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Ampeg V4 bias question

denton57

denton57

Nov 1, 2005
3,967
2,991
Arkansas
#1
I have a 74 VT22 (v4) and a 72 V4B. I just bought a bias probe from Eurotubes because I wanted to make sure the fixed bias current was in spec before putting in some NOS Sylvania 7027's in.
I ran home at lunch, and hooked it up to both amps. I set the meter to 200ma DCA as instructed, and here's the results:

VT22 - 23.5

V4B - 29.8

I know very little about this stuff and since they're basically the same amp, I assumed they both would read around the same.

What do these numbers mean? Is one out of spec? They both fluctuated at first, with the VT22 heading down and stabilizing in the mid 23 range. The V4B went down, then climbed back up to the 29's.

Experts, help me out!
 
#3
I don't know my plate voltage but I read online that it is around 540.

Using 540 and 25 watts for the tube, I guess the v4B is running at 64%, and the VT22 is running at 51%.

How cold are they supposed to run? That's around a 20% difference between the two amps.
 
#4
It is important to let the amp warm up in playing mode (standby off) for at least half an hour, before checking the bias. This will allow all the components to reach a steady state and the readings will be consistent. The number you are reading with the probe represents the cathode current through the tube. The bias setting will affect the cathode current. On these amps, the bias is set by changing a resistor, R49, 75K ohm, in the early V4B. Some amps are modified to add a bias pot which makes setting the bias easier. Adjusting the resistor set the bias for all four of the power tubes. The bias setting needs to keep the tubes within their save operating region and should be optimized so that the amp sounds its best.

You want to attach the bias probe to each of the four 7027A tubes intern, and check the cathode currents. With a matched tube set, the cathode currents should ideally be the same. They never are so you want them close.

So what cathode current do you want? At lot of people like to set their V4B's at around 60-65% plate dissipation. You should measure your plate voltage, not rely on what is on the schematic just to be sure. The wall voltage as well as other things, will affect the plate voltage and the bias. Based on your measured plate voltage, you can use the formula to calculate how many mA per tube you should have for a plate dissipation in the range of 60-65%.

The important question is, which amp sounds the best given their different bias settings? I would put the same set of tubes in each mp when you take the bias readings. There are a lot of factors that will affect how an amp will sound. The same tubes will help level the playing field.
 
#5
Thanks for the detailed explanation. I really just want it to be a stock setting without changing the resistor. I mainly bought the probe to see if they both were operating in a normal range.


So you suggest running the amp off standby for 30 minutes, then taking the reading for a tube, then repeating for each tube. Will start that tonight. With a fixed bias, shouldn't they all have the same current value?

I bought the cheap bias probe, so I am unsure on how to read the plate voltage.
 
#8
Yes, if you want you can leave the probe in one socket and swap tubes. Or you can move the probe to each power tube position and test each tube in the socket that it will be in. All the power tubes need to be installed when you do this test. You can't run the amp with just one tube in it.

When you use the bias probe on one tube, you are measuring the cathode current through just that tube. A typical current for the 60-65% range is around 34mA but this will vary depending on your plate voltage. Depending on how well your tubes are matched, the other tubes will draw a little more or a little less. Expect that the cathode current is going to be a bit different for each tube.

Please only do this if you feel comfortable taking the measurement. Be very careful doing this, the voltages are dangerous.

To measure the plate voltage, you want to measure from pin-3 on the power tube socket to a ground point. Rather than taking the reading at the tube socket, there are four plate resistors that you can see from the top of the amp chassis if you remove the large metal cover, R39, R46, and R40, R45. They are 5W cement block resistors marked PR1. Each of these resistors is connected to pin-3 of a power tube. You can measure the plate voltage on the lead that is closest to the edge of the board because that connects to pin-3 on the sockets. See below. For a ground point, there is a single can capacitor next to the output transformer, you can use the case (NOT the two that are together).

The 30 minutes is only at the beginning, you don't need to wait 30 minutes each time. But when the amp is hot and you plug in a new tube, wait a couple of minutes. The reading are taken with the amp at idle in playing mode, no instrument plugged in. I like to turn off any boosts and set the tones at 12 o'clock just to be consistent. Don't burn yourself on a hot tube. An oven mitt helps.

 
#11
I put the Sylvanias in the VT22 and plugged in my Les Paul Standard. Sounded fantastic for about 5 minutes... Then started getting heavy crackling noises :(. I moved the tubes to the V4B and put the JJ's in, and as soon as it gets flipped out of standby, it goes into heavy crackling. The Sylvanias are working fine in the V4B.

I guess something failed in the VT-22. Sucks, but at least the Sylvanias are OK. Back to the shop :(
 
#12
There are a number of possibilities for the crackling. Since it only started when you changed the tubes, one possibility is that the tube sockets have stretched out and need to be retensioned. Tubes can have different sized diameter pins. If you put a tube set into an amp with larger pins, the tube socket contacts can be bent open a small amount. If you install another set of tubes with smaller diameter pins, they don't fit as snugly into the tube socket contacts. The result can be arcing between the tube socket contact and the tube pin. The solution is to use a small needle, jewelers screwdriver, or something similar to carefully close the tube socket contacts. You don't want to bend them too much or bend them out of round. This might fix the problem. As I said, there are other possibilities.

You can compare the JJ, Sylvania, and bias tester pins and compare them for size to see. Also, if you are inserting the tubes and they feel solid going into the socket, the tension might be fine. If they feel loose in any way, the tension might be the problem.

If you take the amp in, it would help the tech to tell them what you did so he can check out the sockets.
 
#13
Sounds good. It did sound fine for about 10-15 min before the crackle happened. I figured it was the cord, so I switched it in standby and grabbed another. Flipped it off standby and looked at the tubes.. It crackled immediately and all 4 power tubes looked like power was
being turned on and then off. Like dim to bright and very quickly. All of them. Very strange. Thankful the Sylvanias are acting and sounding normal in the V4B.
 
#14
Something is obviously loose making a bad connection.

If all the tubes are going on and off, maybe your power cord has an intermittent connection. If you bend the spades of the plug out it bit it might help to make a better contact with the wall socket. Often this happens at the plug. If a wire in the cable is bad at the plug, holding the wire straight at the plug might help you see if that's the problem. Try the same thing where the power cable enters the amp.
 
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#15
The power cord is the longest cord I've ever seen on an amp. It's 3 prong (so it's a replacement), but it has to be at least 20 ft long. It looks old, so hopefully it's the issue.


What's odd to me is that all was fine for 10-15 min.. sounded great actually.. and then the popping and crackling. There was a storm that blew through while I was playing, so maybe there was a surge or something that blew something internally? I would think the fuse would blow if that happened, but who knows.

Sucks that I have to drag this 90lb beast to the shop AGAIN. I guess while it's there, I can have the bias resistor replaced.
 
#16
denton57 said:
The power cord is the longest cord I've ever seen on an amp. It's 3 prong (so it's a replacement), but it has to be at least 20 ft long. It looks old, so hopefully it's the issue.

What's odd to me is that all was fine for 10-15 min.. sounded great actually.. and then the popping and crackling. There was a storm that blew through while I was playing, so maybe there was a surge or something that blew something internally? I would think the fuse would blow if that happened, but who knows.

I guess while it's there, I can have the bias resistor replaced.
Click to expand...

With such a long cord, it must be a very manly amp. :laugh:

There is some latitude in the bias setting. As I mentioned, some people bias these amps at 60-65% of maximum plate dissipation, others will bias it hotter which will mean shorter tube life. For bass, I like the 60-65% level. For a guitar if you want distortion, hotter might be better depending on you taste. Because there are different levels that you can set the bias at, it is better if you can be there when the tech is selecting the level so it can be adjusted the way that you hear the amp playing best. You might find it interesting to see what he does on the amp.

Sometimes an intermittent issue will arise only when the amp is hot. That might account for the 10-15 min where it worked well.

No question that a surge in the power can affect things in the amp. Voltage spikes are not good. Newer components can withstand pulses easier than older ones. Best to mention the storm to your tech.

Let us know what happens when you get it serviced.
 
#17
Will do, and thanks Beans. It is definitely a manly amp :)

I mainly want it to produce thick and glassy cleans with a little breakup.. So a factory bias probably would work for me. It has the tone I'm looking for with my humbuckers (Les Paul Standard w/ 57 classic pups and 50's wiring).

Plugged the guitar into my V4B today and man does it sound good with those Sylvanias! Even into my sealed 4x10 LDS bass cab.

I know this is a bass forum, so I want to say it is P bass heaven as well with my 63 RI P and broken in Labella flats :)
 
#18
I have a Les Paul with those pickups as well so I know what you mean. The guitar put out a good signal which drives the amp well. Add a pedal and the overdrive into the amp sounds great. Jimmy Page wouldn't say not that setup.
 
#19
I think these amps are so underrated for guitar..I see them on CL for like $350-$400. I have thought of buying another just because.

I can't wait to bring it to a jam with this 3 piece band I play with. I'll let my guitar player play the VT22, and I'll play the V4B. We do a lot of 70's rock. I will turn us into an Ampeg band :)
 

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