Blues junior's plate voltage acting strange. Problem solved!

[Dingleberry]

Hi.
This one is kind of hard to describe but I'll try to do my best to get you guys some clue what's going on inside that little bastard.

I just did a bias mod to Cream board Blues Junior and when I started to rebias it and took some measurements I noticed that the power tube (V4) close to P.I tube is acting very strange.
I could not get a steady voltage readings from it's plate.
My multimeter sort of freaked out and there were some weird readings jumping on the display.

Other power tube ok. Plate supply ok, about 335Volts. All other voltages ok.
But when I take reading from the V4, either across the OT primary or from plate to ground weird things start to happen on the multimeter display. Voltage rises slowly till it reaches something like 331V and then the readings start to roll randomly like in a lucky seven jackpot and then my meter starts to beep error and after few seconds of freaking it shuts out.

I got steady bias voltage reading from both tubes, now about -13V so that can't be the problem. OT primaries from center tap to plate leads both read 99,8 ohms so that's a very colse match.
Also every component around both power tubes tested ok. What could cause that kind of behaviour and why just for the one tube. That power tube don't glow red or anything visible and amp sounds good. That's kind of freaky.

Changed battery to multimeter, no luck.
Tried three different sets of power tubes and same problem still goes on.
Also changed preamp tubes but no no no. Tried another multimeter and that poltergeist still exists. Anyone have any clue? I'm out of ideas.

-T

[selloutrr]

test the components out of the circuit. If they are resistors clip on the DVM meters test leads and heat the component with a hair dryer and see if it's a failure that only happens under heat / current.

Have you tried resoldering all the contacts in that area of the board?
Sometimes it faster to just do it before you trouble shoot.


Confirm any modifications are correct.

Any arching burn marks on the sockets?

I just want to make sure I'm understanding correctly, the meter does work on other parts of the circuit of after acting up it's no longer reading correctly?

[Dingleberry]

Meter works fine acter acting up.
Reads everything else but V4 plate correctly before and after freaking out.
I resoldered the power tube socket.
Nothing I see looks overheated and I see no marks of arcs or sparks.
There are only two resistors around that tube and both are ok.
Screen resistor is 100 ohms and grid stopper is 1K5 as they should be.
There is one diode from plate to ground and it gave reasonable reading, about 886mV when I checked it with a diode tester.

Could it be the power tube socket pins that are loose?
Or if the coupling cap is bleeding trough DC, but I assume that then it would have some affects on the other tube as well.

-T

[selloutrr]

while you are in it it wouldn't hurt to tension the socket pins but doubtful, that usually gives an intermittant popping sound or shows signs of arching burn around the pin socket.

meter the coupling cap and see if it's leaking.

[martin manning]

There is one diode from plate to ground and it gave reasonable reading, about 886mV when I checked it with a diode tester

A diode tester is not like putting 100's of volts across it. Maybe lift one end and see if you still get the erratic voltage reading?

[Dingleberry]

That's a good point.
Got to get some sleep, it's 01.30 am here right now...

-T

[Dingleberry]

OK.

Now it starts to get weird. I just opened that BJ again and measured nearly everything I could think that would cause the weird acting.
The flyback diode wasnt the case.

I noticed that there is AC on the other tubes plate that doesn't work properly.
When I put my meter to AC and measure from plate to ground I get sort of pulsing AC from 0V and slowly (about 2-3 seconds) rising to 32V and then the cycle starst again from 0V...

And that's with the V4 (other power tube) only.

So I assume something is oscillating there or am I losing my mind?

-T

[Dingleberry]

Ok.

Thank's for you patience guys.
Problem actually was oscillating phase inverter tube.
I started to lose my mind but then I remembered the kind words of my guru friend who taught me: "Sometimes when nothing seems to make sense is preferable to take off tube by tube starting from P.I...".

When I realized that the P.I is the problem I remembered some articles about BJ P.I oscillating. I stuck a piece of aluminum with alligator clips to chassis behind the P.I ribbon wire and voilá! No oscillation! Yippee!
With the backpanel on the amp works as it should and with the back panel off... it's a pain in the a**!

How people should service these amps? With the back panel on?
I don't know to laugh or cry.
They are made not to service.
I'm probably gonna go and start to sell "Ultimate Blues Junior Bias Kit". A piece of aluminum and couple alligator clips... Only 49,90€

Let this be the warning example.
And to all of you: Remember the "aluminum piece trick" if you start to mess around with a Blues Junior.

It actually sounded very nice with Cannabis Rex in it.

-T

[PCollen]

Ok.

Thank's for you patience guys.
Problem actually was oscillating phase inverter tube.
I started to lose my mind but then I remembered the kind words of my guru friend who taught me: "Sometimes when nothing seems to make sense is preferable to take off tube by tube starting from P.I...".

When I realized that the P.I is the problem I remembered some articles about BJ P.I oscillating. I stuck a piece of aluminum with alligator clips to chassis behind the P.I ribbon wire and voilá! No oscillation! Yippee!
With the backpanel on the amp works as it should and with the back panel off... it's a pain in the a**!

How people should service these amps? With the back panel on?
I don't know to laugh or cry.
They are made not to service.
I'm probably gonna go and start to sell "Ultimate Blues Junior Bias Kit". A piece of aluminum and couple alligator clips... Only 49,90€

Let this be the warning example.
And to all of you: Remember the "aluminum piece trick" if you start to mess around with a Blues Junior.

It actually sounded very nice with Cannabis Rex in it.

-T

I've had my green-board BJ apart several times, even taking the entire board out to make a mod and re-installing it, and never have seen the problem you describe.

[blackstone]

Thanks a lot for posting this! I too was trying to bias my blues jr and had the exact problem. Sure got me scratching my head, this post has saved me a lot of time, cheers!

[C Moore]

,,,,,,,,

[xtian]

With the backpanel on the amp works as it should and with the back panel off...

Yes, I have experienced this!

[Edgar Sterling]

Had to post… I experienced this exactly today taking voltages after changing a bias resistor on a Blues Junior. Same tube, V4. Couldn't get a b+ reading off the plate, meter beeping and giving no readout, but the current looked fine. Read this about the oscillating phase inverter and pulled the tube… voltage read perfect. But the best part was plugging the P.I. back in, folding up a piece of aluminum foil and laying it over the ribbon… unbelievable. Guess that sector of the B.Jr likes a little shielded privacy before giving up its vital stats. I thought for sure earlier that one of those diodes had gone bad. Glad I found this thread, I'd have been up all night trying to figure out what was up! This was a new one for me.

[Stevem]

When one of these amps does this,reverse the install of one or both of PI coupling caps to the output tubes grid!

[smilan]

Thank you very much for this post, it saved me a lot of time while having the exact same problem with a blues jr.
Would it be a good idea to warp the ribbon cables with a plastic coated wire and grounding one end of it?
Or the shielding of the back panel if enough?