Marshall Jubilee bias: am I overlooking something?

[guitarjb44]

Before I resort to changing a resistor value, I want to be sure I haven't overlooked anything.

I measured the plate voltage of my Marshall Jubilee 2550 (50-watt head) at pin 3: 470vdc. I did this with the tubes in. By my calculations, 470 plate voltage means bias reading at 70% dissipation is around 37mA. Problem is the bias adjustment pot, at it's lowest setting, gets the bias down to only about 40 or 41mV (I am using a Groove Tubes Bias Tool to check the readings, which reads in mV)

The 50/25 switch is on 50 watts, all controls set to zero, nothing plugged into the input.

I have tried an different pair of new, perfectly matched EL34 power tubes and got the same results.

Am I not doing something right? Maybe I should try a third pair of tubes- although unlikely, it could be that coincidentally both pairs of tubes are just drawing too much current (right??)

Ideas, advice??

-Joe

[dazco]

I wonder if the GT unit is at fault. One thing you can try is lift the cathode ground on one of the tubes and insert a 1 ohm resistor and read the mV across it. It'll be around 5mV less that actual due to screen draw. Take the mV reading, add about 5 mV to it and thats your current draw. 40 mV=about 45 mA. This way you can see if the GT unit is reading correctly. Using a 1 ohm resistor on each tube's cathode is a good cheap way to do it w/o and fancy bias tools too. I have test probe sockets on my amps and the bias pot all mounted on the back panel so i can read and set bias w/o removing the chassis in seconds.

[guitarjb44]

I wonder if the GT unit is at fault. One thing you can try is lift the cathode ground on one of the tubes and insert a 1 ohm resistor and read the mV across it. It'll be around 5mV less that actual due to screen draw. Take the mV reading, add about 5 mV to it and thats your current draw. 40 mV=about 45 mA. This way you can see if the GT unit is reading correctly. Using a 1 ohm resistor on each tube's cathode is a good cheap way to do it w/o and fancy bias tools too. I have test probe sockets on my amps and the bias pot all mounted on the back panel so i can read and set bias w/o removing the chassis in seconds.



OK, I will try this. Thanks for the info.

Also...since the bias tool is designed to do the same thing as the 1-ohm resistor method you describe above, shouldn't I be adding 5mA to what the probe reads anyway (assuming the bias tool IS working correctly)?

-Joe

[donnyjaguar]

Maybe the amplifier was modified to use another type of tubes?

[big mike]

Could've been stock with 6550s.

check the bias resistors. 150K or 220K?

This should tell you for sure.

You just need to find the range resistor and change it to give the pot a range better suited to your EL34's.

[guitarjb44]

Could've been stock with 6550s.

check the bias resistors. 150K or 220K?

This should tell you for sure.

You just need to find the range resistor and change it to give the pot a range better suited to your EL34's.



I have been thinking this as well: that it might be time to change a resistor. I have never heard of any of the Silver Jubilee amps coming from the factory stock with 6550 tubes, but for some reason I am not getting the correct range out of the bias pot.

Would the 150k resistors mean it is setup for 6550 tubes, or would the 220k be for 6550?

-Joe

[phsyconoodler]

Different brands of tubes can draw very different currents.The best way to get you in the ballpark is change the range resistor until you get a bias voltage range of about -50v to -25v.If you can get in that range you will be good to go.
You might find that changing the bias trimmer to a 50k would help give you a wider range so you won't run into this issue again.

[big mike]

I have been thinking this as well: that it might be time to change a resistor. I have never heard of any of the Silver Jubilee amps coming from the factory stock with 6550 tubes, but for some reason I am not getting the correct range out of the bias pot.

Would the 150k resistors mean it is setup for 6550 tubes, or would the 220k be for 6550?

-Joe

220K is the El34. 6550 loaded marshalls came with 150K.
Not 100% sure if Jube's ever did though, but I know Slash is running KT88's, and lots of guys modded to 6550 in those.

Different brands of tubes can draw very different currents.The best way to get you in the ballpark is change the range resistor until you get a bias voltage range of about -50v to -25v.If you can get in that range you will be good to go.
You might find that changing the bias trimmer to a 50k would help give you a wider range so you won't run into this issue again.

Excellent advise, I agree.

[guitarjb44]

A few weeks ago I changed the resistor that leads to the bias pot from 47k to 56k- and now all is good.

-Joe

[dazco]

Thats is odd tho. I'm 99.9% sure jubilees never had 6550's. Then again, unless you are the original owner it could be someone before you swapped the bias set resistor for a different value to use other tubes. I'm pretty sure marshall used only EL34's at the time the jubilees came out in i believe '87.

[Jerry Glass]

If that amp were set up for 6550s, your EL34s would have biased from cold to ice cold.

[guitarjb44]

If that amp were set up for 6550s, your EL34s would have biased from cold to ice cold.



My Jubilee has EL34s, not 6550. All good.

-Joe