200S questions

[bonosurf]

I have a 200S which appears to have problems. the gain of the amp has decreased over time and there is clipping at levels which were previously undistorted. All tubes have been replaced but the problems of lower gain/clipping are still there.

I have had the amp for over 20 years but the previous owner had some "mods" done. The amp has diode rectifiers (though it doesn't look like there ever was a tube rectifier - no socket??) A slave amp/direct line has also been added to the output of the amp.

The amp has been getting weaker over time and I assume that the original capacitors are some if not all of the problem. The large can capaictor (multi-element) looks like it would be togh to find a replacement for. The other axial leaded capacitors are probably not so difficult to find.

Any suggestions on where to start (I know whot to discharge the caps so I don't hurt myslef) and I have a schematic.

Thanks

george

[Ryan_77uk]

I would say that your capacitors have gone. The other guitarist in my band has an old wierd Marshall and his capacitors went about 6 months ago and it sounded like what you described.

Good luck!

[66tele]

I agree that it's probably your multi-section filter cap (the upright, round metal can on top of the chassis). Contact Antique Electronic Supply at tubesandmore.com for a correct replacement.

Good Luck!
Ryan

[TubeMan]

also, if you have room in the chassis and youd like to go a lower cost route, you can match the values contained in the can capacitor, and replace them with standard axial caps. the only real problem with that is finding somewhere to mount them in the amp. when i had to do it i just found a nice shady spot and siliconed them in there.

also, if youre going to go to the trouble of replacing the power filter caps then it would be wise to replace ALL of the electrolytic caps. Also the mylar and ceramic caps that are under HT (high tension) voltage.

pay special attention to the interstage coupling caps between the preamp stages, and the bias coupling caps on the output tubes, as these are under constant HT voltage and control the biasing of each tube stage. concequently they are very important to the life of your tubes.
The bias caps on the output tubes in particular, because if one starts leaking, it will pass positive voltage, causing the bias to drift and the tube to go into thermal runaway.

the extra cost is minimal compared to the peace of mind.

[Anonymous]

Ok, I give.  How can a leaky cap acting as a filter in a negative rectified environment somehow change the polarity to positive?

[TubeMan]

Guest> I will ignore the obvious sarcastic tone of your post and answer your question.  

A leaking bias coupling cap does not "somehow change the polarity to positive." The positive voltage is already there. I have taken the liberty, and will demonstrate using a portion of the Model T schematic available on this site.

observe:



one terminal of the cap is connected to the B+ supply, and the other terminal is connected to the negative bias supply. The B+ supply is the reference to positive you need to draw a negative voltage from ground (or pass a positive voltage TO ground, which is negative relative to the B+ supply. however youd rather look at it.   )

If this capacitor is leaking, it will allow a portion of the B+ supply to leak through causing the voltage at the bias side of the cap to be in an increased positive state. In this state you would need an increased negative voltage to compensate for the leak. causing one tube, or tube set, to be biased too hot if there is no adjustment, or one tube or tube set to be biased too cold if you adjust the bias without replacing the cap.

[JoeArthur]

I think the point of confusion here is the use of the term "bias coupling caps".  I must admit I have never heard that term before, as C12 and C13 are merely "coupling caps" as far as I'm concerned, as they serve the same purpose where ever they are found in an amp.  They don't "couple the bias", they isolate DC voltage from one stage to the next - plate voltage on the previous and bias voltage on the next, as Tubeman points out in his explanation.  The only thing that is actually coupled is AC - meaning the signal voltage.

A bad coupling cap is rare, but I have seen it happen.  The easiest way to check for it is to disconnect the coupling cap end that is closest to the grid of the next stage (electronically, not necessarily physically), connect a voltmeter (positive side to the grid, negative side to ground), then turn on the amp.  After an initial charging "kick" of the meter, there should be absolutely zero volts.  Replace the cap if you measure any positive voltage at all.

A bad coupling cap can occur in the preamp stages as well, so don't forget to test them all.  Don't stop at the first bad one found either - if one goes the chances of having a few others that are bad just increased.

BTW... in the amps I have seen this occur, the tube rectifier had been replaced with solid state diodes.  The overall voltage stress on coupling caps is the sum of the voltages between the two stages (absolute values).  So, remember to add the bias voltage to the B+ voltage of the previous stage.  When a tube rectifier is replaced with solid state diodes, it will increase the B+ voltage of the amp.  This can destroy power supply electrolytics as well.

[TubeMan]

Joe,

I was naming the caps "bias coupling caps" to narrow down their identification by association. I know they dont "couple the bias."  

The reason that i suggested they be replaced are:
-theyre cheap and easy.. might as well
-if one decides to leak in a fixed bias amp, you can lose a tube and not really know what happened unless you know what to look for.
-they are under constant HT and usually forgotten.