Help with Transformer Voltage Drop Method

[twinrider1]

I'd really appreciate a walk-through on figuring bias by measuring voltage drop on the output transformer. The patient is a '89 Marshall 2210 (pics available). I would put my skill level at being mechanically able, but not yet experienced enough to know the reasons why things happen. (Edit: What I mean is I know Ohm's Laws type things, but I can't voice an amp, decide what to change to change the tone, etc.)

Thanks.

[stoo]

Power disconnected ..caps discharged!!!
You'd measure the resistance in each half of the primary winding on the opt. So that'd be from center tap to the plate of the tube.
Then power up the amp and measure the voltage drop on each half of the transformer. Ohms law says I= E/R. So if you had a 5 volt drop over a 100 ohms...5/100 =.050 amps = 50 ma for the tube associated with that side of the tranny.
Read all about it here bout 1/3 way down . http://www.aikenamps.com/Biasing.html
Stew
Stew

[twinrider1]

Thanks. This matches up with the excerpts from this thread...
http://www.thegearpage.net/board/showthread.php?t=323260&highlight=transformer


hasserl
Seems like a lot of work to me, plus you're relying on the accuracy of the 1 ohm resistors which you won't be able to accurately measure.

Make this simple on yourself. With the amp off measure the resistance between the center tap of the output transformer and either side of the OT primary. Write these measurements down, making sure to identify which is which. Now fire the amp up and allow it to warm up until it stabilizes. Now measure the voltage between the OT center tap and either side of the OT primary. Now divide the voltage by the resistance measured earlier, voilla! You have the idle current dissipation of the tubes.

Just be careful measure the voltages as it will be quite high WRT ground, even though the actual voltage you measure will be quite low. Also, you wouldn't want to slip with a probe and ground out a winding. I like to use clip on type probes so I can clip them on and then remove them with the power off.

John Phillips
When set to read voltage as in the transformer-voltage-drop method hasserl described, there is much less risk of an accident because the meter now has a very high resistance so it can't pass a high current, and it's less likely to make the amp unstable because you aren't diverting the current via the meter. Actually this is the most accurate method too.


I have questions about the practical application. Where do I physically probe? I can see the OT and the wires, but not an open point to probe.

Also, Doyle's HoM has a table indicating White=CT and Brown & Red=Plate. But on the schematic, it looks like Brown is the center tap with White and Red going to the plates. What's going on here?

I have to say, most of my limited real-world experience is with DC circuits. Comprehending these amp circuits is a challenge. It feels like a balancing act on paths of a big parallel circuit.

[stoo]

If you look at the power supply the B+ should be the first node. ie the first voltage "take off point" after the rectifier. That should be where the CT of the opt is connected. On some ccts this is where the standby switch is located. It's probably where the first filter cap is connected too. The outer ends of the opt are connected to the plates of the o/p tubes (pin3).
Stew
ps. Just had a look at the schematic.. Is there a H/T fuse? That'll be in the B+.

[twinrider1]

Thanks, that helps. I've been spending some extra time comparing the actual wiring to the schematic and it's becoming clearer. Embarassed to say it took a minute to figure out my digital meter. I'm used to the Rx1, Rx10, etc. on my old Simpson. I don't see the logic in the 200, 2k, 20k, etc. (it's not autosensing). I pulled a few resistance readings, but with the kids under foot the live measurments will have to wait 'til tonight.