ENGL Savage troubles

[InDecember]

Hi,

Some issues here with the Engl Savage 120 MK1.

1. Both depth boost switches are not working, no difference in low end when pushing the buttons.
2. Presence control is (almost) unusable. Only on 0 the tone is bearable, everything above 0 results in an ear piercing high end fizz.
3. According to the schematic (depending on which one you look) bias voltages between points A,B and C should be 1.1-1.4 DCV, I measure 0.2 DCV (with bias trimpot maxed out), so not even close to half of what it should be.

A tech installed new powertubes about half a year ago, but just recently started using the amp again, so only now noticed the problems. Everything points to the poweramp section of the amp, but when I slave another amp into the Engl (fx return) the Presence control acts normal (depth switches still make no difference though), so I'm not sure where to start looking.

Any ideas?
Cheers.

Schematic 1:
https://el34world.com/charts/Schematics/files/Engl/Engl_savage_120_sch.pdf

Schematic 2 (does not include depth boost switches?):
http://milas.spb.ru/~kmg/files/schematics/Engl/engl-savage-120-amplifier-schematic.pdf

[InDecember]

So I've added a 22K resistor in parallel with the 22K resistor before the trimpot (to change its range), and I'm up to 0.7V (from the original 0.2V) between points A and C and B and C. Amp sounds already much warmer, louder and better. I now start hearing a change when the depth boost is used (though still not like I remember it). Presence starts to become somewhat more manageable too.

[InDecember]

Removed the resistor in parallel cause I wanted to do some measurements of the amp without any modifications.

I have:

6550 tube (x2) 42watts
Plate voltage (pin 3): 525V
Cathode Voltage (pin 8): 40V
Bias current (with trim pot on max): 17mA.

I don't get it, I should get at least 40mA and with 70% of max dissipation even 60mA.

[PRR]

> Cathode Voltage (pin 8): 40V

6550 cathodes are grounded through fuses. How can they be 40 volts up?

[WimWalther]

> Cathode Voltage (pin 8): 40V

6550 cathodes are grounded through fuses. How can they be 40 volts up?

Blown fuse?

[InDecember]

> Cathode Voltage (pin 8): 40V

6550 cathodes are grounded through fuses. How can they be 40 volts up?

Yeah 40V with stand by off (no sound), 20V with standy on (as in sound is coming out of the amp), not sure why I measured that though, what I meant to measure was pin 8 to pin 3 which is 513V. Also, them 6550's from JJ seem to output 35W instead of the 42watts mentioned earlier. Still, that bias current is way too low.

[WimWalther]

Either you are not measuring the cathode voltage correctly (like, using the wrong ground reference) or something has gone wrong with the standby switch, the fuse(s) or both.

Also, take a look at the protection diodes on the 6550s.. just for drill.

When the amp isn't in standby, the 6550 cathodes should be tied to ground with a very low resistance, like in the milliohm range.

[InDecember]

Blown fuse?

One fuse was blown indeed. Should've checked them first. I'll search for a replacement and report back.

[InDecember]

Replaced fuse, getting about 39mA, all is well now. Great tech I have. Luckily you guys were here, much appreciated.