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#1 2011-11-23 19:37:11

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

V4 Bias woes....

Hey all,

I just joined the forum because I recently got a V4 from a friend that was in need of some TLC. I got it for $300, it's a 70's (NJ) with metal knobs.  No reverb tank.  Bad hum.  2 prong.  No recap. Pretty much completely original except for what he said was a few replaced resistors that had smoked in the bias supply.  His tech threw new Tung Sol 6l6 strs in and he said it was cutting in and out. 

I have built marshalls, fenders, soldanos from the ground up, so I figured I could bring this old puppy back to life with a recap and a little clean up.  WOW, was I wrong!!

So what I have done so far. 

1) Installed a 3 prong, and bypassed the ac outlet so it's not in the circuit anymore. I followed some of the suggested wiring layouts on here after using the search engine.  Death cap gone.  Fuse and mains switch on hot side.  No issues.

2) Replaced almost everything on the bias board (larger board).  Did not replaced R49 R50 in bias supply.  All caps replaced except can caps (I ordered from fliptops, but the package took a while, so I started with bias board) Did not replace any diodes (I count 6, one larger that the others...is the larger one the flyback?) Did not replace Hum balance, but it seems to only work all the way to the left, which means it may be shot, correct?

So after finding one of my 10ohm 5w resistors was open, i figured that would have been the culprit for one of my 6l6's seeing more voltage that the rest when I powered up with my lightbulb limiter.  I was getting 208v on v1-3.  V4 was showing 300v on pin 3.  After replacing the resistor, all was well.  So I bumped up the wattage on the ole lightbulb and everything seemed good.  Checked voltages around pins and all seemed good.

I decided since I was showing no shorts with the limiter, and voltages were seeming right, I would do a power up.  Plug straight to wall and power up, no problems.  I check voltages around bias and pins on tubes and all seems good.  Play through it and it's not sounding right.  Get up to grab a chopstick to probe around and I hear a hum develop.  Weird?  Look at the back and v1 and 2 are lit up like christmas lights. Power off.  Sigh.  There goes another set of decent tubes. 

I guess my question from this long winded post is......

If I replace diodes, hum balance, recap, and tubes, is there anywhere else I should look before giving this thing full juice again.  I'm assuming the issue was in the 2 bias resistors that I hadn't replaced heating up and dropping after running at full voltage and sending the bias on a downward spiral.  Or could it be something else?  Any help would be much appreciated.  Seems like everyone is very helpful, and I would love to get this going.

Anyway, thanks in advance for any assistance.

Daryl

Oh, and the schematic that matches my amp is here...
http://drtube.com/schematics/ampeg/vt22-v4-70.gif

Last edited by Drabidoux (2011-11-23 19:38:48)

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#2 2011-11-24 00:36:39

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

Re: V4 Bias woes....

the hum that developed was probably due to the two tubes that were redplating.   Now what caused the redplating is another story... could be a number of things...

because you had an open plate resistor for one of the power tubes, it is safe to say that at some point one of the old power tubes shorted pretty good.   I would check the coupling caps to the power tubes.  and the bias feed resistor for the side that was redplating.

before any of that.. I would inspect the power tube sockets, and make sure there that there is no burn mark from some sort of arcing.

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#3 2011-11-24 03:07:09

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

I have replaced all coupling caps on the rear board, so i may just replace the sockets and see what happens.  I will check back when I do. 

Ordered up a hum balance pot, and maybe I'll replace the wiring to the power tubes while I'm at it.

Thanks,
Daryl

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#4 2011-11-24 03:57:14

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

Re: V4 Bias woes....

the rear board meaning the main board?

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#5 2011-11-24 16:00:33

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

The board closest to the power tubes with the bias circuit on it.

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#6 2011-11-24 19:24:18

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

Re: V4 Bias woes....

gotcha,  well check the bias feed resistors R35 R36. check tube sockets on the outside and in.

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#7 2011-11-24 20:52:39

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Def replaced r35 r36 100k resistors. I literally replaced most components if i had them on that board. So i'll check the sockets and report back. Can i test without tubes?  Im a little hesitant incase they redplate again.

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#8 2011-11-24 23:29:19

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Repaced outer sockets.  Pins 7 and 3 looked suspect on both. Also, by hum pot wont read anything on my dmm, no matter which lug i'm on. Hope thats the problem.

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#9 2011-11-30 19:39:24

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Well. Got my hum bal pot today. Popped it in and replaced bias resistors. Powered up. Lots of hum. Tubes started to red plate. Turned off amp and swore a lot. Checked the hum pot. It was turned all the way to the left. Oops!!!  Set halfway. Turned on amp and it was dead quiet. I actually thought it wasnt working!!!  Played at low volume while i watched the tubes and all seems good. Input 1 needs some attention, and the reverb isnt working. So thats my next challenge. Ill play full volume tomorrow and make sure its good, but looks like shorted hum bal pot was the culprit. Thanks for the help.

Daryl

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#10 2011-12-01 03:37:18

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

Re: V4 Bias woes....

the hum balance would explain hum... but not the redplating....  keep an eye on it

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#11 2011-12-03 05:10:54

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Well, still having a problem with the V4.  It's running great until I turn it up!?!?

Once i get the thing pushing some air, it seems to be too much for the tubes.  It starts off fine, but after about 10 min, it ends up sending one of the tubes into redplating.  Not bad, but as I hit the front end with distortion and get the volume up to about 11:00 I can see a pulsing red line start to form on my tube.  I have got jj 6l6gc as well as SED's, and all have the same problem. If I turn the amp off and swap the tube to a different socket, that same tube starts redplating. It seems to start on the socket closest to the center of the chassis with both JJ and SED, but as I said I swapped the outside pair to the center and sure enough, starts to get that dim red line on the same tube. 

Measuring voltage off of the 1k resistors, I am getting 570v.  Am I just pushing my luck with 6l6's at this point?  Any idea if I would have the same problem with 6550's?  Seems all the info I've read people are running 6l6's, but I don't want keep buying new tubes and ending up having the same issue. 

Voltages are good from what I gather.  I have replaced all sockets and everything in the bis supply except the diodes.  I haven't replaced any of the diodes.  Could this be the problem?  Any other ideas of what to look for?

I also noticed the 1k 5w were getting very hot since they are so close together.  I moved every other resistor to the bottom side of the board and that seemed to help with the heat issue. 

Hangman.  Thanks for the help.  This thing sounds amazing for the 10 min I can play through it, so any help would be much appreciated. 

Daryl

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#12 2011-12-06 18:27:34

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Wow, this is not going well for you, eh?

I'll re-read through what you have here and see what else I can come up with, but let us know where you stand as of now, or if it's the same as last post...


Matthew

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#13 2011-12-06 19:01:37

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Kinda confused where you stand with the work you've done even this far, so forgive me for any redundancy.   If you search my posts I had a lot of similar problems early on.  I never narrowed down what it was, I just went for a lot of component swaps and that got me feeling more confident and got the amp stable.  I don't even know what model V4 you have, but I did every coupling cap, C11 and C13 being the most critical coupling caps (and related to the bias network).

At this point you've blown enough tubes and/or cash to be meticulous and measure and/or replace 'every' component.  I have a 'why not?" mentality about components with this amp, keep that in mind.


Some thoughts in no particular order:

What's the status of the reverb?  It may not be causing stuff but you want to make sure the circuit is safe (inputs referenced to ground, etc) See this, for example:  http://akavalve.blogspot.com/2009/07/wh … e-amp.html

Get the chinese/sino quad of 6L6GCs from tubesandmore.com.  They're cheaper to blow while you're blowing tubes and getting stuff stable.

But your plate voltage is 570 and this is a 4-tube V4?  That's a little high, I wouldn't put any 6L6s but the SEDs in there at that voltage thought they're mad expensive (see the former).

Measure the value of all your important resistors.  Make sure they are what you THINK they are.  It's easy to overlook a simple mistake like 100R for 1K or something.

Measure your screen resistors and even make them 1k (I did, and why not)?  Get rid of the bias pot and make it a 75k or 82k resistor for now, please.  Bias it cold right now!  Really cold!!  60% of a 6L6GC with 6L6s or 7027s is HOT in this amp by the way.  You don't need it biased hot to sound good after it's stable anyway.  And colder bias will not only sound better (IMO) but keep the amp stable and prolong tube life, which it sounds like you could use a little bit of.

Replace the whole bias network, caps, diodes, resistors, etc.    Measure the value of all your plate resistors.  Are you plate resistors fresh, a multiple of 1 (1ohm, 10 ohm?)  IF so, are you measuring each power tubes bias via the voltage across the resistors and/or calculations?  You should if not right away, eventually.  See what each individual tube is pulling and just how 'balanced' they are.  Be careful though, that's a lot of voltage...  If you're getting significantly more plate current from one tube or one side, that's gonna cause hum, as does redplating.

Be sure you got your impedance math down...and that the switch is working properly...there's so much to check and confirm, I'm just getting started...


Matthew

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#14 2011-12-07 00:22:39

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

I will head out to the space, put an 82k in the bias, and drop in my new JJ's and get some readings for you.  I read your thread and that seems exactly like my problem. 

My V4 is an early metal switch/knob linden NJ model.  It matches the early v4/vt22 schematic I referenced early on in this thread. 

I have replaced everything except the diode in the bias section.  All resistors in the entire power section.  All electros in the amp, all coupling caps on the board that includes the bias section.  Replaced all power tube sockets.  Replaced hum balance. 

I essentially have a new amp aside from the other board.  I did replace all electros on that board too. 

I have a new quad of JJ's.  If they do anything funny while testing, I'm going with 6550s.  This thing is driving me nuts, and I'm losing faith in 6l6's, since all parts are essentially new.

Thanks for your input.  Ill check back with some voltages.

Daryl.

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#15 2011-12-07 02:16:45

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

Re: V4 Bias woes....

daryl, 

I would go with kt-88s before 6550s. (but not the JJ kt-88) 

-steve

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#16 2011-12-07 03:25:00

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

So.....

Had a fresh quad of jj 6l6gcs. Changed all 6 diodes w 1n4007's (looks like mine doesn't have flybacks?). 82k bias resistor. Tried the impedance at 4 ohms. Played for 1/2 hour.

No issues!!!

Not sure if it was tubes or the 8 ohm setting, but the amp seems stable. Ill be practicing end of the month, so that will be the test.

Noticed my voltage was around 560 on the 1k resistors now. Is that a byproduct of replacing diodes, or the colder bias?

Just curious, am i safe with 1n4007's? I noticed d5 was much larger than the others d1-4 and d6.

Anyway, thanks for the tips. Hopefully i won't have to worry about this thing anymore.

Daryl

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#17 2011-12-07 09:26:41

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

What do you get at the wall?   It could be a product of the time of day or where you are - wall voltage varies, and hence your primary voltage will too.  But it COULD also be other things going on, that's just a start.  It's always good to reference the wall voltage if your comparing primary voltage and isolate the variables.

You are getting 560v at which 1k resistors?  Are you talking about R42 R41 R47 R48?  Those are screen resistors.  Or do you mean the 100k at R56 (and R57)?

You've replaced R55 with a 10 Watt, correct?  Just checking.


Matthew

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#18 2011-12-07 15:51:24

teledeluxe
Member
From: Chicago, IL
Registered: 2008-09-24
Posts: 467

Re: V4 Bias woes....

Liquids wrote:

You are getting 560v at which 1k resistors?  Are you talking about R42 R41 R47 R48?

Don't 6L6GC and 7027A amps typically use 470 ohms resistors for these? 1K are more common for EL34 or 6550... Maybe this one was already converted to 6550.


2 wrongs don't make a right, but 3 lefts do!

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#19 2011-12-07 16:31:48

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Yeah.  I mentioned that I put 1ks in there anyway, and that's the only 1k I can think of that may be being referenced here...

Otherwise, I fear that maybe Drabidoux is using 1k's for the plate resistors.  Hopefully it's just a typo

Last edited by Liquids (2011-12-07 16:32:26)


Matthew

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#20 2011-12-07 19:53:20

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

I upped the value of the 4 470ohm screen resistors to 1k5w. Left the 10ohm the same, but upped to 5w. Sorry, i thought i had mentioned that.

I upped them to 1k since it seems like standard procedure around here.

Daryl

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#21 2011-12-07 19:57:32

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Got it.  No harm done.  So you are getting 560v at the junction of the screen resistors and the tube screens, or at the other side of the screen resistors?
And you are getting what voltage DC at the plates and what voltage AC at the wall?

Last edited by Liquids (2011-12-07 19:58:06)


Matthew

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#22 2011-12-07 20:26:27

Drabidoux
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Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Ill check tomorrow.

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#23 2011-12-08 01:50:06

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

Re: V4 Bias woes....

yeah,  the 1k screen resistors is pretty standard... I don't think it has to be done... but the amp does seem much more stable.

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#24 2011-12-08 15:54:31

teledeluxe
Member
From: Chicago, IL
Registered: 2008-09-24
Posts: 467

Re: V4 Bias woes....

The reason I asked is because switching from 470 to 1k in a Fender results in a strangled sounding power section, even after a rebias. My admittedly limited experience in doing that has affected things tonally.

Either way, that wouldn't be the cause of his woes. Carry on.


2 wrongs don't make a right, but 3 lefts do!

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#25 2011-12-09 03:37:47

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

Re: V4 Bias woes....

it does change things a little... I've never tried upping the value in a fender.  so I can't comment there... but in the V4 I've found very little change.

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