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Old 3rd November 2010, 09:34 PM   #1
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Default Building an B-15-inspired amp

Hello, everyone, new kid on the block here

After some tube amp reparations, fixes and minor upgrades (re-capping, fixing bias problems, various troubleshoutings etc.), I am going to build an amp from scratch!

As a model I am taking an Ampeg B-15 "portaflex" bass-guitar combo, simplifying it a bit. I will be using it just like the original - plugging the bass

Now, the original schematics is here:

Click the image to open in full size.

I am getting rid of hum control circuit, "ground lift" switch and Channel Two, adding proper 3 prong mains with true ground. I will certainly add a bias balance pot. Other than that I am keeping the schematics intact. I am not messing with the "most recorded bass amp in the history" too much...

This project will stay budget-concious so there will be no high end components or ridiculously prices NOS parts!
I am most probably going for Hammond chassis and transformers.

Be ready for me asking many questions
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Old 3rd November 2010, 11:45 PM   #2
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Now, the first one: I am planning to add a line output, when should I tap from? Just add resistive voltage divider after OT maybe?

Oh yes, and "ultra high" and "ultra low" switches and "bright" input will have to go to. Never used them on Ampeg amps
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Old 4th November 2010, 02:21 PM   #3
DF96 is offline DF96  England
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The humdinger pot will help reduce hum - do you prefer hum?

I can't see a ground lift switch, just one to swap an RF ground from one mains line to the other. If you directly ground the chassis then you may have to be careful about hum loops.

You may be able to extend the life of your valves (especially the rectifier) by removing the standby switch.

Before you add a bias balance pot it might be advisable to add a bias adjust pot, or is crossover distortion not a problem for bass guitars?
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Old 4th November 2010, 08:17 PM   #4
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Quote:
is crossover distortion not a problem for bass guitars?
Crossover is only a problem when you turn it down. You don't hear it when the volume knob is set on eleven......Oh, wait this one doesn't go to eleven.

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I can't see a ground lift switch, just one to swap an RF ground from one mains line to the other.
That is the one. Otherwise known as the shock switch or the death switch.
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Old 4th November 2010, 08:46 PM   #5
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I'm puzzled by the 'death' switch. If set wrong, it will inject mains noise and hum into the system ground, and probably some tingling. If the capacitor fails it could inject mains into a guitarist!
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Old 4th November 2010, 09:48 PM   #6
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Quote:
Originally Posted by DF96 View Post
The humdinger pot will help reduce hum - do you prefer hum?
Properly designed and assembled amp doesn't hum by itself This was supposed to compensate for hum from 6.3 heaters I guess, and by experience, using the 6.3 secondary winding with central tap eliminates pretty much all the hum...


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Originally Posted by DF96 View Post
You may be able to extend the life of your valves (especially the rectifier) by removing the standby switch.

Before you add a bias balance pot it might be advisable to add a bias adjust pot, or is crossover distortion not a problem for bass guitars?
Yes, I don't see the point in having standby switch here anyway.
Good idea for bias adjust pot, will add it too.
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Old 5th November 2010, 02:41 AM   #7
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If set wrong, it will inject mains noise and hum into the system ground, and probably some tingling. If the capacitor fails it could inject mains into a guitarist!
At least one death has been attributed to a bad cap. With the switch in the wrong position the "tingling" turns into a temporary loss of vocabulary control, when the sweaty guitar player touches his guitar and his lips touch the grounded microphone. about 4000 people heard some rude stuff at an outdoor concert in the Florida summer.

This cap and switch was common in most amps up through the 60's.
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Old 5th November 2010, 10:57 AM   #8
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A few months ago I began to realise that many guitar amps contain design and construction errors (such as poor layout, tone control via huge grid stoppers). Now I have learnt that they are dangerous too. Why is this? Were the original guitar amps designed by people who didn't understand electronics as well as they thought they did, then everyone just copied them for the next sixty years?
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Old 5th November 2010, 05:25 PM   #9
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Not only guitar amps. There were tons of the radios and record player with "universal" transformerless power supplies, and they were even more dangerous.
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Old 5th November 2010, 07:40 PM   #10
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But at least with them the user was usually several feet away out of contact during normal operation, and no external metal was allowed.
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