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#1 2008-02-08 02:38:38

aaronh
Member
Registered: 2008-02-08
Posts: 19

think i may have messed up my v-4b

As a background, I don't have a lot of experience working on amps but I have some tech skills and do a lot of reading and asking questions before tackling anything complicated.  I don't have much diagnostic equipment but I know how to use a meter, read schematics, and can solder adequately.

Ok, so I got a V-4B head recently, and it needed a little work to get going.  New preamp tubes, replaced the cracked fuse socket, cleaned out all the pots and switches.  It was pretty crackly and was tending to cut out intermittently before I performed these fixes.  I tried playing through it again and it sounded pretty good, nice smooth response, good tone.  I'm running this at 4Ohms through an SVT-410hlf cab.

I had noticed earlier that the standby pilot lamp wasn't lighting, thought the bulb was dead, so I left it alone as it wasn't a big deal to me.  I then discovered through some research that the bulb was probably fine as it is common for the 180k resistor that is wired in series with it to blow.  I figured this was a pretty easy fix so I got the appropriate replacement resistor and set to work.  Before doing this I drained the filter caps as always since I don't feel like dying over a piece of gear.

Here is where I think I may have screwed it up big time.  I left a lead clipped across the two big caps that are near the center of the board (not the one by the OT) and turned the power on after doing my repair to test the soldering job (doesn't everyone say not to drain the caps this way specifically because this can happen? yeesh).  When I flipped the standby switch I saw a little puff of smoke come out from the board, not sure where, maybe the power transformer area (eck) and immediately turned the amp off.  I unclipped the lead and looked around quickly but didn't notice anything that was obviously fried. 

I tried playing through the amp but it is distorting harshly, especially at weak volumes, and the volume was dropping off irregularly (poor sustain, lousy tone).  It sort of sounds like the way failing tubes sound, but since it sounded fine before that last bit of work that I did I don't know what to check.  What kind of damage would occur to the amp by jumpering across the caps in this way?  Could I have blown the tubes?  They don't look too abnormal, maybe a little bit dimmer than before but it was hard to tell. I'm hoping I didn't fry the power supply as that is quite an expensive fix.

I will probably throw in the towel and take it to a tech but I sure wouldn't mind hearing some opinions before I do anything else.

***update #1 - I left it on standby for an hour or so and came back to try it again.  there was some improvement in the response but still some inconsistencies in tone/sustain.  I will be taking it to a practice tonight to give it a good workout and will have more to report later.

Last edited by aaronh (2008-02-08 03:53:30)

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#2 2008-02-08 08:39:44

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

yikes... that doesn't sound good.

lets get a few things clear.   you had a wire clipped onto the positive terminal of C20 (the first filter cap)?  what was the other end clipped on to? 

if there was smoke... something went up.  find where the burn is.  is it on the board?  is it on the chassis?   is there a part that is scorched?  that may shed some light on the situation.

measure your DC voltages.   are they low?
test them and let me know.


Also... as a good rule of thumb,  if you're having a problem,  leaving it on or trying to work out the problem through playing the amp is probably not a good idea.   it is not likely that whatever is wrong will simply undo itself and go back to normal... it is more likely that whatever is wrong,  will just get worse.

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#3 2008-02-08 09:48:56

aaronh
Member
Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

You will probably appreciate that I ended up deciding not to use the amp for the rest of the night.  I thought better of trying to do something foolish like that.  I will open the amp up in the morning and take some photos of what I had done and any damage that I find.  I feel like a pretty big goof but I suppose one has to learn one way or another...

Last edited by aaronh (2008-02-08 10:04:49)

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#4 2008-02-08 10:04:23

aaronh
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Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

What's the best procedure for measuring the DC voltages?

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#5 2008-02-08 18:25:49

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

well,  turn your amp on.  take your meter,  set it to DC Voltage.  apply the black test lead to ground,  and the red one to whichever DC voltage you are measuring.

at this point,  lets just see what the power supply is doing.  so measure the DC voltage on each side of R55, then at each of the positive terminals of the filter capacitor by the output transformer (c17). 
lets also measure both pin 3 and pin 4 of the power tubes.
once you've taken those measurements,  turn it off, and report back.
-steve

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#6 2008-02-08 22:27:55

aaronh
Member
Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

Thanks, I just needed to know the test points.

Okay, so my meter won't read more than 500v, but I did what I could.


R55 is reading over 500v on one side, the other side give a reading up in the high 200's that falls steadily and seems to settle around 210.  Same with The power tubes, Pin 3 is over 500, Pin 4 settles around 210 or so.

C17 Has one term reading 370, the other 472.

I didn't find anything that looked burnt anywhere, but I noticed a few other things that I will point out.

Here's some photos:

Overall board

http://farm3.static.flickr.com/2039/2251469660_a9f9e2eb91_o_d.jpg


Here, the red line indicates where I accidentally left a jumper while turning the amp on yesterday:

http://farm3.static.flickr.com/2268/2250674931_be33f15855_o_d.jpg


I believe the filter caps were redone at some point and R52 and R53 were moved off the board to a small terminal strip next to C17

http://farm3.static.flickr.com/2305/2250674575_a3d2a262b6_d.jpg


Also, I noticed this - assuming this is the infamous death cap, and should be removed considering I have a 3-prong cord?

http://farm3.static.flickr.com/2054/2250673459_03a2d4f120.jpg


And here is a damaged point on the board - not sure what's going on here, the white lead at the top of the photos is from one of the inputs that splits and connects to this point, with what looks like a repair jumpered to another point.  There is nothing soldered on the other side at the point of damage (circled in third photo)

http://farm3.static.flickr.com/2019/2251470334_351c496804_d.jpg

http://farm3.static.flickr.com/2409/2251470658_3d43936dc3_d.jpg

http://farm3.static.flickr.com/2415/2250673703_30156a3948_d.jpg

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#7 2008-02-09 06:56:49

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

alright.   I'm concerned with your pin 4 reading.  That should be reading over 500 also. 

lift one leg of R55 and measure it.   because it is a rather low value, and was essentially shorted to ground,  it saw some intense current.  I would not be surprised if you took a closer look at that resistor and found evidence of that.   

let me know what you find.   if it measures fine,  try measuring R41,42,47,48

the board repair is sloppy, but nothing to be very concerned about.  it looks like the trace lifted,  and they used bus wire to repair the trace.   If I were you,  i would try to find a more sturdy way of doing that.  it looks like thats kinda flopping around.

-steve

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#8 2008-02-09 11:52:05

aaronh
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Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

I took a look at R55 and it didn't look fried.  I didn't have time to check all the values on the resistors you asked about tonight, but I did notice that the 6K11 tube looked like it got pretty blown up.  I hadn't been looking around there because I figured I burnt something in the power supply or the output.  There's a big black spot on one side of the tube wall and the paint from the logo flaking off in small bits.  Looks like that tube took a little too much heat.  If I've read the right info, that tube is driving part of the mid eq circuit.  I did notice that besides from the awful distortion and the weak volume that the EQ still had a pretty pronounced effect on the tone and response.  At the very least I fried this tube.  It was an Emerson, probably original the amp or at least not a recent retube.  Don't know if this burnt up in the original incident or as a secondary effect of the initial damage.

I'm taking the whole shebang to a good tech tomorrow -  it would make me feel better to have someone with pro experience actually get this into their hands.  I will definitely be able to use the info I've picked up here so far when I'm discussing the situation with the amp doctor.  I'll report back with the diagnosis in the near future.

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#9 2008-02-09 22:17:58

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

getting a pro to look at it is certainly a good idea.

the 6K11 is probably fine if the EQ is working.  plus DC voltages are good down that point.
I'm guessing that the problems are either those resistors I mentioned or the powertubes.

-steve

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#10 2008-02-22 19:32:53

aaronh
Member
Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

Well, I had my tech take a look at it.  All the caps seemed to check out, and the transformers were fine (whew).  He said the power tubes were pretty well shot, which explains some of the problems.  The biggest issue, probably the result of my unfortunate repair attempt, was that R55 was fried.  This was replaced with a new 10 watt resistor (spec is 7 watt).  Other than that, there were a few cracked solder joints here and there, and the funky repair on that lifted trace was redone in a more secure fashion.

Only issue now is that I ordered some NOS Philips tubes from Angela, had my tech install them, and was almost ready to go when something very odd happened.  He was checking out all the power tubes to make sure they were doing what they were supposed to, when one of the tubes shot out a bright flash and smoke, along with a hissing sound (probably from somewhere around the base of the tube).  Nothing else looked burnt and the tube actually looked ok (still has lots of silver, not all white inside) but he didn't have time to check it out this morning, so we don't know what's up with that tube yet.  He's been working on tube amps for a long time and says he's never seen anything like this.  Possibly a defective tube since all the voltages in the amp were where they should be.  Has anyone dealt with Angela as far as defective tubes and returns go?  They seem to have a pretty restrictive return policy but do guarantee against DOA tubes within 30 days of purchase.

Looks like this amp won't be gigging this weekend after all....

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#11 2008-02-22 19:49:34

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

I thought it might be R55.  its a shame the tubes were also bad.

I'd imagine Angela would want to do you right,  but only if they feel at fault. 
I imagine we'll have a better idea of whats up once your tech takes a closer look.

I hope everything works out.
-steve

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#12 2008-02-23 17:45:48

aaronh
Member
Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

Ok, my tech got to looking at the tubes and determined they're ok.  He thinks we saw a plate/screen arc of unprecedented size (at least in his experiences) due to a loose tube socket.  Now that he's got that sorted out, he traced some of the nasty buzz to the phase inverter, and also seems to think there's some odd work evident in the power supply.  That's as far as he got last night; it seems like this amp has layers of problems originating from multiple places.

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#13 2008-02-23 20:59:58

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

it wouldn't be surprising to find that somebody did weird things to your amp. 
I'm working on a B-25 that has all sorts of terrible things done to the power supply.   quadrupling the size of the filter caps... adding some kind of strange zener diode regulator circuit.    weird stuff man.    its crazy how people hack up their amps thinking that they're making them work better.
hope everything works out
-steve

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#14 2008-03-19 05:08:26

aaronh
Member
Registered: 2008-02-08
Posts: 19

Re: think i may have messed up my v-4b

Got my amp back from the tech today.  Have yet to play it as it's too late at night to plug in, but I will be using it in a rehearsal tomorrow night.  Here's the diagnosis from my tech anyhow:

Low screen voltage on the power tubes -  likely due to frying R55 during my "repair"

Bad socket-to-board connections on the 6K11 tube.

Lifted trace on R51 (as seen in photos)

Hum pot burned out, replaced with ground reference resistors.

Bad/broken caps in the tone stack (C113, C114)

Bypass caps replaced (C106, C112)

Bad tubes, probably just old, one losing some vacuum.  Replaced with NOS JAN 7027s.

Also, a big one, whoever did the cap job wired them up real weird, though I didn't get detailed information on how they were miswired.  My tech said it looked like someone was just reading the schematic without really understanding the functionality of the circuit.

Lastly, there is a little bit of noise in the signal probably traceable to the 6K11, as it is totally quiet with that tube pulled.  I'll be tracking a new one of those down in the near future, but for now it's not a big deal.

So, too bad the power tubes were bunk, but at least the caps and the transformers are fine.  We'll see how it goes tomorrow....

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#15 2008-03-20 04:03:51

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: think i may have messed up my v-4b

as I said before,  people do strange things to old tube amps thinking they're doing good or making improvements. 

I'm glad our transformers are fine.  You're gonna love this thing once its up and running.

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