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New Ampeg V4-B Tube Biasing

FiveG

FiveG

Mar 1, 2013
45
8
Vermont
#1
Question for the group...........
I have a new Ampeg V4-B. I love everything about this amp so far except one thing. The manual says every time I want to change the power tubes I have to take it to an Ampeg authorized service guy to do it. Now I know the old V4-B's had fixed non-adjustable biasing. So I assume the new ones must have a couple of bias pots in there for the service guy to adjust. I'm perfectly capable of biasing my own tubes with a meter. What I need to know is where the test points are, what mV reading I am looking for, and the easiest way to get to the pots.
This is all assuming the bias is adjustable at all, and according to the manual it is. I can't find anything on line about the new V4-B, only the old ones. Anyone have a clue about this?
I would appreciate being enlightened.
Thanks.
 
#4
They're likely internal. Should be fairly obvious if they're there or not when/if you open the amp. Ampeg shared a few pictures of the insides of the amp but none in high enough resolution to get a good sense of what's going on in terms of bias pots.
 
#5
The adjustment pot for the bias is internal. And I'm pretty sure the procedure is straightforward and you could probably learn how to do it with a multimeter. But honestly, I question the need for it every single time you change power tubes. I've been known to swap out power tubes a bit indiscriminately at times, but I figure as long as they're matching and the amp still sounds good, eh...close enough ;)
 
#6
Loud doesn't want to encourage anybody to poke around inside their V4B. For liability reasons and because they don't want people mucking up the amp while it is under warranty, they tell you to have the amp serviced by an authorized service center. This isn't unreasonable.

Since your amp is new, there's a simple way to determine what the factory bias voltage setting is. Have a tech open the amp and take a DC voltage reading of the bias voltage with the mains voltage set at 120 VAC.
 
#7
Well thanks for the replies.
I will call Ampeg for a schematic (they dont post them on line apparently) and ask a tech about the procedure. I bias my home audio tubes all the time. I'm tech savvy enough. I built a couple of mono blocks from scratch last year and managed not to blow myself up. Of course those amps have external test points and pots. I understand Loud's not wanting anyone inside their boxes. I was just hoping someone out there in Talkbass land had already opened one up and done it. But I'm not afraid to go where no man has gone before. Warrantee or not.

I've only had the amp about two weeks now but so far its been really nice. Long story......I bought a Fender 100T and returned it because of an issue with the self biasing system and replaced it with the V4-B. The V4-B is lighter, less boomy, and more natural sounding (sorry Fender heads) IMHO. I really like being able to pick the center point of the mid-boost/cut too. You can really dial in some amazing tone with this feature. I also like the simplicity of the controls. KISS-keep it simple stupid.
 
#8
I hear ya...my V4B just became my go-to amp for most of my gigs. Don't know much about the Fender, though it looks like a nice amp, but I prefer simplicity as well.
 
#9
FiveG said:
I will call Ampeg for a schematic (they dont post them on line apparently) and ask a tech about the procedure. I bias my home audio tubes all the time. I'm tech savvy enough. I built a couple of mono blocks from scratch last year and managed not to blow myself up. Of course those amps have external test points and pots. I understand Loud's not wanting anyone inside their boxes. I was just hoping someone out there in Talkbass land had already opened one up and done it. But I'm not afraid to go where no man has gone before. Warrantee or not.
Click to expand...

I understand what you are saying. Setting the bias, if you know how to do it, is not difficult and will allow you to save on costs. A sympathetic tech can show you how to perform the procedure and how to optimally adjust the bias. It is possible that you might prefer the tone of the amp with the bias set at a voltage that is different than what the factory sets it at. The bias voltage is not set in stone. It can be anywhere within a range. You want to ensure that the bias selected is within the operating parameters of the tubes as specified in the tube data sheets. The tech can work with you on this to ensure that it is done properly.

When you contact Loud and request a schematic, they normally send you an email outlining their terms and conditions. You have to agree to this and supply your serial number. If they can determine that a non authorized person has serviced the amp, your warranty will be void.
 
#10
So I got a schematic from Ampeg. It looks like the power tube circuit has a fixed bias governed by one 47 ohm resistor for each tube. No little easy adjustment pots in here.
Adjusting the bias would entail experimenting by replacing the resistor with a little lower or higher one. CRAP! No wonder they want you to bring it to a service center. It stands to reason that if you only replaced the tubes with exact replacements that the bias would stay the same. Either that or if ALL tubes are slightly different, then Ampeg is choosing the bias resistor for each build. But I dont think so. That would take way too much time in production. Also, there are no measurement shunts so you have to do all of your measurements at around 540V DC. No room for messing up there.
I'm sure there is a trick. So if there are any tube heads out there that would like to share it I'm all ears.
 
#11
If there's no adjustment then that means you need to make sure the tubes you put in there are operating at the correct voltage spec and go unless you wanted to be swapping resistors. Or you can add an adjustment control.
 
#13
Google "Bias King" for easy measurement. Beyond that the old V4-B was the same way, you had to swap resistors. I used a 5 watt pot sub'ed in and when I had the proper bias I would measure the value at the pot for the resistance needed.

But techs are overpaid crooks right? I really don't know why I'm sharing this stuff......
 
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#15
B-string said:
Google "Bias King" for easy measurement. Beyond that the old V4-B was the same way, you had to swap resistors. I used a 5 watt pot sub'ed in and when I had the proper bias I would measure the value at the pot for the resistance needed.

But techs are overpaid crooks right? I really don't know why I'm sharing this stuff......
Click to expand...
Oh no, techs are really nice helpful people that find joy in helping others. That being said, is the master bias resistor R49? That one looks to feed all the others. Mine is 75K. Maybe a 50K-100K pot in place of the 75K is the magic trick.
 
#16
BassmanPaul said:
I'm sorry but your post is making little sense to me. 47Ω resistors in the bias circuit? I'm beginning to feel that you may be reading the schematic incorrectly. I fear for your safety.
Click to expand...
Yeah, you are right. on closer inspection the 47K's are in line with two 100K's split and fed by a 75K. So I assume the 75K is the master control of all 4.
 
#18
FiveG said:
Yeah, you are right. on closer inspection the 47K's are in line with two 100K's split and fed by a 75K. So I assume the 75K is the master control of all 4.
Click to expand...

Thing is, the the bias circuit is a simple scheme. If you understand what it does and how it works, no assumptions should be necessary. If you get it wrong and kill the bias you could well damage the amp permanently. As the amp is new why not just play the thing for a couple of years and leave it as is.
 

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