Small temp voltage on phono input-problem?

trombone · 18th January 2006, 08:10 PM

This may seem like a stupid question. A long while ago, I fried a phono cartridge after one of my preamp mods because I left a short between the B+ and the cartridge input. Ever since, I've always checked for DC voltage between cartridge input and ground after doing a mod. Normally, I turn on the preamp, let it reach operating voltage, and check. Always 0 DC.

For some reason, this time I connected the meter to the input and ground before turning the preamp on. (The preamp is a MFA Luminesence with tube regulated DC. It takes about 20 seconds or so to reach 330 VDC. ) Watching the meter from turn on, I saw it go up to about .9 VDC for a few seconds before settling down to 0. I assume this is what it always does, but now feel compelled to check. Is this normal? Or is my cartridge in some danger?

hagtech · 19th January 2006, 04:36 AM

Could just be grid current as the tube warms up. Depending on the power up sequence, maybe the B+ is applied too early, before heaters are warm.

May also be a leakage path of some contamination like solder flux or dust.

jh

trombone · 19th January 2006, 01:41 PM

Thanks. I thought the delay in the regulator powering up would slow this down, but I guess not.

Do you know if this few seconds of .9 VDC is dangerous to my cartridge. As I said, for all I know, it's always been this way, but now that I know it's happening....

hagtech · 19th January 2006, 05:51 PM

Well, if it is 0.9V with the cartridge connected, it could be a problem. If it is 0.9V into 47k ohm only, then maybe not so bad.

jh

trombone · 19th January 2006, 06:24 PM

Hey, good point. I forgot. I measured it without the cartrige attached. Just into 47K.

18th January 2006, 08:10 PM	#1
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	Small temp voltage on phono input-problem? This may seem like a stupid question. A long while ago, I fried a phono cartridge after one of my preamp mods because I left a short between the B+ and the cartridge input. Ever since, I've always checked for DC voltage between cartridge input and ground after doing a mod. Normally, I turn on the preamp, let it reach operating voltage, and check. Always 0 DC. For some reason, this time I connected the meter to the input and ground before turning the preamp on. (The preamp is a MFA Luminesence with tube regulated DC. It takes about 20 seconds or so to reach 330 VDC. ) Watching the meter from turn on, I saw it go up to about .9 VDC for a few seconds before settling down to 0. I assume this is what it always does, but now feel compelled to check. Is this normal? Or is my cartridge in some danger? __________________ Trombone

19th January 2006, 01:41 PM	#3
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	Thanks. I thought the delay in the regulator powering up would slow this down, but I guess not. Do you know if this few seconds of .9 VDC is dangerous to my cartridge. As I said, for all I know, it's always been this way, but now that I know it's happening.... __________________ Trombone

19th January 2006, 06:24 PM	#5
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	Hey, good point. I forgot. I measured it without the cartrige attached. Just into 47K. __________________ Trombone

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19th January 2006, 04:36 AM	#2
hagtech diyAudio Member Join Date: Mar 2002 Location: Hawaii	Could just be grid current as the tube warms up. Depending on the power up sequence, maybe the B+ is applied too early, before heaters are warm. May also be a leakage path of some contamination like solder flux or dust. jh

19th January 2006, 05:51 PM	#4
hagtech diyAudio Member Join Date: Mar 2002 Location: Hawaii	Well, if it is 0.9V with the cartridge connected, it could be a problem. If it is 0.9V into 47k ohm only, then maybe not so bad. jh

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