JCM 800 2204 Bias Problem

HHwarner

Hi Everyone, Sorry to be posting a question as soon as joining up. Iv just found this forum, I was a member on the Music Radar forum before it went down & Iv been a bit lost without it tbh. My name is Scott & I live in Pershore, Worcs. Anyway back on topic. I have a Marshall JCM 800 2204. I have just treated it to some NOS Mullard ECC83 and a matched pair of Winged C EL34. I have a problem getting the Bias current low enough on the EL34s. I have measured Plate Voltage at 435v measuring at pin 3 and ground but I cant get the bias current below about 55mA with the pot wound right down as far as it will go. My calcs are (25x0.7/435=0.040) are my calcs right? If so a tad too hot. Also Im getting a variation of 5mA between tubes. Does the amp need looking at (maybe a resistor value has wandered) or could it be the tubes. Im wondering if its time for a good service on the old beast. In the mean time should I get in touch with the seller of the EL34s or just take the amp in. I thought about contacting the seller to see if he has a cooler pair, would that fix it, Thanks in advance Scott

ICBM

I would take a bet that one or both of the two bias caps is leaking, This is a *very* common problem on Marshalls of this age and the result is lowered bias voltage and the valves running too hot. It's actually more common than failing main filter caps. Change both caps.

Some techs will bodge the resistor values (the feed resistor to the diode in particular), but this is absolutely the wrong thing to do and will speed up the demise of the caps anyway. These resistors don't actually drift much and they were the right values in the first place, so don't change them!

40mA is too hot in these amps really. Aim for 25 to 30mA, it will sound better and those expensive valves will last much longer. 5mA difference between the valves is OK (and may come down when both of them are running cooler), but always put the hotter one nearest the PT/furthest from the preamp valves, since this one gets driven less hard when the power stage is overdriven.

Do the main filters too if there's any hint they need it or you're feeling like spending a bit on it.

HHwarner

thanks for the quick reply ICBM, is there a way to check the bias caps for leaks

ICBM

Measure the bias voltage. If it isn't over 40V with the trimmer maxed, the caps need changing.

Actually I would just change them. This is the single most common problem I find on these amps, it's more of a surprise if they *aren't* leaking.

HHwarner

Ill check to bias voltage tomorrow. Is that measured over pin 5 to ground and should I do that with tubes removed (suppose im being dim here, if the pot has to be maxed, im guessing the tube need to be removed ha ha)? Im still learning all this amp bias stuff. The caps are the 2 small ones by the trim pot?

Cirrus

Many wisdoms for ICBM.

ICBM

It sounds like you have the pot maxed anyway if the valves are as cold as you can get them. Yes, those are the caps. You can measure from the top of the second cap (the one nearest the trimmer) to ground, it's safer than poking about between the high voltage pins on the power valve sockets! You should be able to get at least 40V (-40V or *more negative*, to be absolutely clear) at the highest setting. The correct bias voltage will probably be more like -32 to -35V when adjusted for 25-30mA.

HHwarner

Thanks for your help, its very much appreciated. I will check tomorrow and let you know what I have. Yes im at 55mA and cant get the tubes any cooler so trim pot is maxed out. Just for reference the tubes I took out were EH el34 biased at 30mA with the trim pot about half way round. Loaded plate voltage was a bit higher at 455.

jpfamps

I never even bother checking these caps in Marshalls; I always change them.

If they are not leaky now, they soon will be.

The problem is caused by the bias feed resistor from the HT being large, usually 220k. 

With such a large resistance in the bias feed, even a small amount of leakage will cause significant voltage drop and hence loss of bias voltage.

Current leakage in electrolytic caps increases with temperature, so if you are unlucky your amp can go into thermal run away.

HHwarner

Thanks for your help. Iv just measured the bias caps and I'm getting a reading of -43v with the pot maxed out. Does this sound low

ICBM

No... oddly enough. I'd still probably change the caps to be on the safe side, but it doesn't actually sound like the problem here - 43V should be plenty - and particularly not if the EHs biased OK. Looks like duff Winged Cs, I'm afraid.

There is also another slight possibility that the coupling caps from the PI to the power valves could be leaking, but it would be highly unlikely for both to do so to roughly the same extent (without which the two valves would bias wildly differently), and anyway the EHs tend to rule that out.

HHwarner

Mmmm strange, iv got a set of jjs which I can just about get in but slightly hot at around 33 and that's with the trim pot maxed. I'll get onto the seller see what they say. Bugger. I was hoping to gig with em this weekend

ICBM

HHwarner said:

Mmmm strange, iv got a set of jjs which I can just about get in but slightly hot at around 33 and that's with the trim pot maxed. I'll get onto the seller see what they say. Bugger. I was hoping to gig with em this weekend

Why not just put the EHs back in? They sound good in Marshalls. I'm not really a fan of the JJs and the last Winged Cs (they've stopped production now) seem to have been a bit variable for quality.

HHwarner

Yes I think that's what I will do for now. The reason I was going to change them is I was getting loud crackling and popping when I was attenuating with a power break. No crackling without or with a thd. Is there any way I can check the power break. It was still doing it with jjs so that rules out tubes

ICBM

That does sound like the Powerbrake might be iffy. Normally they put a bit less stress on the valves than the Hotplate does, but that doesn't sound good at all - don't test it any further, if it's causing arcing it could seriously damage the amp.

It might be worth sending it to Marshall, the main component in them is a proprietary auto-transformer which is difficult to test and probably impossible to replace with anything other than the stock part. It could be some other connection inside but you'd need to find a tech who is good at troubleshooting this sort of thing.

HHwarner

Bugger, I much prefer the Power break over the THD, Its way more transparent and I don't lose those dynamics which I do a bit with the THD. I was worried it may be arking or something. Im getting bluish flashes in the tubes when it does crackle or pop. I certainly don't want to damage my old JCM. I don't attenuate a lot just enough to take the edge off in smaller venues as I love to run the Marshall quite hard. I tend to run it like a non MV amp, flat out ha ha. Ill look at sending it to Marshall see what they say, They discontinued those a few years back I think

HHwarner

Well I contacted the seller and he sent me a new pair that very same day and they arrived the day after. This pair were cooler and Biased easy. They also sound superb. Thank you all for your help.  Im not sure if this is allowed on here but I cant recommend this seller enough, ebay seller yitry and his ebay shop is LANGREX. I will deffo buy from this seller again. In the mean time Iv ordered some new Caps so will have a go at fitting those when they arrive. Happy Days. Thanks once again