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DC Balance

 
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Bootsypratt



Joined: 01 Dec 2006
Posts: 1236
Location: Columbia, SC

PostPosted: Tue Aug 17, 2010 11:12 am    Post subject: DC Balance Reply with quote

Let me know if I have this right. The old tube PA I have has a DC Balance pot. Is this used to get the bais read outs between the two power tubes the same? So if you have two unmatched tubes you could actually match them by adjusting the DC Balance Pot?

If so, cool.
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Bootsypratt



Joined: 01 Dec 2006
Posts: 1236
Location: Columbia, SC

PostPosted: Wed Aug 18, 2010 6:42 am    Post subject: Reply with quote

Messed around with the DC Balance last night and it did what I thought. It's really a cool feature, surprised not more newer amps have them, or maybe they do and I'm not aware.
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TresM (Sir)



Joined: 02 Aug 2009
Posts: 472
Location: South Carolina - Humidity Central

PostPosted: Wed Aug 18, 2010 7:45 am    Post subject: Reply with quote

I think thats probably another name for a 'bias pot'.
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Vince



Joined: 10 May 2007
Posts: 11429
Location: Decatur IL

PostPosted: Wed Aug 18, 2010 7:48 am    Post subject: Reply with quote

No... a bias pot sets the bias....
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Travst



Joined: 15 Jul 2008
Posts: 9535
Location: Birmingham, AL

PostPosted: Wed Aug 18, 2010 8:24 am    Post subject: Reply with quote

I'd guess the DC balance is for reduction of hum. A lot of old Ampegs had a pot that could be turned to reduce the unwanted noise.
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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Wed Aug 18, 2010 9:52 am    Post subject: Reply with quote

John has it right, it is to achieve the lowest hum from the amp.
Your amp actually has test points to set the bias.
It has 10 ohm resistors soldered to the power tube cathodes.
Something not usually seen in old amps.

With no signal input and the volume turned down, adjust each tube until you have .4v DC on each.
What this does is bias each tube to 40ma. Due to the 10 ohm resistors you measure 400ma instead.
I=E/R
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Bootsypratt



Joined: 01 Dec 2006
Posts: 1236
Location: Columbia, SC

PostPosted: Wed Aug 18, 2010 10:12 am    Post subject: Reply with quote

When I checked the bias of the tubes, they were way unmatced (20mA & 40mA). I adjusted the DC Balance and the bais readings got closer and got them matched at about 30mA. Then after they were matched I brought the bais up so each tube was at about 40mA. Is what I did correct, it made the most sense to me?
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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Wed Aug 18, 2010 12:37 pm    Post subject: Reply with quote

I would say that the 40ma is max for that amp.
You can set it colder at around 32-35ma for longer tube life.

I think you probably did it right.
I think I will revise my opinion and say that the DC balance control is to balance the bias voltage, but at the same time when tubes are balanced, there is less chance that they will hum.

So like you did, adjust both for the right voltage and balance of that voltage.

That is actually a pretty neat design.

Does the amp hum at all?
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Bootsypratt



Joined: 01 Dec 2006
Posts: 1236
Location: Columbia, SC

PostPosted: Wed Aug 18, 2010 12:49 pm    Post subject: Reply with quote

It's quiet as a mouse, even when the volume is up all the way, but the amp is not loud at all, maybe 10%. I have some new power tubes coming in today, and I'll see if that will help out, next will be changing the caps, as you already know.
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Thu Aug 19, 2010 5:25 pm    Post subject: Reply with quote

Your intuition was right. This was a common feature on old PA and hi fi amps, before "matched tubes" was even a concept.
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