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Vibro King mods (headroom / reverb)

9/14/1999 5:13 AM
Steve Vessels
Vibro King mods (headroom / reverb)
I just traded for a Vibro King which sounds great. Anyone know of modification info for vibro kings or related amps. Specifially:  
1. I need a little more headroom--that is, the ability to get louder, but stay cleaner.  
2. There is a Torres Engineering mod regarding how to prevent the reverb from eating up the EL84.  
3. The vibrato is noisier than I prefer.  
9/14/1999 6:25 AM

How comfortable are you with working on this type of amp Steve?  
They run with no negatve feedback and could use a little.  
Replace the old reverb OT with a reissue one and drop the B+ voltage supplied to the Reverb OT down to a more reasonable value!!  
I changed the last one down to 350vdc and used a SE 5K3ohm/8ohm 50ma OT with very good results.  
There are a few very minor things you can do like replace the cathode bypass caps with 10uF and look at the schematic and see how the fat switch works.  
Most guys run with the fat switch on all the time but what it really does is remove the normal 25uF cap from the cathode on the first preamp tube.  
The switch really should say "brighter" because it took lows away from what is normal for a Fender and gave it back to you as a feature instead!  
9/14/1999 5:15 PM
Jim S.

As Bruce suggested, I think introducing a bit of negative feedback is probably the key to improving headroom. I've never worked on these amps, but I've seen the schematic. I noticed that the long-tail phase inverter uses a 39K resistor in it's cathode circuit. This is much higher than the usual 10K-22K that most amps use. I would guess this is to compensate for the no negative feedback by reducing the gain quite a bit in the phase inverter, to prevent the tone from getting *really* mushy. So, if you do go ahead and add in some negative feedback, you might also want to experiment with changing that cathode resistor to a lower value as well, to maintain the overall gain of the amp.
9/14/1999 6:34 PM

First things first: Iím NOT a technician and second we may have totally different ideas about what constitutes good tone, so with that in mind I offer you my observations on the NFB loop.  
The short version: With the loop added, the amp sounded a lot tighter, with much less distortion, but at the expense of volume. Overall I liked it and played the amp that way for a year or so. I just recently took it out, because I began playing my Tele almost exclusively (see comments below). It may not be for everybody but since itís so inexpensive and easy to install I would say give it a try.  
The longer version goes like this: The loop consists of a 56K Feed back resistor, a 22K resistor where the 39K was, (thatís the resistor to the ďleftĒ of the two 1M resistors. Sorry I donít know the descriptive names of this part of the circuit). I also added a 10K resistor at the ďtailĒ end of it all (if this description is less than clear just look up practically any BF Fender schematic and it should be obvious). Once we discovered that the secondary was IN-phase with the phase inverter, and corrected that , the loop worked exactly as promised. Sound-wise, itís just like I stated above, there is obviously less distortion and less volume at the same control setting, although itís still plenty loud enough for my situation. The biggest benefit though is that the percussive attack at the start of each note is diminished. It also really tightens up the lower end, itís not as ďboomyĒ. For guitars I started with my Ric, itís an í86 360/6 that, if Iím completely objective, has never really sounded great through this amp. As soon as I plugged in though, I new something good was going on. My Ric sounded 100% better, more note detail when cranked but no more harshness. Even the neck pickup could be made to sound good, which has always been a difficult. task. Next in was the Tele, a í52 RI, that, has sounded great at everything except cranked Ėvolume levels. Previously when you started turning the volume past 6 the sound thinned out and I could hear what I think was a lot of speaker distortion. Whatever it was though, it wasnít very musical. With the NFB the Tele wasnít as dramatically ďimprovedĒ. It definitely sounded great cranked, but at lower volumes it didnít have as much ďbounceĒ, it seemed maybe, a little constricted. I think the Tele might sound better with the amp a little looser. I still havenít played it enough to decide conclusively though. And finally the last guitar I tried was a friendís old (Ďlate 50ís/early 60ís) Guild hollowbody with a neck-position P-90. In the past when Iíve tried this guitar it hasnít gone well, way to much distortion and no real way to clean it up. In fact this guitar turns most amps tone to mud. Normally I get to hear it through a showman or Twin and then itís a truly stunning wall of sound. Now Iíd love to say that the Vibro-King absolutely killed with it, but that would be a little less than truthful. It definitely sounded better than it ever did, much cleaner and more dynamic. But it was also quite thick, maybe be too thick, especially in the mids. Iím not sure what it is about this pickup (or maybe all P-90ís) but I donít think it will ever sound as good as it does through the Showman. As a final comment, my friend (owner/player of the Guild) thinks it sounds more like a Brownface amp now and less like a Tweed. I havenít played enough different amps to be able to offer more insight.  
So there you have it my rather un-scientific, somewhat biased, opinion. Like I said earlier, it may not be for everyone, but if you know what youíre doing (I didnít) it should take about an hour and cost less than $5.00. And if youíre not happy you can yank it out in even less time.  
9/14/1999 11:32 AM

Hey Steve,  
I've been in the same spot you're in right now, I made some changes and have been pretty happy ever since. Although part of it was realizing what I could resonably expect from the amp. The changes are on there way via email, I would have posted it here but it really is just basic stuff and I was really long winded aboout it.  
Stephen Prouty
9/14/1999 7:36 PM
Steve A.

I would have posted it here but it really is just basic stuff and I was really long winded about it.  
    Thanks for posting the details on your second post in this thread! I'm sure that other people will find it helpful, too.  
Steve Ahola
9/27/1999 3:59 AM
Steve Vessels

Thanks to all who have responded with mod info. I will send it to my amp tech, who will be doing any mods.  
By the way, anyone know of a Vibro King schematic online? I have an early model--#375 or so. Thanks again.

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