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]

Issue number ?

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]

Bias voltage source and path

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]

Happy to help!

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

[davman]

JCM900 biassing suggestion

I hope my experience in repairing a JCM900 100W dual reverb may be useful, as the bias cct seems very similar for all the JCM900 models.

i) Two of the 4x EL34's had gone white, so a new set of quad EL34's were fitted.

ii) The bias current would go no lower than 84mA per valve (should be 44mA), & it was found that the grid bias voltage could not go more negative than -31v.
Note, I measured the current between one cathode & 0v.
Grid Bias volts were measured on a 10Mohm DVM across grid to chassis.

iii) With main caps discharged, I removed the two wires to the driver stage O/P coupling capacitors - just in case these were leaky - but did not affect the grid bias volts, so were reconnected.

iv) The actual fault was the 47nF, X1 coupling capacitor C15, which joins to the transformer bias winding. See attached cct.
This item measured only 27nF - indicating a problem with metallisation damage.
The cap showed no sign of exterior bulging or cracking.

I was rather surprised by the fact that this 275v ac capacitor had 363v ac fed to it from the bias winding of the transformer (with the Standby switch off) - as the main smoothing caps are initially at 0v (Standby off) - pls see attached cct, point A is initially at a diode drop from 0v.

The other end of the bias cap (C15, 47nF) can only go to a neg voltage, dictated by C14 charging - so as C14 starts to charge from 0v, the other end of C15 can go to peak negative swing of the transformer bias winding.
For 363v ac on the bias winding, this is 363 x 1.414 = 513v pk.

I know that the specified X-rated capacitor can withstand overload voltage spikes, but the charging of C14 gives a maximum of 32% voltage overload on C15 during initial charging. This happens every time the mains power is turned on (with standby OFF).

Could this be why my 47nF cap has deteriorated over time?

Interestingly, at mains switchon, the voltage across the main smoothing caps climbs from 0v to about 117v (in standby) - taking about 50secs. I realised that this was due to a charging path via C14,D1 & C15 - effectively soft-clamping one end of the bias winding so that the other end can source current into D5.
The corresponding initial transient current at switchon can be high, which also stresses C15.

My suggested alternative
Why not use a capacitor whose working voltage is much higher than any momentary overload voltage?

In my case, I used four paralleled 10nF, 2kv disc capacitors, giving a 40nF, 2kv capacitor - which can fit in the original C15 space (using two stagerred-pair arrangement).

This allowed me to set the correct bias level (44mA) and had a wide bias range with no change in other components. The operating grid votage on the EL34's was -42v.

I would be grateful if anyone can forsee any problems with this approach.

[g1]

I don't see a problem. Where I worked when these models were current, we noticed the same issue with the standby voltage on the bias feed caps. When they failed and the users overfused the cathodes the OT's would go too.
We used 600V philips 341 series (yellow chiclet) for replacement and never saw any failures.

[minim]

I know we're on an old thread here, but give this guy a read:

http://atomiumamps.tumblr.com/post/9...or-jcm900-4100

He's a bit of a genius. The last paragraph on the JCM900 is relevant here.