of damaging the ECL86 tubes.

I've checked my wiring - it is as in the schematic - what way should I proceed?

JOhn

jkeny · 10th February 2007, 08:16 PM

Here's a better schematic

ray_moth · 10th February 2007, 09:05 PM

The squeal you heard is most likely positive feedback, which you need to cure by reversing the connections to either the primary or the secondary winding (whichever is more convenient, but not both!).

The reason you measured such a high plate voltage is that, with no tubes in place, there is no current flowing through the plate resistors and the power supply. No current = no voltage drop, so the voltage measured at a plate pin will be the same as that generated in the unloaded power supply, probably around 1.4 x the power transformer's secondary voltage. Once the tubes are in place and warmed up, the voltages should be as expected.

jkeny · 10th February 2007, 09:19 PM

Thanks ray,
I'll disconnect the feedback & put in the tubes on one channel.

What's odd is that this is the feedback originally used by the Rogers Cadet i.e taken from the 16ohm output tap.

One other thing - I've used a LM317 to provide the CCS for the diff stage - using 1.2k res to give 1mA current. If I disconnect this temporarily can I replace with a resistor to ground? How would I calculate this?

John

jkeny · 11th February 2007, 09:59 AM

It is the feedback alright - now when connected I get a ticking, when disconnected none (this is without any speakers attached).

With speakers attached I get 5V of offset at the 8 ohm output tap (10V at the 16ohm tap) & loud hum when feedback connected.

Ray, what do you mean by reverse the connections? - feedback comes from 16ohm tap on OPT secondary through 100K R with 22pF bypass to grid of input Tube. How can this be reversed?

Output tubes anodes connect to either end of the OPT's primary - I will try reversing these but don't see how that would change anything.

John

Svein_B · 11th February 2007, 10:48 AM

The first thing you need to fix is finding where the 10V is coming from. Assuming this is with the global feedback connected, it would appear that the grid of the input tube is not at GND level.

Check if it is + or - 10V. Check that the 6.8K to the input tube grid is connected to GND, and that the CCS is connected correctly.

SveinB.

jkeny · 11th February 2007, 08:28 PM

I disconnected the global feedback leaving local shunt feedback in place and it sounds stunning - far better than the Cadet's output stage.

With global feedback disconnected I don't get 180V at anodes of Diff stage but 124 & 133V.

I can't balance this with the 1K pot on the CCS between the cathodes - is this the problem?.

When I connect GF I get -11V at one of the Diff amp anodes (133-124=11V) & lots of hum & distortion.

My CCS is just LM317 with 1.2K between Adj & Out. I initially had In pin connected to wiper of 1K pot (with a 470ohm R connected to wiper & to each leg of pot & then to anode of diff tube) & out pin connected to ground but I thought it might be wrong way around and connected In to ground & Out to wiper but this made no difference.

Any/all help greatly appreciated

John

Svein_B · 11th February 2007, 08:50 PM

Please do check that the grid of both triodes are at 0 Volt, without the global feedback connected.

SveinB.

jkeny · 11th February 2007, 08:54 PM

Yea checked that already I think and they both were at 0V. I may have confused matters - I don't think I did have a 5V offsett at the output like I thought

Svein_B · 11th February 2007, 09:10 PM

OK, if everyting measures OK, and sounds good without the GFB, I guess it is only left to sort out the phase to get the GFB working (if you need it).

This can be done by swapping the grid connections to the input triodes, or the primaries of the OTP, or the secondary of the OTP.

It is however a little worrying that the input anodes will not balance by adjusting the pot. Not that a few volts difference matter, but more the reason why it can not be adjusted.

SveinB.

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
OMG! This slipped! So after bugging everybody maybe it is time.....	Penguin	Introductions	6	2nd February 2002 03:05 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

10th February 2007, 09:05 PM	#3
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	The squeal you heard is most likely positive feedback, which you need to cure by reversing the connections to either the primary or the secondary winding (whichever is more convenient, but not both!). The reason you measured such a high plate voltage is that, with no tubes in place, there is no current flowing through the plate resistors and the power supply. No current = no voltage drop, so the voltage measured at a plate pin will be the same as that generated in the unloaded power supply, probably around 1.4 x the power transformer's secondary voltage. Once the tubes are in place and warmed up, the voltages should be as expected.

10th February 2007, 09:19 PM	#4
jkeny Banned Join Date: Feb 2007 Location: Dublin	Thanks ray, I'll disconnect the feedback & put in the tubes on one channel. What's odd is that this is the feedback originally used by the Rogers Cadet i.e taken from the 16ohm output tap. One other thing - I've used a LM317 to provide the CCS for the diff stage - using 1.2k res to give 1mA current. If I disconnect this temporarily can I replace with a resistor to ground? How would I calculate this? John

11th February 2007, 09:59 AM	#5
jkeny Banned Join Date: Feb 2007 Location: Dublin	It is the feedback alright - now when connected I get a ticking, when disconnected none (this is without any speakers attached). With speakers attached I get 5V of offset at the 8 ohm output tap (10V at the 16ohm tap) & loud hum when feedback connected. Ray, what do you mean by reverse the connections? - feedback comes from 16ohm tap on OPT secondary through 100K R with 22pF bypass to grid of input Tube. How can this be reversed? Output tubes anodes connect to either end of the OPT's primary - I will try reversing these but don't see how that would change anything. John

11th February 2007, 10:48 AM	#6
Svein_B diyAudio Member Join Date: Oct 2005	The first thing you need to fix is finding where the 10V is coming from. Assuming this is with the global feedback connected, it would appear that the grid of the input tube is not at GND level. Check if it is + or - 10V. Check that the 6.8K to the input tube grid is connected to GND, and that the CCS is connected correctly. SveinB.

11th February 2007, 08:28 PM	#7
jkeny Banned Join Date: Feb 2007 Location: Dublin	I disconnected the global feedback leaving local shunt feedback in place and it sounds stunning - far better than the Cadet's output stage. With global feedback disconnected I don't get 180V at anodes of Diff stage but 124 & 133V. I can't balance this with the 1K pot on the CCS between the cathodes - is this the problem?. When I connect GF I get -11V at one of the Diff amp anodes (133-124=11V) & lots of hum & distortion. My CCS is just LM317 with 1.2K between Adj & Out. I initially had In pin connected to wiper of 1K pot (with a 470ohm R connected to wiper & to each leg of pot & then to anode of diff tube) & out pin connected to ground but I thought it might be wrong way around and connected In to ground & Out to wiper but this made no difference. Any/all help greatly appreciated John

11th February 2007, 08:50 PM	#8
Svein_B diyAudio Member Join Date: Oct 2005	Please do check that the grid of both triodes are at 0 Volt, without the global feedback connected. SveinB.

11th February 2007, 08:54 PM	#9
jkeny Banned Join Date: Feb 2007 Location: Dublin	Yea checked that already I think and they both were at 0V. I may have confused matters - I don't think I did have a 5V offsett at the output like I thought

11th February 2007, 09:10 PM	#10
Svein_B diyAudio Member Join Date: Oct 2005	OK, if everyting measures OK, and sounds good without the GFB, I guess it is only left to sort out the phase to get the GFB working (if you need it). This can be done by swapping the grid connections to the input triodes, or the primaries of the OTP, or the secondary of the OTP. It is however a little worrying that the input anodes will not balance by adjusting the pot. Not that a few volts difference matter, but more the reason why it can not be adjusted. SveinB.

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