Measuring NFB resistor in circuit?

[Fresh_Start]

Since I have a vintage Princeton Reverb on the bench, I figured it would be interesting to measure various resistors in the circuit to see if/how far their values have drifted over the past 45 or so years.

The 2.7K negative feedback resistor (circled in attachment) appears to measure 47 ohms.  Probably not a coincidence since there is a 47 ohm resistor going to ground from one end of the NFB resistor.  I just can't figure out how the other end of the 2.7K resistor is grounded?  Is it through the OT secondary?

Other measurements surprised me a bit.  The plate resistors are generally very close to spec.  Some of the cathode resistors, however, have drifted upward in value.  The resistors on the power rail are all within 20% of spec or even closer.

Please let me know what's up with the measurement of the NFB resistor.

Thanks,

Chip

[stingray_65]

Hey Chip!
Is the speaker plugged in?

If so it seems to me like it would be like reading 2 resistors in parallel. 2.7k and 47+8 (for the speaker)

If you want to see the circuit more clearly, eliminate the ground symbols and draw a line connecting them

See?

Ray
****EDIT****

One handy little item I have is a radioshack cheapo 1/4" plug with metal shield.
 You can plug them in and open switches on jacks. you wouldn't use one to repair a cable or nuthin cause they're lousy plugs. mine has blue heat shrink on the barrel to remind me its a tool.

Oh! you can also remove the barrel and insert it into your input. a light touch with your fingertip works to inject a 60hz signal. make it easier to hear if you have 60 or 120hz buzz.

*** DOUBLE EDIT***
The above tool may be necessary when blueprinting those 68K's on the other end

[eleventeen]

Thru da speaker!! Or, even with speaker plug pulled, via the shorting action of the primary spkr jack.  

I don't see a convenient (non-solder) way to open up the eastern side of the 2.7K NFB resistor. It will connect to ground either by the shorting action of the spkr jack or via the output winding of the OT, which is a pretty small resistance.

[Fresh_Start]

Thanks guys - I kinda thought it was through the speaker/OT secondary.

Chip

[PRR]

Measure the speaker winding of the nearest loose OT.

It's under 1 ohm. (It has to be much less than the nominal speaker load or you'd waste a lot of power.) 0.8 ohms might be typical; you can get a very good reading IF your meter is accurate sub-Ohm (most aren't).

So you have a loop with 2,700, 47, and 0.8 ohms in series. You can't get a clear view of the 2700.

The 2700||(47+.08) works out to very close to 47 ohms. 46.968

In theory you could read all three legs to very high accuracy, and do some math. A common DMM won't give a very accurate result.

You can get a clue by driving the output to 1V, then reading the ACV across the 47. This is neary the ratio of the "2700" to the 47. It is somewhat skewed by cathode signal. You could kill the amp and use another amp to drive the OT secondary to 1V, then read the ACV at the 47.

[tubeswell]

Otherwise just grab your iron and unsolder one side of the resistor from the board, then measure it