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Old 11th June 2012, 06:59 PM   #1
p0ultry is offline p0ultry  United States
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Default 5150 II tube head with hot bias

Hello everybody. I'm new to the forum, so thanks in advance if anyone can offer some help with this question.

I am approaching the need to re-tube one of my amp heads. The amp is a 5150 II biased to 37 mA in the poweramp section. When I change the tubes, will I have to re-bias or does this not change from tube to tube based on how much that specific tube is drawing?

Thanks again!
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Old 11th June 2012, 07:12 PM   #2
7N7 is offline 7N7  United Kingdom
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Quote:
Originally Posted by p0ultry View Post
Hello everybody. I'm new to the forum, so thanks in advance if anyone can offer some help with this question.

I am approaching the need to re-tube one of my amp heads. The amp is a 5150 II biased to 37 mA in the poweramp section. When I change the tubes, will I have to re-bias or does this not change from tube to tube based on how much that specific tube is drawing?

Thanks again!
First does the amplifier have fixed or cathode bias?
Second, what type are the output valves?

Paul
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Old 11th June 2012, 07:25 PM   #3
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What or who is 5150 II ?
My point is that not everybody here is a bass amp tech. A schematic would help greatly, too

Last edited by costis_n; 11th June 2012 at 07:25 PM. Reason: typo
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Old 11th June 2012, 07:30 PM   #4
p0ultry is offline p0ultry  United States
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It is a fixed bias of 37 mA with 6L6 output tubes. Thanks for getting back to me so quickly!
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Old 11th June 2012, 07:32 PM   #5
p0ultry is offline p0ultry  United States
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Quote:
Originally Posted by costis_n View Post
What or who is 5150 II ?
My point is that not everybody here is a bass amp tech. A schematic would help greatly, too
No problem. It is a Peavey 5150 II 120w guitar amp head. I'll try to dig up a schematic from Peavey.
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Old 11th June 2012, 07:40 PM   #6
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A 5150 is a common Peavy guitar tube amp. Aka 6505 Plus.

You should be able to find the manual online, see what Peavy recommends.

You will have to check your new tubes too be certain, after you put them in. That is the point which you will adjust between performance vs tube life.

Your new tubes might sound better a little below or above 37mA depending on what you get. Experiment a little to taste.

Modern guitar store tubes may not like to be biased hot whereas some vintage NOS tubes might let you run a little hotter without too much compromise to longevity.
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Old 11th June 2012, 07:41 PM   #7
p0ultry is offline p0ultry  United States
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Originally Posted by costis_n View Post
What or who is 5150 II ?
My point is that not everybody here is a bass amp tech. A schematic would help greatly, too
http://www.blueguitar.org/new/schem/peavey/5150evh.pdf

the power amp section is page 3 I believe.
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Old 11th June 2012, 07:52 PM   #8
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Isn't that the schematic for the older fixed bias model?

Mind you I'm too lazy too trace it out.
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Old 11th June 2012, 07:54 PM   #9
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After killing my eyes to trace it out, it seems that this amp is fixed bias, shared by all output tubes, and not adjustable.

Also they have a fuse in the bias supply !
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Old 11th June 2012, 08:02 PM   #10
p0ultry is offline p0ultry  United States
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Originally Posted by costis_n View Post
After killing my eyes to trace it out, it seems that this amp is fixed bias, shared by all output tubes, and not adjustable.

Also they have a fuse in the bias supply !
Right, but like I said it is modded to allow adjustment on the bias as they come biased very cold stock. This was done by an amp tech of the previous owner, not by myself, so unfortunately I dont have those specific schematics. But in general, if my tubes are a fixed bias of 37mA, and I used the same tubes to replace the 6L6's currently in there, would I have to adjust the bias again when the new tubes are installed? Its basically the difference between taking it in to have someone pop new tubes in for me at substantial cost, or just removing them and replacing them myself?
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