Cathode bias voltage ( UL84 specific ) - UK Vintage Radio Repair and Restoration Discussion Forum

Reproman · 20th Feb 2006, 12:59 am

Hello everyone

What is the correct voltage for cathode bias using UL84 output valve
I measured it at 17.70 volts, is this too high?, the bias resistor is a 220R
and the bias cap is 22uf, both are new.

This amp is out of a Fidelity HF 16 record player, the max sound output is not as loud as I would expected,even though there is a new output valve fitted along with new capacitors (except the
electrolytic smoothing caps).

Also a new cartridge has been fitted, I know these cheapo sets don't give the best sound in the world but this one seems quiter than others that I have worked on, am I missing something?

I really hope you can help

Station X · 20th Feb 2006, 9:21 am

According to my data book the correct grid voltage for a UL84 is -12.5 volts.

Combined anode and screen voltage is 75 mA, so the calculated cathode resistor value is 12.5/0.075 or about 167 ohms. Nearest preferred value is 180 ohms and it will need to be rated at one watt.

With a 220 ohm cathode resistor the calculated grid voltage is 220 * 0.075 or -16.5 volts.

Aub · 20th Feb 2006, 10:14 am

Is the cartridge the correct type? I think this kind of set up, with only one valve, requires the high output crystal cartridge. If the cartridge is the ceramic type then the output is a fair bit lower.

regards

Aub

Ed_Dinning · 20th Feb 2006, 1:39 pm

Hi Gents, the other possibility is shorted turns on the op trans. This will both reduce the volume and increase the current in the valve, which would push up the cathode voltage.

Ed

Reproman · 20th Feb 2006, 2:18 pm

Hello everyone,

Many thanks for your help.The resistor I replaced was rated at 270 ohms
so I naturally assumed to replace it with a value as close to the original as possible.Regarding the cartridge used, at the moment I am using a BSR X5M.

I also still have the original crystal cartridge for it but the sound output is exactly the same,I also tried a compatible output transformer
and result was the same. I will replace the resistor with the one you recommend and see how it goes...........

Thanks again

P.S. Anyone got the circuit diagram of this record player as I suspect that it has been tinkered with before I got hold of it!

zak · 20th Feb 2006, 3:02 pm

Quote:

Originally Posted by G4ILN

According to my data book the correct grid voltage for a UL84 is -12.5 volts.

Combined anode and screen voltage is 75 mA, so the calculated cathode resistor value is 12.5/0.075 or about 167 ohms. Nearest preferred value is 180 ohms and it will need to be rated at one watt.

With a 220 ohm cathode resistor the calculated grid voltage is 220 * 0.075 or -16.5 volts.

Different manufactures use different values for the same valve,if you increase the value of the resistor you will increase the bias,this will reduce the cathode current,thereby reducing the volts drop,so you can end up with the same condition!

mickjjo · 20th Feb 2006, 3:02 pm

The circuit diagram, (attached), for the HF16 shows the bias resistor as 330R

. You really need the BSR X5H high output cartridge with these single stage amps. Also note that there is no isolation between the mains and the pickup! Nasty!

.

Regards, Mick.

Sideband · 20th Feb 2006, 3:24 pm

Quote:

Originally Posted by mickjjo

You really need the BSR X5H high output cartridge with these single stage amps.

Yep!! Agree with that. I seem to remember that there was also an extra high output cartridge available at one time by BSR for the teenager who required just that little more output!

Rich.

Reproman · 21st Feb 2006, 1:01 am

Hello,

Thanks guys for your valuable input.

Cheers everybody

20th Feb 2006, 12:59 am	#1
Reproman Retired Dormant Member Join Date: Nov 2005 Location: Southampton Posts: 17	Cathode bias voltage ( UL84 specific ) Hello everyone What is the correct voltage for cathode bias using UL84 output valve I measured it at 17.70 volts, is this too high?, the bias resistor is a 220R and the bias cap is 22uf, both are new. This amp is out of a Fidelity HF 16 record player, the max sound output is not as loud as I would expected,even though there is a new output valve fitted along with new capacitors (except the electrolytic smoothing caps). Also a new cartridge has been fitted, I know these cheapo sets don't give the best sound in the world but this one seems quiter than others that I have worked on, am I missing something? I really hope you can help Last edited by Darren-UK; 16th Jun 2007 at 1:09 pm. Reason: General tidying.

20th Feb 2006, 9:21 am	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: UL84 Cathode bias voltage According to my data book the correct grid voltage for a UL84 is -12.5 volts. Combined anode and screen voltage is 75 mA, so the calculated cathode resistor value is 12.5/0.075 or about 167 ohms. Nearest preferred value is 180 ohms and it will need to be rated at one watt. With a 220 ohm cathode resistor the calculated grid voltage is 220 * 0.075 or -16.5 volts. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Feb 2006, 10:14 am	#3
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: Cathode bias voltage Is the cartridge the correct type? I think this kind of set up, with only one valve, requires the high output crystal cartridge. If the cartridge is the ceramic type then the output is a fair bit lower. regards Aub

20th Feb 2006, 1:39 pm	#4
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Cathode bias voltage Hi Gents, the other possibility is shorted turns on the op trans. This will both reduce the volume and increase the current in the valve, which would push up the cathode voltage. Ed

20th Feb 2006, 2:18 pm	#5
Reproman Retired Dormant Member Join Date: Nov 2005 Location: Southampton Posts: 17	Re: Cathode bias voltage Hello everyone, Many thanks for your help.The resistor I replaced was rated at 270 ohms so I naturally assumed to replace it with a value as close to the original as possible.Regarding the cartridge used, at the moment I am using a BSR X5M. I also still have the original crystal cartridge for it but the sound output is exactly the same,I also tried a compatible output transformer and result was the same. I will replace the resistor with the one you recommend and see how it goes........... Thanks again P.S. Anyone got the circuit diagram of this record player as I suspect that it has been tinkered with before I got hold of it! Last edited by Darren-UK; 16th Jun 2007 at 1:11 pm. Reason: General tidying.

20th Feb 2006, 3:02 pm	#7
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: Cathode bias voltage The circuit diagram, (attached), for the HF16 shows the bias resistor as 330R . You really need the BSR X5H high output cartridge with these single stage amps. Also note that there is no isolation between the mains and the pickup! Nasty! . Regards, Mick. Attached Thumbnails

21st Feb 2006, 1:01 am	#9
Reproman Retired Dormant Member Join Date: Nov 2005 Location: Southampton Posts: 17	Re: Cathode bias voltage Hello, Thanks guys for your valuable input. Cheers everybody Last edited by Darren-UK; 16th Jun 2007 at 1:04 pm. Reason: Irrelevant comments removed.