low volume output

[anthillrich]

hi , im new to the forum, and i was wondering if anyone can give me some advice. i have a marshall jcm 900 4501. it has two el34 pwr tubes. i have had years of trouble free use out of it untill today. i was playing at a nice & loud volume level( like i normally do), when suddenly the volume went way down. real weak output with a hefty amount of distortion( on clean & dirty chan). first thing i thought was "its a bad tube." but after i tried two different sets of el34's, i realized that i may have a bigger problem. i did check a few things tho, like: different cab, cable,& guitar. so i open the amp to take a few readings with my meter. i have 450v on the plates, about the same as always, -45v on the bias supply. i also took an AC reading on the bias, it was less than a volt. all of my readings were about normal i think. also, no blown fuses or anything burnt. i did plug into the effects return to make sure it wasnt a pre-amp fault & replaced 12ax7's with my spares. same result. so.... do i have a bad ot? also, if i did need to replace the ot, would i need to specify that its for el34 and not 6l6, or do they use the same ot? thanks for any help you could give me.

[Chuck H]

The first thing I would try is cleaning the effects loop jacks. But because you have distortion on the clean channel also I suspect it's something else. Probably not the OT. But it could be a bad power tube. If you can just swap in a new pair and rebias to about 40mA per tube. You will also want to check the bias circuit. The bias circuit in those amps seems to fail commonly and that could cause a tube failure.

Chuck

[anthillrich]

thanks for the reply chuck. much appreciated. i actually did try that already(cleaning effects loop jacks), i know these amps are notorius for bad effects loop connections. also, i did try a couple different sets of pwr tubes( i always carry spares, as this is my only gigging amp). the tubes i used to swap were ones that i had used for a little while, then took out to keep as spares. the thing is, it will bias up just fine with the bias probe, there's no red plates going on or anything that i can see. by the way, i would like to thank the learned members on this forum, i have learned a great deal from reading the posts on here. a great resource to be sure. seems like the more i learn, the more i realize there's so much more to learn. thanks again.

[Chuck H]

Well...There's a bunch of non tube amplifiers in there too. You may have to do some signal tracing to isolate the problem. If you have a way to generate a signal it should go pretty well. Just follow the schematic and test for AC along the signal path.

From the description so far I would guess that the problem is past the channel switching and behind the phase inverter.

HTH

Chuck

[anthillrich]

thanks again for your reply. i was able to try running my signal(guitar) out of the direct out of the amp. i believe(if im reading the schem right) its past the pre amp & before the pi. i ran the signal into a powered monitor and the sound seemed to be clear(at least "clearer"). also, the controls seem to be working, if i turn gain up, it responds like i would expect, same with the eq, and master volumes. so i agree with your diag. i do have a scope at work that i could use(with permission from my boss), and a cheap signal generator i built from a kit. but as you pointed out, there is alot of ss components in this thing. would my elenco function generator work well enough in this situation?

[MWJB]

You've taken plate & grid voltages for the EL34, what about screens at pin 4? What about dc voltages at the PI pins?

[anthillrich]

i did manage to measure that(pin4 screen), it was very close to whats on the plates,(i cant recall exact voltages, but within a few volts), i believe thats a good thing, right? i also checked the 2k2 5w's for ohms, it read ok. i didnt lift a leg before reading them tho, should i take 'em out & check again? they dont look burnt or stressed even. also, im not sure what to look for on the phase inverter. i did replace that tube also, just in case. thanx again guys.

[MWJB]

voltages at pins 1 & 6 of the PI would be handy, may as well check 3 & 8 (joined) whilst you're there.

Screen voltages sound OK if a few volts off the plates.

[anthillrich]

i will check 'em and report back later w/the results(if i dont get completely snowed in). i know my original post sounded completely stupid, and you guys are way cool for not calling me out on it, but i still am curious about something. i think i understand the ots role in an amp. meaning...it takes the high voltage/ impedance signal from tubes and steps it down to a low voltage & Z/higher amperage signal the speaker uses to move air. i've read all the typical guitar amp handbook stuff. i guess what i meant to ask(hopefully without appearing more foolish) is: did marshall make a distinction between their el34 vs 6l6 amps on this model, or did they just throw a drake in there that could deal with both sets of tubes. i realize there are different Z's for each tube type, but how critical is that when choosing an ot for guitar amps. im sure marshall took the most cost effective route.

[MWJB]

Primary Z is rarely an exact requirement, there's a fair range over what will "work" depending on tonal/financial requirements/parts availability (important when you're building thousands of amps & might save a couple of dollars on each OT).

A typical EL34 amp might have an OT with a 3.6K primary Z, a 6L6 Fender style OT might be from 3.8K to 4.2K (middle of the road primary Z for 6L6)...either tube will work between 2.5K and 5K+.

Many manufacturers (I'm not sure about Marshall - there's others here who are more hip to Marshal OTs specifically) used off the shelf OTs, some UK amps used 6L6 with 2.5K, Masco used 6L6 with 7.5K in some cases.

The important thing to bear in mind is that unless you know that your OT will tolerate a mismatch (like Fender OT in an amp with a parallel ext. jack, like BF/SF amps) then don't try and second guess primary Z, as there may already be a degree of mismatch already.
That 2.5K into 16ohms might work fine, but sub the cab for 8ohms and you're asking for trouble.

Do a search on establishing "turns ratio".

Sorry, got a bit carried away...it's likely Marshall did specify different OTs for different amps but the tubes will be happy enough in either case, realistically I'd be surprised if they did. Exact primary Z that the tube wanted to see may not have been the deciding factor.

[anthillrich]

thanks for the info. i did google turns ratio, and it seems everyone is selling xformer testers. can i use my sine wave generator, it can put out about 4v peak two peak as read on my scope. is that sufficient? i cant find it, but i seem to remember reading on this forum you could use a scope & generator to test. if my math is right, i think i could determin the turns ratio, but i would have know idea if its at spec, since i dont know what that spec should be. but i guess it could give me a ball park idea. reason i ask is i have a silvertone1484 i would also like to check. i know...i know, i have the ugly duckling collection of tube amps, but i love them anyway. plus, i just would like to be able to xformers in general. am i off topic? im not up on forum etiquett, or spelling for that matter.

[anthillrich]

i managed to check the pi pins 1&6 , 3&8. the plates have 240v, and the cathodes are 35v with respect to gnd. this sounds right, yes? this amp has me scratching my head.

[Chuck H]

So far you've done just fine. Your signal gen should be fine for this app. You really don't need a scope for this but you can use it anyway to good effect. I would check the IC outputs just prior to the PI tube next. Use your DMM to test your guitars output voltage (probably between .25 and .5 volts on a power chord). Set your signal generator to that voltage. Following the AC path you will find the place where it drops out. Don't confuse the pad for the effects loop for a dop out. Once you find the AC drop out you'll know that the amplifier (probably an IC) preceding that point is the culprit. Testing with a scope in the same way would also show the waveform distortions you describe. Which would further confirm your findings.

Chuck

[anthillrich]

thanks for the reply. i guess IC8 (5201) is the master vol switching IC right before the pi. i tried to put up a link to the schem im reading, but my internet skills are a bit weak. sorry. anyway, i got it from drtube.com. there are a few of these 5201 switching(?) IC's. (fyi, i dont use the reverb or effects loop normally). my question is, do these amplify? so when i test with my signal generator & volt meter as you suggested, should the signal be bigger at the outs of these IC's ? i know the feed back loop(i cant remember if thats what its called) sets the gain on these things, but its been a long time since i had to figure that out. sorry if my question is unclear, im not sure im even asking the right questions sometimes. thanx again , rich.

[anthillrich]

i feel a bit overwhelmed with this pre amp schematic. im trying to trace the signal, but it is a bit confusing for me. im not sure if what im reading/seeing on the scope is even correct or not. i will keep at it tho. i did however discover something else,& im not sure if it is a big deal or not, or if it relates to my amps problem, but here goes: i found the tube amp debugging page at geofex( great site btw), and decided to do the simple (in amp) ot check with the ohm meter. i removed the tubes as instucted & discharged the caps, and the reading from the b+ to the plate primary( forgive my terminology please) on one side is about 40 ohms, when i measure to the other plate primary i am getting 2 ohms.( is there too much disparity there?) i followed the instuctions described there & checked the primary windings to grnd, they read infinitey, the secondary's were at about 20-30 ohms each w/respect to gnd. between the primarys & secondarys read in the mega ohms ,and slowly went lower & lower like a cap was charging or discharging. any of this abnormal? thanx, rich.