5150 III Bias

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bri5150

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I have a 5150 III 120watt half stack that I bought when they first came out. The serial # is in the low 400's. Its never left the basement has maybe 100 hours on it. Last week, no sound. Had a look inside and found a few loose screws on the power tube sockets. Also found tube V1 blown. The top of the tube was white. I had a spare used 12AX7 tube and tried it. Amp worked. While I had the case open, I checked the bias. It was FACTORY set at 54! From what I've read, the first batch of heads should be around 15. And the newer one's around 30. How could this be correct? Could that be why I blew that tube? I set the bias to 30. Should I change all the tubes just to be safe? Its been a while since I've used this amp since I picked up a Carol Ann OD3r combo. But the amp sounded very harsh to my ears. Again, the CarolAnn amp is setup for Allman Brothers, SRV, Black Crowes... Not EVH so maybe I'm just not used to it. :confused:

What do you think?
 
I have your amps sister (mine is also in the low 400's). I've played mine extensively since 2007 and am very happy with it. If you bought yours new than I find it very odd the bias came out of the factory that high. Maybe that amp got through the QC somehow. Yes, its probably why your tubes blew with so low playing hours. Also could be why you thought the amp sounded harsh, though many feel the EVH is a harsh sounding amp anyway. I have found that I have to keep the highs around 9-10:00. I'm curious as to what pre and power tubes yours came with. I've also read that many people have noticed loose tube sockets. Its odd though that yours would be loose since you stated you rarely play/move it.
 
The amp came with JJ tubes. I did buy it new from Musicians Friend, I think. When I took the head out of the box I heard something loose. When I did open the head up I found a nut rolling around inside and a screw that holds the 120V cord input missing. Then I noticed a few loose screws that held the power tube sockets and one of them was out a full 1/4". As I said, this amp has never moved since day 1.

As for the bias, I set it back down to 30 and just ordered all new JJ tubes. I think when I open it again for the new tubes, I'll set it at 28 and start with that.
 
30? Are you measuring across the bias resistor in the amp? If so you are measuring all 4 tubes. It should read about 120 which is 30 ma Per tube. 54 was about what the factory use to set them at. Way to low. 30 for all 4 is just wrong.
 
When I pulled out the head and laid it upside down(tubes facing down) on the circuit board about in the middle, it says right on the board "bias + -" or something close to that. The two points are about 1 1/2" apart. I think it was right in the middle of the power tubes. Only saw this marking in one spot. Maybe its beside all 4 power tubes and I missed it?
 
That's where you measure. The value should read about 120 mA since it reads all four tubes at the same time. 4 tubes * 30 mA per tube = 120 mA
 
As stated above, the bias point is the mA reading for all four tubes. Mine was set at the factory at around 85mA. I adjusted to 112mA to bring the tubes to about 28mA each. I noticed that the amp warmed up a bit, but the feel of the amp got alot better as well.
 
Now I'm wondering if I was reading voltage. .0054 volts. Not amperage. When you test, do you set the meter to amps and just touch the bias + & - points on the bottom of the board to get the reading? Amp on standby or on? Do you need a guitar plugged in and strummed to get a real reading? Man, this is getting deep.
 
Amp needs to be plugged into a cab (or load device) and turned on. Your DMM should be set to read mV. Touch the probes to the appropriate points on that bias resistor to get the reading and adjust the trim pot according.
 
... and master should be set to "0" and the amp should be warmed up.

I guess you'll be surprised at how different the amp will sound compared 15mA to 28-30mA. It will be a lot fuller and much more alive.
 
My 100W came set at 44mA on the bias point readings. When adjusting the trim pot, the most I can get out of it is 68mA. Ideas? Is something wrong?
 
According to the factory schematic he factory setting is 80 mVDC which is 80 millivolts. What you are measuring is voltage drop across a 1 ohm precision resistor. To determine current using ohms law.
Voltage divided by resistance = current. So 80 /1= 80ma.

I am not saying the factory setting is correct or ideal.

But I have seen several posts in this thread on setting the multimeter to current or milliamps which is incorrect. It should be set to volts and or millivolts. Basically that resistor is just a bias probe for all 4 tubes.

So in this application millivolts = milliamps. Just make sure your meter is set to millivolts.
 
kgsweb":1dr6kf7l said:
My 100W came set at 44mA on the bias point readings. When adjusting the trim pot, the most I can get out of it is 68mA. Ideas? Is something wrong?

The question you need to ask, is all 4 tubes drawing current? Do you have a blown screen resistor or resistors. Also it is very possible the bias range is limited and you have a set of very cold tubes.


Most mass produced amps limit the bias adjustment to the cold range of the amp.
 
I thought all you want is 28-30mV at the bias check point (which is marked right on the board as "+ Bias -") since that point is running thru all 4 power tubes. I'm going to replace all my tubes and start at 28.


I just ordered these new tubes for my 5150 III

4 - JJ 6L6GC Power Vacuum JJ-6L6GC = Standard Tube Matching
1 - Sovtek 12AX7LPS Testing = Matched+Balanced Triodes
2 - JJ ECC83 / 12AX7 Gold Pin JJ-ECC83G Testing = Matched+Balanced Triodes
5 - JJ ECC83s / 12AX7 Preamp JJ-ECC83 Testing = Standard Test Included


Sovtek in V8 (PI)
Gold Pin's in V1 & V4
standard pin ECC83s in V2, 3, 5, 6, 7
 
bri5150":1reamy0r said:
The amp came with JJ tubes. I did buy it new from Musicians Friend, I think. When I took the head out of the box I heard something loose. When I did open the head up I found a nut rolling around inside and a screw that holds the 120V cord input missing. Then I noticed a few loose screws that held the power tube sockets and one of them was out a full 1/4". As I said, this amp has never moved since day 1.

As for the bias, I set it back down to 30 and just ordered all new JJ tubes. I think when I open it again for the new tubes, I'll set it at 28 and start with that.
Curious as to when you bought it. If you have a serial number that low you should have purchased it summer/fall of 2007. If it was way later than that I would be worried the amp may have been refurbished or sold as "new" when it may not have been. Also, after speaking with many 5150III owners, amps with serial numbers that low (< 1000) came from the factory with EH/Sovtek mix tubes. JJ's came later so it sounds like it may have been retubed before you bought it. Something doesn't seem right.
 
the factory is setting them to 30 ma on the 50 watts i retubed my head and just set it to 30 did nothing else and it sounds fine
 
mniel8195":b34hewrh said:
the factory is setting them to 30 ma on the 50 watts i retubed my head and just set it to 30 did nothing else and it sounds fine
The factory setting is wrong. According to there own schematics http://support.evhgear.com/schematics/E ... matics.pdf it should be set at:

AT RATED LINE VOLTAGE
IMPEDANCE SWITCH AT 16 OHMS
BIAS SET TO 60MV AT TP45
PRESENCE FULL CCW
RESONANCE FULL CCW
150 MV, 1 KHZ INPUT AT RETURN JACK
OUTPUT 5OW (28.3 VRMS) < 5% THD

When i checked mine it was @ 38 mv from the factory :no: I set it at 60 and it seemed to warm the amp up some and IMO sounded better.
 
bri5150":1yaui7pm said:
I thought all you want is 28-30mV at the bias check point (which is marked right on the board as "+ Bias -") since that point is running thru all 4 power tubes. I'm going to replace all my tubes and start at 28.


I just ordered these new tubes for my 5150 III

4 - JJ 6L6GC Power Vacuum JJ-6L6GC = Standard Tube Matching
1 - Sovtek 12AX7LPS Testing = Matched+Balanced Triodes
2 - JJ ECC83 / 12AX7 Gold Pin JJ-ECC83G Testing = Matched+Balanced Triodes
5 - JJ ECC83s / 12AX7 Preamp JJ-ECC83 Testing = Standard Test Included


Sovtek in V8 (PI)
Gold Pin's in V1 & V4
standard pin ECC83s in V2, 3, 5, 6, 7
Is that D25? The 100 watt schematic says to set the bias at 80 mv across D25. http://support.evhgear.com/schematics/E ... matics.pdf
 
Just want to correct one thing. I said set your DMM to mA, but I should have said set it to mV. baron55 caught my typo. I'll edit the post so it won't confuse others. No clue why I said mA when I just did this like a month or so ago.
 
Code001":32yj9240 said:
Just want to correct one thing. I said set your DMM to mA, but I should have said set it to mV. baron55 caught my typo. I'll edit the post so it won't confuse others. No clue why I said mA when I just did this like a month or so ago.


It can be confusing :)


Most mass produced amp don't even check bias! They set the bias pretty low and just thrown tubes in them, and the fact they don't really use good matched or burned in tubes.
 
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