Why am I not getting a bias reading on my Matamp?

[Soundstorm]

Hi, I'm very new to biasing my own amp. I'm aware of the dangerous voltages present inside and have taken precautions. I do work with electricity on a daily basis, just not inside tube amplifiers (just running my disclaimer).

Here's the problem, I had one of my EL34's starting to redplate. I contacted the Matamp dealer in Canada (who may have not been the best person to get this info from) to find out how to bias my Matamp GT1 (it's about 6 months old, btw).

I was checking the plate voltage between pin 3 and gnd and getting 505 Volts DC. The weber calculator told me that a hot tube would be in the 35mA range.

Each power tube on the Matamp has it's own bias trim pot. The dealer told me I can get the bias reading from pin 1 or 8 (they're visibly jumped) and ground on my mA setting on my multimeter. I can get a milliamp reading from only one tube. The rest read zero. The one tube I can get a reading from is the one that was redplating. I can also adjust it's bias with the trimpot. I can get sound from the amp but I'm afraid to turn it up too loud.

Someone recommended checking the screen resistor. After the jumper from pins 1 and 8 it's connected to a nearby resistor. I'm not sure of the value or anything so I wanted to check here first. Would pics maybe help? Can you guys give me any idea as to what might be going on because I'm lost.

Also, my name's Trey, and thanks in advance!

[roberto]

I think he said to set the multimeter on mV, and that 35mV are 35mA (across a 1 Ohm resistor from pins 1-8 to ground). Try this way.

[ampgeek]

Welcome Trey!

It is highly unlikely that you will get a dependable "bias" reading with you meter set on mA and reading between 1+8 and ground.

Current, at mA levels, exit the tube at 1+8 when jumpered and goes to ground. You set-up a parallel path with your meter when you measured it. So....current is flowing through both paths. How much in each depends on the resistance in each path.

It is common to place a 1 ohm resistor between 1+8 and ground. Measuring mV across that resistor and using ohms law (V/R=I, where R=1) will tell you the current draw of the tube in question.

Bias voltage can be measured between pin 5 (g1, control grid) and ground. That controls the amount of current flowing between 1+8 and ground.

Set your meter to VDC and measure pin 5 to ground. It should be something like negative 30'ish VDC. That voltage is what is being changed with the bias adj pots. More negative=less current flow. Less negative (e.g., getting closer to zero)=more current flow and redplating in extreme cases.

Do you have a schematic for the amp in question?

Dave O.

[Soundstorm]

[IMG:640:478]http://i36.photobucket.com/albums/e7/Mu ... G_1117.jpg[/img]

Maybe this pic will help. The tube placement indicator is pointing straight down at 6 o'clock, and I was led to believe pins 1 and 8 are on either side of the indicator, and that the numbers progress in a clockwise motion. Meaning the pin with the red jumper would be pin 3, where I'd read my plate voltage (which is currently around 510VDC). When I go two pins to the right of that, set to volts DC on my meter I get an OL symbol and some static through my speaker.


Ok, so after leaving my lead on pin 5 for a second I was getting readings from 45 to 55 V **EDIT after re-reading your post, I realized it was in fact reading negative voltage** DC on the meter, which I was able to adjust with the bias trim pots. Though I'm not sure how this relates to the final bias reading in milliamps.

[Soundstorm]

And thanks for the welcome and help, guys. I can see myself coming here as often as possible. It's also reminding me that I need some basic electrical classes, lol.

[Soundstorm]

FYI, I'm trying to set the tubes (which are 4 EL34's in a 120 watt amp) to 35mA. The Weber bias calculator sets this at hot with the plate voltage at 500VDC, of which I'm reading 510. The Matamps are known for running hot and power tube breakup is a huge part of their sound, but I wouldn't want to have to buy a new set of tubes more often than I have to, lol.

[Soundstorm]

I took a reading on either side of that big grey resistor connected to pins 1 and 8 and had a 100+mV reading on the third tube (the one that was red plating). I used the bias pot to get it to around 35mV, but when I went to read the others I get a 0 reading.

Thought I had no problem getting a reading between pin 5 and GND on all of the tubes.

Well, I'm gonna give it a rest for tonight and check back in tomorrow. Thanks all

-Trey

[ampgeek]

The voltage at pin 5 is what controls the current at 1+8. It is the bias voltage.

The terminology is confusing..especially to those just starting off in all of this.

The current at 1+8 is simply the current flowing the EL34 tube and is the combination of the current flowing through the tube plate and tube screen grid (g2).

Tube biasing is done with the amp at a quiescent condition (e.g., no signal coming to the control grid-g1). You set the bias voltage on pin 5 to get a certain amount of current flowing the the tube...the plate specifically...so that it operates at an "optimum" condition. Kinda like setting the idle speed in an engine.

To be uber-accurate in the biasing process, you can subtract the contribution from the screen.

A good rule of thumb is that the screen grid contribution is ~10% of the total flow. But...10%, generally, doesn't mean squat in the tube realm. So... close enough if you choose to ignore it.

You should carefully measure and report back the bias voltage at pin 5 on all power tubes as that is usually the most reavealing data point when troubleshooting a bias misbehaving amp.

Dave O.

[Reeltarded]

I think bias at -35/500-ish is not the best plan if the amp is *very* dynamic.

I use a meter to get me in range, then I finish by ear playing at three volume ranges. As you change each bias the voltage will change from 510 overall though. It's hard to get a multiple bias rig straight. Balancing a coin on your nose..

Try for -32.5 or so when you get back to it. The amp is hard and hot. Let's moderate it a little? at least til you get comfortable with what it's likely to do.

[Structo]

I can't read the value but if the resistor that is connected to pins 1&8 is 1 ohm, then you set your meter to millivolts (mv) and read the value with the red probe on pins 1&8 and the black probe clipped to chassis ground.

The 1 ohm resistor converts the current to volts via ohms law.

R x I = V and since R is 1, any current number will result in voltage of that number.

So if you read 35 mv that is actually 35 ma.

Confusing?

Just read the bias with the mv setting and pretend it is ma.

The screens on EL34 will be at around 5-8 ma so you can either subtract that from the reading you get at pins 1&8 from the total current to get plate current or leave it for a buffer.

If you are red plating a tube then you are most likely exceeding 70% bias unless a component has failed (like a screen resistor).

I don't use EL34's so not sure what is a good setting in that amp.

I find modern 6L6 tubes sound better, last longer around 55-60%.

Here is the formula:

(W * .7) / V = I

Here W is the max wattage of the tube multiplied by .7 to give us 70% of the tube's maximum plate dissipation.

This number is divided by the actual B+ plate voltage in the amp to determine what idle cathode current to adjust the tubes for in AMPS.

I hope this helps clear it up a bit for you.

[Soundstorm]

Ok, been super busy at work, but was finally able to get a better look inside.

When I went to check the value of the GBR's, I noticed that three were unsoldered on one side. I'm guessing the amp got that hot. I unsoldered the fourth one and checked the resistance across all of them which all read 1K. I resoldered each of them.

I put an old set of boogie/sovtek tubes in at the shop (had it plugged into a load, but wasn't able to test it with a guitar) and was successfully able to trim the bias to 35mA for each tube (four bias pots). I get home, and while it sounds good (actually great) cranked, I noticed it sounded kinda farty when I'd try to clean it up. It sounded to me that they were running much too hot in that I couldn't get a clean tone out of even the cleanest setting (it was on NMV mode).

Since these were older tubes I decided to put in my fresher set of Russian Winged C's and check the bias. First, plate voltage read 510 across all tubes, wall power was good.

Bias reading (from pin 1/8 to chassis) was:

V4: 135mV
V3: 45mV
V2:115mV
V1 100mV

(Edit: I'm trying to get these =C='s at about 35mV)

This is the lowest reading I could get per tube. I couldn't physically turn the bias pot any lower. So once again, I'm at a wall.

Thanks in advance for the help guys!

[ampgeek]

Assuming that the "GBR" is the resistor between pins 1+8 and ground, 1K ohms doesn't sound right. That is WAY high. 1 ohm, 10 ohm or maybe even 100 ohm would be more like it.

Assuming it really is 1K ohm, your reported mV readings indicated an extremely overbiased (extremely low current) condition. Run the math (V/R=I) and you will come up with only fractions of a mA.

Again, carefully measure the bias voltage at pin 5 and report back with what you find. Consider excercising the bias pot for each and observe/record the highest and lowest reading that you can attain. Remember, these will be negative voltages.

Dave O.

[Soundstorm]

Oops,

Forgive my ignorance. I meant the screen grid resistors, or at least the ones tied to pin 4.

[Soundstorm]

These readings were taken between pins 1/8 and chassis ground on the mV section of my Fluke meter. If i turn the pot the other way it'll go into OL, and I don't want to cook anything.

I've been told time and time again to take the bias reading between 1/8 and chassis ground.

[Soundstorm]

The reading at pin 5 is -60V on all four EL34's