• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

New Ampeg V4-B Tube Biasing

#21
So although the amp sounds great as it is, there is always the possibility of optimizing the sound. Some folks just have to tinker. :p

Why look at the bias?

The amp is bounced around by the shipping guy. With a bias pot, unless the adjustment shaft is glued into position (which is sometimes done) , the shipping could have knocked the bias out of its factory set position. We assume that it wasn't but you never know.

The bias will change as the tubes burn in so it doesn't hurt if you are using the amp a lot, to have the bias checked every 6 to 12 months.

If the bias does need to be set down the road, what bias voltage should be used? Checking to see what the bias was set to at the factory while the amp is still new is a good way to find out.

Finally, the factory bias may not be the optimal bias as far as the player is concerned. Some companies like to give the amp a conservative bias setting to prolong tube life. A player might want to run the bias a little hotter to give them the tone that they want. Generally dialing in an amp's tone is done by working with a tech. That way, based on their notes, they can recreate the same performance whenever the amp is serviced.
 
#22
No, its true. Its new and its fine. But, one of the ways you know its time to replace your tubes is when you can no longer dial in the bias to spec. I would like to be able to monitor that as I go before a tube fails and possibly does some damage. I play 1 to 2 hours a day, so she is getting a lot of use. Just would like the ability to keep track.
 
#23
beans-on-toast said:
So although the amp sounds great as it is, there is always the possibility of optimizing the sound. Some folks just have to tinker. :p

Why look at the bias?

The amp is bounced around by the shipping guy. With a bias pot, unless the adjustment shaft is glued into position (which is sometimes done) , the shipping could have knocked the bias out of its factory set position. We assume that it wasn't but you never know.

The bias will change as the tubes burn in so it doesn't hurt if you are using the amp a lot, to have the bias checked every 6 to 12 months.

If the bias does need to be set down the road, what bias voltage should be used? Checking to see what the bias was set to at the factory while the amp is still new is a good way to find out.

Finally, the factory bias may not be the optimal bias as far as the player is concerned. Some companies like to give the amp a conservative bias setting to prolong tube life. A player might want to run the bias a little hotter to give them the tone that they want. Generally dialing in an amp's tone is done by working with a tech. That way, based on their notes, they can recreate the same performance whenever the amp is serviced.
Click to expand...
My sentiments exactly.
 
#24
beans-on-toast said:
We were told that there is a pot inside the amp for adjusting the bias. A bias circuit with an adjustable pot looks something like the image below. See anything similar, with different values, on your schematic.

Click to expand...
Yes, this is very close to my circuit. But, there is no pot in my schematic. I have not opened her up to take a look yet. They very well may have added one thats not on the prints. I'd have to take a look under the hood.
 
#25
V-4B-Bia-Instructions_zps06996e1a.jpg


Our amazing enginner, Dawn, read this and kindly instructed me to clear this up. The image here are the bias instructions for the new V-4B. Maybe there was a mix-up when the schematics were sent, but there is certainly a bias pot. Check with our support guys to get the full schematics. They have a disclaimer you have to agree to before they will send.
 
#31
AmpegUSA said:


Our amazing enginner, Dawn, read this and kindly instructed me to clear this up. The image here are the bias instructions for the new V-4B. Maybe there was a mix-up when the schematics were sent, but there is certainly a bias pot. Check with our support guys to get the full schematics. They have a disclaimer you have to agree to before they will send.
Click to expand...
Wow! This is so great. Thank you sooooo much!
 
#35
Hello everyone. Would it be possible to have the V4-B schematics? I'm seriously in the market for one, but I would prefer an XLR DI out pre 'master' volume control. I would like to know if I could easily modify it. Thanks for any help!
 
#36
FiveG said:
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.
Click to expand...

Since the amp is from Washington, there is a little pot in each one. :laugh:

​
 
  • Like
Reactions: TheRealKong
#38
Folks, I've been using a Weber bias checker for years with dozens of my vintage amps.

Mine has the plate volt option so all you do is flick a switch and it displays BOTH cathode current draw AND plate volts.

This makes it very easy to calculate total power dissipation!

Just throwing in a random set of power tubes is very much ill advised without checking current draw immediately.

A power tube can be pulling WAY too much current without redplating and its life will diminish rapidly if not eventually damaging an amp.
 
#39
If your Ampeg V4B is around 540 plate volts the lowest current draw would be;

33ma of current draw at about 50% total power dissipation.

46ma of current draw at about 70% total power dissipation.

The plate volts will vary according to how much current is being pulled at idle, but not a lot....usually not more than 10 to 20 plate volt variation.

If you pull MORE current, the plate volts go down.

If you pull LESS current, the plate volts go up.

Keep in mind 6550 can be all over the map.....sometimes you do not have enough bias adjustment to get the 6550's in the proper range.

ALWAYS better to be cool bias than HOT.

Do not exceed 70% total power dissipation.....V4 and SVT sound fine at 60% dissipation and the tubes will last a long time.

Don't pay any attention to folks that quote Eddie Van Halen biasing his tubes very hot in the studio (which Eddie did).....his tubes were toast in no time and he blew transformers left and right in the early days.

Otoh, don't let the bias get too cool as this can cause the already murderously high plate volts in Ampegs to swing too high.

40% is too cool of bias......it will create crossover distortion and the amp will lack dynamics....plate volts will creep too high as well.

You need to be right at 55 to 60% total dissipation imo.
 
#40
Fair Warning said:
Im glad to hear all this got resolved. No bias pot would have been a deal breaker for me. Thanks Ampeg.....You will get my order this April.
Click to expand...

That's about the silliest thing I've read here. Bias adjustment is easy, with or without a pot, plus adjustment is seldom necessary once set correctly as long as you use similar tubes. Rejecting a fine piece of equipment because of a very minor service item is very short sighted.

Few years ago I refurbished a friend's original V4. It had all original bias resistors and measured right at 70% with new tubes. Honestly, anywhere between 60 and 75% wouldn't need any adjustment.
 

Latest posts

Top Bottom
Back