Ferranti U1040 distortion. - UK Vintage Radio Repair and Restoration Discussion Forum

nostalgia · 7th Jul 2007, 10:42 am

Re a ferranti U1040 valve radio.this set works fine and loud apart from a higher than normal distortion Level the Anode voltage of the output pentode is low at 158v (192v).The screen grid is low at 90v (156v).The cathode is at 12.5v,The input grid is at 10.5v relative to earth 0.5 v negative of the cathode not 2v negative,meter effect?What should the grid read relative to earth?

There is a 42v drop accros the resistor between the smoothing caps indicating a current of 127ma is this normal?

The smoothers are ok as are the other electrolytics and the input coupling cap.I can't find any high resistors.

The speaker checks out. The problem is in the amp as the distortion is present with a good audio source connected to the volume pot.

I can't figure out what is causing the output valve to load the PSU if that is what's happening.

Could a faulty valve cause this?

paulsherwin · 7th Jul 2007, 10:54 am

You have postive voltage on the output valve grid. This is causing the valve to draw excess current, hence the low HT volts and distortion.

The most likely cause is a leaky grid coupling capacitor. If that has been changed, the output valve itself is probably leaky internally.

Paul

nostalgia · 7th Jul 2007, 11:00 am

Hi,thanks for the reply.I changed the grid coupler first off.Must be the valve as the tone control pot to earth is'nt open circuit and the cap is ok. Presumably the grid should be ov to earth? I'm new to valve gear having been a TV engineer up to 2005,now repairing valve sets as a hobby!

paulsherwin · 7th Jul 2007, 11:14 am

The grid should be around zero volts WRT chassis.

What is the output valve? If it's a UL41, these are notorious for developing internal leaks. Unfortunately they're scarce and expensive now. These valves and their problems are often discussed: https://www.vintage-radio.net/forum/...earchid=400103

Paul

nostalgia · 7th Jul 2007, 11:35 am

Hi,thankfully it's a UL84 I found a Polish one for £7.00 on ebay.Hoppefully this will cure it as the radio only cost £7.00!

Tim · 7th Jul 2007, 3:41 pm

It is uncommon but not unknown for valveholders to conduct between pins.

nostalgia · 11th Jul 2007, 2:26 pm

Thanks for the assist,I bought a UL84 from Colomor and this has curred the problem,the Voltages are now O.K. and the distortion has dropped to acceptable levels!. looks like it had an internal short somewhere.

7th Jul 2007, 10:42 am	#1
nostalgia Retired Dormant Member Join Date: Dec 2006 Location: Accrington Posts: 19	Ferranti U1040 distortion. Re a ferranti U1040 valve radio.this set works fine and loud apart from a higher than normal distortion Level the Anode voltage of the output pentode is low at 158v (192v).The screen grid is low at 90v (156v).The cathode is at 12.5v,The input grid is at 10.5v relative to earth 0.5 v negative of the cathode not 2v negative,meter effect?What should the grid read relative to earth? There is a 42v drop accros the resistor between the smoothing caps indicating a current of 127ma is this normal? The smoothers are ok as are the other electrolytics and the input coupling cap.I can't find any high resistors. The speaker checks out. The problem is in the amp as the distortion is present with a good audio source connected to the volume pot. I can't figure out what is causing the output valve to load the PSU if that is what's happening. Could a faulty valve cause this?

7th Jul 2007, 10:54 am	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Ferranti U1040 distortion. You have postive voltage on the output valve grid. This is causing the valve to draw excess current, hence the low HT volts and distortion. The most likely cause is a leaky grid coupling capacitor. If that has been changed, the output valve itself is probably leaky internally. Paul

7th Jul 2007, 11:00 am	#3
nostalgia Retired Dormant Member Join Date: Dec 2006 Location: Accrington Posts: 19	Re: Ferranti U1040 distortion. Hi,thanks for the reply.I changed the grid coupler first off.Must be the valve as the tone control pot to earth is'nt open circuit and the cap is ok. Presumably the grid should be ov to earth? I'm new to valve gear having been a TV engineer up to 2005,now repairing valve sets as a hobby!

7th Jul 2007, 11:14 am	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Ferranti U1040 distortion. The grid should be around zero volts WRT chassis. What is the output valve? If it's a UL41, these are notorious for developing internal leaks. Unfortunately they're scarce and expensive now. These valves and their problems are often discussed: https://www.vintage-radio.net/forum/...earchid=400103 Paul

7th Jul 2007, 11:35 am	#5
nostalgia Retired Dormant Member Join Date: Dec 2006 Location: Accrington Posts: 19	Re: Ferranti U1040 distortion. Hi,thankfully it's a UL84 I found a Polish one for £7.00 on ebay.Hoppefully this will cure it as the radio only cost £7.00!

7th Jul 2007, 3:41 pm	#6
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Ferranti U1040 distortion. It is uncommon but not unknown for valveholders to conduct between pins. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

11th Jul 2007, 2:26 pm	#7
nostalgia Retired Dormant Member Join Date: Dec 2006 Location: Accrington Posts: 19	Re: Ferranti U1040 distortion. Thanks for the assist,I bought a UL84 from Colomor and this has curred the problem,the Voltages are now O.K. and the distortion has dropped to acceptable levels!. looks like it had an internal short somewhere.