Biasing issues

recently I noticed when checking the bias setting that there's one output tube that seems not to be able to hold a steady bias.
I say 'seems' because I have the feeling the wandering bias is not caused by the tube itself. I did switch the output tubes and it didn't follow the tube.
when biasing this specific location I can see the value going up (and down) between 100mV and 1100mV. only when I hold the red pin of the multimeter a certain way does the voltage stop wandering around.
has anyone experienced this before and knows of places I should check first? to me, it seems that this could be related to a losse solder connection or something of that nature, but may it well be a faulty resistor?
I appreciate any pointers you can give me.

What amplifier?

yes, that may be useful information

it's a VTA ST70

nerpissad wrote:only when I hold the red pin of the multimeter a certain way does the voltage stop wandering around.....
it seems that this could be related to a losse solder connection or something of that nature, but may it well be a faulty resistor?

If you are measuring the bias voltage from the front using the "BIASET" pin on the octal socket on the front, check the solder connection to that pin.

hawaii.ken wrote:
If you are measuring the bias voltage from the front using the "BIASET" pin on the octal socket on the front, check the solder connection to that pin.

that's exactly how I'm measuring. will check the soldered connection when I get home tonight.
thanks for your reply!

nerpissad wrote:recently I noticed when checking the bias setting that there's one output tube that seems not to be able to hold a steady bias.
I say 'seems' because I have the feeling the wandering bias is not caused by the tube itself. I did switch the output tubes and it didn't follow the tube.
when biasing this specific location I can see the value going up (and down) between 100mV and 1100mV. only when I hold the red pin of the multimeter a certain way does the voltage stop wandering around.
has anyone experienced this before and knows of places I should check first?  to me, it seems that this could be related to a losse solder connection or something of that nature, but may it well be a faulty resistor?
I appreciate any pointers you can give me.

As others have said, check solders ( or reflow them) . Another possibility is the coupling cap that has problems.

yesterday I checked all the solder joints and redid all of them (relating to the rear left tube socket where I have the biasing issue). it doesn't seem to have any effect. of course there could still be a bad solder connection somewhere I did not look, but what about the coupling cap? would a faulty coupling cap manifest itself only on one tube socket?

nerpissad wrote:yesterday I checked all the solder joints and redid all of them (relating to the rear left tube socket where I have the biasing issue). it doesn't seem to have any effect.  of course there could still be a bad solder connection somewhere I did not look, but what about the coupling cap?  would a faulty coupling cap manifest itself only on one tube socket?  

YES

GP49 wrote:

nerpissad wrote:yesterday I checked all the solder joints and redid all of them (relating to the rear left tube socket where I have the biasing issue). it doesn't seem to have any effect.  of course there could still be a bad solder connection somewhere I did not look, but what about the coupling cap?  would a faulty coupling cap manifest itself only on one tube socket?  

YES

o man...  I just realized that a coupling cap is not a quad cap .
that's why I wondered how a faulty quad cap could influence just a single tube socket.  
my ST70 uses the russian PIO coupling caps, is there a way to make sure it's actually this coupling cap? should I switch one coupling cap with another and see if the problem follows?
anyhow, these PIO caps are not expensive, so I'll just order a new set of 4 to be sure.  I don't think these should be matched at all, do they?

The Russian PIO sets that Bob sells are hand matched ...

I got a hold of 4 new K40Y-9 PIO's locally here in holland and swapped the problem child with one of the new ones.
at first I was surprised to see the bias still not being steady but wandering about...  but after a couple of minutes it became stable again and still is up until now.
thanks for the help, guys.  changing the coupling cap was exactly what was needed!

it turns out changing the coupling cap only very temporarily fixed my issue. today I don't seem to get the rear right tube to hold a steady bias at all. it goes from anywhere between 80 to 1500mV.
it's not always responding to the bias pot when I try to adjust it.
sometimes I bias it all the way down and it looks like it holds a steady bias at 100mV but when slowly raising the voltage it becomes unstable again and may start wandering to >1V

I just swapped the PIO in the rear right location the other day so I'm pretty sure it's not the coupling cap anymore that's causing this. does anyone have any ideas left as to where I should be looking?

thanks for that!

I checked all of the things you mentioned:

bias pots are centered, output tubes taken out --> pins 5 & 6 on all tubes show a steady +/- 30VDC
then I tried checking the 1000Ω resistor simply by putting the black and red pins of the multimeter into pin 5 & 6 of the tube socket. all four output tube sockets measure +/- 980Ω. so I think that's right as well, but please tell me if I'm doing the measuring wrong

that leaves me with checking for loose pins on the sockets which I will do when the amp is cooled down and my kid has gone to bed.

nerpissad wrote:thanks for that!

I checked all of the things you mentioned:

bias pots are centered, output tubes taken out --> pins 5 & 6 on all tubes show a steady +/- 30VDC
then I tried checking the 1000Ω resistor simply by putting the black and red pins of the multimeter into pin 5 & 6 of the tube socket.  all four output tube sockets measure +/- 980Ω. so I think that's right as well, but please tell me if I'm doing the measuring wrong

that leaves me with checking for loose pins on the sockets which I will do when the amp is cooled down and my kid has gone to bed.  

Do check tubeholder pins!.
Another possible source is the adjustment pot itself. Try to measure the centrail viper to either side
simultaneously adjusting the pot. It should be smooth changes. Also check solders around this pot for cold-solders ( you did not use leadfree i hope ??)

I think I may have found the culprit. there was a loose lead from the bias socket to pin 1 & 8 where the lead was not making a solid connection to pin 1 of the right rear tube socket. I resoldered it and for now at least it holds a steady bias.

@peterh: no I'm not using lead-free solder

nerpissad wrote:I think I may have found the culprit. there was a loose lead from the bias socket to pin 1 & 8 where the lead was not making a solid connection to pin 1 of the right rear tube socket.  I resoldered it and for now at least it holds a steady bias.

@peterh:  no I'm not using lead-free solder

Nice to hear, the best is that your amp is working again !

yesterday I received the quad of genalex KT66's I ordered because one of my EL34's was red plating. this red plating was happening on the same socket that has the biasing issue.
I said has, because the wandering bias is still happening.
I think I may have found something regarding this though. it seems the bias remains steady if I touch the bias pot with a screwdriver in a specific way. my farmer's logic (as we say) tells me that the real issue may be with the pot itself instead of the coupling cap (which I already replaced).
found another thread where Bob recommends the following to be checked:

1. The coupling capacitor on the driver board for that tube. You should read an "open" (no continuity) across that cap.
2. The 1000 ohm resistor between pins 5 and 6
3. The 270,000 ohm resistor on the edge of the driver board
4. The 10 ohm bias resistor for that tube

does the idea of the bias pot itself being the culprit sound plausible at all?
regarding the four points above, I'm assuming these need to be checked with the amp powered on with all tubes inserted, right?
do I need to pay attention to polarity at all when measuring points 2,3 & 4?
also, what constitutes no continuity when measuring the coupling cap?

could very well be a bad pot, maybe it has a "spot" in it's track where there's no continuity, it's not common, but it has happened!

those 4 checks you listed should be done with the amp OFF !!! Just use an ohmmeter

I had a bad pot on a new build ... worked for a while, had a tube red plate on start and started digging. Replaced the pot and all is well.

sKiZo wrote:I had a bad pot on a new build ... worked for a while, had a tube red plate on start and started digging. Replaced the pot and all is well.

that's exactly what was happening with my set of EL34's. sometimes one specific tube would start to red plate. if I turn the amp off and start again after a couple of minutes it would be fine.
I'll look into replacing the bias pot as well. don't think they cost much at all. so it'd be an easy fix I assume.

nerpissad wrote:
1. The coupling capacitor on the driver board for that tube. You should read an "open" (no continuity) across that cap.
2. The 1000 ohm resistor between pins 5 and 6
3. The 270,000 ohm resistor on the edge of the driver board
4. The 10 ohm bias resistor for that tube

got round to checking the values just now:

1. I believe no continuity means no reading? if so, that's what I'm seeing.
2. 988 ohm
3. wasn't sure how to measure it, so I put the meter in the 20K ohm position where I measured 6.37.  not sure if I did that correctly.
4. 10.1 ohm  

so the odd one would be #3. to be sure, we're talking about the resistor (R39) located to the right of the UF4007 diode on the bottom of the VTA board:

thanks for your answer Bob. while I was measuring I quickly checked all other resistors including the four outer 270K resistors you mention and they all measured the same value.

when I put the amp back in its place and turned it on again, it decided to nicely bias all 4 tubes.
that's one more point for suspecting a bad tube pin contact. I will take a closer look at the tube pins and also all the solder joints (again )

I measured everything again since the good biasing didn't last very long.
now, the following is going on:

the four 270K resistors all measure around 290K except for the problematic tube socket (V7). I get no reading at all on that resistor. same goes for the 1000 ohm resistor between pin 5 and 6.
I was under the impression bad resistors have a much higher/lower value instead of no value at all?

while inspecting solder joints I decided to replace the PIO coupling cap for V7 with the one I had previously used. decided to use an ohm meter on that too and saw 15,000 ohm. so I switched back to the other again right after that..

my guess would be that it's too unlikely that both resistors have failed. does no reading on a resistor automatically mean it has failed? where should I go from here?