Engl Blackmore Sig E650 screen resistor failed - preamp sounds weak. Need Help

[ChriG]

Hi, I´m Chris.

I stopped building amplifiers as a hobby a few years ago when the components became very expensive in Europe and I didn't have the time.

Now I realize that I need your help because I've lost a lot of knowledge.

A friend asked me if I can take a look at his Engl-Blackmore 100 watt amp. The amp broke during a gig.

What I've noticed so far:

One of the power tubes failed, also the 470R 1.4 watt screen grid resistor of that tube and the 470R 5 Watt voltage dropping resistor in the psu ( marked with a red star in the schematic attached ). So I only had ~489V on the plates, screens and the whole preamp had no voltage.

So I replaced both resistors, measured the voltage without tubes, which looked good.
I had ~489 volts on the plates and ~480 volts on  screens. Preamp looked good too.

I put in tubes (the power tubes were older, but working tubes just for testing).

At this point the power amp was "working" again (I was afraid that the output transformer had been damaged, but no!)
The clean channel sounded very thin and quiet, the overdrive channel just sounded clean but louder than the clean channel.
I replaced all the preamp tubes, but that didn't make any difference.

In the meantime, my friend (the owner) brought over a set of new 6L6GCs.
We installed them, switched the amp on, and immediately another screen grid resistor burned out.

Maybe I should have replaced all four resistors at once.

That's what's happened so far and I'm running out of ideas.

Maybe I can get it right with your help.

Greetings from Germany

[Latole]

IMO this tube is worn out mostly because bias is too hot or normal use.

To be safe and to do a reliable job :
Check bias and adjust at not more the 60% power dissipation.  Never at 70% or higher
5881 tubes are rated at 23 watts . 60% is 13.8 watts

Bias ; for any new tubes, new bias is a must.

You may also have a burn / damaged contact on tube socket, the one who is worn out . The original tube 5881 destroy a contact in the socket.

Can't tell more without seeing the amp.

From Canada, I went 3 times in Germany, a lovely country

[ChriG]

thank you for your answer!

The screen grid resistor that burned out this time was on a different socket than the first resistor that burned out and with NEW tubes. I will test the sockets!

For now I replaced all the screen grid resistors with new ones, tomorrow I will put in tubes and check the bias.

Could this weak sound/almost no distortion at the dirt channel and almost no sound at the clean channel be caused by open caps?









I'm glad you enjoyed it in Germany! I have been to the States 4 times but unfortunately never to Canada, but I will!

[Latole]

Could this weak sound/almost no distortion at the dirt channel and almost no sound at the clean channel be caused by open caps?
I'm glad you enjoyed it in Germany! I have been to the States 4 times but unfortunately never to Canada, but I will!

It is because you have no voltage on preamps

[ChriG]

Power amp is working at the moment.

The preamp voltages looking quite good, but same issue...weak thin sound at clean channel and "good sounding" clean sound at the lead channel.

Also the clean channel gain Pot and the lead channel gain pot doing nothing.

Tonestack seems to work, also the Masters.



Thats what I meassure:

V1
Pin1: 193,4V
Pin2: 0V
Pin3 3,09V
Pin6: 257V
Pin7: 0V
Pin8: 3,02V

V2
Pin1: 260V
Pin2: 0V
Pin3 2,05
Pin6: 197V
Pin7: 0V
Pin8: 3,02V

V3
Pin1: 268V
Pin2: 0V
Pin3 2,25V
Pin6: 253V
Pin7: 0V
Pin8: 1,8V

V4
Pin1: 278V
Pin2: 72,5V
Pin3 83,1V
Pin6: 287V
Pin7: 74,5V
Pin8: 83,1V

V4 Pin 2 and Pin 7 looking a little high Form what I see in other amps, If I remember correctly


 

[Latole]

I will check all the switching channel circuit and is power supply

[ChriG]

I will try to do that, but this amp is a pain to work on....all components are mounted upside down on a big pcb. You have to desolder all leads coming from OT and PT to look where are all the components are.

Lets say there is a issue with one of the relays somewhere...one of gain pots should work anyway, because you can switch between clean channel and low gain/high gain....but low gain and high gain sharing one gain pot.

So if I get sound, one of the pots must work but they doesn't. 

[ChriG]

I solved that Problem.
The reason for that weak clean channel and not working gain pots was a broken ribbon cable, coming from the main pcb going to the pcb where the pots are mounted. There was no ground connection.

Now the amp is working just fine 

Greetings

[Latole]

Congratulation !