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In Reply to: RE: Assistance Request PH3 pinout / voltage measurement posted by Triode_Kingdom on November 09, 2016 at 21:21:48
Triode, would you then think the 128 volts is measured pin 1 and another of the pins ? Maybe to the grid?
Thanks john
Follow Ups:
I'm sure it's measured with respect to ground. FWIW, this is a common cathode stage (lower tube) driving a grounded grid stage (upper tube). Audio Research seems to have used this general arrangement in a number of their products. Must be a pet topology of the chief engineer. :)
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Buy Chinese. Bury freedom.
Gosh then I'm confused. I think you said not to worry, all is ok, but I am reading 210 volts pin 1 to chassis, not 128 volts as shown in the diagram.
From the schematic would you expect 128 volts on pin1 to ground ?
Thanks mate, John
If you are reading 210V on the plate of the upper tube then it is not passing any current or there would be a voltage drop across the 50k resistor. It might be helpful measure the voltages at relevant points (pins 2 & 7 of the upper tube in particular) and record them on the schematic. Post the schematic with the voltage readings. It should be possible to understand what's happening if all the voltage are shown.
Thank you Palustrus, I will do that. Gosh I've been measuring the point without a tube inserted, perhaps that's the whole issue. Well I always like to get smarter about this stuff, thank you for your help. I'll post it later mate.
John
Yes, that could be the problem: without a tube there is no current flow through the resistor.
Palistrus, You were right. The voltage measurements require the tube to be in the circuit. I plugged in some old (used) 6922 tubes and measured. Just watched the Pin1 voltage drop from 210 volts to 153 volts as she warmed up.Yesterday I got through to Audio Research and with a little bit of repeated requests they did take the time to help me out. The engineer annotated a copy of the PH3 schematic that gave updated voltage readings for the "new" 6922EH tubes. They explained that older 6922s will have even higher voltage readings. They did explain that we should try to get the "OP" (not defined but refers to voltage readings) within 10% for best balanced sound.
So: In a nutshell - The tubes must be plugged in for proper readings. I was worried about nothing. Now with my personality I'll start worrying about voltage aberations between the pins - Then again I've been running this unit for 15+ years without a problem so I'll just go along for the ride :).
Thanks again for your help. An updated / annotated copy of the PH3 schematic voltage readings at critical points for everyones' future reference is in the next post in this thread. Looks like I accidently selected the orginal and pressed Post. So, above is original, next post is annotated by engineers.
Edits: 11/11/16
This time with proper JPEG. Please excuse previous post, I attached the original schmatic by accident.
John
I'm happy all is well now and very impressed by ARC's tech willing to help you resolve your problem. ARC has always been a company friendly to the DIY community, ever since the days of 'Audio Amateur' magazine.
Yes I was lucky. Truth is I refrained from telling then about the regulated heater power supply other the fact that I've changed (upgraded) just about every capacitor and resistor in the unit. I was afraid they would just shut down on me. Well thanks again for your help.
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