Studio .22 red plating stays in same socket

[texwest]

Hey guys I'm a bit stumped.  I have a studio .22 I bought a few months ago.  Am el84 was redplating so I put in a set of newer tubes.

Now I have a pretty bad red plating on one tube again.  I pulled the amp apart to check things.  I resoldered all the socket connections.  Checked all the resistors to make sure they aren't burnt.

Fired her back up and redplating again.  Switched the tubes around.  Took longer to redplate but started up on the same socket.  The tube that I thought was bad does not redplate now.

I checked all the voltages on the el84 sockets and they seem pretty good.

The only thing I noticed is that the 470 screen resistors measure around 590 ohm.  But both of them are the same reading.  When fired up the voltages on each socket are the same.  Doesn't make sense to me why one socket is causing this, when all the voltages read the same.

Need your help!

[PRR]

> I checked all the voltages on the el84 sockets and they seem pretty good.

Pin 2 is less than 1V over the long term?

[texwest]

Not sure about long term on pin 2.  This amp is non adjustable fixed bias like so many other boogies.  The schematic says pin two should read -12v.  I checked both when I turned it on and they both around -11.85v.  Should I start it up again and keep checking it?

I just thoroughly  cleaned the sockets and am about to retension them.

I'm also wondering if I should replace those screen resistors?  But they both read nearly 592 and 594.  Not sure if something else in the circuit is making them read high.

[shooter]

15min with NOT signal, OR until your see red

retention n clean is good, look with magnifying glass for micro cracks, larger G2 R values I believe run the tube "safer" 

[texwest]

That's what I thought about the screen resistors being high.

Guess I will start it up again and see what happens with the negative voltage on pin 2

[texwest]

Ok now it's really weird.

I cleaned and retensioned the tube sockets.  Put the tubes back in. Checked the pin two voltage over time.

Now its redplating on the other socket but not on the original redplating socket!!  As it started to redplate the negative grid voltage slowly went down from the 11 volts and I finally turned it off around -6.85 volts.  Also the plate voltage on both tubes started to lower also.

Will switch the tubes and see if it follows the tube now.

[shooter]

Will switch the tubes and see if it follows the tube now.

leave the tubes OUT and repeat for another 15 minutes, monitor the -12vdc AT tube side of socket pin2

[texwest]

Will switch the tubes and see if it follows the tube now.

leave the tubes OUT and repeat for another 15 minutes, monitor the -12vdc AT tube side of socket pin2

I will try that. 

I'm starting to think that I had a dirty connection in the tube socket and I ruined one tube in the process of figuring it out.  But I will try this out to verify there aren't any other problems.

[shooter]

I had a dirty connection in the tube socket

sometimes it's just easier to put in new than mess with old, especially if you don't like repairing the same thing over n over n ..... 

[texwest]

Will switch the tubes and see if it follows the tube now.

leave the tubes OUT and repeat for another 15 minutes, monitor the -12vdc AT tube side of socket pin2

OK I did what you suggested. As an experiment I did it  with 3 different PI tubes.

The tungsram that was in it hovered at -11.18 volts

A JJ   -10.76v

Sovtec  -10.40v

That's for both tubes but one side was consistently different by .02v on all 3

I was surprised that the different tubes created different negative voltages.  It made me wonder if the coupling caps have a problem but the fact that both pin 2's have a consistent voltage that is nearly exact and consistent makes me think otherwise.

[shooter]

I did it  with 3 different PI tubes.

not sure where the PI tubes come in 

so you have a steady bias with no tubes, the 2$ question, what do you want, working, out the door move on, or Never see this problem in the next few years.
me, I'd rebuild the bias supply, replace couplers and use new tubes.  I would use my breadboard to sort out the why and what's still good for troubleshooting the future

[texwest]

To the super experienced guys - is it normal for different PI tubes to vary the bias grid voltage?

The experiment I conducted was with no power tubes but with preamp tubes installed.  Three different PI tubes created three different negative bias voltages.

Is this normal when the power tubes are removed? 

If the bias voltages on the two power tube sockets weren't the same I would be worried and thinking about replacing the coupling caps from the PI.  Its just such a PITA to access them on this studio .22.

[sluckey]

is it normal for different PI tubes to vary the bias grid voltage?

It is on this amp. Look at the schematic.

[DummyLoad]

try replacing C10 - 220uF 63V electrolytic.


--pete

[texwest]

is it normal for different PI tubes to vary the bias grid voltage?

It is on this amp. Look at the schematic.

Yes you are right!  Last night I looked at the schematic and realized that the bias for the pi is connected to the power tube bias supply.

After seeing that I realized that running the amp without the power tubes probably wasn't good for the pi tube.  I'm sure it ran it really hot so I checked and yes the pi tube was being run at about 360 v on the plates while I did that test.