Resistance Check Questions

[TadS]

Hi All--
I finally got a chance over the holidays to wire up my SEX2.1 kit.  I have a few resistance values that don't agree with the manual and I wanted to check the WOG before plugging this thing into the wall.  All the values are within a few percent of the manual except the ones that I discuss below.

Terminal(s)       Manual Value          My Value(s)
 14, 24              11.5k ohms            2.3, 2.4 ohms
 A4, B4              11.5k ohms            217k, 224k ohms

 A7, B7              174 ohms              760, 762 ohms
C1,C2,C4,C5        *                        773 ohms for all, but no fluctuation as I saw on the other * values.

For the heaters, I am getting ~770 ohms on all the terminals and it does not appear as if I am charging a cap.  This "problem" clears up for terminals C1-C5 if I remove the tubes.  But even without the tubes, the resistance of the A/B7 terminals never drops into the 174 ohm range (750 ohms maybe).  I am not too worried about the heater circuit as I have seen posts from others with similar readings where everything seemed to work okay.

For the terminals surrounding the 220 ohm carbon resistors between terminals 14/24 and A/B4, I am more concerned. The values for 14/24 swing from 2.3 ohms up to 99k ohms when the wiper of the volume pot is turned all the way, whether the tubes are installed or not.  I have checked and reheated all of these solder joints (14, 24, A4, and B4) and the readings have not changed.  Is it safe to disregard these and go with the voltage checks?

Thanks for any help!
--Tad

[TadS]

Actually, there is another aberrant set of readings that I failed to record in my previous post.  That is, the input jack center pins have 106k/99k ohms resistance regardless of the wiper position.   

Thanks,
Tad

[Grainger49]

Tad, I don't have a SEX 2.1 manual so I can't help out on the other readings. 

Unless the volume control is wired as a FP III, a change from SEX 2.0, then the resistance should not change as the volume pot is turned.

To verify this you can look at the volume pot, one lug, an outside lug, will be grounded.  The input wire should go to the other outside lug and the center lug should go to the tube socket.  It might go through a terminal strip and through a 220 ohm resistor, but it will end up on the tube socket.

If this is the case the resistance will be constant and those measurements are fine.

[TadS]

Thanks, for the help, Grainger! 

I don't have a FP-III but input looks pretty standard to me. It goes across the volume pot, to wiper, through an input resistor, to the grid.  From looking at the schematic, I thought the same thing that you did.  It appears that it should not matter where the wiper is, the resistance to ground should be the full value of the pot.  Since the wiper goes through the input resistor to the grid, there should not be any ground except across the pot.  This is why I did not mention it in my original post.  But since I can't figure out why I am not getting the 11.5K ohm value in the manual for terminals 14/24 (which come directly from the wipers), I figured that I must be missing something.  Also, the manual does state that the input center-pin resistances should go from 0 to 100k ohms with the wiper going from min to max.  I guess that is a typo? I have triple checked the wiring to the volume pots and everything looks correct.

I am about to plug this thing in anyway.  When I was a kid, my dad always told me that my big problem (with building models) was that I could never wait for the glue to dry. 

--Tad

[Grainger49]

Tad,

The resistance from the wiper (T14/24) to ground WILL change with the position of the volume control.  I think those values got swapped.

[TadS]

Thanks, again. That was a quick response.

The value at 14/24 does change with the wiper position as I would expect.  The question I have is why at Min volume, it should be 11.5K ohms.  I am getting  only 2.3 ohms at the Min position.  T14 connects to the screen via a 220-ohm resistor and the value I get at the screen is 220 ohms.  The values listed in the manual for both 14/15 and for A/B4 (screen) is 11.5k ohms (at min volume).  Because of these numbers in the manual, I am guessing that there should be some 11.5k resistance to ground that I am missing somehow. 

--Tad

[Grainger49]

That value in the manual for the volume control at minimum isn't right.  The pot, volume control, should be shorted to ground at minimum.  You essentially have that with 2.3 ohms. 

Then screen to ground at minimum should be 222.3 ohms but 220 ohms is still just fine. 

Don't worry about these measurements at all.

[TadS]

Thanks, Grainger, for your help and confirmation.  I decided to go ahead and do the voltage checks.  Everything (except C2) was fine.  I think C2 is mis-abelled in the manual, also.  It is the heater plus voltage terminal from the PT and is listed as zero volts.  It connects to A8 which is +3.15V, so I am sure it should be the same.

With everything to spec or explainable, I started using the amp, and it sounds ABSOLUTELY GORGEOUS.  I have been breaking it in for about six hours so far.  It sounds solid, rich, clean, with that SE clarity or realism that I was looking for.  I added a couple switches in the OT transformer wiring so I can switch between  8 and 32 ohms.  I have mostly been using the 32 ohm setting with my HD650 (300ohm) headphones.  Using the headphones and feeding the amp with a Musicstreamer II+ dac, it has excessive gain so I will need to figure out how to pad it.  I am not currently using the headphone resistors shown in the schematic.  Using the 8-ohm setting and a pair of speakers I made from Fostex FE126 drivers, the gain with a standard line-level source is fine.  I think I saw a couple posts that addressed methods for doing this, so I will look into it.  But, for now, I am very happy with it.

Thanks again, for your help!
--Tad