alter 436c trouble shooting

[Nashanalog]

I have been working on an alter 436c. pretty much a total rebuild. replaced the transformers with new sowter transformers, re capped it, and used diodes in favor of the old rectifier. checking voltages this morning everything looked pretty good except the voltage to the plates of the 6BC8 tube. instead of being 65v, as stated on the schematic it was at 315V!!!!!
now I know the voltages should be a little higher because this was originally built for 117v, but this is way off!
any thoughts and help would be much appreciated.

thank you
John

 

[Rob Flinn]

Normally when this happens there is not a connection between 0v and the cathodes of the valve in question.  I suggest you check the grid and cathode connections of the 6bc8

 

[Nashanalog]

Hey I rechecked the connections and it seems to be all fine. Still getting about 280v, which is considerably less than before, but it the schematic does say with no signal passing it should be around 60v and at 20db of compression it would be around the 280 reading.  :'(

 

[Rob Flinn]

Hey I rechecked the connections and it seems to be all fine. Still getting about 280v, which is considerably less than before, but it the schematic does say with no signal passing it should be around 60v and at 20db of compression it would be around the 280 reading.  :'(

If it were all fine the voltages would be right.  I would check again until you find the issue.    Use your meter on continuity to verify connections from component leg to component leg, don't rely on visual verification.

 

[PRR]

Is the cathode also up around 300V?

Is the tube getting hot?

 

[Nashanalog]

I re checked all the connections with a multimeter. There was one cold joint so I fixed that. Now that the solder joint is repaired the tube is lighting up.... progress, but I am still getting the high voltage

 

[Nashanalog]

I also measure the caps and resistors again, just to make sure everything was okay there, and those all check out. 

 

[scott2000]

and used diodes in favor of the old rectifier.

??

What exactly does this mean?  Is the drawing correct or have you made some changes to it?

 

[Nashanalog]

I replaced the selenium rectifier with two 1N4007 diodes. 

 

[scott2000]

I replaced the selenium rectifier with two 1N4007 diodes.

Gotcha. Thanks.

What are you doing with the  pot marked release time? I see the lines squiggled out.  Is that just a fixed resistor? Is there a line missing there?

 

[PRR]

Is the cathode also up around 300V?

 

[Nashanalog]

0 volts across the cathode

 

[Nashanalog]

I check all the connections again, and they are fine. Is it possible I have a bad tube. I can’t think of what Else it would be.

 

[industrialarts]

Sound to me like the tube isn't turning on hard enough. You didn't answer about the components that scratched out. It looks like they would have an effect on the tube bias.