JCM 900 Problems

[willshep]

Hi - been checking the forum, can't find this specific problem.

Ok - I have a 4101 JCM 900 112 combo 100W (w/ el34s) - I bought it from a friend - maybe 8 months ago - I noticed about 3 months ago - that rarely, it would start quitely popping, it would almost be like a crescendo, up to really loud popping - and then it would just stop. Though it's got worse recently - took it back to my friend who knows a good deal about audio amps in general and we eventually noticed that two of the tubes in the back of the amp were glowing cherry red (2,3) - and the others were not (1,4). So thinking it was simply in need of new valves, I bought 4 el34s from Watford (both measured at 44ma but one pair was at 5.4V the other pair at 5.6V) and replaced - all fine - however after 20 mins - same thing happened - two valves started glowing (after some popping) - I swapped 1,2 and 3,4 - and again after some time the same two valves started glowing.

THis is where i think it gets quite confusing because i don't see how it could be just a mis match - not only because watford are pretty renown at getting that sort of thing right, but surely the mismatch would happen immediately not after 20 mins and so suddenly glow red like they have been. Also at no point does the amp (so far) go dead - it always plays, the volume just goes up and down.

The only ideas i have are to check for components that are delosdering themselves when it gets hot - or an CAP that's arcing - has anyone expericed this before - your help would be much appreciated.

Thanks,

Will

incidentially i will be testing values or componsnents and checking again for any burns etc... although recently i checked and saw none.

[Alex R]

...problems in the bias supply circuit I should think. First thing is take out the EL34s and check for bias voltage on pin 5 of each one. From the tube side that's the fifth pin counting around in a counterclockwise direction from the lug in the centre hole.

[Amp Kat]

It is definitely not a mismatch and you have a problem. Either a coupling cap is bad passing DC and causing the bias to fluctuate or a bad tube socket connection or an open resistor maybe in the cathode circuit if there is a current sense resistor. Do what Alex said and monitor that bias on pin 5 and see if it's dropping less negative after a few minutes or when they start to red plate. Matched tubes are not always matched but even largely mismatched tubes will not cause red plating they will just create crossover distortion. Something is causing the tube to pull excessive current and when you find it they will stop red plating.

[willshep]

Thanks for the quick and informed response guys; just to make absolutely sure, i'm testing the pins on the valve sockets on the PCB - not the actual valves themselves (just to avoid confusion)?

Thanks

Will.

[Alex R]

Well, either really - slightly different recommendations. With the valves out you're looking at pin 5 from outside the chassis to see if the bias voltage is being supplied in the first place. With them in, you're inside the chassis checking what happens when the redplating starts. Easier (and safer) to do the outside-chassis check first... but you're going in there anyway if you want to fix it...

No, not checking valves, seeing if the amp is supplying the bias voltage correctly (which it likely isn't). My post was ambiguous, sorry about that.

[willshep]

Thanks for the reply,

I'll have to wait until the weekend to do the checking - i think it would be a good idea to get some caps and resistors though - presumably the large blue caps visible next to the valves are the de-coupling ones, as opposed to the coupling ones which will be much smaller and inside - as far as i can tell i'll be looking for some 47nF@400V and 2k2 resistors - any idea where is best to get them from in UK - I'm unsure what cap type to get with regards to Maplin..

Thanks again,

Will.

[willshep]

Little update - (i found all components i might need btw.)

I've had a chat with a friend of mine - we're thinking that the two 50uF+50uF 500v Filter caps are the most likely component to be failing as not much else is going to let enough current through to essentially melt glass! (the sides of the old tubes were actually bending very slightly...) - and it always seems to be a consistent delay while things warm up before 2 of the vales red-plate. Yesterday i simply turned on the amp no guitar plugged in and settings all quite low - and simply after about 13 minutes the popping starts suddenly and the 2 tubes start glowing.

It can't hurt to replace them anyway - none of the other components look damaged but I will of course test all that i can.

Thanks, will update after this weekend.

W.

[Alex R]

Bad main filters won't cause the redplating that has melted the glass. That will be due to some kind of bias supply failure - back to pin 5...

Current doesn't flow though the filters into the rest of the amp - they just filter out ripple by passing AC, not DC, to earth. If they fail, current can pass through them to earth and destroy them, but that won't make the valves redplate.

[willshep]

OK (quickly cancelling the order to WAtford for 2 filter caps ) - so it's the bias that controls current flow in the Valve - so i'll be expecting a complete dropout at pin 5 - thus causing the red-plating (full current running through?) - I'm not going to be able to test until the weekend - it's really frustrating...

Thanks for the continued advice.

W.

[Alex R]

That's right. Pin 5 is the grid, where there will be a signal (when you're playing), and also a negative bias voltage, like about minus 50v. These two voltages between them control the flow through the valve from plate to earth (actually the electrons go the other way but NEVER MIND THAT). The signal fluctuations are then present in amplified form at the plate (pin 3), where there will be 450 or more volts.

Yes, the bias voltage stops a runaway current flowing through the valve and causing it to overheat and redplate, blow fuses, melt glass and output transformers etc etc. Without a bias voltage, or with an intermittent one, you will see the symptoms you've been describing. As has already been said, there is probably a problem inside the amp, one of two or three possibilities, but we'll get to that later. Get your digital camera ready....

Pin 8, the cathode, is earthed on your amp and should be at zero volts. Pin 4 is the screen, another control element, at a similar voltage to pin 3. Taking out the power valves and - with no input - checking the voltages between the chassis and pins 3, 4 and 5, is a good first step in diagnosis.

Remember that from outside the chassis you should count anticlockwise from the locating lug in the centre hole.

[willshep]

So impatience got the better of me and I went and bought a multimeter and did all the checking myself...

I found that the idling voltage was at around -34/35 - which is presumably running the valves quite hot - i found the trim pot and adjusted it to stay at around -38 - amp has now been on for some time and there's no problems yet.

There's a large drop from about -63 in standby to -38 in 'active' mode - though I would guess that's normal. Anyway the amp isn't causing any problems as yet..

The chassis is not yet back inside the speaker cab (as it's a combo) so I guess if the original problem wasn't because the bias was set too 'low' (or high depends on what you mean..) and may have been heat related, I won't know - but this has certainly been a positive experience

Is there anything else you can recommend other than just wait?

Thanks for all your help and guidance -

W.

[willshep]

On putting the chassis back in the speaker box, I find that the same problems occur after it's heated up a bit... so frustrating!!

I had it running for over an hour with no problems with the chassis open! One little discrepancy i found was that the coupling caps 47nF were rated as 250v inside the box but 400v on the schematic; wonder if that's anything to do with it?

Rergards

Will.

[Steve Conner]

If those capacitors are only 250V, replacing them for 400V units won't hurt. They can see at least 400V until the PI tube heats up, if the power switch is turned on with the standby already on. This could have made them go leaky.

However, I've found that EL34s can sometimes just go into thermal runaway and redplate if they feel like it, especially if cooling is poor. Thermal runaway, BTW, is when the tube's grid gets so hot that it starts to emit electrons and drag its own bias voltage towards zero, making everything even hotter, and so on.

I once worked on an amp that would have the uppermost tube run away and redplate when the chassis was stood on its side on the bench, but work normally otherwise. The fact that your amp works on the bench but not in the box suggests that you have a similar runaway problem, since tubes-down combos run hotter in the cabinet. Your two middle tubes run hotter because they get heat radiated onto them from the outer ones, so they run away first.

Did you check the 220k resistors that feed bias to the grids? Maybe these are bad, and in any case 220k is a marginal value. The higher this resistor, the easier the tubes run away. If you just can't get the amp to behave, you could try replacing them for 100k. There was nothing wrong with that amp I was working on: it just seems that EL34s are close to thermal runaway with 220k grid resistors, and 100k buys you more headroom against it.

[willshep]

Hi,

I checked the bias again in the amp today - and found that the pin 5 readings (in standby) were fluctuating by around 10V from -52 to about -62 - when i switched the amp on the bias sunk right down to -33 on the valves and I switched it off. That's far too low

Also found that on one of the big de-coupling caps (50uF+50uF) the voltage was reading above 500v (and they are only rated at 500vMAX - as is my DMM so I switched it off...)

Also - C14 - a 10uF 100v electrolytic (axial) cap which forms part of the bias circuit where the trim pot is - looks like it's been affected adversly by heat as the outer plastic covering has essentially 'shrunk' exposing about 3mm of the metal casing of the cap - i'm not sure that it hasn't always been this way though - but all the other caps appear normal... though it seems really likely that if this cap was sizzling away inside that the bias woudl be affected...

So i'm going to replace that cap and see what happens this weekend...

The other thing i could do is install a quiet fan inside... hope that doesn't affect the sound.

Regards,

Will.

[52 Bill]

C14 is the filter (de-coupling) cap for the bias supply, it could be cause the fluctuation in bias voltage that you are reporting. Also check all of the connections and the resistors in the bias circuit.

Something that gets overlooked sometimes is a bad or dirty bias adjustment pot. These can get dirty (oxidized) just like a volume or tone pot can and can cause the bias voltage to jump up and down.

Adding a fan isn't a bad idea, but it is treating a symptom not curing the problem. There are thousands of these exact same amps out there working just fine without needing a fan to make them work properly. I think that you should first continue investigating the original source of the problem.