AC Voltage Reading Problem

[astroimage2002]

Hi All,

We now have the correct resistance on the terminals. What we are seeing now is an issue with the AC. The amp has been configured for 45's NOT 2a3's. Here are the AC voltage issues we are seeing:

Pad          Expected   Actual
HV+         350          434

Terminal   Expected   Actual
1              340         436  
4              0             430
5              57           0
11            57           0
12            0             430
16            57           0
20            57           0
A1            57           0
A2            340         433
A3            0             434
A4            57           0
C1            57           0
C2            340         433
C3            0             433
C4            60           0

PC Board checks fine.... I should also let you know that I have 2a3's mounted in the amp when checking the voltages.

HELP!!!

Brian

[Paul Joppa]

Very disturbing!

The first explanation that occurs to me is that you may have installed the 4-pin tubes or sockets rotated 180 degrees.

I just now checked page 32 of the manual, and sure enough there is no warning or caution about orienting the socket correctly. We'll have to fix that! Looking at the socket, you will see that two of the holes are slightly larger, these are pins 1 and 4. (They are larger because they carry the filament current, which is much greater than the other connections.) They should be the two holes closes to the hum-balance pot. On page 46 of the manual you can see where the filament power is wired to those pins, and on page 63 the hum pot is connected through the 22 ohm cement resistors. When the tube is inserted into the socket, the larger (fatter) pins should go into those holes. Occasionally someone will force the tube into the socket incorrectly - it is possible, though fairly difficult. I suppose this is one of the reasons that later tubes have a more unequivocal orientation!

If you have reversed the socket, you'll have to fix that. You'll want some solder-sucking copper braid to get the wires off the pins; other than that it should be easy. If you've inserted the tubes incorrectly, the small socket pin connectors may be stretched out - make sure you have secure connections before testing again. If the socket is too badly damaged to provide reliable connections, Eileen can fix you up with replacements.

If neither of these is the problem, then we'll have to re-visit the situation, and perhaps ask for a few more measurements.

[astroimage2002]

Hi Paul,

I thank you for the information! As you suspected I have the sockets 180 degrees out of rotation. Soldering iron is warming up... Copper is at the ready... Hope that I don't need to bug Eileen (what a sweet heart she is!)

I'll report back after they are rotated and re-soldered. Fingers crossed  

[astroimage2002]

Paul,

You nailed it. Tube sockets have been rotated 180 degrees and the amp is now alive.

Many thanks!

Brian

[jtkarma]

Hehehehe.

Interesting thread.

What a great place the Bottlehead Forum is.

Please post your results once your sort out your issues. Love to hear what your thoughts are on the amp.

[astroimage2002]

jtkarma,

The amp was up and going in just over two hours after Paul's post. Once Paul Genius Joppa figured out where I messed up it was just a matter of flippin' the tube sockets around I was off to the races!

As for how it sounds... I will do so in the coming weeks.

Brian