The the OD sound of my amp sounded cracking up when the power section starts to clip.

[Alan0354]

This is pretty much the continuation of my amp that started out under power. I think I solve the problem using the new PT that has balance secondary AND using the 8 ohm tap instead the supposedly 4 ohm tap to match a 4K OT to 6V6 on a 8 ohm speaker. I got 24W from a pair of 6V6 at +B=420V. I don't think I can complain anymore. Thanks for the help.


Now I run into another strange problem that I need opinion from you guys.

The OD channel of the amp sounds great......until I crank the master volume (MV) above 7. Then the sound start cracking up and it is not good. I look at the waveform, it is the point the power amp output starts clipping. From the test in the last post, that relate to the the input hitting 0 grid voltage.

The 6V6 is bias to about 25mA per tube. The grid voltage is about -38V for +B=420. As soon as the grid reach 0V, the power amp starts to clip. That's when the amp starting to sound funny.

I measure the screen voltage, I saw signal reaching about 50Vpp at the point of clipping as before.

The +B is very quiet, it droop about 30V when hit hard, but there is not strange waveform, just droop down and slowly rise back up as the input signal decay.

Is this normal, I don't believe I encountered this problem before, but at the same time, I did not really check the signal at each stage before. Maybe I never drive the other amp this hard.

I know I lot of other amps put a resistor in series with the MV pot to cut the max signal down. I have the tone stack driving the MV pot directly without any resistor to lower the signal.

1) How do you guys match the output of the PI to the input of the power tube?

2) Do you match the max drive signal so it just hits 0V at the grid? Or this is only a problem with my amp?

Here's the schematic of the power amp, it's a run of the mill power amp circuit!!!

Thanks

[Alan0354]

I have to specify, what I meant cracking is the distortion has a hairy sound, definitely not anything loose. It remind me of my Marshall Plexi 100W stack back in the days. It sounded very good at 6. But if I crank it all the way to 10, it had the same cracking sound that did not sound good.

[J M Fahey]

It remind me of my Marshall Plexi 100W stack back in the days. It sounded very good at 6. But if I crank it all the way to 10, it had the same cracking sound that did not sound good.

Maybe you just don't like power tube clipping?

[jazbo8]

Is the crackling at "10" normal in the Marshall Plexi? Could it be the OPT saturating?

[Alan0354]

Is the crackling at "10" normal in the Marshall Plexi? Could it be the OPT saturating?

I don't know, it was 1973, I only have mine to play. It was brand spanking new. That was way before my days of electronics , I just never turn it to 10, kept it at 6 max.

[Alan0354]

Sorry guys, I think it's a false alarm. I was testing late last night and I was testing using an attenuator at high attenuation to get very low volume. I was going to do the recording and I got rid of the attenuator and go loud. It is not apparent anymore.

Lesson learn, you really cannot test amps at low volume using attenuator.

Even using the attenuator, going at less attenuation sounds fine.