6L6GC Tube problem

[omriba53]

Hi guys,
I've run into a Marshall amp that it's bias current is around 0mv.
I've check the specifications against the amp values and I didn't see a significant reason for that so maybe I'm missing something...

At the schematics Page 3 (Attached bellow), at the bias adjust, the schematics says it should be around -50V, but after adjusting the lowest value is around -65v...

Any idea what can it be ?

Tube: 6L6GC
Plate voltage: 480-490V
Screen: 470V
Grid: -32v
Heater: 6.3V

http://www.thetubestore.com/lib/the...rshall-30-Anniversary-6100-100W-Schematic.pdf

 

[Duke58]

I might be wrong, but isn't -65V lower than -50V?

 

[lcsaszar]

Where did you measure the bias current? I suppose 0 mV is a typo, but if you measure zero cathode current, it means your tubes are in cutoff, that is the grid bias is -50V or whatewer is the negative bias supply voltage. There might be a broken connection of the bias adjustment to GND.

Check RV201 and R212

 

[omriba53]

I might be wrong, but isn't -65V lower than -50V?

Hi Duke! Thanks for replying me 🙂

You're not wrong, it actually makes sense because according to the tube characteristics it's somewhere around the cut-off point but there is still some circuitry after, that may make the voltage more positive (I saw other amps that even -60v at the same point is ok) .. I'll check it tomorrow...

Can you think about something else?

Where did you measure the bias current? I suppose 0 mV is a typo, but if you measure zero cathode current, it means your tubes are in cutoff, that is the grid bias is -50V or whatewer is the negative bias supply voltage. There might be a broken connection of the bias adjustment to GND.

Check RV201 and R212

Thanks for replying me lcsaszar!
It's not a typo I'm reading 0 mv 🙂 I thought maybe it's my bias prob but it worked on my other amps perfectly
I'll check all the bias section and keep you updated

 

[JMFahey]

0mV is to be expected with -65V bias which puts tube into full cutoff.
The tube is doing just what it´s being ordered: pass NO current!!!!
There must be a problem with your bias adjustment, please post the relevant schematic so we can suggest some tests.

"A Marshall amp" is not exactly specific 😉

 

[omriba53]

Hi JMFahey 🙂 thanks for the reply!
I've added the schematics at the bottom of first message

Yeah I've got it now I'll check what's the voltage at the grid pin because when I said -65V I meant at where the schematic says -50V but the is still some circuitry after that so maybe it's not hitting the cut-off point but it sounds like the problem

 

[Koonw]

Hi JMFahey 🙂 thanks for the reply!
I've added the schematics at the bottom of first message

Yeah I've got it now I'll check what's the voltage at the grid pin because when I said -65V I meant at where the schematic says -50V but the is still some circuitry after that so maybe it's not hitting the cut-off point but it sounds like the problem

I think the wiper of RV201 is opened as it's turned towards minimum, so it is 22k instead of zero ohms at that point, and that is why u get -65v instead of -50v (assume that the voltage on the other end of bias supply R212 is -100v) as shown in the schematic as -50V is the voltage divider formed by R212(22k) and R213(22k) but if RV201 is 22k instead of zero you get -65V.