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515A Trigger Cathode Followers (V410) Questions


mda231
 

Hello.

I have two questions regarding the Trigger Cathode Followers (V410) in the 515A:

1) I noticed blue "sparks" inside V410 (Trigger Cathode Follower; a 6DJ8 valve) at power on only.

2) This valve was replaced recently and went bad just a few days later.

3) I have now replaced it with yet another.

4) Inspecting the circuitry on the underside of the 'scope chassis, I have noticed that neon lamp B410 between pins 2 and 3 (grid and cathode) of V410 does not illuminate at power on. (- The other neon (B416) between the other grid and cathode does light up).

5) As I understand it, the purpose of the neon lamp is to protect against the grid voltage going too positive wrt the cathode when the scope is first switched on.

Now, my questions:

Are the blue "sparks" a sign that the valve is in distress?

and,

Could the failure of this valve be caused by the neon lamp not turning on when the 'scope is first powered up?

Thanks in advance.

MDA.


 

I am no expert on tubes but I have to assume that the 515 is designed to power
up the tubes safely. Your descriptions sounds like arcing which could be caused
by debris in the tube, the tube being gassy, or excessive cathode current being
drawn before the space charge around the cathode is established.

The replacement 6DJ8 might have been bad or the neon bulb could have failed.

On 14 Sep 2014 14:13:32 -0700, you wrote:

Hello.

I have two question regarding the Trigger Cathode Followers (V410) in the 515A:

1) I noticed blue "sparks" inside V410 (Trigger Cathode Follower; a 6DJ8 valve) at power on only.

2) This valve was replaced recently and went bad just a few days later.

3) I have now replaced it with yet another.

4) Inspecting the circuitry on the underside of the 'scope chassis, I have noticed that neon lamp B410 between pins 2 and 3 (grid and cathode) of V410 does not illuminate at power on. (- The other neon (B416) between the other grid and cathode does light up).

5) As I understand it, the purpose of the neon lamp is to protect against the grid voltage going too positive wrt the cathode when the scope is first switched on.

Now, my questions:

Are the blue "sparks" a sign that the valve is in distress?

and,

Could the failure of this valve be caused by the neon lamp not turning on when the 'scope is first powered up?

Thanks in advance.

MDA.


mda231
 

Thanks, David.

- Both of the 6DJ8s, the recent failure and its replacement, have the blue "sparks" when the scope is first powered up. - They're made by the same company and were bought at the same time so may be from the same batch. It could be that they were both bad valves.

I am leaning toward a failed neon because it doesn't illuminate.

- Do you think the neon malfunctioning could kill the tube?

MDA.


 

The neon bulbs are used to limit the grid to cathode voltages so I certainly
think it is possible that a bad one would allow the tube to be damaged.

I looked them up in the parts book and they are NE-2 style bulbs with a maximum
firing voltage of 90 volts and an operating current of 0.5 milliamps. I would
change them both.

On 15 Sep 2014 04:32:57 -0700, you wrote:

Thanks, David.

- Both of the 6DJ8s, the recent failure and its replacement, have the blue "sparks" when the scope is first powered up. - They're made by the same company and were bought at the same time so may be from the same batch. It could be that they were both bad valves.

I am leaning toward a failed neon because it doesn't illuminate.

- Do you think the neon malfunctioning could kill the tube?

MDA.


Albert Otten
 

What do the similar neons in the Hor Amp do during start-up? The circuit description of the Hor Amp mentions explicitly the protecting function of these neons.

Albert


mda231
 

Hello David.

I think you're right: Unfortunately, the replacement valve has died now and the scope is malfunctioning again.

Thanks for looking up the details.

MDA.


mda231
 

Hello Albert.

Thanks for your reply.

Both neons, B253 and B280, in the Horizontal Amp light up and then turn off.

Regarding B160 and B161 in the Time Base Circuit, I'm having difficulty telling them apart. However, the one on the right hand side (looking from the screen end) lights up and stays on, while the one on the left hand side doesn't light at all.

According to my manual (515 / 515A, PDF) there are also a pair of neons (B375 and B385) associated with V370 in the Vertical Amp circuit, but these are not fitted in my scope.

MDA.


Albert Otten
 

B160 really has the function of DC level shift, see the circuit description. So will lit all or most of the time.
B375 and B385 (schematic) seem to correspond to B373 and B383 in the Parts List; there are no B373 and B383 in the schematic (my 515/515A pdf). These neons were dropped after S/N 1613.
Anyway, neons cost near nothing so I would replace them if suspected.

Albert


===
Regarding B160 and B161 in the Time Base Circuit, I'm having difficulty telling them apart. However, the one on the right hand side (looking from the screen end) lights up and stays on, while the one on the left hand side doesn't light at all.

According to my manual (515 / 515A, PDF) there are also a pair of neons (B375 and B385) associated with V370 in the Vertical Amp circuit, but these are not fitted in my scope.

MDA.


mda231
 

Thanks for pointing that out, Albert.

You're right, neons are inexpensive (much less so than valves) so it's probably better to be safe than sorry.

MDA.


Craig
 

I doubt that this pertains to your problem, but I had a recent issue with some 6922/6DJ8 tubes. This is a very popular tube in audio equipment and I recently had a problem with Tesla brand tubes in a Audio Research power amp. The tubes were breaking down with plate to grid shorts (visible sparking) when balancing the low level section of the amp. Switched out to Electro-Harmonix brand and no more problem.

This was a very specific circumstance because setting up the low level biases is done with no output tubes plugged in. As a result the high voltage (400V) remains for a long time after the power (tube filaments) is switched off. It was during this time when the high voltage was present and the filaments were off that the internal arcing in the tube happened.

These same tubes perform perfectly in less demanding preamp applications.

There are differences in the 6DJ8 tubes from different eastern european manufacturers.

Craig

- Both of the 6DJ8s, the recent failure and its replacement,
have the blue "sparks" when the scope is first
powered up. - They're made by the same company and were
bought at the same time so may be from the same batch. It
could be that they were both bad valves.