Philips Saturn stereo B8D51A - High plate current on the output tube ELL80.

[svetolik]

Dear all,
After low power and distorzed sound and changed 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 deep and clean.

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.

[kalee20]

First, there is unlikely to be anything wrong with the ELL80 valves. But I would expect these to be expensive, so don't run with the anodes glowing red for any longer than you need to make a quick measurement, as it will be bad for them.

If anything, I would suspect C164, C168, C165, C169 coupling to the ELL80 grids. Best way to check is remove the ELL80's, and measure across R150, R152, R151, R153 (the grid resistors). You should get zero volts. If there is anything positive, then replace the capacitors and you should be OK.

[Michael Maurice]

Kalee beat me to it as I was preparing the diagram.

Replace the four capacitors indicated by the red arrows.

[svetolik]

I have checked out again. DC voltage is zero (grids to ground)

AC voltage with input signal on the grid is 15-20 volts.
I have already buy those caps 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.
Thanks for help.

[PJL]

The AC measurement of 15-20V indicates it is oscillating. Try disconnecting the capacitors marked with the red arrows.

[svetolik]

Originally Posted by PJL

The AC measurement of 15-20V indicates it is oscillating. Try disconnecting the capacitors marked with the red arrows.

I have plan for rehabilitation of this 44 yeras old gentlman. He is a old school guy buth replacing some parts at least once in a centyry is reasonable effort.

If there is oscilation how is possible to have correct sound?

[Herald1360]

Did you measure this AC voltage when there was signal present, or under no signal conditions?

It is possible for an amplifier to sound OK even if it is oscillating at high frequency (above audio range), though commonly there would be some suggestion of a problem.

Are the 47K grid stopper resistors (R154 - R157) still good (correct value and same type of construction as originally)?

[svetolik]

They have right values and they are origin as everything else in this radio who is eyewitness of many deaths in new technology.

Now i have measured 100mA current instead of 55mA reference value in the high voltage line that comes from fuse z3, thry R 166 and than goes to the pra amp anodes resistors.
i will put new resistor with same value as R166 in serial connection after C172 because I can not reach other point.

[Alistair D]

Can you repeat the cathode voltage measurement and the AC voltage on the grid(s) with loudspeakers connected and no signal applied to the amplifier?

Al

[svetolik]

DC cathode voltage on the resistor is 10V and zero for AC grid voltage. Both measurements are without signal.
I have put one old ELL80 which have lost metalic top (air inside?), working normally!
There is no glowing plates and DC cathode current is 30mA.

Has anybody experienced faulty valves with these symptoms?

[Tractorfan]

Hi,
If the old ELL80 has lost its metallic coating in the top, or it's turned a white colour, then the tube has lost its vacuum and cannot not work and is now useless. No vacuum means no electron flow and, therefore, no output.
Cheers, Pete

[svetolik]

Now I have it done!
We are singing again:
Added two more resistors 47, 680 in both high voltage lines before and after fuse Z3 and finaly get reference value in the second line(245V/50mA) with 290V plate voltage in the first line.

Missing sound with 50mA plate current.

Thanks for help!
Testing further.

[svetolik]

Originally Posted by Tractorfan

Hi,
If the old ELL80 has lost its metallic coating in the top, or it's turned a white colour, then the tube has lost its vacuum and cannot not work and is now useless. No vacuum means no electron flow and, therefore, no output.
Cheers, Pete

It is no white like other old one. Maybe there is small amount of air inside and resistance is increased?

Anyway my old budy sing and dance again!

[svetolik]

Originally Posted by PJL

The AC measurement of 15-20V indicates it is oscillating. Try disconnecting the capacitors marked with the red arrows.

How to check this?
Please be more specific with other indications for oscilating.
I have no osciloscop to check.
Thanks.

[PJL]

I thought it was OK now? You have replaced the valve and the current is now normal?

[love kt3]

hi,if you were useing new chinees valves-that might be the only problem.

[svetolik]

ELL80 cathode current is now 4.5mA and I am on the tube FM fullrange poetic again.


I have old pair of ELL80 without metalic coating.
One of them works under 50mA without gloving what was maybe reason for misunderstanding.
Other 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.
Those output trafs are very good for mids and high buth bass is slow-soft and without impact.

Thanks again for care and help.

[PJL]

The two output valves share a common cathode resistor, R158 and R159. If one of the valves is not working properly, the other valve will take almost twice the design current.

It is therefore important you use pairs of ELL80 valves that are quite well matched.

[radiotircoed]

I have a pair of functional (presumably matched) ELL 80's removed from a functional Grundig Radio. Might these help?

Bob W

[svetolik]

Originally Posted by radiotircoed

I have a pair of functional (presumably matched) ELL 80's removed from a functional Grundig Radio. Might these help?

Bob W

Thank you Bob!

I belleve that your pair is for sure better match.
I can hear difference between chanels. Is sending problem for you?