low volume output

[Chuck H]

The schem is redily available at schematic heaven. Your findings on the OT are suspect. I don't know what kind of meter you use but with mine anything in the tens of ohms is really rough reading. But the difference between two ohms and fourty ohms is big enough to question. Do the test again to see if it reads the same. If it does I'll go over how to use your signal generator to diagnose your OT.

Chuck

[anthillrich]

i just checked again the OT ohms test with two different meters. i used a fluke and a radio shack digital meter. They both show that same disparity between B+ winding and plate windings. one side reads 40 ohms, the other is 2 ohms.
a buddy of mine has a 100watt version of this same amp, and he graciously allowed me to do the same test on his(simple in amp test found on geofex page). i promised him i wouldnt even turn his amp on to do the test. lord knows i cant afford to have two amps fixed.
on his amp i found B+ to each plate winding to be at about 20 ohms.( lower resistance than mine, but both equal).
i also ran my pre amp out - into his fx return, and it sounded like it should(loud& clear). any thoughts, suggestions are greatly appreciated(as always).
thanks again, rich.

[Chuck H]

Sounds like the OT to me.

You can do a another test by putting your signal generator into the secondary (doesn't matter which ohm tap) and measure the voltage going in. Then measure the voltage on the primary from the center tap (B+ lead) to each end lead. If they are dramatically different then I would say the OT is bad.

FWIW it's rare to have an initial guess that the OT is bad ACTUALLY turn out to be a bad OT. You must have your finger on the pulse.

With your plate voltage almost any OT made for a two tube Marshall would "work" fine. Going by rep and personal experience I would say your best value for tone out vs. $$$ in would be the Hammond unit. Heyboer also has a good rep but their trannies are harder to find because they only sell proprietery designs through distributors. But I've heard great reports on the Hammond and had good experience with Heyboer. Both fall into the "affordable" price range as OT's go.

You may even get a tone improvement as a bonus for your trouble.

Chuck

[anthillrich]

chuck, thanks again for the tips. i'll def look into the hammond if my OT is infact toasted. i will do the test you suggested for me. btw, i like the way you described it. i have read posts on how to check the ot, but i was always alittle unclear of exactly what should go where, and there seem to be a variety of different methods, but this test seems to be within my abilities.

i was wondering, do you think it would be a good idea to change filter caps and some coupling caps as well, if i need to replace the ot?

thanks again, rich.

[MWJB]

the 100W amp has an OT wound on bigger wire so dc resistance will be less than a two output tube amp, 20ohms doesn't sound unreasonable.

However, your OT sounds like it is shot, you should only get a difference of 10-15% (in practice it is often less than this).

[anthillrich]

hey guys, thanx for the reply.
my ot is infact toasted. oh well, not the worst thing in the world to happen.
i dont want to ask why it blew up, cause im sure there are numerous reasons it could have happened. What i am wondering tho is, why did it go out with a wimper instead of a bang? i mean , it wasnt hot( or even warm) to the touch when it blew up, no puff of smoke, no flames.... it just died.

the amp was cranked up when it happend, but i run this little box quite hard most of the time i use it(and have, since i got it years ago).

i guess i was expecting a more grandios failure. not complaining, just not what i was expecting. i will replace it(and a few caps-n-such) and report back.

i hope to someday contribute more to the forum than just questions, but questions are all i have to offer right now.
thanks again for the help. rich.

[MWJB]

most likely causes are improper speaker load - wrong impedance, bad connection? Too much plate curent on the power tubes - check bias.

Be sure to get these checked otherwise the new OT may go the same way.

[anthillrich]

i will certainly check bias supply and HT supply, as recommended.
as far as wrong impedance of speaker goes, it is the same 8ohm thats been in there since i got it.( its a 1 12 combo).

i used to run an ext 1 12 cab in paralell that was 16ohm, that did create a miss match( this was before i even knew what an ohm was).
however, this set up quikly fell out of favor due to my lazy take on carrying extra stuff to a gig.
point is, i didnt run a miss match very long, and i havent for several years now.
the speaker cable i use is very sturdy, and does not take much if any abuse, because of its " tucked away" location in the amp. i will make another just incase, but im not convinced thats the problem tho. but ofcourse why take a chance.

im still a bit puzzled as to why it quit.(the ot).
i am also confused about why a static ohm requirement is neccessary for matching a speaker to an OT, the impedance of the speaker changes with freq--right?
so why is it so important for some amps, and some amps dont seem to care what load you put on them(ie. mesa mark 1,2,3 ect.) is the quality of the irons made for some amps that much better than others?
i've read up on the importance/or not of speaker matching, and it seems even the experts cant agree on this one.

MWJB, you mentioned in an earlier reply that some OTs put in by a company might already be mismatched to the tube compliment they used, and that putting an extension speaker on may push it over the edge. this certainly makes sense, but surely people mismatch their stuff all the time, without even knowing or caring.( i did for awhile, before i knew better). its actually funny in a not so funny way, that the amp i own, didnt really give me any trouble untill i started to give a d@#% about it. oh well.
thanks for taking the time to educate me on some things guys, i always appreciate it. rich.

[MWJB]

"i am also confused about why a static ohm requirement is neccessary for matching a speaker to an OT, the impedance of the speaker changes with freq--right? " - Look at it as your nominal datum line, yes impedance swings around which is why you have an "acceptable margin" of mismatch, but if it swings too far, for too long something has to give (tubes, OT).

"so why is it so important for some amps, and some amps dont seem to care what load you put on them (ie. mesa mark 1,2,3 ect.) is the quality of the irons made for some amps that much better than others?" Again, because some amps use middle of the road primary impedances that allow you to mismatch speaker loads to some degree (e.g. for a typical BF Fender you can double or half the rated load - except for Super Reverbs & 4x10" Concerts, you don't really want to halve a 2ohm load). If you start with a degree of mismatch, then your options are limited. The Mesas you mention (MkI RI, MKII, MKIII) have multiple secondary taps, suggesting that they DO care what speaker load they see.

"i've read up on the importance/or not of speaker matching, and it seems even the experts cant agree on this one." Re-read the previous paragraph & the previous post on OT impedances. The experts DO agree...if anyone tells you that speaker load doesn't matter in a tube amp, then they are NOT an expert.

"People" do mismatch loads all the time, they also change tubes without checking bias...some get away with it, some get big repair bills. Some "people" play Russian roulette, does that mean you should too?

Usually Fender & Marshall OTs are up to the job, you really have to do something bad to make them pack up. The 2 most common ways to do this are to overload the primaries with high plate current (too hot a bias), or severely mismatch speaker loads. Running an amp permanently dimed can make it more susceptible these 2 issues.

[anthillrich]

thanks for the reply.
i ordered some replacement parts(ot,caps filter/ coupling, and some diodes).
there are two diodes im not sure about in this amp.
http://www.drtube.com/schematics/mar...0112-iss10.gif
they are d6 & d7.
they are paralled with 22ohm res, and a .1uf cap tied to chassis.
they read shorted in the ckt. have not pulled em out to check yet. they are 1n5408 gen purpose diodes. big fat diodes. they are not present on the 100watt version of this amp. (why?) what do these do exactly?

i was also gonna put a bulb limiter on the amp for pwr up after i swap parts, then it occurred to me that i have more than one meter to use, so i'll put one in series with the pwr line in as a current meter.
what is typical current draw on these amps. the mains fuse is 3amp i believe, but what should i expect to see if all is well?

[guitar]

amp sound

I have a marshall jcm 800, and I have no power and dont know why. It worked fine the time before, the next time I go and turn it on I dont get a power light. but if its quiet enough in room you can hear a quiet hum. The fuse looks fine. I am just ready to quit playing.

[anthillrich]

hey guitar, nonsense.....dont quit playing or learning.
most anything can be fixed. im not the guy to ask for help on here, as im trying to figure this stuff out as well, but there are lots of nice folks on here that probably can and will offer some good advice.
you may want to start a new thread tho, as i think this one was "demoted" to the childrens forum.
good luck, dont quit, just get some good advice & a good tech if your able to find one in your area. they can be hard to find, but not impossible.

[anthillrich]

replaced ot, replaced some caps & all is well with my amp. thanks to Chuck H,& MWJB for your tips and insight. as always, i appreciate the help this forum provides. rich

[guitar]

Thanks. Most can be fixed, but at what cost.

hey guitar, nonsense.....dont quit playing or learning.
most anything can be fixed. im not the guy to ask for help on here, as im trying to figure this stuff out as well, but there are lots of nice folks on here that probably can and will offer some good advice.
you may want to start a new thread tho, as i think this one was "demoted" to the childrens forum.
good luck, dont quit, just get some good advice & a good tech if your able to find one in your area. they can be hard to find, but not impossible.

To know what it might be helps me determine the cost and coin is something I dont have much of anymore, I developed Epilepsy and unable to work