1987ptp : too much voltage on preamp valves

fedachris · Post by **fedachris** » Sun Aug 27, 2006 10:25 pm

Hello
After replacing my board, first check shows more than 400 volts between pins 1 and earth on V1 to V3.
Any clues or areas to seek ?
Thanks to everyone who has placed a topic here. This forum is a gold mine. Thumbs up !
Christian

Flames1950 · Post by **Flames1950** » Mon Aug 28, 2006 12:19 am

Are your measurements done with no tubes installed? If so, put in at least the preamp tubes and re-measure.....there is no voltage dropping taking place with no tubes installed and you'll read full voltage all the way across!!!

fedachris · Post by **fedachris** » Mon Aug 28, 2006 7:14 am

I did made the measures without tubes....
After putting some in (v1&2 NOS GE 12ax7a JAN, v3 brand new MESA 12ax7 of unknown origin but probably chinese)
I measured this :

v1 :
pin 1/ 160 v, pin 6/ 219 v

v2 :
pin 1/ 177 v, pin 6/ 303 v, pins 7&8/ 179 volts

v3 :
pin 1/ 229 volts, pin 6/ 229 volts

by the way valves 4&5 not inserted I found :

pin 3&4/ 428 volts, pin 5/ -37 volts with bias pot fully counter clockwise

What's your opinion ?

Bainzy · Post by **Bainzy** » Mon Aug 28, 2006 7:35 am

That looks normal to me, you should expect V2 pin 6 to be higher than the rest of the plates in the preamp as there's no 100k voltage dropper resistor before it (unlike the other plates).

fedachris · Post by **fedachris** » Tue Aug 29, 2006 12:24 pm

Thank you guys :

Just plugged : it works perfectly, no strange noise, nor funny things ...
All apprehensions disappeared at the moment I played a few riffs !

'All right now' & 'Let there be rock'

And a big thank you to everyone from France

PTP rules : time for tweaking now !