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#1 2012-12-06 22:59:35

nitraus
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Registered: 2012-02-22
Posts: 55

Problem

So, last saturday I lent my trustworthy V4 to a friend of mine who was playing a gig (I was doing FOH). Suddenly the volume went dramatically down and the clean sound became distorted. I mean as there'd be a fuzz constantly on, but *really* low on volume.
I tried to cool it down, change speakers/cable, change preamp tubes but nothing changed. I didn't check power tubes, but they are definately not microphonic. And last time my tech checked it up in september they were nearly new.
What could be the problem? Thanks in advance for any hint!

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#2 2012-12-07 03:42:57

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

Re: Problem

Which v4 version?

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#3 2012-12-07 11:09:55

nitraus
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Registered: 2012-02-22
Posts: 55

Re: Problem

I have a '77 master volume/non distorsion V4

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#4 2012-12-10 18:41:17

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

Re: Problem

Flyback diodes?


Matthew

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#5 2012-12-10 21:08:27

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

Re: Problem

I would check the preamp out,  see if everything is normal there first.

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#6 2012-12-11 18:58:34

nitraus
Member
Registered: 2012-02-22
Posts: 55

Re: Problem

hangman wrote:

I would check the preamp out,  see if everything is normal there first.

You mean by simply connecting it to another power amplifier or by making any measurement on the output?

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#7 2012-12-11 22:27:57

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

Re: Problem

You could plug the preamp out into a mixer, or a power amp or check it with a scope....  Listening to it would tell us if the problem is pre or post power amp

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#8 2012-12-12 00:50:07

nitraus
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Registered: 2012-02-22
Posts: 55

Re: Problem

I connected the preamp out to the poweramp in of another amplifier and it worked perfectly, so I guess the problem is in the power section.

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#9 2012-12-12 01:23:59

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

Re: Problem

Try plugging your guitar into something clean with a volume control (or turn down your instruments volume, etc), and plug directly into the EXT (beware of the volume), or from the preamp out of one amp into the power amp in of another...that will confirm it, but from what you told, it's the power section.

Are all your power amp tubes and the final two tube stages' tubes 'lighting up' for sure?

Here's some thoughts....

If you're not handy with amps and know how to work safely, bring it to a tech and tell him what you've already done (to explain that the preamp section should be ignored), and suggest something like this, or do this yourself if your handy with amps and know how to work safely: with the amp off/unplugged, and the voltage drained from the caps, test the flyback diodes with your meter; measure the resistance of the plate and screen resistors (ensure they're not open circuits, write down the measured resistance of the plate resistors without tubes in there), test the bias diode, and capacitor, and test the rectification diodes (or just replace any of those compoents if they're not new/recent).  IF any of those aren't spot on, replace them, and then see what you have with the amp fired up but ready to be tested further when 'running'...

...if you still have issues, put your amp on standby, prepare to probe, flip back off standby...measure bias voltage on the grids of the power tubes (quick, you could be frying your power tubes!), flip back to standby...take it off standby, now measure plate current via the voltage drop in single-digit volts or mV across each of the plate resistors (feel free to turn the amp on standby after doing this each time for the sake of the tubes and the high voltages on said resistors when the amp is not on standby)....also, measure your screen voltages.

Just some ways to find out what's wrong.


Matthew

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#10 2012-12-12 02:21:21

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

Re: Problem

If you don't have A multi-meter and are not prepared to open the amp up,  I would suggest swapping out the 12au7 to see if that changes anything. 

Also,  another thing you could check is the output transformer secondary dc resistance.  might not be a bad idea.   
If you have a multi meter,  (with the amp off) plug a speaker cable into the speaker jack,  have no speakers hooked up.  Just the speaker cable,  measure the resistance of the tip of the 1/4 inch plug to the ground.   It should measure something like 1 ohm or less.   

If it is much higher than that,  you got a Output transformer problem.

Those are the two simple things I can think of that don't require opening up the amp,  and minimal tools.

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#11 2012-12-12 23:56:57

nitraus
Member
Registered: 2012-02-22
Posts: 55

Re: Problem

@Liquids: I'm not familiar with high voltages, so I wouldn't do anything risky inside my amp. Thanks anyway for your suggestion!

@Hangman: I didn't have a new 12AU7, but I measured the output transformer and it is 0.7 ohms, so it seems to be ok.
Anyway the 12AU7 in my amp is quite old (maybe 6 years?) so it could be part of the problem.

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#12 2012-12-14 23:10:32

nitraus
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Registered: 2012-02-22
Posts: 55

Re: Problem

I tried to swap the 12AU7 but the problem lasts.

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#13 2012-12-15 09:17:54

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

Re: Problem

At this point, I would say take it to a tech.   The problem is in the power amp somewhere and If you aren't comfortable with high voltages we can't really go any further.

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#14 2012-12-18 12:05:05

nitraus
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Registered: 2012-02-22
Posts: 55

Re: Problem

I'll do that for sure. Thanks a lot anyway for all your tips!

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#15 2012-12-18 14:46:38

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

Re: Problem

Bring all the info here to your tech!  He may find some of it frivilous, but a lot of techs are marshall/fender familiar, and the ampegs are a different beast - PCB, but far from modern fender PCB.  Different issues arise with these Ampegs than one might expect after the 'same old same old' vintage/classic fender and marshall repair issues.  For some of us who've worked on them to varying degrees of familiarity and with varying degrees of knowledge (such as mine which is in the area of sophomoric, and steve's which is professional-level), this info may provide the tech a lot of assistance.

If the tech is too arrogant to refer to your offering this info - "this is what I found on a forum for tips, how far I got with troubleshooting, and what may be worth checking first..." which may save him time, save you money, and build him (her) a customer who trusts and respects him (etc.) - if he responds well or graciously, that's a good sign, and even if he ignores it all, he may might end up surprised to find out how close Steve and I are (and others who may have or may yet chime in here) to pinpointing the problem area, and he'd probably kick himself for ignoring the info if he ends up burning up tubes on his own dime when he could have avoided stuff like that if he had not ignored useful info a customer offered - your own troubleshooting and info from a forum dedicated to this quirky amp series with countless posts from more than one professional tech such as Steve (Hangman) and Steelyman, etc.

If they blow you off , than either find a new tech who doesn't think he know's it all, or don't be shy to gently quiz him on Ampegs/V-series amps, without a schematic in front of him ('what voltage range does the V-series amps run,' 'what power tubes do V-series amps typically call for/ what power tubes do you think are best in this amp,' 'what kind of tone stack and phase inverter does the V-series use,' etc.)... ask questions, see just how familiar he/she is.  I wouldn't hand my car to just any guy with a garage and mechanic's suit without a great referral or a little inquiry,  let alone my amp....

IF you ask questions in that regard: see if he back peddles, or can back up claim of experience/knowledge on the fly of stuff that is basic to those who've worked on Ampegs!   Also, be sure to express interest in getting all the details of what he plans to do, and later what he claims he did, afterward...one thing you can probably do is unplug the amp from the wall while it is running full on...flip it upside down, let the tubes cool for 30 minutes or so or put a fan on them until they cool....unscrew the large bolts on the side of the amp that hold the chassis, and carefully (it's heavy), carefully take off the nuts that hold down the two metal plates on the underside/tube side of the chassis over the preamp.  Careful!  The voltage, if it hasn't drained from the unplugging or wait time, is 'present' to hurt you if you touch the wrong component or the PCB under these metal plates.  But, if you are methodical and careful, even if voltage is still being held by the caps, you can avoid being shocked.  This like anything is at your own risk, and you have to know how careful and clumsy you are, and what to do to be prepared in case you make a dangerous mistake (plenty of info on the net for that).  At the same time, those plates are easily accessible, and it's a tad safer than opening the top plate of the amp chassis, and safer than removing the chassis on a fender amp, which have no chassis cover most if not all of the time, just a filter cap 'block.'

Then, you can safely/carefully take pictures of what you see (the PCB) after removing those plates.  You'll now be able to go back and see (or show) what the amp looks like 'before,' then can carefully, put the plates back, and put the amp back in the cabinet the same way (can be a pain, but upside down can help make it easier. 

You can post those pictures here...you can show the tech that you have those pictures, for reference, to show him you'll be paying attention...  You can do the same - or ask the tech to take digital pictures before/after of the inside of the amp as well  where you can't see by doing the aforementioned - so you can compare and review any and all the work that's potentially been done. 

If you do all or any of this, and it pisses him off before you hand over the amp - unless he's the only game in driving distance (a bad place to be with an Ampeg...), find someone else.  People too arrogant to work on a customer's belongings and who bring/wants information about what is going on is rarely if ever the kind of person you (at least, I) want working on my amp.   Really, even beyond pro techs, around here, there are plenty of guys who can be found who do amp work on the side just as well or better than the full time/have an LLC, etc, if you ask around enough among those who know such 'techs' and that they can be trusted, they can often be found. 

Otherwise, when something later potentially goes wrong shortly or immediate after those kind of arrogant, 'drop it off and I'll look at it! [end of conversation]" type techs do work, they blame you (and bill accordingly) since they're typically also too arrogant to admit something was due to their oversight "it worked fine when it left the shop" - and yeah...but maybe that was when you were playing harmonica, watching the scope while it was hooked it up to a scope, a signal generator at 1khz, and feeding a dummy load, rather than even SPEAKERS, and paying no attention to the power tube plates that were glowing red and drawing over max current, and there were HF oscillations...etc.

But as a customer, they're right, the problem is something you did, and they 'know' it, and now you're in the hands of an arrogant tech who's mysterious work is shrouded in know it all attitude and arbitrary billing...if you dare bring it back to them the second time...

There's at least a handful of postings around here over the years of people who took their ampegs to techs and don't have all the finer details (since there's no way they could know what they were), but these kind of things likely happened...a good tech is gold to the player who owns a V-series amp but doesn't do amp work.

I've posted elsewhere about those who are 'not' DIY, and what to do when you finally bring it to a tech for checkup, and what else is worth replacing 'blind,' beyond the standard power supply filtering section - where fresh parts are a notable cost;  I've forgotten and not re-reading posts at the moment so you may well have already done that.  Anyhow, the rest is labor/bench time costs more than parts, and since the amp is unbuttoned, and this is not a fender/marshall/vox 'carbon comp/magic capacitor' amp where most tech's I know spend their time trying to pinpoint ONE problem and fix it, hand you back the faulty 'original' part so that if you re-sell it you can have it put back to stock/original/not-working condition for the sake of best resale value, and cross fingers about the timing between the returned amp and the next repair need.  Instead, here (if not elsewhere too), amp bench time spend testing out components and pinpointing the exact faulty component is better spent ($$), IMO, by the tech first testing/hearing how things are 'before,' then (removing everything to do the work), blindly replacing all the 'usual suspect' components for long-term reliability which I have more or less compiled elsewhere, and then then (the tech) hearing what they now have by comparison to before, via more audio.  This may bother them if they want to know 'exactly' what component was the problem. The test/find/replace/re-plug everything back in/fire up/ listen approach often takes more time ($$), but has the satisfaction of the tech knowing and being able to tell you what exact thing(s) were the CURRENT problem.  IMO, who cares...fix the amp, and everything that could be or become a problem in the near future.  All fixed now?  Great. See you in 5+ years, or when I have the $$ to fix my other amp!

So, from the 'inside,'  you'd rather pay for bench time at 50+/hr that is predominantly spent soldering, rather than just 'discovering' what one or two part were the culprit THIS TIME, replacing them, and then handing the amp back to you as 'fixed' with or without a fresh set of tubes...which are often marked up in cost and sometimes were not even needed; tech sometimes do this (did this to me) as a kind of 'blanket' approach at offering a customer what will be perceived as a like-new/clean slate amp, and it mitigate the cost charged to the customer by it not being what it is - almost exclusively X hours of labor costs incurred from finding/replacing a $1 or less component. So, I fixed your amp, re-tubed it [even though it didn't need it, thanks for the spares], the bill is $XXX, thanks for coming!

My opinion is, in this amp especially, if you're not handy with DIY, get the running like a well-tuned machine all in one shot. Many of even the preamp-area components (coupling caps, tone caps) and bias network components are liable to go at 'any point' down the road, even if they 'test out fine' (been told that before....) and MIGHT hold up for a few years, IMO, I say, screw that mindset, replace all the components that is EVER and REPEATEDLY suspect around these parts among V-series amp users, get them replaced with superior quality components, and take it home having spent more all at once and up front, but knowing that barring bad workmanship or lackluster assumptions/work by the tech, you're amp will be golden for a while to come, without another bench charge/troublshooting time/pinpointing one area of components whenever the next problem rears it's head, and you're back in the shop without your amp for a week or more (for example).

Techs I've talked to and work with don't like this philosophy, but that doesn't make them bad techs - it just that it only makes sense for those who want to play their amps, and own ampegs.  Sure, save the replaced components in case V-series amps with 'all original' parts down to resistors and caps become a hot commodity, but even if still, get that incomprable amp happy, stable, and ready for years of tonal bliss.

Consider buying and bringing Steelyman's PCB, which that eliminates the 6K11 tube quite beautifully, and is available on ebay, along with to-spec replacement standoffs along with you to your chosen tech, if you have the money to spend available and long term vision for this amp.  Don't get rid of the 'stock' PCB if you do (again, maybe someday an 'all original' V-series amp will be incredibly 'valuable' - and if so, having a working 6K11 and the populated 'stock' PCB for it all will be an asset as the 6K11 sources dry up.

As aforementioned, check out previous posting I have done where I outline what may as well be overhauled/replaced blind, *IMO*, if someone is not a DIYer and brings their V-series to a tech.  I really should get the link to that posting on hand since I say that so much.

I always tip my hat to Steve  and, to Steelyman, if not others also who know their stuff but post far less frequently than Steve), since he knows more than I do in the theoretical realm and the practical realm with the amp. 

However, I still do tend to speak up - it's the internet, and a forum, not a printed reference book written by a professional, so it's a place my semi-knowledge might be helpful and contribute accurate information and/or a valid opinion even if or where it veers from the opinions of others who are more experienced. 

I can say that while I am humbly a hack amatuer DIYer, I do have experience with these amps, and I spent a lot of time before that working with techs, some of whom appreciated my understanding of schematics and the research I brought to them, other's who would've rather a customer who dropped off his equipment with a tag that said 'broken - please fix' and nothing more.

I do have a local friend who is a VERY knowledgeable late 50-something/early 60-something age, multiple degrees in the engineering field, reads old tube manuals/books like it's zombie apocalypse books to him, admires old ampeg amps in general, has become a full time amp/tube/Leslie/hammond/tech, is a contracted consultant in the amp field and an amp 'designer'/custom built tube amp guy who is THE MAN, has taught me a LOT via his generosity of time for free even when I was just a young kid who wanted his 1968 super reverb made road-ready and present when biased to my own taste by the tech;  he encouraged me that I was knowledgable and skilled enough to venture into DIY HV tube work eventually, and when I had a V-series amp in hand that needed help but was too scared to go beyond low-mid voltage semiconductor work at the time.  He runs circles around me until I'm WAY over my head in the math of tubes, circuits, calculators, frequency, etc., etc.  But he calls ME when he can't locate the info he needs, or needs someone to evaluate the tonal compromises of engineering/circuit option A vs option B & C from an audio/player perspective or a second opinion. I take that as one of the highest compliments I've gotten in the area.   smile 

So I'm a music lover and player with a  spotty, sophomoric, experience/article based knowledge of this stuff, with a good resource to help me, and hope to help you and/or others, much like others have helped me, from 'I know nothing, I need a tech to fix my amp that has something wrong' on up. 

Good luck, be saavy, don't get ripped off!

Last edited by Liquids (2012-12-18 15:50:21)


Matthew

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#16 2012-12-18 16:06:27

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

Re: Problem


Matthew

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#17 2012-12-19 10:04:36

nitraus
Member
Registered: 2012-02-22
Posts: 55

Re: Problem

Fortunately my tech is a good one, the only problem with him actually is that he lives in a town 40 miles from here and he's moving to another location, so he's not ready to work in his new location yet.

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#18 2012-12-19 20:11:57

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

Re: Problem

A good tech is to find.  40 miles is worth it.

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