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Headroom in Ampeg V-4B

9/6/2004 10:17 PM
Paul Drexhage Headroom in Ampeg V-4B
My friend picked up a great looking (and sounding) late 70s Ampeg V-4B. The thing had been taken care of and had been worked on before (filter caps changed within the last ten years). It came with 6L6GC's rather than the 7027A's as drop-in replacements (no resistor changes, etc.) The thing sounded great, and was reasonably loud, but broke up at around "3" (9/10 o clock) on the volume knob. I replaced all the electrolytics (coupling caps) in the preamp for him, gave the power tubes sockets a quick cleaning. The thing is biased to stock (-52V). He also replaced all the tubes in the preamp. Still broke up. Finally he put new 7027A's in there and still the distortion. Now, my question is, is this actually a "problem"? Did vintage V-4B's and the like always break up around this point? My friend and I were sort of hoping that we could crank the volume knob a bit more before it breaks up. He is a bassist, after all. Mind you, we're running reasonably hot basses into this thing through an Ampeg 6x10.  
Thanks guys,  
9/7/2004 5:59 PM

"He is a bassist, after all. Mind you, we're running reasonably hot basses into this thing"
As long as it's not active, I would say it should distort. I had a '73 for years. The V4 and V4-B preamps are set up differently gain-wise and someone may have modded it for guitar(easier to overload). At three, a guitar should sound like an electric piano, assuming good speakers. You should determine the actual bias current in case it's something that simple.
9/7/2004 10:15 PM
Paul Drexhage
Thanks for your reply. I take it from your post that you mean it SHOULDN'T distort as long as the basses aren't active. Well, one bass has a Seymour Duncan quarter pound p-bass pick-up (passive, though hot), while the other is a Musicman/Ernie Ball Stingray (active). Both sound the same (in terms of distortion).  
As for the bias, with 6L6's, it had been set to about 35 mA per tube (70 mA per side using the transformer shunt method).  
Thanks again,  
9/7/2004 11:51 PM

Using Duncan's load.xls file, and assuming a 540V B+, 35mA/tube is good, if not perfect.  
If the wrong variant of 12__7 is in any given socket, this could happen too.  
It seems like it shouldn't be distorting. Bet it's loud though :D . Do you have a scope so you can tell how much clean power it's delivering?  
Also, I kind of remember some jacks in the back. You might be able to bypass the preamp and inject a signal into the power amp from a function generator or a synth.
9/8/2004 3:30 PM
Wesley R.

Every Ampeg bass amp I have owned or used (and its been a bunch of them), broke up just after 10:00 on the volume knob. All of the Ampeg fanatics I knew called it the magic 10. As a side note I have not used active PUPs with an Ampeg. No particular reason, just haven't.  
Best Of Luck,  
Wesley R.
9/10/2004 3:35 AM
For what it's worth, I have a '72 V4, and I wrestled with the same problem of distortion at about 10 o'clock. I replaced all the caps, some resistors, all tubes, etc... and it's still like that. I thought there was something WRONG with it, even though I like the sound of it. Magic 10, I like that. It is definately where the magic happens. I think it's just a V4/B thing that should be loved for what it is :D  
9/10/2004 5:11 PM
Magic 10
Where do you have the sensitivity rocker set for that? That switch changes the cathode resistor and can add a bypass cap, which doubles the gain. A V4-B has no such switch or bypass cap, ftm. It's possible that someone added one to the amp in question.
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