JCM900 2100 Power Tube Bias question

[ThKind]

Hey guys I've got a Marshall JCM900 Slx 2100 that i just did Power Tubes in. I got a matched set which when checked with a meter are all within reasonable ratings of one another, but when i got to check the bias for them, they are on average 5 mA hotter than what an el34 should run (should be 35-40 mA they are @ 45mA) this is after the bias control has been turn all the way down. All 4 tubes are runnning in the same rang.... My question comes down to how can i fix it for them to run on the cooler side so i don't torch the tubes, i would like for them to last longer seeing as how i am using this amp as a touring rig and can't nor want to be tracking down tubes while on the road. if anyone can help that would be greatly appreciated.

Thanks Tom

[soundguruman]

The major issue with 900 is oscillation in the effects loop.
Once you get the oscillation out, the output runs quite a bit better.
The plastic 50/100 power level switch may have failed. Or the connections on the board for the screen resistors crack and fail.
Or the tracks on that rear board tend to burn and fail.
This explains why two tubes may be running much hotter.

[ThKind]

Thanks for the reply Sgm ... i wasn't totally clear on that... i am using the 1k cathode shunt method to bias firstly, and it seems all four tubes are at least 4.8-5.1 mA higher. i have no signs of any other problems on the amp although when i do flip the power switch (the 50/100w not on/off) there isn't any true difference to my ear, and not to sound like an ass but i have a fairly sensitive ear from doing recording and sound engineering for the last 15 years.

[J M Fahey]

All 4 tubes are runnning in the same rang

Just sayin'
Please post the SLX schematic (which is not "just another JCM900", despite the label) and maybe we can suggest something.

[soundguruman]

Thanks for the reply Sgm ... i wasn't totally clear on that... i am using the 1k cathode shunt method to bias firstly, and it seems all four tubes are at least 4.8-5.1 mA higher. i have no signs of any other problems on the amp although when i do flip the power switch (the 50/100w not on/off) there isn't any true difference to my ear, and not to sound like an ass but i have a fairly sensitive ear from doing recording and sound engineering for the last 15 years.

If you are flipping the power switch and there is no difference, then something else is probably wrong.
Of course we are talking playing the amp very loud, as it should be.
Yes that rear board has problems...
Yes, there are several models of 900.


Look closely, do you see the problem?
You are thinking bias problem, and I am telling you, it's a circuit board problem.

[ThKind]

Google Image Result for http://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_jcm900_slx_100w_2100.pdf_1.png

by power switch i mean the 50w/100w (i hope that was apparent and not the on/off lol) it's the jcm 900 slx 2100 I have rung out the switch it self and that seems fine and nothing seems wrong with the board. I pulled that and checked it visually and nothing seems burnt, haven't quite checked every thing on it with my meter but all seems well. the schematic indicates a 48mV (with the shunt method it would be mA = mV) bias is right but the last set of tubes i put in were torched to high heaven in 3-4 months and through every thing else i have read it seems like the EL34 would really prefer something around 35-40 mV...

[ThKind]

certainly no holes blown through the board... i wish it was that easy

[ThKind]

I'm thinking that a resistor change should be in order but not 100% sure which ones to change on the schematic there are two one before the the pot that is a 10k and one after that is a 47k (jcm 900 slx 2100 100watt... positions r28 and r29 respectively) i'm thinking that the r28 could be changed to lower the bias a hair but just want to make sure before i blow it up

[Jazz P Bass]

Here is the schematic.
It appears that you have well matched tubes that are 'hot'.
It is important that there be minimal ripple on the bias rail.
So check that with your meter set to Vac.
Beyond that, you could try changing the voltage divider (R29) to get a higher negative voltage at the tubes.
Attached JCM Bias.

[J M Fahey]

Well, those schematics are both horrible and incomplete, like al late Marshall "tech support".
None of them shows clearly where the original bias voltage comes from.
Probably lowering R29 may cause biase voltage to raise a little, but I think raising R28 to 68K or 100K will have a larger effect.
Of course, only actual trying it will do.
Anyway you don't need much more than you have now, maybe 4 or 5 Volts extra will do.

[soundguruman]

I'm thinking that a resistor change should be in order but not 100% sure which ones to change on the schematic there are two one before the the pot that is a 10k and one after that is a 47k (jcm 900 slx 2100 100watt... positions r28 and r29 respectively) i'm thinking that the r28 could be changed to lower the bias a hair but just want to make sure before i blow it up

Well, Marshall is supposed to run hot, or else it sounds like doo doo.
But again if one side runs hotter than the other, it's not usually a bias issue on this amp.

[g1]

Could you please explain what you mean by "1K cathode shunt method"? Is it possible you are seeing the current for 2 tubes?
The "OPV fail" fuses are a convenient place to insert a current meter and measure mA. If you do it this way you will get a value for 2 tubes at each fuse.

[ThKind]

Thanks guys, I switched out the 47k in r28 and dropped in a 100k and it brought me down to 30mV which is right where i wanted to be, again just for longevity i don't want to be on the road tracking tubes down in some garage sale or something cause this thing decided to get hot enough to cook some steak on. still sounds solid with plenty of power (i mean really 100w is excessive anyway) and clean (for a dirty marshall that is, ie no hum or buzz)) i do greatly appreciate the help everyone.

thanks again Tom

[ThKind]

i have a amp head tube bias tester Dual Bias Tester Probe for Vacuum Tube Guitar Amplifier by AMP-HEAD which uses a 1k resistor to shunt the cathode for a 1mA=1mV ... or so they say ... shrug. i tested a few other amps and it seems to work fine on them so i trust it on this

[J M Fahey]

Fine but your "1K" resistor is actually "1 ohm", the proper value to turn 1 mA through it into a 1 mV reading.
In fact the brochure you linked to states:

Probes have matched 1-Ohm 1% Tolerance Shunt Resistor.

Good luck and happy that you could solve your problem.