Correct Voltage to bias my Ampeg VT120

[skot10]

Hello,
I have a Ampeg VT120 Head and on the back there are points to put in a multimeter to set the bias (via a bias adj screw). I see on the schematics that it says to set the bias at 0.1 volts. Now I assume that is DC but since there are 4 power tubes I want to know, is this an average? It then goes on to state a setting of 0.02 volts. Which one is right and what type of bias reading is this giving me?

[mikeboone]

Measure the voltage at the junction of R9 & R10. It should be around -46 according to the schematic. This voltage should be referenced to ground. That should put you in the ball park.

The .1VDC +/- .02 that is referenced is a voltage difference between test points TP1 & TP2. So your range will be .08 to .12 VDC.

In my experience, if you are changing out your tubes with the same make & model AND the are a Matched Quad Set, you shouldn't need to make a bias adjustment unless you have reason to believe is is out of tolerance. Your ear can be used to tell if they are biased too hot as you will hear a hiss.

I would check for the -46VDC as mentioned above. Then check voltage between TP1 & TP2 for the .1VDC +/- .02.

Setting bias current is a somewhat involved process that can be done in a few different ways and there are a variety of opinions on which way is best.

Don't take my word as gospel, only as a suggestion. Wait a while to see what some of these more expereinced techs and engineers have to say.

Read this article by Randall Smith at Mesa Boogie. Its pretty interesting, maybe a bit over-simplified, but interesting info just the same.

http://mesaboogie.com/US/Smith/biasadjust.html

[52 Bill]

If you check the schematic, the test points are probably attached to a small value resistor that is in ground path of the output tube cathodes. By reading the voltage across this resistor, you are actually reading the current that the output tubes are drawing at idle. If all 4 tubes are grounded thru this resistor, then the current figure will be for all 4 tubes. Ideally if it's a matched set, each tube will draw an equal amount of current.

[skot10]

Okay,
So are you guys saying that if I set the voltage to between .08VDC and .12 VDC that I have the tubes properl biased? Or is this more of a fine tune thing after setting it somewhere else?

[skot10]

Also,
When I bought the amp it was shipped to me and when I fisrt played it it sounded like crap (no overdrive on the A (overdrive) channel). I replaced the preamp tubes with groovetube SHG-MHG tubes (high gain ecc83's and a both sides matched phase inverter tube) which made an incredible difference. Now before you say that 1 or more of the preamp tubes were probably weak or shot it should be known that I first replaced them all with new sovtek 12AX7's and while it helped it still wasn't quite right. Anyhoo, the reason I am even asking about the bias at all is because when using the preamp out into another amp as a slave it sounds great but through the amps own poweramp section (in otherwords with the speaker hooked up to the head) there is a weird distortion that by what I've read seems like an output tube distortion. It may need new powertubes but I figured I'd make sure the bias was correct first before shelling out the cash for a matched quad set.
One more thing, I wonder if I would be able (without too much modification) be able to replace the 6L6's with EL34's.
Thanks
Skot

[52 Bill]

The "weird distortion" could be caused by any number of things including bad output tubes or output tubes that are biased too cold.

Have you set the bias yet? What voltage did you set it to?

Replacing the 6L6's with EL34's can be done, how easily depends on the original circuit. In most cases, the screen grid resistors will need to be changed, and maybe a simple re-wire of the tube sockets. You will also need to see if the bias voltage will adjust to a level that is ok with the EL34's.

[skot10]

Hey,
I set it (at the point we talked about earlyier) to 0.1VDC. The manual I have for the amp says you can set the bias using the screw on the back by hooking a multimeter to those two previously discussed points.
If it's wrong then tell me what to set my meter at (VDC, VAC,etc) including the value (2, 200, 200ma etc) and the end value.
The distortion just kinda sounds wierd and as I stated before it doesnt do it when I use the preamp out and use another amp as the power amp. Also it stays constant even if I lower the preamp gain.
Thanks,
Skot

[r3gularJuan]

I just bought one of these baby's...... whoa momma!! best $400 I've ever spent on an amp..... tube at that!!! I sold my line 6 spider 3 to get this.... bonus!!!

Mine had a loud hum also.. but I bypassed the speaker (original celestion sidewinder) plugged straight into an ampeg v-412 via 8ohm @ 60watts and that eliminated the hum drastically. I plan on taking into a shop to figure exactly whats going on. in the mean time try using an external cab.


I have an owners manual, let me know if you need a copy.