having some problems with this thing distorting -

wondering if anyone here has any knowledge of these units?

i know op-amps alright, but theres a lot of *s!!t* going on here......

specifically: im trying to figure out if U103 is powered okay. its got -12V at pin 4 and +17V at pin 7. shouldnt those both be +/-12V?

schematic: http://rdn.harmanpro.com/product_documents/documents/602_1324058489/160X%20Schematic_original.pdf

why relay in this amp and not other similar SS ones?

is there something particularly volatile about the STK chip?

okay new STK chip works -

only issue is the relay/protect circuit was maybe also fried in the process of the output section going.

i bypassed the relay and amp works....bad idea?

okay clarification:

there is -50V on pins 10, 12 and 13.

and the low voltage reference on pin 7 is only 8V, half of what it should be.

pins 11 and 12 are not shorted together, but 10, 12 and 13, are, at least when tested in circuit - but there is nothing i can see that'd be shorting them externally, so im guessing internal short.

new chip!

hey peeps -

long time no post!

quick question: im fixing an acoustic B200. everything looks good until the last power stage, where i get no signal on the STK404 output.

i'm getting +/- 50VDC on pins 12 and 13 (output pins). looks like they are spiking up to the rails. clear sign that the IC needs to go? should be 0VDC there, right? just not 100% sure i understand the layout of this particular chip....

thanks!

true -

put in a 1458 and good as new.

this is one situation where the original part does have the 'mojo,' even though its technically inferior, i guess!

damn i think it works well in there but i cant seem to get rid of the oscillation -

seems like it makes more sense to get a 1458 and see if that fixes the issue, rather than messing around with shunting caps here and there (already fried a couple LM358s touching my alligator clamps where they arent supposed to go!) -

ill let you know if that works!

thanks JM!

cool - ill track one down -

i tried an LM358 and it biased up fine but there was still a tiny oscillation - coming out at like 1 V p-p on the output but still there....

just to clarify, are you suggesting a 100pF across BOTH R48 and R50 (so two caps total), or somehow between them?

oh boy - this amp is so confusing! at least starting with the PI onwards....

brief rundown: amp was working OK but had some weird distortion. opened it up and saw one of the caps across the plates had a broken lead - figured there was maybe some HF oscillation or something - replaced that cap - amp was working but had quite a large very high frequency (100KHz) oscillation on the output. after some soul searching i saw someone had put an additional 68k resistor across the existing one (R49) coming back for NFB from the load. not sure why.....removed it and the oscillation was gone! amp worked great and biased up fine - ~25mV across those little emitter resistors on the transistors feeding the tubes.

all good - lots of play for a while - then one day signal dies very low power....?? open it up and see that the voltages look good but the signal looks weird around the LM1458 - the little chip feeding the PI. i replace it with a 5532 and we are back up to power! EXCEPT (THIS IS THE CURRENT ISSUE) now the amp won't bias up right....im getting no less than 80mV across those little emitter resistors....ive tried a couple diff chips - the TL082 got me down to like 60mV but still tooo high. is it possible i need to replace it with an LM1458? thought any of those dual opamps would be about the same.......

other details is there is a very small about 1 Vp-p very high freq oscillation that disappears by the tube plates but is definitely present around that last opamp section....figure that might be related.......this amp is a bit confusing to me - anyone able to demystify??

ive been using this schematic: http://www.music-in.de/pics/hd130a.gif

i know with tube amps 2:1 impedance mismatch is OK -

too high stresses transformer a bit more
too low stresses tubes a bit more

what about with SS amps? if it can be turned on with no load at all, seems like going too high with the load impedance is not a problem? what about too low? stresses output transistors risks overcurrent?

just curious -

well, bulb-limiter aside -

i popped in a separate 47uF cap for the time being instead of the first section of the can and things are looking and sounding good -

looks like it was a shorted filter cap - didnt realize part of the big multi-section can could short....but i guess it can!

yeah, its only red-plating when driven really hard -

i reduced idle current even further (about 50mA per tube - at 550V) and the red-plating seems to have abated.

loads are matched - the PI coupling caps were okay but i replaced them anyway -

i think it was just running the tubes too hot - we'll see what happens once its tested for a while under real-world conditions -

the 47k im referring to is the one in parallel with the 10k biasing the grid -

i increased that resistance to about 89k or so to decrease idle current -

i havent checked the coupling caps off the PI for leakage - that might cause red plating, right? if that DC voltage sneaks onto the grids?

is any red-plating bad? always?

this amp sounds great but when its cranked that evil red glow starts to come out a bit from deep within one of the tubes.....

tubes are brand new!

the story: when i got it the amp had EL34s - the stock is 6550s, so the owner asked me to return it to stock. i did - not much had been changed by whoever put the EL34's in, except they had changed the 47K resistor in the bias network to a 23K and a 10K trimpot, and they had 470 ohm CONTROL grid resistors (??) on both tubes -

(also the tremolo wasnt working but i figured that out and its not relevant to my question here)

i returned things to stock (i took off those 470 grid resistors.....whats their deal, anyway?) and put in 6550s - with a 47K resistor in the bias network they were running like 80+ mA's each though, and the B+ is like 550V (!!) so i tamed them down to about 55mA (by putting a 68K resistor in series with a 20K trimpot instead of the 47K resistor in the bias network) each so like 30W each - that should be fine for 6550s, riight?

BUT i saw some red-plating - noticed there was a large-ish 20Vp-p ripple on the B+ so i replaced the first filter caps but the ripple is still there....hmm?? could that cause the red-plating?

someone added 220k bleeder resistors to the big caps, too, but i dont think those are related......

any thoughts?

oh yeah: https://www.flickr.com/photos/73628360@N00/14628046201/

its four caps in one can!

a 40/40 for the first filter and 20/20 for later on.....

a light bulb limiter is a great device for finding shorts! and for saving fuses! hellooo!!