Problem biasing

[Stretchy]

I've built a 100 Watt plexi a while ago (8 months ago), everything was fine. Recently it sounded kinda different, I thought that since I built it, I've never rebias the amp yet, no change of tubes either, so I said to myself that maybe it was time to rebias it.

Per my reading on pin 3 of the power tubes, (roughly 490 watts), I need to bias it so that I got a reading of 36 mV on pin 8. To my suprise I read around 8 mV. Even after turning to pot all the way, I was able to max at around 12.

I then closed the amp and retried, I was able to set the bias correctly at 36 on my hottest tube. Then suddenly it was down to 7 once again. After a few retries, it was obvious that somehow the current on pin 8 was fluctuating between a "high" (which was the correct bias) to a "low" around 7. When I play it, I can feel or ear when the amp is in a "low" mode or a correct one.

I've checked the current on pin 3 and I dont see this fluctuation. I have a stable reading of 489 to 491 Volts. Tried also to move the patch cord to the load, once it turned the amp in correctly. Could it be a bad contact somewhere ? Since voltage on pin 3 is stable, how could that be that I have "two" values reading pin 8 ?

This happens on all the power tubes, not only the hottest one.

[toner]

Check the negative voltage on pin 5 to see if it fluctuates. It probably is since all 4 tubes act the same. If it is, see if the AC bias feed from the PT fluctuates also (before the first resistor and diode in the bias circuit). If pin 5 fluctuates but the AC is steady, you may have a bad ground connection, resistor or bias pot in the bias circuit.

[Stretchy]

Sorry for my newbiness, I'm still learning, what path is the "biasing circuit" ? What parts are affected or concerned by it ?

[toner]

Here's an image of the bias area. It's near the right end of the board when facing the front of the amp. The ground connection should be coming from somewhere in the green area.

Check the DC voltage on pin 5 of one of the power tubes to see if it fluctuates. If it does, check the AC voltage at the red circled turret also. The reason to check the AC is to see if the PT is the problem (hopefully not).

Since the bias current fluctuates on ALL of the power tubes, it's likely that the negative voltage on pin 5 is the cause. If it were just one tube, I'd suspect either the tube or the ground connection on it's 1 ohm resistor.

[Stretchy]

Ok, thanks for the info. I'll give it a try tomorrow and let you know what I found.

Played with the amp today and it was really nice. I've noticed that when I mess with the socket pluging the amp to the cab, it looks like a bad connection, if I shake it left to right the sound stops for a second. Could this be it ? Could a bad connection there (those two outputs to the cab) have this effect ?

[Stretchy]

Ok, here it goes... I think I need an expert on that one.

That fluctuation seems to have stopped... but it is always on the "low" state (7 mV). Low readings on the pin 8. I noticed that if I squeeze one of the output jack (still plugged in... and from the outside of the map) I got a ok reading. After some time... no reading at all. Just no more juice were feeding pin 8. Nor the high voltage pins (like pin 3). Checked the fuse and it turned out the HT fuse have blown. I changed it an after it, even after many on and off cycle, I always got a good bias reading and was able to bias the amp correctly at 36 mV.

I've checked everything on the voltage charts and also checked as you told me the reading I have on AC voltage on the turret board at the 27k resistor, and I have 103V. That one never fluctuated and was always stable, like every reading I did everywhere but on pin 8.

Could this be a fuse that was "going out" doing all this ?

Usually, a HT blow means a faulty tube, should I change them ? They are not old, 8 months of use, JJs.

Your take on that would be greatly appreciated!

[toner]

I'm not sure if a bad connection at the speaker jack could cause the bias current to vary at idle. Someone else will have to answer that.

I would touch up all the speaker jack solder joints and the tab on the impedance switch that connects to the speaker positive.

The HT fuse wouldn't affect bias but the opposite could be true. In other words, if the bias current went very high for some reason that could blow the fuse. If the fuse is a slow-blow, it could have partially gone while you were playing. I really don't think it's a tube problem because it would be very unusual for all of them to develop the same problem at the same time.

[Stretchy]

I've checked the connection joints on the speaker jacks and the ohm selection rotary switch. They are solid and shiny. I think the problem with the speaker jack is inside of it, like something broke inside of it or has a loose connection. Only one of the speaker jack does this. It's not a big problem cause I'm only using a single 2X12 cab, but I will change it and see!

I'll check the biasing once again tonight see if it holds!

Thanks.

[toner]

Yeah, I was wondering if the jack itself was broke or the contacts were bent. I'm pretty sure a loose speaker connection wouldn't affect the bias though.

[Stretchy]

Ok... I give up. Ever since I changed the fuse, the amp is dead steady. I can bias the amp on spot at 36 mV (34 on the coldest tube and 36 on the hottest, the other two are at 35... so pretty even). Never saw any other fluctuation and the amp sound very sweet.

I think I'll just enjoy the amp now... would like to know what happened!

Thanks for the support!

[BAinFL]

Something that wonky and I'd suspect the meter might be the issue...especially if the amp sounds good.

I'd try a new 9V battery in the DMM and go from there...

[Stretchy]

Ok... new development here. I need help!

The amp started to sound different, so I've check the bias and it was again turned down to 7 instead of 36. If I grab a hold of the cab cable and squeeze it like to torque it a bit, I got a 36 mv reading. Played a little and I can here a very faint crackle and the sound is less defined. New reading... 7 mv.

When the amp is in the "low state" I took a reading of the plate voltage and I got 416 V. If I shake the speaker cable a bit and be able to have a "high state" I have a plate voltage of 480 V which is "nornal". 416 is a bit low.

In shortm, when I have a bias of 36mv the plate voltage is at 480V and the amp sounds good. If it shifts to 7 mv, the plate voltage is at 416V. Seems to be a "power" problem. I haven't checked the juice coming in the amp, but what puzzles me is that if I shake those speaker cables hard enough, the "power" problem vanishes! So it looks like it doesn't come from the mains.

I've rechecked the soldering at the speaker connectors and they look good.

The only thing I can suspect is that brown cable coming from the OT! If you refer to the metro step by step guide, it says at step 8 on page 17 to "Scrape off the red enamel coating and solder into place as shown." I recall that no mater how hard I look I never saw that "red enamel". I did try to "scrape off" whatever was there, but I never saw a single thing.

This cable from the OT hooks on those suspects speaker connectors. Could it be that ? Could the stuff coming from the OT affect in some way the plate voltage ?

Need help now... dunno where to look. Could the problem be the PT and that when I'm shaking those connectors, I "correct" this problem is some strange way ?

This problem really p**ses me off, come and go, even though now it looks like it is easily stuck on the "low state". As soon as I play a bit, it goes down!

[JimiJames]

I feel ya, man. I'm no expert but, have you troubleshot by checking your wall voltage and played in another room that's off a different breaker? Also, check across all four 1 ohm'rs for resistance/continuity as mentioned earlier by toner. An extra set of spare tubes is always good...

... I had 2 pre's & 1 power tube in the same build that almost put me in a padded room

[mathd]

I'd suspect either the tube or the ground connection on it's 1 ohm resistor.

I had a bad cathode ground problem in past, and bias reading was going high(around 140 mV) not low.
I guess if someone use the OT shunt bias method it will yeld a lower reading, but since we are talkng about the 1 Ohm resistor bias method.

Now about the problem, if you move/squeeze the speaker cable/jack and the problem go away, id try another speaker cable.. and if that dont make it change the speaker jack.