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Topic: My Custom Vibrasonic is not very loud... |
Yann Obergfell
From:
Gottenhouse, France - Soon in Bloomington, Indiana
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Posted 2 Oct 2005 1:35 am |
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Hi,
I just got myself a '95 Custom Vibrasonic (100W, reverb and tremolo) but after playing it for a while it seems that the amp is not as loud as it should be. On 3 on the volume knob it is as loud as my CVR on 1,5. You can run the Vibrasonic at full blast and it won't blow your head off. It is still loud though, and it sounds great, but not as loud as I would have expected from a 100 watter.
Looking at the schematic, it seems that the amp has no negative feedback loop. Is it the reason why it doesn't sound very loud? Can a cold or lukewarm biasing have that effect? The amp otherwise sounds fine. Any ideas?
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 2 Oct 2005 5:34 am
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Yann, a negative feedback loop actually reduces the output, so that wouldn't be the problem. The first thing I'd check would be the preamp tubes. Players often put the wrong ones in (like a 12AY7 in place of a 12AX7) to change the breakup characteristics. After verifying that the tubes are correct and in good shape, the next thing would be to check the bias and plate voltages throughout. That should reveal the problem, if there is one.
Don't forget to check the speaker for the proper impedance, and remember that Fender amps with the "tone-stack" tone controls are very sensitive to tone settings. In other words, if you set all the tone controls at "3" to get the tone you like, you really won't have much power available.[This message was edited by Donny Hinson on 02 October 2005 at 06:40 AM.] |
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Jim Peters
From:
St. Louis, Missouri, USA, R.I.P.
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Posted 2 Oct 2005 6:48 am
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| Does it have the correct ohm speaker? JP |
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Yann Obergfell
From:
Gottenhouse, France - Soon in Bloomington, Indiana
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Posted 2 Oct 2005 7:04 am
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The preamp tubes (except the reverb driver tube, which is a GT 12AT7) are all ruby relabeled sovtek 12AX7WB. I like the sound of the tone controls at about 6.
The speaker is the original 4 ohm Eminence "special design".
Something is worrying me. It looks like the frame of the speaker is broken, or was reglued together. I've never seen anything like that. Could my problem be a bad speaker??? Yann
[This message was edited by Yann Obergfell on 02 October 2005 at 08:07 AM.] |
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Jon Light (deceased)
From:
Saugerties, NY
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Posted 2 Oct 2005 7:15 am
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| Could my problem be a bad speaker??? |
Yes it could. Also, I am forgetting now----the original Vibrosonic Reverb from the 70's had an 8 ohm output. But I don't know about the CS reissue.
[This message was edited by Jon Light on 02 October 2005 at 08:16 AM.] |
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Yann Obergfell
From:
Gottenhouse, France - Soon in Bloomington, Indiana
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Posted 2 Oct 2005 7:30 am
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The back of the amp reads "100W 4 ohms total"
What could have cause the frame of the speaker to split? It looks like cast steel or something... The amp doesn't seem to have been roughly handled, since the tolex is in very good codition.
What gives?
Yann |
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Jim Sliff
From:
Lawndale California, USA
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Posted 2 Oct 2005 7:55 am
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Cast frames can crack fro a sharp impact - maybe the package was dropped in shipping, and if there was a weak point in the cast a crack is possible.
But I think most Eminences are stamped frames. I know the one in my Vibroverb is. Regardleess, that could be the problem, but doubtful. If the speaker was bad you'd have a "ripped bedsheet" sound when the amp was cranked, most likely.
I'd look at the following:
1. Power tubes and biasing.
2. Possible bad filter cap.
3. Bad driver tube (the one right before the power tubes - a critical tube in the circuit most people don't think about).
4. Incorrect preamp tube (V2, the second one from the right).
5. Bad filter caps - I've had new amps with defective Sprague Atom filters that gave me about half-power.
First try it with a different speaker setup with the same impedance - like a Twin Reverb's 22x12's. If that doesn't blow the doors down, try substituting new power AND driver tubes - be sure to rebias it (using an exterrnal gadget like a Bias Riite, Bias King etc. Iff you don't have onee and don't know how to do it inside the chassis, take it to a tech). If that doesn't work, take it to a tech unless you are well-versed in amp maintenance and especially safety.
Hope that helps -
PS - comparing amp volume control settings at low volumes is an invalid test. Tube amps and speakers react in different ways at low volume. If you are pllaying at low volume, don't use that amp - get a smaller amp that you can turn up and drive fairly hard to get the best tone. Tube amps turned up to 2 or 3 universally sound lousy - the circuit and speaker havee to bee driven fairly hard for the amp to open up and sound like what it was designed to do. For most home practice playing, a Deluxe Reverb is about as big as you can reasonably go unless you have a soundproofed practice room.[This message was edited by Jim Sliff on 02 October 2005 at 08:59 AM.] |
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Yann Obergfell
From:
Gottenhouse, France - Soon in Bloomington, Indiana
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Posted 2 Oct 2005 8:57 am
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i'll try to do that.
I actually bought the amp to play on stage, that's why loudness is a concern to me. The amp strats breaking up on half-power, and I'lm not sure that I could compete with my heavy handed drummer with the amp as it is. Thanks for your help.
Yann |
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Ken Fox
From:
Nashville GA USA
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Posted 2 Oct 2005 9:27 am
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If both channels are equally weak then the problem could be the driver tube V#6, the 12AT7 just before the power tubes. Swap it with V#3 (12AT7, reverb driver tube).
Beyond that I would suspect poor biasing of the power tubes, weak power tubes. That amp should be darn good and loud! Poor biasing will result in low output and early clipping. The tubes should be in the 30-35 ma range at minimum.
[This message was edited by Ken Fox on 02 October 2005 at 10:28 AM.] |
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Daniel Gillett
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Posted 2 Oct 2005 5:17 pm
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I Have Two Of These Amps. They are loud at 3. Both have the same volume and tonal qualties. Both are sweet sounding for my Emmons and my tele. They Don't break up at half volume.
Dan |
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James Pennebaker
From:
Mt. Juliet, TN
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Posted 2 Oct 2005 8:13 pm
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I have one of the "Custom" Vibrasonics. I took it out on a job recently after not using it for a few years. It didn't seem to have the volume nor the headroom it once had. I swapped all the tubes and checked biasing but no change. Replaced the filter caps and bingo! Good as new. The amp is 10 years old and that's about the life expectancy of most electrolitics.
Warning. If you haven't any experience in such things, don't attempt a cap job yourself. Get a professional repair person to do it. Those caps hold lethal voltages that can kill you.
JP |
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Ken Fox
From:
Nashville GA USA
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Posted 3 Oct 2005 4:59 am
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The primary B plus caps, two largest ones probably suffered leakage to DC in James amp! Caps can become a resistive path to DC, loading the B plus supply down and resulting in lower plate voltages for all the tubes. I have seen them eventually short out and take a fuse with them on every power up. The other condition is the cap will not pass AC, in other words it is no longer a filter cap. Such an amp will have a hum in the audio output. Any competant tech can located either with a meter and/or O-Scope.
Bypass caps (electrolytics) on the cathode of preamp tubes can open up and result in very low gain in the output as well. Audio coupling caps can pass DC when they start to fail and cause the next stage of a tube to be biased into saturation, resulting in low gain and distortion.
Again, look at the tubes first. Best of luck and keep us posted on your final results.
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Yann Obergfell
From:
Gottenhouse, France - Soon in Bloomington, Indiana
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Posted 3 Oct 2005 6:11 am
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My particular amp hums a lot. I think I'll have to take my amp to tech.
geez, the seller had told me that the amp had been serviced & retubed recently! He used to play it in his flat, so I suppose he didn't norice the problem... |
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