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Old 10th February 2007, 12:32 AM   #1
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Default Amplifier heat issue

I have a Crate head and have done a lot of mods to it inside and out.
Anyway on the preamp board, there is a trim pot that if I crank clockwise, the amp gets BURNING hot. So hot with this cranked all the way to the right that I'm sure it could start a fire.

Can anyone explain what this trim pot is likely doing?

I'm pretty sure I noticed a bit of a volume increase when I had the amp opened up and was tinkering around and cranked it up but didn't experiment enough with it wide open to be certain.
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Old 10th February 2007, 12:49 AM   #2
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Do you get the same issue regardless of the music you're playing ?

ie. I had an overheat problem that only occurred playing "Black Magic Woman" from a cheap 'best of' recording 5 or so times in a row and the consensus was the level of distortion Santana was using combined with the lousy recording and my Gale 401's being a difficult load caused the amp to overheat.

Graeme
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Old 10th February 2007, 12:49 AM   #3
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It should be a BIAS adjust pot.
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Old 10th February 2007, 01:06 AM   #4
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a bias adjust pot?
What would it be biasing?

Graeme it has nothing to do with what I play through it. This is a solid state, I believe push pull power amp stage, guitar amp. If I crank this trim pot all the way clockwise, it just gets burning hot as long as it's on, even if the volume is at 0.
I'm pretty sure I noticed a volume increase when I cranked it open. I had the amp apart to add some finishing touches on circuits I've added and to change up some of the op amps. I cranked it wide open and seemed to notice a bit more fullness and volume while I played to decide which op amps I wanted to switch to so I left it that way until I started experiencing the heat issue.
I've done this in the past when tinkering around inside and forgot that this is what happens from turning it all the way to the right.

I'm mostly curious to know exactly what it's doing and if there may be some benefit of cranking it up a bit. I have it turned completely counter clockwise right now and there is no more heat. It has a lot of range so if there is any benefit to opening it up a bit, I'll do so a tiny bit.
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Old 10th February 2007, 01:09 AM   #5
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Not to put too fine a point on it, but do you also pull the trigger of every gun you pick up?
Yes, indeed, it is almost certainly the bias pot. Bias is not something you want to play with lightly, at least not in a situation like a guitar amp where there's minimal heatsinking and power supply.
My suggestion is this: Get a service manual for that amp. Find out what the bias is supposed to be and set it properly. Then leave it alone, at least until you have some idea of where you're going with it.
Home stereo gear is one thing. If it pops, you're out of music, but nobody cares but you. Road gear is another matter entirely.
That amp is almost certainly class B. If you start biasing it class A or even AB (hotter heat sinks) you're likely to destroy it.

Grey
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Old 10th February 2007, 01:28 AM   #6
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I'm no expert but I thought biasing was in relation to tubes, this isn't a tube amp.
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Old 10th February 2007, 01:32 AM   #7
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All amps are biased in one way or another, regardless of gain device.

Grey
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Old 10th February 2007, 01:55 AM   #8
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I know roughly how it was set before I fiddled with it. Unfortunately, Crate isn't very reliable when it comes to tech questions. Years ago I asked them which capacitor I needed to change to get a different midrange band on the mid control and they dead walled me making one excuse after another. I finally asked what the value of the capacitor used in the midrange control and they wouldn't even tell me that, bunch of jerks they are.

I've since modified the amp out the wahzoo and due to the location of this trim pot, suspected it worked in relation to the distortion stage or it's active 3 band EQ. After cranking it almost all the way counter clockwise, which I believe is about where it was set before fiddling with it, the heat issue is solved. The chassis itself serves as the heat sink but it never really got more than slightly warm before this tinkering. The power transformer's size and quality is pretty impressive though and it's likely overkill for this units needs.

I tired hunting down the service manual a few years ago but ended up on a wild goose chase. I was only able to track down the schematic for $15 but by then I had done a lot of circuit modification and no longer cared much about changing the midband capacitor. I emailed them and hopefully they will send me the service manual but I won't hold my breath.
I don't think much of the people at St. Louis Amplification anymore.

This amp will likely never see road action, I'm building one of my own design from the ground up as it is. If I ever do desire it's tone for the road, I'll just buy the newer version of the same model, give it a couple of the mods this one has and I'll have my patented tone. I suspect the one I'm currently designing/building will become my main rig anyway and if I'm that pleased with it, I'll have a few boards printed up so I'll have at least a pair that are road worthy.
Thanks for the input man.
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Old 10th February 2007, 02:30 AM   #9
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Crate is a business. Telling random phone callers the values of capacitors in their tone controls falls under the heading of intellectual property. I'm not surprised that they didn't want to talk about it. Blaming them for this simple reality reflects poorly on you.
If they're using the chassis as a heatsink, it only reinforces what I said earlier--there's not much leeway. I strongly recommend that you call them back and ask politely if they will give you the bias setting. No, I'm not kidding. I've been playing bass for thirty-five years and doing electronics at some level or another for even longer. I have a hard and fast rule: I will play all day with stereo gear. I never rearrange my bass rig. Period. If I want a different sound, I build an external circuit. Taking this to its logical conclusion, I long ago began building my own instruments--not kit crap, but a fully equipped woodworking shop. I wind my own pickups and design my own active electronics. But I leave the rig strictly alone.
Your feeling that the power transformer is overkill doesn't quite reassure me. There's also the question of whether the rectifiers can handle the added current, the possibility of hum on the rails due to inadequate filtering, the possibility that the power supply caps might fail due to ripple current beyond their rated capacity (power supply caps in stage gear are very, very small compared to stereo caps)...etc.
You want the most likely failure mode? That you'll blow the output devices by exceeding their SOA.
Don't guess.
Know.
Find out what the bias is supposed to be.
Set it.
Leave it alone.
For your own sake, and that of your amplifier, please learn some electronics before you go twisting pots again. I doubt that your amp design will be successful if you are unfamiliar with the concept of bias.
You can ask questions here, you can attempt to find a decent tech and see if he will teach you, you can buy books; The Art of Electronics by Horowitz and Hill is a good starter book...
But please...pretty please...don't touch anything else once you get that bias reset. Not until you get the fundamentals in place.

Grey
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Old 10th February 2007, 02:38 AM   #10
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Quote:
I'm building one of my own design from the ground up as it is.
I strongly suggest you read up on transistor basics on proper biasing. This may save you a lot of burnt up transistors.
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