ENGL Savage 120 low bias

[Travis8]

Hi!

I check my engl savage 120 bias and is 18ma for one KT88 and 17ma for the other one, they are news and I changue it becouse I notice that the amp was a little bit quieter than It should be.

Did someone experiment something similar?

Thanks

[Travis8]

Red leds are on In the tubes, the manual said that:

Power Tube Monitor
This red LED lights up to indicate one of the internal power tube fuses has blown. You can continue playing,
but the amp‘s performance will be diminished. Normally the loss of a power tube results in an unbalanced
signal. Be sure to have a specialist look over the power amp as soon as possible; the fuse probably blew
because of a defective power tube. Once a fuse has blown, it must be replaced by a new fuse.

I will check it..

[Latole]

Schematic

If you can read 18 and 17 ma fuses are good

https://elektrotanya.com/engl_savage-120_sch.pdf/download.html#dl

or here ;  https://el34world.com/charts/Schematics/files/Engl/Engl_savage_120.pdf

[Travis8]

Schematic

If you can read 18 and 17 ma fuses are good

https://elektrotanya.com/engl_savage-120_sch.pdf/download.html#dl

or here ;  https://el34world.com/charts/Schematics/files/Engl/Engl_savage_120.pdf

But is normal 18ma of bias in a KT88?¿

[Latole]

Bias ma reading work with voltage. Only ma mean nothing
What is the plate voltage ?

  KT88 = 42 watts maximum tube dissipation.
I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts

Let say : If tube plate voltage is 500 volts ( check your amp )
500 volts X 18 ma = 9 watts,  A way too low. A weak tube may read that. Put new one.
With good tube ;  you must adjust bias ( grid) negative voltage

[Travis8]

Bias ma reading work with voltage. Only ma mean nothing
What is the plate voltage ?

  KT88 = 42 watts maximum tube dissipation.
I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts

Let say : If tube plate voltage is 500 volts ( check your amp )
500 volts X 18 ma = 9 watts,  A way too low. A weak tube may read that. Put new one.
With good tube ;  you must adjust bias ( grid) negative voltage

I tried with new tubes and bias is 18ma, let me read plate volts..

[Travis8]

Bias ma reading work with voltage. Only ma mean nothing
What is the plate voltage ?

  KT88 = 42 watts maximum tube dissipation.
I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts

Let say : If tube plate voltage is 500 volts ( check your amp )
500 volts X 18 ma = 9 watts,  A way too low. A weak tube may read that. Put new one.
With good tube ;  you must adjust bias ( grid) negative voltage

around 580 volts but just 18ma

[sluckey]

Check bias according to the manufacturer's procedure. What have you?

[Latole]

I tried with new tubes and bias is 18ma, let me read plate volts..

580 X 18 = 10.4 watts.

 Tubes will last 100 years and more

Follow Sluckey advice.

[Travis8]

Check bias according to the manufacturer's procedure. What have you?

0,8 volts and 0,680.

0,680 side starts coming down little by little, after 10 minutes I read 0,614. Then no more drop and still 0,614... I check it for 20 minutes.

[JPK]

Well there's your problem. Now how to raise it.

[Travis8]

Well there's your problem. Now how to raise it.

I´m thinking about it, could be a output transformer issue, bias trimmer in middle position; 0,222 volts

[sluckey]

Check bias according to the manufacturer's procedure. What have you?

0,8 volts and 0,680.

0,680 side starts coming down little by little, after 10 minutes I read 0,614. Then no more drop and still 0,614... I check it for 20 minutes.

OK, that confirms the mA readings from your first post. Now measure the negative dc voltage on pin 5 of each KT88. Also measure the dc voltage on pin 4 of each KT88. What have you?

[Travis8]

Check bias according to the manufacturer's procedure. What have you?

0,8 volts and 0,680.

0,680 side starts coming down little by little, after 10 minutes I read 0,614. Then no more drop and still 0,614... I check it for 20 minutes.

OK, that confirms the mA readings from your first post. Now measure the negative dc voltage on pin 5 of each KT88. Also measure the dc voltage on pin 4 of each KT88. What have you?

Around 410v on pin 4 and -59v on pin 5

[sluckey]

Around 410v on pin 4 and -59v on pin 5

Those voltages look fine. Now measure the voltage on pin 8 of both KT88s. What have you?

[Travis8]

Around 410v on pin 4 and -59v on pin 5

Those voltages look fine. Now measure the voltage on pin 8 of both KT88s. What have you?

0 volts on pin 8

Power tubes fuses are ok

[sluckey]

That rules out everything except the tubes and the OT.

[Travis8]

That rules out everything except the tubes and the OT.

Tubes are new...

[shooter]

0 volts on pin 8

just to cover the odd things;
NO power
ohm pin 8 (tube side) to the ground side of the STBY switch, both sockets
do it with the switch closed and open

[sluckey]

That rules out everything except the tubes and the OT.

Tubes are new...

Put the old tubes back in and recheck the bias.

[Deric]

I´m thinking about it, could be a output transformer issue, bias trimmer in middle position; 0,222 volts

It's not very clear so just to be sure...  Here you say "bias trimmer in middle position". Have you actually adjusted the bias trimmer?  What's the DC voltage on pin 5 when the bias trimmer is all the way CW (clockwise)?  Voltage on pin 5 all the wy CCW (counter clockwise)?

[Latole]

I´m thinking about it, could be a output transformer issue, bias trimmer in middle position; 0,222 volts

It's not very clear so just to be sure...  Here you say "bias trimmer in middle position". Have you actually adjusted the bias trimmer?  What's the DC voltage on pin 5 when the bias trimmer is all the way CW (clockwise)?  Voltage on pin 5 all the wy CCW (counter clockwise)?

Yes, turn this trimmer to reduce negative bias voltage

[Travis8]

0 volts on pin 8

just to cover the odd things;
NO power
ohm pin 8 (tube side) to the ground side of the STBY switch, both sockets
do it with the switch closed and open

pin 8 of both sockets to ground stby switch 0 ohms, to the other side of the stby switch 82.6ohms

[Travis8]

I´m thinking about it, could be a output transformer issue, bias trimmer in middle position; 0,222 volts

It's not very clear so just to be sure...  Here you say "bias trimmer in middle position". Have you actually adjusted the bias trimmer?  What's the DC voltage on pin 5 when the bias trimmer is all the way CW (clockwise)?  Voltage on pin 5 all the wy CCW (counter clockwise)?

If I use old tubes with bias trim all the way down, pin 5  read 0 volts.

With new tubes, PIN 5:

All the way up:  -49.0
All the way down:-62.8

[Latole]

At -49 how many ma did you read ?

More than 18 ma ?
From -59 you read first, you may go over 20/ 25 ma

[Travis8]

At -49 how many ma did you read ?

More than 18 ma ?
From -59 you read first, you may go over 20/ 25 ma

I read with all the trimmer way down around 7/9ma and with the trimmer all the way up I read around 33/35ma

[Latole]

With these readings you can have the right bias.

Do calculation as I show you.

Dont forget ; more ma = less plate voltage
check after each adjustment


__________________________

Bias ma reading work with voltage. Only ma mean nothing
What is the plate voltage ?

  KT88 = 42 watts maximum tube dissipation.
I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts

Let say : If tube plate voltage is 500 volts ( check your amp )
500 volts X 18 ma = 9 watts,  A way too low. A weak tube may read that. Put new one.
With good tube ;  you must adjust bias ( grid) negative voltage

[Travis8]

I read the bias like the schematic said and I can get 1,5volt max...

I assume is ok?

I get around 19 watt if I calculate with plate and ma, is less than 50%.

[Travis8]

With these readings you can have the right bias.

Do calculation as I show you.

Dont forget ; more ma = less plate voltage
check after each adjustment


__________________________

Bias ma reading work with voltage. Only ma mean nothing
What is the plate voltage ?

  KT88 = 42 watts maximum tube dissipation.
I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts

Let say : If tube plate voltage is 500 volts ( check your amp )
500 volts X 18 ma = 9 watts,  A way too low. A weak tube may read that. Put new one.
With good tube ;  you must adjust bias ( grid) negative voltage

is I read bias like schematic said I move the trimmer to get right now: 1.447v

with this position I get:


34ma and 563v, so it´s 19.142 watts

its fine?

[Latole]

That is very good  ! On both tubes ?

Did you read carefully my answer ?
"I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts"


19.42 watts Is very good

[Latole]

How did amp sound now ?

[Travis8]

That is very good  ! On both tubes ?

Did you read carefully my answer ?
"I mostly bias at 50 to 60 % power dissipation ; 50 % = 21 watts"


19.42 watts Is very good

I read the other tube, is: 18.938w

and following the schematic is: 1.37v

[Latole]

Il bias the higher tube to 21%

How is the amp sound now ? 

After the right bias, if amp sound powerful and good, job is done

[Travis8]

Il bias the higher tube to 21%

How is the amp sound now ? 

After the right bias, if amp sound powerful and good, job is done

Its look fine, I think the problem was the range of the bias pot is all in 1/2 mm. and the rest is under volt.

Thanks!!

[Latole]

Said thank to Deric , post #20.

It is him write to turn the pot.

If pot is so sensitive ( show picture) I will replace it by a less sensitive one , a better one.
Your pot may move with time. This is what happened

[Latole]

I sometimes see these pots, they are not good ;