Unusual voltage drop in the bias circuitry.

[Mark]

Again I have to humbly admit I’m somewhat confounded by what I’m seeing in a ENGL Classic Tube 50 amp.

The overall issue is lack of bias voltage which is simple enough to understand.

My lack of understanding is why do I get -48.2VDC at the top of the bias pot, but once the voltage travels across the 330K grid leak resistors (not drawn in my notes) on pin 5 of the output valve sockets the voltage is now -46.8VDC?

I measure across the grid leak resistors and there is no voltage drop across them (as I would expect as no current should be being drawn.)

I don’t have output valves (6L6’s) plugged in so there should be nothing drawing current.

I have tried two different meters, always used the earth the bias circuit uses when taking voltage measurements.

Okay, what am I not understanding and where am I going wrong?

Thanks for taking the time to read my post and respond.

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https://el34world.com/charts/Schematics ... l_e350.pdf

[martin manning]

Are the voltages at the valve ends of the 330k’s exactly the same? A leaky coupling cap could cause the voltage to rise, but two coupling caps leaking exactly the same amount seems unlikely.

[Mark]

Are the voltages at the valve ends of the 330k’s exactly the same? A leaky coupling cap could cause the voltage to rise, but two coupling caps leaking exactly the same amount seems unlikely.

Thanks for taking time to reply Martin, I appreciate your assistance.

Funny you should mention the leaky caps, I had that fault on my early 60’s Bandmaster. The voltage is the same across both 330K resistors in this ENGL amp.

Here is a dodgy video to show my mate Barry what is happening. It might demonstrate what I’m trying to say.

https://youtube.com/shorts/bWrIcd1GFc4? ... yP_v8_Ohw7

[martin manning]

I think I would try lifting the bias voltage supply ends of the 330k’s and see what voltage appears on the grid ends.

[Stevem]

Put one output tube at a time and see if the results change.

[sluckey]

My lack of understanding is why do I get -48.2VDC at the top of the bias pot, but once the voltage travels across the 330K grid leak resistors (not drawn in my notes) on pin 5 of the output valve sockets the voltage is now -46.8VDC?

What you are seeing is perfectly normal. Your meter looks like a 10M resistor. When you connect the meter to pin 5, you now have a simple voltage divider (between the bias voltage and ground) consisting of the 330K resistor and the 10M meter. Applying 48.2V to the 330K will result in 46.66V (calculated value) across the 10M meter. Just use simple math to solve. Your actual measured voltage verifies the math.

When you connect your meter across the 330K you should measure zero volts because there is no longer any voltage divider and zero current flows, therefore zero voltage drop.

[martin manning]

Doh!

[Mark]

My lack of understanding is why do I get -48.2VDC at the top of the bias pot, but once the voltage travels across the 330K grid leak resistors (not drawn in my notes) on pin 5 of the output valve sockets the voltage is now -46.8VDC?

What you are seeing is perfectly normal. Your meter looks like a 10M resistor. When you connect the meter to pin 5, you now have a simple voltage divider (between the bias voltage and ground) consisting of the 330K resistor and the 10M meter. Applying 48.2V to the 330K will result in 46.66V (calculated value) across the 10M meter. Just use simple math to solve. Your actual measured voltage verifies the math.

When you connect your meter across the 330K you should measure zero volts because there is no longer any voltage divider and zero current flows, therefore zero voltage drop.

Thanks for explaining that to me. I thought it was something that I should have known.

Thank you everyone for your assistance. I appreciated it.

Now I can get back to working out how I can get enough bias voltage out of the circuit. I’m inclined to try increasing the 22K in series with the 25K pot. If that doesn’t work I will have to try the Trainwreck method where bias is derived from the supply tapping and a 220K resistor is used to
lower the voltage.

[pdf64]

It's dropping >7V across R114 6k8, perhaps consider reducing its value, eg 2k2, 1k.

But with <430V on the screen grids, I would have thought that -48V would be sufficient for a reasonable idle current for most 6L6 family valves?