JCM900 running real hot!

[delmonte]

Hello there,

At the moment i'm stuck with a problem with my Marshall JCM900 combo (4101 model 100W. build in 1990 with 4 EL34's).
I got it recently, second hand.
The problem is that the tubes are getting realy hot, but I thougt if I reset the bias, things would be OK.
No such thing. I can not get the tubes under 66 mV, the biaspot is turned all to the left. The plate voltage reads 430 V and the bias voltage is -32 V(Marshall recomends -38 to -40 V).

I got the marshall scematics. the power Amp PCB reads JMP50B wich according to the doc's is the pcb for the 50W model (4501 etc). ???

An other question; there is no wire comming from the 4ohm (? not 100% shure) left outer pin of the OT. this should go to a resistor/cap and then to the PI. Is this OK?

I hope someone can help me with this, because it is a real nice amp. Good "warm" sound.......

[Enzo]

And when you turn the control the other way?

What range does the bias voltage have when the power tubes are all removed?

If the bias voltage is always too low, then flip your meter to AC volts and measure the bias at the power tube sockets. There should be just about zero volts AC. If there are several volts or more of AC, then most likely a bias filter cap has failed. Possibly a resustor in that supply has driffted way off.

They often use the same board for both 50 and 100 watt versions of amps, the only difference is the couple extra tube sockets and some small parts. The transformers are different too of course, but they are not on the board.

The negative feedback "wire" does not connect at the OT terminals, it connects over on the circuit board near the speaker jacks. It is a copper trace instead of a wire then.

[delmonte]

Thank you for the relpy.
If i turn the bias pot the other way, it rises to 95mV, bias voltage drops a little. -28 or -29 V.

I'll check the values without the power tubes, and you other reconmendations tonight.

About the 4ohm wire; there should be a wire running from the OT to the PCB?

[Enzo]

Only if it was intended on that model.

[Robert M. Martinelli]

Hi,
have you verified the issue of both amp and schematics? I know it is a dumb question, but the 4100 series has been built at least in three versions ( to my knowledge ), issue 50 52 and 60, and some issues had 5881s instead of EL34s.

If the versions match, and the schematic you have is the right one, another thing I would check is the bias adjustment network, you stated you can' t go under -32 V, so I would advice to check carefully the trimpot and resistors' values, the diode and the filter cap on the negative supply. if one of them is out of spec the attainable voltage range will differ from the design, and the plate quiescent current could rise to a dangerous level. the bias adjustment should be able to move between, say, -35 / - 45 VDC.

Oh, you stated "66 mV" so I guess you' re using the "1 Ohm cathode resistor" biasing method. If so you already know 66 mV would mean 66 mAmps, but since you' re measuring at the cathode, you are measuring the total current flowing through the tube, including the 8-9 mAmps flowing through the screen grid. Remember to subtract the screen grid current ( which you can calculate measuring the voltage drop across the screen grid resistor ) from your reading. This way you can have a correct plate quiescent current measurement.

Hope this helps

Regards

Bob

[delmonte]

Thanks for the replies.

Well, i tried a few things.
Without the power tubes values were almost the same.
What i noticed; On stand by the bias voltage is -52 volts and when I put it off stand by the voltage drops to -32 v, but maybe that's normal.
There is no AC voltage.

Also compared the resistor and cap values to the scematics. The bias network values read and measure OK.
How do I check these filter caps?

About the issue nr of the amp;
The only thing I can find is that's a 4101 STD (says so 3 times) production date 2-10-1990 (handwritten, not 100% 1990 is sertain)

[Robert M. Martinelli]

Hi,
seems to me the bias voltage on that series was obtained from the HT sec, so it makes sense that the voltage drops with HT supplied to the center tap of the OT ( and thus to the anodes ), your problem seems to be an excessive voltage drop; as you stated the bias voltage should be -38/-40 VDC, and with such voltage your reading across the 1 ohmmers should be 40-50 mAmps, which, subtracting the screen grid current would mean a plate current of, say, 40 MAmps, which, multiplied by 430 VDC of plate voltage, would mean 17,2 W ( still safe for an EL34 ).

My advice would be to follow the "voltage path" for the bias circuit. On the 4101 they used a different approach to obtain the bias voltage : instead of using a series resistor ( 220 K ) from the HT of the power tranny, they used a series capacitor ( using its capacitive reactance Xc at 50 Hz to drop the voltage ), this cap is located before the diode which rectifies the negative portion of the AC voltage; if the cap is defective the voltage drop across it could be excessive and could be the reason for your problem.

Hope this helps

Best regards

Bob

[delmonte]

Dear Mister Bob,

You were absolutly wright!!
I changed the cap and all is well now.
Baised my new tubes...
and off I go, never sounded better (of course you'd say).

Thank You Verry Much!

[Robert M. Martinelli]

Hi Delmonte,
I 'm very glad my help' s been useful; one of the two reasons I joined this forum was to share the little knowledge I have ( the other was to increase my knowledge, which is never enough.... ).

One last word about biasing... there are a lot of theories about how far you can go biasing the output tubes....some say hot biased tubes sound better; I always try to find the minimum value at which cross-over distortion doesn' t occur, and I bias the tubes right above that point, if you have an oscilloscope and a sinewave generator you can determine this point pretty easily, especially "magnifying" the zero crossing region, if you don' t have them, then let your ear be the judge....your tubes will be very grateful if the bias is not so hot, and they will last longer.
Regards
Bob