Crate BV120H Output Waveform

[TomCarlos]

I am testing out a Crate BV120H. Specs call for 100W @ 5% THD. So I figure I should be getting somewhere around 85 watts before clipping.

I am looking at the waveform just before clipping. I have a Triplett Voltmeter (not a True RMS) and that is reading 28 Vac at the output. 28 squared divided by 8 ohms = 98 watts. Take a look at the attached photo. The sine wave at the top looks right. The wave at the bottom looks odd. Any ideas on what might be causing this?

I checked the Function Generator straight into the scope. The signal looks ok.

[Enzo]

Yes, it is a guitar amp, not a hifi.


If you want to know just where in the amp that happens, just trace the signal stage by stage and see.

[g1]

Are you going in to the FX return? That's the best place if you want to see clean output. Otherwise, you will need to play around with the tone controls and level controls to get clean output.

[TomCarlos]

I am going to follow the signal from the very beginning stages. Good exercise in learning.

This amp may have a problem with Channel 2. It seems like I don't have the same volume coming out of the dirty channel as opposed to the clean.

[km6xz]

The waveform you posted looks like current limiting in the output section so check the 6L6's. A problem to look out for on that amp is dead heaters, are they are all hot? If known good tubes are not getting hot, check the octal socket pc board that connects to the main pc board with light weight header pins and sockets on either side, there are two of them. Any looseness or corrosion on the pins, the pin receivers or tube socket receivers on pins 4 and 5 and you will lose heater to 1 or more tubes.
I have seen that as a weakness in the design that causes problems repeatedly.
If they are all getting hot and the waveform looks like that, look for an open 5 watt 560 ohm screen resistor (R205-208) or just bad tube. If a screen resistor went, it is probably a bad tube it is connected to.

[TomCarlos]

KM -

I think it was the biasing... that is now set properly. This amp is rated at 100 watts RMS @ 5%. I get 80 watts rms before clipping. I believe that is "all she will do" before breakup. I am right in the ballpark.

One thing I noticed.... The Clean channel volume is really loud when the volume knob is at 12 o'clock. The distortion channel seems to work differently. I guess it is because of the combination of the Volume and Gain controls- you just can't compare one 12 o'clock to another. Both channels run through the same amp... so it's gotta be as right.

[Enzo]

Knob numbers are only there for positional reference for the player. They are not any sort of calibrated indicator. Imagine the knobs were plain and you just turned them until you liked the sound.

[TomCarlos]

Point well taken. Thanks.