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Post subject: ec vibro-champ volume control problem
Posted: Sat Jul 07, 2012 11:08 am
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Just bought an EC vibro champ a couple of days ago after much consideration. Great sound and looks.

While playing today (using a classic player 60's Strat), I was trying different settings on the amps volume knob and discovered that from 1 to half way between 2 and 3 on the control, there is no sound at all, and I have to turn it up further than I would expect before getting distortion.

I'm concerned because either there is a problem with my amps circuit - which would be bad news as I don't want to send it back and wait ages for a replacement, or the pot is cheapo, which makes me very nervous about it's longevity, and about the other components.
The first one of these amps I tried in the shop was faulty, and they had to ship this one in from another branch.

This is an EXPENSIVE amp, is this kind of behaviour in the volume control normal?
Should I take it back, or just accept it?

cheers

Col


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sat Jul 07, 2012 2:06 pm
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Hate to say it but I would return it if it is not working correctly. I know you don't want to wait, but for the amount you paid for it, I would be demanding it worked perfectly!

As for the distortion coming later than expected, can't help you there. It could be related or it is just different from your expectation.

HTH

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sat Jul 07, 2012 3:38 pm
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Actually, the volume control operation sounds perfectly normal to me. The volume control goes from "1" to "12". "1" is off, no volume. The knob has to be turned sufficiently to get into the range of the pot curve where sound will begin to be produced. On both my Blues Deluxe Reissue and 5F6A clone, I get no sound out of either amp until the volume control is set to at least "2".

As far as getting distortion, how loud do you have the guitar set? You could have the amp set at "12" and if the guitar is not turned up sufficiently, you will not get any overdrive. If you tell me that you have both controls set to "12" and are not getting any overdrive, then you may have a bad tube.

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sat Jul 07, 2012 4:42 pm
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bluesky636 wrote:
As far as getting distortion, how loud do you have the guitar set? You could have the amp set at "12" and if the guitar is not turned up sufficiently, you will not get any overdrive. If you tell me that you have both controls set to "12" and are not getting any overdrive, then you may have a bad tube.


+1

The V1 gain stage needs every available millivolt of input signal to help saturate the tube. Likewise, the downstream stages (especially the phase inverter) require the highest possible level to start the compression process going into the final drive. Turn your guitar all the way up.

Arjay

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sat Jul 07, 2012 5:31 pm
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colB, this is ok. Pots change the impedance of the input stage---esp sensitive in simple SE amps. And thus can change the overall gain at certain settings. More important is how the tone is throughout the volume range. Should be a slow onset of OD with volume. Not clean, then super crunch.


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 4:52 am
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The tone is good, and progresses smoothly from clean through to tweedy distortion.
I'm aware of the influence of the guitars controls, and had them maxed out.

My guess is that the volume pot has been manufactured with the track slightly out of position. So I get no sound at all for the first 50 degrees or so - it's not just very quiet, it's off. If this is the case, it would also mean that the knob travel stops before the the wiper reaches the other end of the track, so it never reaches the maximum crazy flappy fuzz that the amp is capable of. This is my experience, comparing my amp with some of the promotional videos on the web. The only way of knowing for sure would be to de-solder the pot and test it, but I don't want to invalidate the warranty.

I've had a look at the schematic, and I can see that this pot is not likely to damage the amp if it fails. I don't need the super fuzz maxed out sound. So it's just the disappointment in the QC for such a premium amp, and the concern that gives me about the rest of the components - particularly when the other one that I tried was faulty.


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 7:06 am
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I have already explained that of my two amps that go from 1 to 12, that there is absolutely no sound until the knob is on "2". I and others have also explained the distortion characteristics of the amp with regard to guitar and amp volume settings. Where does the volume start on your amp; at 2.5 (both of mine start at 2), or 50 degrees? There is a big difference. Trying to compare a live amp sound against a You Tube video is pretty useless due to the amount of compression applied to the recorded signal. If you are not satisfied with the amps performance after reading the responses here, take it back and be done with it.

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 7:07 am
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If you really want to, go for a Canadian-made PEC pot. These are really nice puppies that last.

http://www.tubesandmore.com/products/R-VPEC-1MA


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 7:08 am
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Fender seems to have decided to not build any more "premium" amps. It seems they only wish to charge premium prices for some models. With this site combined with other guitar sites around the web there are QC and reliability complaints about every single model of Fender amp you can name. I suspect this won't be the last time you have an issue with this amp. If it were me I would take the amp back for a refund while you still can and buy something like a Victoria 518 or 512 and use a Supa-Trem in place of the amp's vibrato circuit.

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 7:18 am
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You know, if you dig the tones of a Vibro-Champ... maybe you ought to look into buying a VC? Just a thought... :D

http://www.ebay.com/itm/Vintage-Fender- ... 3f1965099d


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 7:27 am
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BMW-KTM wrote:
Fender seems to have decided to not build any more "premium" amps. It seems they only wish to charge premium prices for some models. With this site combined with other guitar sites around the web there are QC and reliability complaints about every single model of Fender amp you can name.


+1000!

That's why I see no new Fender amps in my future......

Ever!

:roll:

Arjay

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 8:25 am
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The OP says "The tone is good, and progresses smoothly from clean through to tweedy distortion."

Personally, based on the OP's conflicting description of the amp's "problems (Volume come up at 2.5 or 50 degrees. They are NOT the same position on the volume knob.), I don't think there is a single thing wrong with the amp. He also doesn't think that the distortion gets to the same level as he has heard on line, yet he makes the opening statement at top.

The only problem, in my opinion is the OP's understanding of how the amp works and his expectations. He has already said that he doesn't want to void the warranty so I don't see him changing the pot. The chances of ONE pot being bad out of an entire run of pots, and Fender picking that one pot to use in an amp are pretty slim in my opinion. If the OP is dissatisfied with the amp's performance he should return it under warranty and either have it replaced or buy something different.

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 10:35 am
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bluesky636 wrote:
based on the OP's conflicting description of the amp's "problems (Volume come up at 2.5 or 50 degrees. They are NOT the same position on the volume knob.),

You're right, I made an error in my arithmetic. The rotation from the 1 position to 2.5 is more like 42 degrees.
Quote:
He also doesn't think that the distortion gets to the same level as he has heard on line, yet he makes the opening statement at top.

Don't see why that's a contradiction. The tone is great. It doesn't get as distorted as the same amp in promo vids I've watched. Both of these things can be true at the same time.
Quote:

The only problem, in my opinion is the OP's understanding of how the amp works and his expectations.

My understanding is that the volume pot works as a voltage divider between the first two stages of the amp after the coupling capacitor. It is set up to vary the signal voltage from zero to that at the output of the previous stage (If there is a fault in the pot, then it may be that it can never be set to allow the maximum signal voltage through).
My Expectations are that this amp should have top quality components and extremely tight quality control as it is marketed as a premium device, and the price point is very high.
Quote:
The chances of ONE pot being bad out of an entire run of pots, and Fender picking that one pot to use in an amp are pretty slim in my opinion.

Yes, I would have to be really unlucky for that to be the case, however the chances that there are many borderline bad pots in an entire run, and Fender use them because its cheaper, or because their QC is not great, and most folks don't notice seems likely.
That said, the fact that you have similar behavior on two of your amps suggests that it's fairly normal and maybe Fender consider these parts as 'within tolerance'.
I'd really like to hear from a Fender tech. I've made loads of stomp boxes over the years, and Have never seen a pot where the first 11.5% of the rotation does nothing (FWIW, I usually test them with a meter), but those are low current devices, so I suppose there could be some intentional dead-zone for safety or something...

I'll probably hold onto this amp, assuming there are no more problems.

Thanks for your help

Col


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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 12:04 pm
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colB wrote:
I'd really like to hear from a Fender tech.


All of us that have responded to your posts are experienced amp techs and are as good or better than any Fender tech when it comes to diagnosing and repairing amps. The only way you will hear from a Fender tech is to take the amp to a service center.

Take a look at the pot taper graph for an audio taper pot (which the volume control in this amp is) and you will see why for the first 25% or so of the knob rotation, there is very little change in volume level. Depending on the other elements in the circuit and the pot tolerance (typically 20%), it should be obvious why the operation of your volume control can be considered normal. Even the RS Superpots that I use in all of my guitars (special taper, 10% tolerance CTS pots) don't really come into play until at least "2" on the dial.

http://www.geofex.com/Article_Folders/p ... tscret.htm

As far as comparing the drive levels of your amp to those you hear on line, unless the comparisons are made under identical conditions (using the same guitar and mic with the guitar plugged straight into the amp), any conclusions made about the level of overdrive are meaningless. No two amps will ever sound exactly alike even under identical conditions due to the combined tolerances of every component in each amp.

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Post subject: Re: ec vibro-champ volume control problem
Posted: Sun Jul 08, 2012 12:56 pm
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bluesky636 wrote:
Take a look at the pot taper graph for an audio taper pot (which the volume control in this amp is) and you will see why for the first 25% or so of the knob rotation, there is very little change in volume level. Depending on the other elements in the circuit and the pot tolerance (typically 20%), it should be obvious why the operation of your volume control can be considered normal.

I understand audio taper pots - have used them many times. I'm also very familiar with that article you linked to.
Thing is that it's not very low level or very little change from 1 to 2.5. It's absolutely no output at all. Then clear but quiet sound as I would expect at the start of the taper from 2.5 on the dial.
If the 20% tolerance means that for the part of the taper where the resistance is less than 20% of the maximum value, zero resistance is 'correct' because it is within 20%, then this pot is within tolerance. That would mean the pot is not faulty, just really crappy :|
Quote:
As far as comparing the drive levels of your amp to those you hear on line, unless the comparisons are made under identical conditions (using the same guitar and mic with the guitar plugged straight into the amp), any conclusions made about the level of overdrive are meaningless. No two amps will ever sound exactly alike even under identical conditions due to the combined tolerances of every component in each amp.

Yes, you're right, how hot the pickups are would certainly be significant.
I've been playing the amp again today, and it sure does sound sweet :D

btw, are there any other forums you would recommend where experienced amp techs hang out that aren't manufacturer specific? I have a Hughes and Kettner Blues Master that I'm just about to start servicing - now that It's not my only amp - It would be good to have somewhere to ask questions if I get stuck.

cheers

Col


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