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Topics - ilyaa

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Amplifier Discussion / acoustic 150 zobel?
« on: April 15, 2015, 09:20:04 PM »

am i write to say that R404 and C404 make a zobel network across the output?

what would cause R404 to blow? my guess: amp plugged in without a load puts the output wattage across the puny 1W R404 and blows it up -

but then isnt that resistor just asking to be destroyed? ive seen one in another acoustic 150 that was blown literally in half...

Tubes and Hybrids / peavey roadmaster super duper festival
« on: April 03, 2015, 02:38:34 AM »
this amp is huge and cumbersome and a pain to take apart but i think its *mostly* working

i didnt do a road test but just took some measurements for now -

just a few questions (refer to attached schematic):

1) i get +300V at the PI supply, but on the transistor end of the load resistors i get ~10V everywhere i meausure - any of the transistor leads that arent at ground give me about 10.25V. one or more of the transistors bad probably? ive dealt with and fixed a bunch of these hybrid peaveys and almost ALWAYS the PI transistors have to be replaced.....

2) im getting a weird looking ripple at all points on the amp when measured with my scope. kind of a large, jagged wave. even when i touch the scope to ground its there. about 100Vp-p!! what the....??? i thought it may have been death cap related so i removed those caps but that only seemed to make it worse......any ideas?

3) related to that, what do the two caps on the power cord side of the PT actually do?

Tubes and Hybrids / PCB question
« on: February 26, 2015, 03:51:49 PM »
i have a question about circuit board (potential) damage:

someone gave me a hot rod deville to fix up - one of the tubes is red plating the other barely glowing - figured just might be a shorted tube. i opened it up to take a look, though, and saw some bad looking light brown ash kind of residue over the area on the PCB where the red-plating tube is.

i cleaned it with some alcohol but there is still some dark brownish residue i cant quite get off

(see pics:

1) any tips for a better solution to clean this crap?
2) i think Roly mentioned before that a burnt PCB can become resistive. the owner of the amp has said its had problems before and im wondering if a resistive PCB in the power amp section might be shorting tubes for him....any thoughts?

Tubes and Hybrids / magnatone ancient amp
« on: February 08, 2015, 02:56:50 PM »
another one from a friend -

works - speakers are shot (cone-wise) but working - while he's getting those fixed up or getting new ones im trying to see if anything inside needs work

its all pretty old and worn/rusted (see pics) - i think the amp's from 1948!!

its got those cool tubes with the plate tap on top!

two primary concerns (as youll see in the pics):

1) someone put in a new power cable and just thought to leave the safety ground hanging!!!

2) the leads coming out of the OT are kind of weird - each side of the winding comes out as a two, unstranded, wound together, exposed copper wires....these have some green oxidation on the ends right before the terminal where they hook up to the speaker wires - thoughts? i tried to look up this OT - triad 1746 - but not much luck....whats up with these exposed leads? thicker for wattage handling? kind of a small OT, too....anyway, just curious if i should clean them/cut off the green part/replace the OT.....

Tubes and Hybrids / fender champ line out
« on: January 30, 2015, 03:21:30 AM »
i built a champ a while ago and my buddy made me a cab for it!

sounds great - i want to put a line out on it for recording -

anyone have a definitive method??

PS its a 5F1

Tubes and Hybrids / music man hd130 intermittent what the??
« on: December 21, 2014, 07:34:14 PM »

long time no post -

fixing a music man tube/ss hybrid for a buddy

works fine sounds great for about 1 hour and then starts to lose volume and completely crap out, making some weird sounds on the way!

any ideas where to start with something like this? seems to be heat related...whats a good way to diagnoze/troubleshoot problems like that?

im guessing its the power amp or PI, because once it starts to crap out it doesnt matter which channel you plug into -

Tubes and Hybrids / earth g2000/fender dual showman vibrato troubleshoot
« on: October 15, 2014, 04:16:40 PM »
amps fine but vibratos not working....

thought it was the LDR/optocoupler so i replaced that but no dice

so far the only issue i can see is that both cathodes of are at 0 V, instead of 2 and 13DVC.

but ive never really troubleshot this circuit before - good place to start?

its not actually a dual showman, but it seems to be pretty much exactly this schematic:

edit: ive got -44V at the grids of the vibrato tube - is that right?


Tubes and Hybrids / kay model 720 => AB mod
« on: September 09, 2014, 12:39:34 AM »
got this amp the other day because i was intrigued by the cabinet design - its a 1x15 but with a grill on each side and the controls and stuff on the traditional 'rear' to encourage you to face the speaker butt out - and thought itd sound good to use together with my sound city concord -

recapped everything but the main can and its sounding good but its a little quieter than i expected - took another look - didnt realize it was a class A design!

i was thinking itd be an interesting and fun learning experience/experiment to modify it to run AB instead. ive looked into tapping off a negative bias supply ( - thats a useful thread!) and am deciding between the ampeg style capacitive divider and this simpler one: what kind of diode is appropriate for this - just any ole high voltage silicon style 1N400x?

but before i get into that i want to make sure im not missing something -

1) i cant find any info on the specific output transformer - if its meant to run two 6L6's Class A, should it be able to handle AB operation?
2) and a follow up to that: because of the greater range of input voltage of to AB designs, will the amp actually be making more power - larger voltage swing?
3) is this a stupid idea?

Tubes and Hybrids / carvin x-amp
« on: August 03, 2014, 06:45:37 PM »
heres a weird hybrid one -

R132 was (literally) burnt out - i replaced it and the amp is working fine, but im trying to understand what would make that resistor fry -

i checked the diodes in the bridge that feed it and they seem fine - where does the V- that its filtering/providing even go? i assume to the opamps for a +/-15V swing but i cant quite find evidence of that on the schematic -

what could make that particular resistor burn up? its in a lowish voltage part of the amp so im a bit confused.

note: R132 is in the lower right part of the schematic - off the transformer, through the lower voltage bridge -

Amplifier Discussion / another acoustic 370
« on: July 30, 2014, 03:03:22 AM »
another one of these -

complaint was "farty/buzzy" - i tried it out and heard no such thing, but it shocked the hell out of me so i figured i may as well take care of that and see what else might be the matter in the process.

odd thing was, it had a three prong cord, but jokes on me for thinking that meant it was actually grounded in a safe manner: someone had just put a three prong adaptor onto a two-lead power cord (leaving the earth lead open!). what a dangerously strange move - ill take care of that.

otherwise, visual inspection just yielded one thing: L301 (the inductor on the output) looks a bit charred - there is a burnt looking black spot on the windings....bad?? any way to measure it? what does that inductor and its parallel resistor (R328) do, anyway?

Amplifier Discussion / ampeg 350H got wet
« on: July 09, 2014, 01:45:07 PM »
okay as promised heres a SS amp -

ampeg 350H that was salvaged from a flooded basement. smells a bit musty in there and some of the chalk power resistors look like theyve absorbed a bit of water (the chalk looks darkish greenish kind of).

the good news is it powers up - my limiter tells me no shorts - and things actually look okay in there. the power resistors measure fine (should i be concerned about them) and im getting +/-60 and +/-16 on the main power rails.

edit: i had my scope set wrong. its giving me close to 20V p-p when cranked (into 8 ohms).

im not quite sure where to start - mosfet output stage but its hard to get to them because of an enormous heat sink they are glued to.

anyone give me some good advice for beginning to diagnose this kind of thing? (ive been in tubeland for a while and i have to be honest - my solid state theory needs some fleshing out)

i guess splitting the amp in half power/pre would be a good first step - ill do that and report back!

Tubes and Hybrids / ampeg vt-22 PI
« on: July 08, 2014, 04:15:37 AM »
once again - fixing this one up for a friend.....

symptom was no/low signal and weird sound when turned on.

quick preliminary looked showed that the screen grid resistor on V5 (R42) was fried (low voltage there - resistor tested open and looked a bit burnt). ill change that.

BUT upon looking around the rest of the amp i also found that the PI voltages were bad. ~150V on pin 1 and ~330V on pin 6. now i remember having a similar issue on a deville i worked on. i also know more about long-tailed pair configurations now, enough to say that it looks like pin 6 is not conducting! both heaters are lit. i tried a different tube - same symptoms (a 12ax7, admittedly, not a 12au7 as it says on the schematic, but that shouldnt matter for this issue, correct?). ive resoldered all the pins and adjoining connections and checked the region and it all looks good. does NOT seem mechanical.......any other things i should check or am i just being sloppy with the mechanical stuff most likely?

Tubes and Hybrids / peavey classic B coupling caps
« on: June 11, 2014, 03:57:37 AM »
fixing this one up for a friend -

incidentally i at one point had 3 of these identical amps in my garage -  a popular one in my milieu, i guess....

anyway, this one was doing something weird and i traced the problem to the phase inverter (solid state differential amp), which was giving me wonky voltages. replaced the transistors and all voltages are back to normal.

amp sounded fine - nice and loud - but had an unpleasant kind of congestion and loss of volume still when cranked up.

noticed the bias voltage for the PI (which should have been 330V) was only at 260 or so. saw that the last resistor in the PS filter network on the main cap was not, in fact, 68K but 87K, for some reason. change that out and voltage is closer (now at about 300V) - the first resistor in the HT filtering is 1K (instead of 400), so that might be it, but ill leave that as is for now.

the amp is making power. the slightly low HT voltages seem an unlikely suspect. i have noticed, looking through the amp, that many of the smaller coupling/DC blocking caps have a white residue on them as if they have suffered a tiny explosion. is this a sure sign of age/wear? should any caps with this kind of sign of stress be replaced? just wondering if that's a clear indicator or one of those 'maybe you can change it but it's probably okay' kind of things.


Amplifier Discussion / two prong to three prong
« on: May 30, 2014, 01:48:59 AM »
im fixing up an acoustic 150b head

works great - just had some caps that had desoldered themselves and other ones that seem leaky - just doing a basic clean up and recap

this amp actually sounds awesome! a real growler

anyway, the power cord is kind of deteriorated and also only 2 prong

just wondering: what is standard procedure for retrofitting an older SS amp with a three prong cable?

im particularly confused because the cable it has now is polarity indifferent (equal sized prongs) -

guidance please!

Amplifier Discussion / acoustic 370 rustle rustle
« on: April 28, 2014, 12:31:48 PM »
this amps giving me guff again - have to say im getting a little tired of it.

when on (no input signal) the amp makes a low end kind of rustling noise - almost sounds like rats have made a home in the speaker cones. its, unfortunately, an intermittent problem. sometimes the amp will be on for a while and not make the noise and sometimes the noise will get louder and louder as time goes on.

what ive managed to figure out/try so far:

1) i think its coming after the volume, because the volume has no effect on it (it can even be all the way down and the noise still persists)
2) it might come before the main EQ, because the treble/middle/bass knobs seem to have a *slight* impact on it
3) it definitely comes before the graphic EQ, because that has a huge impact on it. if i turn the graphic EQ all the way down the noise *almost* disappears and if i crank it the noise becomes thunderous
4) pretty sure its in the preamp, because if i send the Line Out to another amp the noise comes with it
5) scoping around in there i dont get too much info.
    scope on the speaker out when its happening: what you'd expect, just some low frequency looking     noise
    DC volts on the speaker out when its happening: a little bit, jumping around between like 15 and 70 mV
   scoping around inside the preamp when its happening: i dont get anything at all really UNTIL the collector of Q105, when the flat line on the scope starts to move up and down (not AC-style, but looking like shifting DC) - it moves along with the rustle. as i move further along the preamp the movement gets more significant but stays the same - just that flat line moving up and down.

my thoughts:

poor filtering feeding the preamp transistors an unsteady/fluctuating bias thats leaking into things like a low frequency AC? only thing is ive checked the main big caps and they seem pretty solid - although of course they are old.....

bad/leaky caps in the preamp? particularly C108, C112, C115, C116, C118, C119, C122, C124, C125, and C127.

am i going down the right path? i dont want to disappoint everyone and resort to shotgunning these caps around but in the absence of a clear indicator of the fault im not sure what to do. unless the shifting DC on the scope points to something specific.

on a side note: im curious about the speaker outs on this amp. with the speaker unplugged, the amp shows like 25V on the speaker outs, but once i plug it in that goes down to near 0. that normal?

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