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Messages - ilyaa

#286
Tubes and Hybrids / sound city clean-up
March 10, 2014, 04:46:46 PM
this might be a long one....

its a Sound City Concord combo

(schematic attached)

i inherited this amp from a friend - "if you can fix it, you can have it!" - another mutual friend had worked on it once upon a time with no success.

its already a complicated, cramped mess in there, but add to that the fact that i think the previous 'tech' may have tried out some voodoo of his own, and i really have no idea what's going on.

with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

here are some brief measurements/observations so far:

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.

i will go in and do a more careful trace to see what's going on and report back. its pretty confusing and hard to pick apart but ill start with the power section and see if i can at least figure that out.
#287
coooool

checked and all V's looked good. plugged it in, turned it on, and everything sounds fine!

i'm going to glue the culprit caps in a little better to make sure it doesn't happen again, but looks like we're good to go!

if something comes up i'll be back but i think we can put this thread to rest!
#288
thanks for the exhaustive response, Roly! it's really helpful for a n00b like me to get as much info as possible.

Quote
I have a collection of lamps ranging from a few watts up to 100 watts, and if it's possible you may still have a fault such as a dead transistor you have missed*, then you need to step up the wattage cautiously, taking measurements as you go and being suspicious, not optimistic - if you see something you don't like power down and have a cup of coffee and a good ponder on the situation.  Can this be explained purely due to the reduced voltage, or could it be an indication of something more serious?  Amplifiers don't have Human Rights and don't get the benefit of the doubt.

what's the real procedure, though, as i approach the real-life test stage? ramping up bulb wattage and just taking measurements, or actually plugging the amp in - to a speaker - and trying it out with a limiter still in-line? if things are working at that stage, presumably im ready to remove the limiter - but if im not getting any sound then back to the drawing board - is that the right process?
#289
wow you guys got it!

one of the -ve filter caps had de-soldered itself. when i measured across it after taking the board out, i got a little spark and 50 volts (amp off). i discharged it, re-soldered it, and now im getting +/- 60 or so volts on the rails and a steady -24 mV on the output.

only thing is, plugged in with a limiter, the bulb glows red - dull but not as dull as it was glowing before. before i could just see the filament glowing, now its glowing bright enough to make the whole bulb kind of dim red. does that indicate too much current or is it okay? should i plug it in for real and see if it works?
#290
Tubes and Hybrids / Re: blues deville got the blues
March 09, 2014, 04:48:28 AM
i dont have a dummy load yet - planning on making one soon.

what if i put a dummy lead into the power amp in? with nothing on the other end - will that be safe? wont that just fool the power amp into thinking its getting no signal in - therefore none out. and then i can take the preamp out and do with it what i wish. is that flawed thinking?
#291
Tubes and Hybrids / Re: blues deville got the blues
March 08, 2014, 06:51:11 PM
when i take a signal from the preamp out, shouldnt it short out the power amp?

its not doing so - makes it difficult to hear if anything is changing because ive got two amps playing....

#292
Tubes and Hybrids / blues deville got the blues
March 07, 2014, 03:43:05 AM
i like this amp a lot (the 2x12 combo) and it sounds good (usually)

this is a 93-ish deville (schematic attached)

lately ive noticed a sort of muffled/congested/sometimes even grainy quality to the drive channel. normally i have no complaints with this amp's tone, but this one is bugging me. it's sometimes crackly verging on staticy. why is it so hard to explain?! bottom line is, i keep wanting to turn it up but it just doesn't seem to break through the graininess. on top of that, sustained notes in particular will kind of crap out into a cheap sounding fuzz.

i got new power tubes. put them in. its fixed bias but i checked it out anyway and they seem fine. running about 37 mA at 474 on the plates so 17.5 watts. that's not too cold is it? (6L6GCs).

control grid voltage is only at -43 or so - should it be more like -50? im deliberating whether or not to put a bias pot in....

most voltages around the amp seem okay EXCEPT:

1) TP26 on the schematic (cathodes of the third pre-amp tube) should read 34 VDC but only reads 22 VDC
2) point Y, off of big filter caps, going to plates of third pre-amp tube and control grid of power tubes read 440 VDC should be 413
3) point X (going to plates of first two pre-amp tubes) reads over 400 VDC and should be only 364 VDC

now i know the high voltages have some leeway, but what about 1) ?? i cant help but feel like the problem is preamp related, seeing as new power tubes that seem to be running at the right bias didnt do much. if so, would a low voltage at preamp cathodes maybe cause the issue? ive swapped around preamp tubes with no luck yet (but i only have one known good one that im switching in and out)

any thoughts?

should i put in a bias pot and try to run the power tubes a bit hotter, see if that fixes it?
#293
plugged it in with a limiter:

initial power on the limiter glowed really bright just for a second - i guess kind of a surge of current - and then quickly went down to just faint reddish glow. after that if i powerd on/off it did not light up bright again, just stayed faint reddish glow.

with limiter inline, i get between 100 and 200 mV on the speaker outs (normal), with occasional spikes up to around a Volt (is that normal)

if i plug the amp in WITHOUT the limiter, i get 0 V on the speaker outs.....

A+ im getting +60
A- im getting -40 (!)

B+ im getting +16
B- im getting -14 (!)

its hard to read the schematic, but im assuming A+/A- should be equal but opposite polarity riiiight?

ill post the other voltages later because ill have to spend some time with the schematic to figure out exactly where they are.....anyone every seen a more legible version?!
#294
im also having a hard time finding a good schematic. the only one around is illegible.

there are no tubes in this thing, so i assume no output transformer. is it safe to turn on without a load?

heres the schematic:

http://img.photobucket.com/albums/v202/xr3drumx13/MV500Schematic.jpg

(i know how can you read that?!?)

whats that coil thing on the output? a relay?
#295
got one of these from a friend to fix

its the mosvalve 500 (942)

i havent done anything yet but the problem as my friend has described it is:

turn it on, VERY loud hum, turning gain all the way down does nothing - hum does not stop.

would checking DC volts on the output be the right place to start, assuming everything inside looks normal once i open it up? is that what normally produces an unstoppable hum? or would DC getting to the output create other symptoms?
#296
Amplifier Discussion / Re: Light Bulb Limiter
February 27, 2014, 02:31:45 AM
Oh yeah I meant 1 amp -

In the case of a tube amp, though, I'm obviously not turning it on without a load, anyway....

But just to be clear, you are confirming my suspicion: an amp with a light bulb limiter inline will not actually SOUND like it should, even if everything in the circuit checks out, correct?
#297
Amplifier Discussion / Re: Light Bulb Limiter
February 26, 2014, 11:42:30 PM
Last question:

is the light bulb limiter only useful in a preliminary testing stage - making sure youve got the right voltage on rails and such things, no shorts, no faulty or open spots - but should be removed before actually testing the amp in a playing situation?

the reason im asking: i was thinking about it today and it occurred to me that if the bulb limits current to below 1 mA, the amp cant actually function properly, can it? depending on the amp, of course, but most need more current than that at least through the primaries, right??? in which case if you had the limiter inline, could you actually test the amp beyond taking voltage measurements?

does this question make sense?
#298
Amplifier Discussion / Re: Light Bulb Limiter
February 23, 2014, 12:33:28 PM
cool - thats what i thought!

so if i turn on this amp thats been sitting around in an unknown condition and start to troubleshoot it and the bulb is glowing brightly (this is all hypothetical right now) it's going to be alright to leave it on as i try and figure out the problem? i was imagining id have to shut it off if there was excess current, but i guess if the bulb has an actual the limiting function things should be all good, riiiight?
#299
Amplifier Discussion / Re: Light Bulb Limiter
February 23, 2014, 02:14:41 AM
just a question about the function/purpose of the light bulb limiter (finally made one after a couple stupid mishaps):

does it function as BOTH an

1) indicator - brightness giving you some info about current draw in amp
2) safety measure - if amp IS drawing too much current, will the bulb actually dissipate it - as power/heat in the filament - and thus prevent dangerous currents from getting into the amp itself? in this case its working as a sort of fuse that doesnt blow?

??
#300
Amplifier Discussion / Re: roland jc120 weird woes
February 20, 2014, 01:32:36 AM
well i guess thats a good and bad solution!

good in that ive got no fixing to do and bad in that i was hoping id be able to get something out of this 'feature'

oh, well. thanks guys!! at least i found the right schematic for future use if i need it.