New 5E5A build, Presence control problems

[russmanr]

Hi there,

I just finished my new 5E5A and it sounds great except for one thing.
The presence control doesn't do anything. I verified the pot does
work by testing the resistance across the terminals (it changes
accordingly). I've re-traced the wiring many times to make sure I
didn't mess anything up. Could I have a bad capacitor on the pot?
How would I test that, and what would be the normal troubleshooting
procedure here?

Thanks in advance!
Rich

[loudthud]

Install a short across the cap. Now the pot should adjust the gain (volume). It should do something across most of the rotation, not just at the max end (12). If that works, the pot and wiring are good, find a new cap. If it doesn't, there is a problem with the wiring, the 100K, the 1.5K or the pot. Is there about +1.5V or so at the cathode of the tube the 1.5K is hooked to and at the pot?

[russmanr]

Thank you for your help.

So, when I short the cap, then I do not get the expected behavior.
I only see a volume drop when I get towards "12".

I do see 1.6V at the cathode of the 12AX7 and at the pot.

However, my multimeter is showing 1.5Kohms across the supposed
100k resistor. Unless there is something else affecting this measurement,
then I'll try and swap that out... Is it common for resistors to be
mislabeled?

[loudthud]

Quote: However, my multimeter is showing 1.5Kohms across the supposed
100k resistor. Unless there is something else affecting this measurement,
then I'll try and swap that out... Is it common for resistors to be
mislabeled?

That's normal. At the output transformer, the resistance to ground is very low, less than one ohm. At the other end you have the 1.5K cathode resistor and the pot in parallel. To accurately measure the 100K you'll have to disconnect one end.

You should also measure 1.5K across the ends of the pot (pot set to "1"). Remove the wire across the cap and trace all the connections with your ohm meter. You must have a broken wire, missing connection or a bad cap.

[MWJB]

"I just finished my new 5E5A and it sounds great except for one thing. The presence control doesn't do anything."

I have heard this many times with respect to 5E5A. I have to wonder whether the peculiarities of this amp override the typical perception of the presence pot's effectiveness?

[loudthud]

I have heard this many times with respect to 5E5A. I have to wonder whether the peculiarities of this amp override the typical perception of the presence pot's effectiveness?

Interesting. That brings up the possibility that the 100K feedback resistor should be 56K as in the 5E8A and 5E4A. The Presence should do something being across the cathode resistor of the 12AX7 gain stage but not like we normally think a Presence control should sound. Perhaps the cap just needs to be a higher value, maybe in the range of .47uf to .68uF.

[russmanr]

Well I replaced both the pot and the cap with no change.
I really think I have this wiring correct, but maybe the
previous post is onto something.... I just can't really
hear the difference when I mess with this control.

I'm open to trying tweaks if you have any suggestions.

[russmanr]

Finally able to bring this to closure. Note the difference in wiring between Weber's layout, and the original 5E5A layout -

https://taweber.powweb.com/store/5e5a_layout.jpg
Fender Pro Amp 5E5-A Layout - KBapps.com

In the original circuit, the cap at the presence knob is tied to both of the 1st 2 terminals.
In Weber's layout, it's only connected to the middle terminal.

Once I made that extra connection, the knob began working.

Now, someone smarter than me can explain why that worked...

[MWJB]

It didn't.

Only the portion of the pot between the wiper & the RH tab works in either scenario. They are the same.

[Wittgenstein]

Now, someone smarter than me can explain why that worked...

My guess is it was a cold/bad joint that was fixed when you re-flowed the solder on the pots.