Mainline Resistance Check

[bdog9]

Need a little help getting pass the resistance check.
I have gone over wiring multiple times and feel confident that it is correct. I lingered however, on the soldering of connections 6 and 8 on the input switch. I am guessing I will need to replace it.
Terminal 1 and 5 are reading OL, 15 and 25 are reading 205.5 and 207.5 respectively. All others are in the 0.1 to 0.3 range.
Advice would be appreciated. Bill

 

[Paul Birkeland]

Could you post some build photos?  Do you get a different resistance reading on 1 and 5 if you turn the fine switch all the way down?

Terminals 15 and 25 are supposed to read around 200 ohms, so I don't see an issue there. 

 

[Paul Birkeland]

Inspecting the photos you sent me, could you let me know what meter you're using?

 

[bdog9]

Thanks for the help. I measured with a Klein CL390

 

[Paul Birkeland]

If you turn the fine control all the way up and the coarse control all the way up, what's the DC resistance reading on the center post of each 6C45 socket?

 

[bdog9]

130.0 for both

 

[bdog9]

Sorry, fat fingers. Should be 139.0 for both.

 

[bdog9]

I made an asumption that could be wrong. I measured from the ground at the power cord to the center of the outer tube sockets. If wrong by all means correct me.

 

[Paul Birkeland]

That doesn't make a lot of sense, was the coarse control turned all the way down by chance?

If the answer to that is yes, then that tells us that the coarse attenuator seems happy.  The next resistance check would be to the lower two lugs on the fine switch with the fine switch turned all the way up. 

 

[bdog9]

PB your patience is admirable. Thanks. Now I know full clockwise is up, and full counter clockwise is down.
The fine lug reading fully up was OL to either of the lower lugs.

 

[Paul Birkeland]

Can you check that resistance reading with the switches in those positions against the resistance reading of the center lug of each 6C45P jack?

 

[bdog9]

Measuring with fine and course all the way up, clockwise, the resistance reading between ground and the center lug of the 6c45p both right and left is OL.

 

[Paul Birkeland]

What I would do is to hold a probe on one center socket and turn the coarse switch back down and stop when you see a resistance reading.  Let us know what that resistance is and what step you're on with the control.

I am somewhat confident that the meter you have just isn't properly reading large resistance values, though it is advertised as being able to do so.  If you lose the DC resistance value on both center pins of both 6C45P sockets at exactly the same step on the coarse switch, I would tend to blame the meter.  If the resistance reading drops out on one side but continues counting up on the other and then drops out, I would suspect flaky solder joints on the coarse switch itself (though doing a ton of resoldering on these switches is not at all recommended).

If the meter is the problem, the ~$20 meter at Harbor Freight can resolve these measurements.

 

[bdog9]

I started seeing resistance reading when I reached -36dB. It registered 394 both socket. At -45dB it dropped to 139.5 both sockets. I then switched to another multimeter and everything improved. 5 and 12 now register 39.5. I am reading resistance all along the coarse switch but the reading between center socket both sides and ground is still 394  at -36dB. It then drops to 139 when at -45dB and I wonder if I should question that or move on to voltage checks ?
Thankyou for your help.

 

[Paul Birkeland]

The resistance readings at the center pins of the 6C45 sockets should go from close to zero up to about 25K as you rotate the coarse switch assuming the fine switch is turned all the way up.  Provided you see those resistances present, you can move onto the voltage checks.

 

[bdog9]

OK. Check resistance as suggested and range form 25 to 0 noted, so moved to voltage test. When I switched on the power the tubes did not glow and I turned it off. I could smell heat and on careful inspection the green resister on the power board had one leg with bubbling solder.
It has cooled now.  Advice?

 

[Paul Birkeland]

Remove the red wire leaving the +275V pad and power up the amp again, measuring what's available on the +275V pad if you don't get the hot smell again.

If the resistor continues to get hot under these conditions, then you have a backwards 100uF/450V cap or you have swapped two of the caps on the power supply board on accident.

 

[bdog9]

Cleared both red wires from 275 connection, powered up and no smoke and 365 volts between 275 + and ground.

 

[Paul Birkeland]

When you put back one of the two red wires, you'll get that issue again, and that can be used to narrow down which half of the amp is misbehaving.  It's also possible that either wire causes the same problem, in which case you've just made the same mistake on both channels.

 

[bdog9]

Thanks for your help ! Back to the build.