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Low B+ in SF Vibrolux Reverb


 
9/22/2005 5:37 PM
Carlo Pipitone
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Low B+ in SF Vibrolux Reverb
My '79 Vibrolux Reverb used to have B+ around 470V (the schematic says 450V).  
Today I read B+ again and it was 418V (415V plate voltage). What happened? What can cause a drop in B+ when everything else has remained the same?  
This reading stays stable with different rectifiers (5U4GB) and different power tubes.  
Power tubes were biased at 45 mA.  
The AC line voltage is around 238V: I keep the VAC selector on 240V.  
 
Should I worry for such low B+, and especially for such a drop if compared with previous readings?  
 
Carlo
 
9/22/2005 8:36 PM
Jack Koochin
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Hi Carlo,  
 
A 5AR4, along with a slight rise in the AC line voltage, will give you around 10% higher B+, perhaps even higher.  
 
I found my fave Fender amp B+ varies a fair bit just due to line voltage, as well as output tube bias. Did you increase the bias current since your last (470v)measurements by any chance?  
 
Were you possibly using a 5AR4 during your previous 470V measurements? Did you note what the line voltage was for the 470V measurement?  
 
HTH,  
Jack
 
9/23/2005 6:34 AM
Carlo Pipitone
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Re: Low B+ in SF Vibrolux Reverb - too low?
Jack,  
 
nothing has changed from my previous (470 V) B+ readings. I've always been using a 5U4GB recto as per the schematics. Further, the AC line voltage has been more or less the same in the last couple of years in my area.  
Only change: a used matched pair of RCA 6L6GC. Unfortunately I did not read voltages before swapping tubes, so the B+ drop might have started with the previous tubes. Anyway, swapping tubes again now doesn't affect the B+.  
One odd thing (at least it sounds odd to me): both tube pairs have almost exactly the same current draw in this amp. But it could be just a coincidence.  
 
Would it be wise to raise artificially the B+ a bit putting the AC selector on 220V instead of 240V?  
 
What kind of troubles can I expect from a lower-than-normal B+? Can it be a symptom of a dying power transformer?  
The amp seems to have a little less volume than before, but it could be me. For the rest it works and sounds fine.  
 
Carlo
 
9/23/2005 11:51 AM
Jack Koochin
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Carlo,  
 
Now that I recall, I actually did seem to experience the same sort of thing on an old Bandmaster of mine. I couldn't find anything wrong at all, but the B+ seemed low, by about 10%. Perhaps a transformer guru could enlighten us on the possibility of the power xformer aging and not delivering the same voltage out as a result... don't know.  
 
Just for grins, did you measure the secondary AC of the power transformer and do the calculations to see if what you're getting in DC is on par to what you're getting in AC at the secondary?  
 
One other thing I did was put a 1.0 ohm resistor in the main B+ path so I could easily measure the current thru it (by measuring voltage drop across it), to see if the amp current draw was what it was supposed to be.  
 
Other than now having a higher idle current in your output tubes, or some other load you've added, I'm sorry I don't have any more suggestions.  
 
I juiced up my amp B+ by about 10% or so by installing a 5AR4 instead.  
 
Just curious... how much power do you get out of the amp?  
 
Jack
 
9/24/2005 9:44 AM
Carlo Pipitone
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Re: Low B+ in SF Vibrolux Reverb
Just for grins, did you measure the secondary AC of the power transformer and do the calculations to see if what you're getting in DC is on par to what you're getting in AC at the secondary?  
 
Jack,  
I have 357VAC on pins 4 and 6 (red wires), and 415VDC on pins 2 and 8 (yellow wires). The latter measurement is quite a bit lower than specs (that is 465VDC).  
 
Any thought?  
 
Carlo
 
9/24/2005 12:32 PM
Chris/CMW amps
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Hi  
 
Could be mistaken but most VibroluxReverbs do run at lower voltages than SuperReverbs and TwinReverbs...  
 
Chris
 
9/24/2005 1:35 PM
Carlo Pipitone
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Maybe... but mine USED to run at about 470-480V...  
 
BTW, I do have 480V on the plate WITHOUT power tubes; then, no matter which 6L6GC pair I use, the plate voltage drops at 415V! This didn't happen before!  
 
AND the grid voltage is -38V, which is way too low, right?  
 
Do I risk to screw everything up if I adjust the grid voltage to around -48V with the bias pot? How will this affect the tubes' current draw?  
 
Carlo
 
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