Re: IGNORE this DAMN FOOL

In article <4909CC00.707EEE2@xxxxxxxxxxxxxxxxxx>,
info@xxxxxxxxxxxxxxxxxx says...


AJ wrote:

In article <MPG.23736c7a5b03f809989a68@xxxxxxxxxxxxxxxxxxxxxx>,
Sorry@xxxxxxxxxx says...
It could be how they set them up at the factory on a regulated supply.
I've being biasing my guitar amps(push/pull, grounded cathode, fixed
bias class AB), by first measuring the resistance of the transformer
secondaries(unplugged, power off, caps discharged).

Sorry - that should be resistance of the "output transformer primaries".
Same terminals as B+ and output tube plate. A standby switch may have to
be set to the operate position to get a resistance reading.

The amp should be turned off and cold to measure the winding resistance
each side of the PP circuit. DVM sometimes won't measure the Rw because
of the inductance, so the sec should be shorted to make the Rw
measuremts. Better is to add 10 ohms R between each anode and the OPT
primary connection. Of follow what I said in my other recent post. The
OPT Rw for each side won't be equal, and to avoid mistakes with
calculations, better if you only have one size of R to measure.

Some use 1 ohm instead of 10 ohms. They need to be accurate.

Patrick Turner.

Actually, I've found that the resistance drifts up an ohm or 2 after the
amp heats up, but that measurement is less critical than the voltage
drop measurement. This probably works best in less extreme 2 or 4 tube
guitar amps with reasonably well-matched tubes and in circuit board amps
where adding cathode resistors would be difficult. On my own amps, once
I find what I like, I just write the numbers down somewhere inside the
chasis. Then next time it's just a matter of verifying the B+ and
biasing to the appropriate voltage drop, no math required.
.

Relevant Pages

  • Re: Shunt resistor in DMM and baising Marshalls
    ... I've decided that some folks here really do talk about amps, ... Both my Fluke and my Protech meters read 11 ohms when set to 40 mA range. ... accounting for 11 ohms internal resistance in the meter at the lower ... Fire up the amp with the tubes in it and measure the voltage drop across the ...
    (alt.guitar.amps)
  • Re: Shunt resistor in DMM and baising Marshalls
    ... Or, another thought, how close to in the ballpark could I be using the 400mA setting on my Protech meter if its internal resistance is 1.9 ohms? ... I've decided that some folks here really do talk about amps, ... One article written by LV suggested that 10 ohms in the meter was too much. ... Actually, if you pull the power tubes, you only need to lift the center tap and you can measure from each tube pin to the center tap. ...
    (alt.guitar.amps)
  • Re: Maximum current for TO-220 package 75-95A, and how to connect heavy leads
    ... resistance as low as 0.0024 ohms which is rated 260 ... An IRFBA90N20D has 0.023 ohms and is limited to 95 amps by its ... "Super-220" package, which does not have a mounting tab, and has ...
    (sci.electronics.design)
  • Re: More on amps.....My exoerience.
    ... IF you have 1 ohm of resistance in the circuit. ... Amps = Volts / Resistance ... at perhaps 8 or 16 ohms resistance in the circuit... ...
    (alt.guitar.beginner)
  • Re: Measuring tiny unknown resistance?
    ... > Anyone know a simple way I can measure a small, unknown resistance? ... less than 100 ohms. ... On the 200 ohm scale the maximum resistance that can be measured ... take you measurement of your 0.5ohm to 1 ohm resistance and ...
    (sci.electronics.basics)
Loading