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ampeg v4-b biasing


 
7/7/2000 9:37 PM
danhei
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ampeg v4-b biasing
I just got his amp (ampeg v4-b) in an auction and had it taken into my local store to make sure it was set up correctly. They said the bias wasn't adjusted correctly and charged $40 to rebias the amp. It is, I believe, a cathode biased amp running four sovtek 7027's (they came with the amp).  
I've noticed that when I play two of the four tubes glow red on the edges of the plates (I think that's what they are called). Most of the plate is dark but along the seams where the two halves are joined together two of the four tubes glow red. Does this mean that these tubes are running too hot? Or is a little red nothing to be concerned about?  
I must say I wasn't very impressed with the music store...that's why I lurk on sites like this so I can learn how to do this stuff on my own!  
Thanks,  
Dan
 
7/7/2000 10:48 PM
Casey4s
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That amp is "Fixed Bias" and is not a cathode biased amp.  
 
I would take the "Glowing" tubes and put them in the other sockets to see if the glow went eith the tubes, or stayed with the sockets. The plates should not glow, perhaps you see the filaments thru openings in/between the plates. You might have another problem in the amp, or poor quality tubes if the plates are glowing.  
 
Casey4s
 
7/7/2000 11:11 PM
danhei
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Seems someone lied to me when they said it was cathode biased then!  
Thanks for the tips, Casey. It is definately the exterior structures (plates) that have the bit of glow to them. I can see that there is an organge glow through the holes inside the plates. But the plates themselves are definately glowing a dull red at the seams. So I'm guessing that the tech who worked on it just wasn't very competent and biased the tubes a little hot.  
Dan
 
7/7/2000 11:59 PM
Casey4s
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There is a schematic for that amp here at Ampage, it is in the resources/schematic index, NOT in the the schematics listed just below the chat room link.  
 
Casey4s
 
7/8/2000 4:06 PM
Tom L
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However, those schematics do not include the voltage readings, but they are available here:  
 
http://www.freeweb.org/hobby/ik2zdf/teknikal/circuit1/v4.htm  
 
--Tom
 
7/8/2000 4:11 PM
Tom L
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Oops, sorry, I thought you were talking about a v-4, not a v-4b. Try this:  
 
http://www.freeweb.org/hobby/ik2zdf/teknikal/circuit1/v4b.htm  
 
They're not too different; the v-4 has a couple of extra tubes.  
 
--Tom
 
7/8/2000 4:16 PM
Tom L
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Ignore those other posts; THIS is the right link
Joe Piazza's (the guy who drew the schematics) site had been down for a while, but I just found his new site:  
 
http://home.rochester.rr.com/piazza/index.html  
 
The voltage charts are linked at the bottom of the page.  
 
--Tom
 
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