Ampeg V4.com

forum

You are not logged in.

#1 2008-04-13 18:24:13

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

V-4 Overheated then Quit, Any Ideas?

Just finished re-capping my V-4 with a Fliptops cap kit.  I tired it out last nite at our gig.  This amp sounded like a million bucks for almost two long sets.  Near the end of the 2nd the volume just dropped out.  I had just used the rocker switch above the volume to add some more drive.
I could feel the amp was extremely hot, and turned it off.  I did not hear any unusual noises before it quit; no burning smells, either. 
This morning I tried it out.  Fuse is fine, all the tubes light up, but the sound is very weak, intermittent and distorted.  Any ideas of where I should look for the problem?

Thanks.

Tonedef

Offline

 

#2 2008-04-14 18:44:05

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

Did it appear to run hotter than normal(before the overhaul)?. If not, I'd suspect either a bad or dirty rocker switch. Try the other channel and see what happens. If the other channel works, shoot a little DeOxIt in the bad channel's gain switch and work it a few times. See if that helps. Otherwise, it's time to start circuit testing.

Best, Paul

Last edited by paully (2008-04-14 18:52:19)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#3 2008-04-14 19:15:59

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Hi Paully, thanks for the response.  I just bought this and re-capped it.  I had never played it longer than 20 minutes, so I can't answer that question.  The rocker switches both seem to still work fine (as far as I can hear, the volume is very low and distorted).  It sounds the same thru both channels.  This has supposedly new Sovtek 6L6GB's in it, any chance I fried one of those? 

Tonedef

Offline

 

#4 2008-04-14 22:37:29

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

Are you gonna try to repair it yourself? If so, start with the basics.

1) Have the tubes tested. I'm not sure the 6L6GB is a drop-in replacement for the 7027a, but somebody'll know. 

2) Run a cable from the line out jack to a seperate guitar amp or stereo. That'll tell you if the problem is in the preamp.

Do you have the tools to circuit trace? List them up, and we'll see what you might need. Basics:

1) A schematic that you can follow.
2) Multimeter. If it can test caps great, otherwise a seperate meter to check them.
3) Scope, preferably dual-trace.
4) Some kind of signal generator. A keyboard that'll give you a sine or square wave will work.
5) Rubber soled sneakers, a good light source, a high tolerance to pain, and nerves of steel!!

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#5 2008-04-14 22:49:01

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Thanks Paully, my bro is the electronics wiz, we'll see what we can do with your checklist before giving up and taking it to a tech.

Tonedef

Offline

 

#6 2008-04-14 23:43:23

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Hey Paully,

I checked the pre-amp section and it works like a champ via both channels.  That has to be a good thing...

Also, this is what my brother says we have:

1. A beckman rms multimeter.
2. A tecktronics signal generator.
3. A Tenma 40mhz dual trace scope
4. A variac.
5. A tenma soldering station.
6. tennis shoes.
7. A goofball just dumb enough to go sticking his hands in this-you    (who, me?).

Tonedef

Last edited by tonedef (2008-04-15 00:38:54)

Offline

 

#7 2008-04-15 01:11:38

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

You still need something to check the caps. Hopefully the mm has that function. OK, here's where I'd start.

* All tubes off to be checked. Mark them so they go back in proper order. Once you have a working set installed, download a schematic from either http://www.drtube.com/guitamp.htm or
http://www.schematicheaven.com/ .
* Open the amp up and do a visual. You know what to look for! If everything looks OK, plug in a speaker.
* Using the variac, bring it up to operating voltage in 20 volt stages. Check DC power supply points at all stages to make sure they're all rising proportionately.
* At full AC, check the tubes' heaters to make sure they're all lit and no tubes are glowing red. If all supply voltages are in spec:
* Plug the generator into one of the channels at .1 Vpp(turn the other channel's controls OFF) and set all controls for flat trace after the first stage. Then do what I suggested earlier. Run a cable from the line out jack to a seperate guitar amp or stereo and check the sound. If it sounds OK and the output from the line out jack looks roughly the same as the input signal from the generator at the input jack, that'll tell you that the problem's probably not in the preamp.
* Turn all preamp controls off, unplug the monitor stereo/amp cable, and re-plug the generator into the line out jack. From there it's a matter of testing signal points in the power amp(tube grids and after plate caps) with the scope. After that, it's component time.

OOP! Wrestling's on. Gotta  run. I'll check in later. Have fun.

Best, Paul

Last edited by paully (2008-04-15 01:21:43)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#8 2008-04-15 18:33:14

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Thanks Paul, we ordered a cap tester.  When it gets here, we'll have at it.

Tonedef

Offline

 

#9 2008-04-15 18:55:47

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

Briefly, I don't know what you ordered, but I'd seriously suggest an LCR meter, which measures inductance, capacitance and resistance. I have a Beckman LM22A, a handheld unit, and I'd be lost without it.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#10 2008-04-17 01:03:28

jacobarber
Member
Registered: 2008-01-22
Posts: 57

Re: V-4 Overheated then Quit, Any Ideas?

I am not an electronics expert in any sense of the term, so i'm going to give you an opinion based on everything I've read about these amps.

The first thing that popped into my mind is that I have never heard of anyone using 6L6GB tubes in a V-4 or VT-22.  The only drop-in tube replacement for the 7027A's is a 6L6GC, which is supposedly the exact same as a 7027A.  They may have caused damage to something.

Offline

 

#11 2008-04-17 01:13:25

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

To add to jacobarber's comment, make that a matched quad-set.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#12 2008-04-17 02:27:17

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Thanks guys.  I'm hoping that is the problem, that a power tube fried.  I have a matched set of JJ's on the way.  The previous owner really cheaped-out on the power tubes--told me there was a new set of Groove Tubes in it.  Maybe Sovtek means Groove Tube in Russian.  I'll still check all the voltages to be sure and (try to) find the source of the problem (if it isn't a tube problem).

I'll check back once I have everything lined up.

Tonedef

Offline

 

#13 2008-04-17 18:09:52

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

Before you pop the new tubes in, make sure you check the current limiting resistors on the output tubes'  plates. They're the 10 ohm, 5 watt jobs. Also verify that you have the proper negative voltage from the supply. Both super important tests. Otherwise it's bye-bye to at least one new tube.

If a tube is bad, something caused it to go besides being a cheapie. The low, distorted sound could indeed be 1 or 2 blown or shorted current limiting resistors.

Personally I'd use old(but working) tubes to do your circuit/componant testing. Once everything's verified, then stick the new stuff in. Again, have the old tubes tested first. Have fun!!

Best, Paul

Last edited by paully (2008-04-17 18:17:41)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#14 2008-04-17 18:15:03

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Thanks Paul, I will do that.

Tonedef

Offline

 

#15 2008-04-30 21:41:22

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Here is where we are at.  We had recently completed a cap replacement and after the replacement the amp work fine for 2 hrs, then lost power.  Now in checking the amp there is correct voltage on pins 3 of the power tubes, 549VDC, but low or no voltage on pins 4.  Nothing is obviously wrong with the amp (other than the power tubes are suspect).  All the new caps were de-soldered and checked (except the one on the pre-amp side), and checked good.  Any ideas of what else I need to check to rectify the voltage problem?

Offline

 

#16 2008-05-01 02:59:51

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: V-4 Overheated then Quit, Any Ideas?

Looking at the schematic, I would trace the voltage back from pin 4 on the power tubes. There are 470ohm - 1W screen resistors. Check the voltage on the other side of those resistors. It's unlikely that they all burnt at the same time. The circuit draws power from a 470ohm 5W or 7W resistor. Check that one too. There are also a pair of 40 mfd caps in series that supply that circuit - they may be shorted. I would pull the output tubes first and test voltage on pin 4. Maybe the grid in each tube is shorted.

Offline

 

#17 2008-05-01 03:36:04

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

Some thoughts. Diablo hit most of them already.

1) Drain the PS, then check the connection from B+(4 section PS cap) to the 470 x 5W resistor for continuity.

2) Disconnect one end of the 470 x 5W resistor and check it. Reconnect.

2) With the output tubes removed, slowly bring your variac up to no more than 75 VAC.
While doing this, check the side of the 470 x 5W resistor that connects to B+(the 4 section PS cap)
for voltage. Both sides of that resistor should rise in step with the B+ voltage. The resistor
will slightly reduce the voltage on the cap side.

3) Drain the supply. Check that resistor's 2 x 40 x 450 filter cap for proper connection and ground.

4) The only things left on the supply side are the current limiting resistors to pins 4. Doubtful that all four
would be taken out, or that all four tubes would go at once either.

Let us know what you find.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#18 2008-05-01 05:27:59

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Thanks Paul, will do, and many thanks to you as well, Diablo!.

Tonedef

Last edited by tonedef (2008-05-01 19:06:51)

Offline

 

#19 2008-05-14 00:13:50

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Just an update, it looks like the worst thing that coulda happened did, after chasing all the voltage readings down, all roads seemed to lead back to the PT.  Found the voltage readings from the power trans are not up to spec, one side in particular is extremely low.  I ordered a new one from Bruce, as he has been a great guy to deal with.  If I want to have the original re-wound, any suggestions?  Or, if I might have missed something and jumped the gun, I'd like to hear about that as well.

Tonedef

Offline

 

#20 2008-05-14 01:38:41

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

Re: V-4 Overheated then Quit, Any Ideas?

how many volts AC did you measure on the secondary of the power transformer?   

Bruce gets his transformers from Heyboer.   I have used their output transformer and found it to be a nice replacement. 
I've also worked directly with Mercury magnetics, and they are very good.

Offline

 

#21 2008-05-14 05:32:26

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Hey Steve,

I don't recall offhand, but one side was down about 100v and the other side was near double digits (best I can recall, it was the end of a long day of checking voltages).  The readings were not anywhere near what they should have been.

Tonedef

Offline

 

#22 2008-05-14 13:27:51

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

tonedef wrote:

Just an update, it looks like the worst thing that coulda happened did, after chasing all the voltage readings down, all roads seemed to lead back to the PT.  Found the voltage readings from the power trans are not up to spec, one side in particular is extremely low.

>>>>>>>>>>>>>>>
That's too bad. Did you actually measure the voltage on the secondaries while decoupled from the filter section? What were the AC readings?

As far as having the original rewound, that only makes sense from a resale POV. Also gets REAL expensive. Get a replacement, keep the original and include it if you sell. Let someone else bear that expense. I know if I was buying an old V4, I'd be much happier with a new replacement PT than an 'iffy' original. It's not like vintage guitars.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#23 2008-05-14 19:03:26

tonedef
Member
From: So Cal
Registered: 2008-02-12
Posts: 59

Re: V-4 Overheated then Quit, Any Ideas?

Hey Paul,

Yes we unsoldered the output wires from the board to check them, as per Bruce's instructions.  One side was around 180 or so, the other was half that.  Oh, well...
Now I'm wondering if those low budget power tubes the previous owner saved a couple of bucks on took out my PT.  Or maybe it was just at the end of its life. 

Tonedef

Offline

 

#24 2008-05-15 01:31:55

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V-4 Overheated then Quit, Any Ideas?

OK, this may sound like a dumb question so bear with me. There are only two secondary taps; one high voltage(~500) and one for the filaments(6.3). Half of tested 180 volts is 90 volts, and a filament tap shouldn't be anywhere near either of the tested voltages. Is that where you're getting the readings? You're not talking about the primary side, are you? The reason I ask(and the dumb question) is I'm wondering if maybe you didn't forget to bypass the variac and use 117 VAC when you tested. Common mistake.

Best, Paul

Last edited by paully (2008-05-15 01:32:53)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

Offline

 

#25 2008-05-15 05:55:28

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

Re: V-4 Overheated then Quit, Any Ideas?

paully wrote:

OK, this may sound like a dumb question so bear with me. There are only two secondary taps; one high voltage(~500) and one for the filaments(6.3). Half of tested 180 volts is 90 volts, and a filament tap shouldn't be anywhere near either of the tested voltages. Is that where you're getting the readings? You're not talking about the primary side, are you? The reason I ask(and the dumb question) is I'm wondering if maybe you didn't forget to bypass the variac and use 117 VAC when you tested. Common mistake.

Best, Paul

paul and I are thinking similar thoughts.   the high voltage secondary should be two red wires.  when you measure the voltage coming off of those you would connect one lead of your multimeter to one and the other lead of your multimeter to the other, and set it for AC.   (it will be some really HIGH VOLTAGE, so be careful!)
I'm confused by what you mean by "half"

-steve

-steve

Offline

 

Board footer

Powered by PunBB
© Copyright 2002–2005 Rickard Andersson