philips_saturn_stereo_b8d51a - High plate current on the output tube ell80

[svetolik]

Dear all,
After low power and distorted sound and replacement of cathode resistors ( correct values from the schematic) I am experiencig red-orange gloving plates of both ell80 output tubes.
All referent voltages acording to schematic are 10% higher except plate (anode) voltage of both ecc808 tubes in the preamp stage which is 120V instead of 80V.
Heating voltage is correct.
Anode currents of both ell80 are 50mA! and I have checked this value measuring voltage on the cathode resistor which is about 15V.
The AC resistsnce of output traf primaries are also correct and the sound is again just amazing, full powered, dynamic and clean.
Newest measurements:
AC voltage with input signal on the grid is 15-20 volts. Without input signal value is zero.
R154 - R157 have correct values and coupling capatitors is also correct.
DC curent in the high voltage line is 100mA instead of reference value of 55mA.
This line comes from R165 thry fuze Z3, R166 and to anode resistors of preamp stage and ecc83 phase invertor.
I will connect resistor in serial connection after C 172 with same value as R166.


I have already buy caps around ell80 and i will replace them anyway.
What will happened if I just repalce higher value cathode resistors and decrease plate current?
Should amplificatin factor of ell80 circuit drop? As I remember this factor depends on the cathode resistance.

Please find values and otput circuit in the attachment.
I am just sweling my brain wondering abut
DC resistance of ell80 tubes.
Is it possible that it has drroped because of some damage!?
I have electrotechnican degree buth it was 20 years ago.
This radio is exactiy my age and friend of mine so your help will make my night.

 

[Tom Bavis]

If plate voltage is much higher than 270V, then see if there's a transformer tap for high line voltage. You can drop supply voltage with a resistor or raise cathode resistor as you mentioned. Absolute maximum rating for ELL80 is 300V.

Any positive voltage on the grids? Could be leaky capacitors (but you changed them) or grid leakage in ELL80 tubes. Grid return resistor value 680K is within rating (must be < 2 meg ) but check them too...

 

[RWood]

So you did replace the grid capacitors, C164-5, 168-9? As Tom said, that would be a likely culprit (although a little unlikely that all four failed at once.) Still a good idea to replace them if you haven't yet.

Yes, you could increase the resistance of the Rk to keep the tubes from redplating but I think that would be masking what your real issue is.

Some questions for you:

Were the ELL80s redplating before you changed out the cathode resistors? And since this tube has two pentodes in one bottle, can you tell if they are both glowing orange, or just one?

Do you have real good connections to ground on both of the cathode resistors? Measure from the power tube socket pin 7 to the chassis and check.

And when calculating the plate dissipation, the plate voltage you use should be the difference between anode and cathode, not anode to ground. Sorry if this is something you already know.

I have a receiver, too, that uses ELL80s, but on mine they all share one Rk and one Ck.

RWood

 

[svetolik]

So you did replace the grid capacitors, C164-5, 168-9? As Tom said, that would be a likely culprit (although a little unlikely that all four failed at once.) Still a good idea to replace them if you haven't yet.

Yes, you could increase the resistance of the Rk to keep the tubes from redplating but I think that would be masking what your real issue is.

Some questions for you:

Were the ELL80s redplating before you changed out the cathode resistors? And since this tube has two pentodes in one bottle, can you tell if they are both glowing orange, or just one?

One tube that has lose metalic top(air isnide?) works without gloving!
DC anode current is 30mA. Having nice blu haze inside after 5 mins and distorzion.
The gloving ones are untested.

Do you have real good connections to ground on both of the cathode resistors? Measure from the power tube socket pin 7 to the chassis and check.

Checked

And when calculating the plate dissipation, the plate voltage you use should be the difference between anode and cathode, not anode to ground. Sorry if this is something you already know.

Checked 300V, that is exactiy what I need.

I have a receiver, too, that uses ELL80s, but on mine they all share one Rk and one Ck.

RWood

Same as here and what model is it?
ELL80 is a old school gem. Sounds like thunder with 50mA plate current.


Now I have it done!
We are singing again:
Added two more resistors 47, 680 in both high vltage lines before and after fuse Z3 and get reference value in the second line with 290V plate voltage.
Thanks for help!
Testing further.

 

[jaymanaa]

ELL80s are getting rare and exspensive. I have a small stash tucked away, but never built anything with them.

 

[svetolik]

ELL80s are getting rare and exspensive. I have a small stash tucked away, but never built anything with them.

My pair is quite used and thah is preventing me to play around more despite that I have ideas.
Are you willing to sell some ones?

 

[svetolik]

Pair that play now is quite used and if someoone has old ELL80 or ECL800 it will be of great help here.
Going for bypassing tone control and making cathode follower and trying to understand what was happerned here.
Any tips for ecc808 preamp modification?