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MikeMcK
12-09-2007, 11:04 AM
Call me a wuss, but I’m not comfortable putting scope probes on the output from some of my amp heads, but I do want to do my own biasing.

So suppose I have an 8-ohm power resistor as a load. And suppose I put that in parallel with 2 resistors in series… say 1k and 10k. Then I put the scope across the 1k resistor.

Will the shape of the waveform be true enough so that it can be used to check for crossover distortion?

Rosewood
12-09-2007, 11:17 AM
I don't use the scope method for biasing but when I'm checking the output waveform I just use the probe on the load resistor, never had a problem. Not sure I understand the reason for the resistors.

VaughnC
12-09-2007, 11:38 AM
I think scoping a guitar amp for biasing is overkill because it's the inherent non-linearities of a guitar amp that makes them sound better than a 100% transparent amp. All you really need is a bias meter, calculate the max safe plate current/tube, and your ears. Play the amp and adjust the bias until the amp sounds the best while keeping the current under your calculated safe value. IMO, the name of the game is what your ears hear, not what a scope sees ;).

MikeMcK
12-09-2007, 12:15 PM
I think scoping a guitar amp for biasing is overkill because it's the inherent non-linearities of a guitar amp that makes them sound better

I understand your point, I'm just thinking about minimizing crossover distortion to get to a starting point. Thanks for the response.

MikeMcK
12-09-2007, 12:17 PM
I don't use the scope method for biasing but when I'm checking the output waveform I just use the probe on the load resistor, never had a problem. Not sure I understand the reason for the resistors.

It's probably more cautious than I need to be, but the idea is to only be messing with a low-power signal. I've fried probes before, and maybe that's why I'm being conservative.

tele_player
12-09-2007, 02:14 PM
It doesn't sound cautious, as much as unfamiliar with your tools.

Chris Scott
12-09-2007, 09:17 PM
Although your concern to get things right is laudable, the way you are going about it is a waste of time, IMHO.

Were you working on some HI-FI gear, it would be a different story altogether. But these are guitar amps.

Find the plate dissipation figures for the tubes you a using, measure the plate voltage of your amp. Set the bias where it sounds the best to YOU within the safe PD range relative to the PV, lock it down and go back to playin' your guitar.

..or bass.
..or accordion.

-just my .02:rolleyes:

antik
12-09-2007, 10:12 PM
Looking at crossover distortion in the output is not a very accurate way to set bias. It is a bit hard to tell the point when most of the crossover distortion goes away. Magnifying the waveform in the areas of interest helps a little, but this can be difficult to do with some scopes. Also the crossover distortion is present at 2 points in the sine wave. It often looks different between the 2 points.

While not too accurate the method can tell you if the bias is in the ballpark. The downside is you can not tell with any accuracy how close the operating point is to the maximum power rating of the tubes.

aleclee
12-09-2007, 10:30 PM
IMO, the name of the game is what your ears hear, not what a scope sees ;).Word. There's no guarantee that the point where crossover distortion goes away is where the amp makes your ears happiest.

vibroverbus
12-10-2007, 08:25 AM
Put probe right on your dummy load - it's absolutely not high voltage, no way you're going to have trouble with that. It shouldn't even be a problem to probe the HV either. I don't think there's anything wrong with finding out where visible/detectable cross-over goes away just as a reference point. That's not the ultimate determiner for me, but it certainly is an interesting datapoint to have if nothing else, not sure why it makes other people cranky that you want to see it, but there's the internet for you...

Lonely Raven
12-10-2007, 12:02 PM
It doesn't sound cautious, as much as unfamiliar with your tools.

While accurate, your snotty elitist digs quickly get tiring.

MikeMcK
12-10-2007, 08:08 PM
Well, thanks for all the responses. I'm not all that worried about plate current... I do the 'measure R from plate to tap, then measure the drop and divide' thing for that. It's just that I got a decent scope cheap and got curious. It'd be nice to be able to relate a certain sound with something definable.

Thanks for the standing up too, but don't worry about me. I'm not particularly thin-skinned, especially on the Internet. Hey, sometimes there's useful information in a post someone dropped in a bad mood. Thanks again, though.

JJman
12-10-2007, 08:15 PM
I will also be looking for crossover distortion using my newly acquired scope. Just for curiosity. I was initially afraid to scope the power tube plates because of the high DC. There’s no need to fear scoping a dummy load since there’s no DC and the AC is not colossal.

The dummy needs to be a real dummy and not a mere “power resistor” (i.e. rated for high enough wattage.)