Bias settings for a MA50C Marshall combo

[bsco]

Hi there. Trying to get a handle on calculating bias settings. The MA50C that I have was biased at 70mV per side....This is the second set of tubes that the customer has used in a matter of a few hours. Does this seem extremely high for this amp?? I set the bias for 40mV and I measured the screen current which was under 5mA....4.6mA to be exact... (by measuring the voltage drop).....plate voltage was 505V...but I am still confused with the calculations to determine the correct bias setting with a certain plate voltage reading....also should I bias for 70% or should I bias for 60% of plate dissapation?? I am trying to get a good compromise between good tube life and good sound.... with the bias set for 40mA what percent of plate dissapation has this given me?? If the max plate dissapation is 25watts for this tube, and i have a plate voltage of 505V and I want to bias at 60%, then .6 times 25 divided by the plate voltage will give me the bias value......29mA.....I can't get the controls to go low enough to get to that reading...I can only get to very high 30's(and with slight drifting to around the 40 mark)... for the bias setting.......am I doing something wrong here...or do I have a circuit problem.....or both???? Any insight greatly appreciated....
Cheers,
Bernie

[Chuck H]

It sounds like a design specific anomoly to me. But if you stick to the specs everything should be fine. You CAN change a resistor in the bias supply to get to the lower mA setting if you have to. Why the adjustment isn't possible with the stock circuit is a mystery. 505Vp @ 70mA is well beyond the max dissapation for the tubes. As for a "correct" setting... Well... Whatever sounds good and works within safe parameters is correct. The Vp will change with bias setting too. Or, at least it should. But with 505Vp @ 40mA the dissapation is around 80% dissapation. With a high-ish Vp I would expect somethig more like 60% to 70% dissaoation at idle. Have you confirmed that the Vp should be over 500V?!? That does seem high for a modern amp. Have you tested the filament voltage? If it's high this could reaveal a short in the PT primary that is causing excessive Vp.

[bsco]

Thanks Chuck...I'll check the filament voltage and let you know...I'll also check the schematics to see what the original source voltage for the plate is actually supposed to be....but if I have a resistor that has changed in value, the readings should be lower...I have worked on a few of these amps lately and they pretty much have high plate voltages...I was mis-informed about the first set of tubes. This amp was purchased used and the new owner could get a metallic ringing sound from the amp when he played it...and apparently when tapping the output tubes, he could get this metallic ringing sound so he replaced them...The new set of tubes only lasted four hours and one of them blew...I checked the new set when it arrived and one was shorted intermittmently. I also checked the original set and they checked fine but were kind of weak on the emission side of things. He provided me with a brand new set and this is where I am....I also noticed that the original tubes had their labels badly dis-colored and they were GT's......the set that he installed were also GT's....the new set that was given to me are also GT's....EL34's.......what if I just temporarly installed the original first set of tubes and check all the voltages???maybe the new tubes that they gave me are drawing too much current anyway....even though they are new and supposedly matched.....and I also noticed an excessive oudor that you would normally get when something is running too hot..If I do have to change out resistors in the bias ckt.....which ones??? and there are two bias voltages.....one for each tube...will increasing the values of the cathode resistors improve anything?? Sorry for all the questions...just trying to get myself as much info as I can....
Cheers,
Bernie

[bsco]

HI Chuck....I just re-installed the original tubes that were in the amp...The bias went down to just under 30mV.....both sides.....also note...the bias controls are backed off as far as they can go as well.....but it is not at 40mV as it was with the brand new set of "matched" tubes. So am I correct in saying that the new tubes are drawing too much current??? I don't have another set of new tubes to try out....at least not until Monday....I'll have to get another set from the store and bring these ones back to them.....what a difference in tubes......anyway, your input (and anybody else's for that matter), is kindly welcome....
Cheers,
Bernie

[Chuck H]

Well it certainly seems that the new tubes ACTUAL spec is the problem. If the old tubes work, and the new tubes don't and/or even fail, what else can we conclude? But the amp should be able to run with the new tubes if the current can be adjusted. I'd need to see a schematic to identify any changes to be made.

Are both new sets of GT's the same AND are they different in construction from the originals. GT isn't a tube maker, but a rebrand. They test and match tubes from several different sources. GT usually offers an el34 from China as well as one from Russia and sometimes one from southers Europe as well. These would all be somewhat different tubes. GT's also have a number on the base that serves as a soft or hard designation. A low number will generally draw more current than a higher number. So if the new tubes are different, base number or manufacturer, that may be the cause.

[g1]

From what I gather, 36mA per side is the factory bias setting. If you could get this with the old tubes, but not with the new ones, it probably has to do with the number grade of the GT's, as Chuck has suggested.

[bsco]

Hi Chuck...Here is some info.....
Original tubes are EH brand...Russian made...dated 08/07 and 08/04 GT coded White (7)...both tubes look indentical when looking through the glass....this set draws a lille less than 30mA...
Set replaced by customer...EL brand...Russian made...dated 08/06 and 08/08....GT coded Red(8)...these were the set that lasted for 4 hours and when checked with a tube tester one of them showed an intermittment short...

New set from store....EH brand...Russian made....dated 08/06 and 08/06...GT coded White(5) these are the ones that are drawing 40mA
I can't get the schematic attached to this thread....if you send me your email address I can send it to you....mine is berniescott2005@yahoo.ca
Cheers,
Bernie

[Chuck H]

This makes some sense now. The tube that failed was probably a bad tube. The ones drawing 40mA have a lower number than the originals. The amp was probably designed with that original GT code #. The new tubes that draw more current have a lower number. If you had a set with the same number (7) they might bias right up. But there should be no trouble adjusting the bias for the new tubes. Provided the bias supply is working correctly now. As in, not broken. I'll PM you my Email.

[g1]

Unfortunately, no voltages shown on schematic.
MA100___MA50_Circuit_diagram.pdf

[bsco]

I'll have to go back to the store Monday and see if I can get another new set...I am going to look for a set coded 10 seeing as to how the higher the number the lower the current... Once I get this resolved, I am going to measure some voltages and write them down on the schematics...I have noticed that a lot of the Marshall stuff have no voltages at all marked in on their schematics....maybe this is their way of making things difficult....I'll post my findings Monday after I get another set of tubes.....Thanks for all the help.
Cheers,
Bernie

[Chuck H]

Rather than go for more tubes, you could change R125 and R82 from 47k to a higher value. 68k or 82k (use the same value for both) would probably get your bias in range. And, as mentioned above, do be certain that the circuit is working correctly. Both bias circuits actually.

[g1]

Chuck: I was wondering about an alternate method, tacking a resistor in parallel with R85. This should raise the entire DC bias supply. In this way you would not have to even flip or remove the circuit board. I think the voltage ratings of C70 & C71 are high enough that you wouldn't need to worry. Not sure what value to start with, maybe 33K?

[Chuck H]

Brilliant. And I would probably do it that way. I HAVE done it that way. But I've seen guys here poo poo that method as a hack job. So I didn't want to admit to it. R85 is only 15k now. Which is a small portion of the voltage division. But R124 and R83 are 47k. You could parallel another 47k across each one and that should get it in range without the need to pull resistors.

[g1]

Hack job? I've seen that done at the factory and marked on the schematic as "factory selected resistor" with no value given. Also have seen many factory mods that are less neat than a properly dressed parallel resistor on the component side. I guess one man's "hack job" is another man's "customized just for you". Somehow reminds me of a sign I saw in a shop once, "organization is for people who are too lazy to look for things".

[bsco]

Thanks g-gone and Chuck...I am going to pick up another set of tubes just to try....the store is only a couple of minutes from me anyway...and if that don't work, I'll do the resistor mod on the component side as you have suggested...I will have to make a note of the serial number and file it for future reference if and when it ever comes back to me again....I'll let you know how I made out... Thanks guys.....
Cheers,
Bernie