have a bad cat bc-50 plexi style amp

[wsaraceni]

not in love with it. I think 90% of the reason why is my komet 60 does everything the bad cat does only better. I think I might send it out to get modded. turn it into something like a slash mod or maybe even a Friedman style. that great 80s lead tone amp. any suggestions?


I also want to put a komet style bias test points in it and possibly order a new transformers because this one makes crazy noises under load. physical noises not through the speaker outs.

any suggestions on where to start? I assume the best thing to do first would be to figure out the differences between this an da stock 50w plexi. I assume its most similar to a 1987x but im not too sure. it's really difficult for me to trace it out inside with my limited amp building knowledge.

[xtian]

Bad power tubes? Try the simple stuff first.

[wsaraceni]

i have new power tubes on order. i just have to figure out how to bias the amp. ive never done that on a marshall style amp before. looking up how to do it is real confusing. thats why i thought it might be best to order the resistors and jacks to wire it up like the komet.

[xtian]

Checking bias is easy, even when no 1R bias sense resistors are installed. Read up here, and let us know if you have questions:

http://www.aikenamps.com/the-last-word-on-biasing

This part specifically:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

[wsaraceni]

seems simple enough. when i get back from my next work trip ill open it up and see if it makes sense when im actually looking at it.

[Reeltarded]

seems simple enough. when i get back from my next work trip ill open it up and see if it makes sense when im actually looking at it.

Modding it will be fun. Let's look at the circuit when you get ready and you describe the difference between what it is and where you want it to be. Will be two or three simple things.

So, you are saying the OT sings? I hate that. My ears are very sensitive to the pick on the strings and anything nasal like a Tweed whining or an OT singing. I can hear it even with amp three times louder than the noise. Pick on string makes my eyes twitch sometimes.

[wsaraceni]

seems simple enough. when i get back from my next work trip ill open it up and see if it makes sense when im actually looking at it.

Modding it will be fun. Let's look at the circuit when you get ready and you describe the difference between what it is and where you want it to be. Will be two or three simple things.

So, you are saying the OT sings? I hate that. My ears are very sensitive to the pick on the strings and anything nasal like a Tweed whining or an OT singing. I can hear it even with amp three times louder than the noise. Pick on string makes my eyes twitch sometimes.

. Hear you can hear the transformer. I have it plugged into a load so ignore the fan sound but the buzzing is 100% coming from the amp. The inside is pretty decent quality too

Mod edit: fixed youtube link.

[Reeltarded]

SOUNDS like a loose piece of metal moving in an OT end bell...

[Reeltarded]

SOUNDS like a loose piece of metal moving in an OT end bell...

Also sounds a bit like EL rattlesnake from an EL34. Get your ear around back and see if you can localize the noise.

[wsaraceni]

SOUNDS like a loose piece of metal moving in an OT end bell...

Also sounds a bit like EL rattlesnake from an EL34. Get your ear around back and see if you can localize the noise.

im almost positive its coming from the transformer but ill confirm its not the tubes once i get a break from my kids trying to kill me.

if it is the transformer, i assume i either have to take the end bells off and see if there is metal in there or i have to replace the transformer if it's not a physical thing. correct?

[pompeiisneaks]

sometimes it just takes tightening the bolts on the transformer, they get loose over time and allow for vibrations.
you can test this by just squeezing the transformer tightly and see if the noise goes away or at least diminishes.

~Phil

[Reeltarded]

Yeah, I would try to confirm that the sound is coming from one side of that transformer then open that side up and let the thing out before it jumps somewhere bad and lets the smoke out.

[wsaraceni]

Checking bias is easy, even when no 1R bias sense resistors are installed. Read up here, and let us know if you have questions:

http://www.aikenamps.com/the-last-word-on-biasing

This part specifically:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

what points am i using to measure the resistance of the primary windings?

if i used this as an example am i finding the resistance between RD and BN and also WH and BN?

then taking the voltage at BN - RD for the one side and BN - WH for the other side?


i 've also started mapping out the layout and values used. seems drastically different than the 1987x schematic which i was told it was based off of.

[wsaraceni]

im pretty sure im measuring correctly but not so sure im getting anything remotely accurate.

i get a little over 500v on the center tap coming off of the 2nd fuse.

i get 485ish volts on the white tap and 425ish volts on the red tap. tried with multiple output tubes. as soon as my lead touches the red tap i get a high pitch squeal and the meter starts jumpin all over the place. the resistance is pretty close across both sides.


also, i get a spark in the 3 way switch when i switch from standby to full power.

[xtian]

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).