have a bad cat bc-50 plexi style amp

[wsaraceni]

am i wrong in assumingi can measure the voltage at the center tap (in relation to ground) and pin 3 on both power amp tubes separately and do a little math for the voltage drop? doing it that way allows me to do it all 1 handed. the numbers are way off from anything that seems normal.


resistance black to red 72ohms
resistance black to white 74 ohms

voltage at center tap 504V
voltage at white 458v (voltage drop 42v) = 582mA
voltage at red 426v (voltage drop 79v) = 1.06A


and like I said, whenever I touch the probe to the red tap it makes a horrible squeal.

[xtian]

am i wrong in assumingi can measure the voltage at the center tap (in relation to ground) and pin 3 on both power amp tubes separately and do a little math for the voltage drop?

Sure.

I can't guess what's going on here. Schematic? Post photos of power tube sockets, OT primary wiring, and B+ filters?

[wsaraceni]

I’ll have to open it up again for better pictures. Will report back

[martin manning]

Your meter leads are adding enough feedback to cause oscillation, and fouling up the measurement. If you want to work one-handed, I suggest clipping one lead to the CT, then probe the plates with the other.

[wsaraceni]

3rd set of power tubes seemed to help. it doesnt sound like crap anymore. but i still need to find a proper set of clip on leads so i can bias it correctly. still not 100% convinced the transformer isnt messed up. at least somewhat.


any suggestion on a shorter set of leads with clips on the end?

[wsaraceni]

With amp off and discharged, measure, "resistance across each side of the output transformer primary" -- so that's from pin 3 (anode) of each power tube to OT center tap. In typical Fender amps, the wire colors are blue, red, and brown. Red is the CT. You'll get resistance readings in the tens of ohms, usually.

Then, when amp is running, measure voltage drop across same points, and you'll get voltage drops of a few volts.

Totally made up example: 1.0 volt across 15 ohms is 67mA of current (use Ohm's Law).

so I remeasured.

I'm getting 2.5 voltage drop on each side. and 33mA/32mA draw per tube at just under 475V plate voltage. so the bias seems right from the few charts I've looked at.

im also drawing up the schematic. im comparing it to this one

https://drtube.com/schematics/marshall/1987u.gif

it's very similar except im trying to figure out the power supply and the bias section. that's seems a little different. im still not sure what the issue is as im getting a few issues such as high pitch squeal when the chassis isn't in the cabinet and also that buzzing noise. once im done I'll post the full schematic.

[wsaraceni]

Some better pictures

[wsaraceni]

Can someone tell me what this switch is doing here?


In the picture you can see three large resistors. Two are 100 ohms going between each side of the heaters and ground. The switch bypasses a 33k resistor that goes between pin 6 on each power tube and a 33uf filter cap. The other side of that filter cap goes to a 221k resistor to ground.

[Roe]

seems like the switch is for increasing screens filtering, useful for a tighter, stiffer response