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Re: Low B+ in SF Vibrolux Reverb


 
9/24/2005 1:55 PM
Chris/CMW amps
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Re: Low B+ in SF Vibrolux Reverb
Hi Carlo  
 
That negative voltage is for biasing (setting the platecurrent at idle).  
Lots of guys do put a 1 ohm resistor between pin 8 and ground to measure the cathodecurrent (slightly more then just the platecurrent but this method is pretty easy/safe).  
It's saturdaynight so don't flame/blame me for errors :D  
Put the negative voltage at the highest (let's say -48V in your case) setting and then measure the voltage between pin 3 and ground (the so called platevoltage).  
Then you know about the voltage, you know the plate-wattage from the tube (30W for sturdy modern 6L6). At idle the tube has to handle about 70% from that wattage (there are other "rules" but this one does work most of the time) so 21 Watt. Power = current x voltage so current = wattage / voltage.  
When you do adjust the bias from a very low current situation the voltage will drop after some adjustments. The current is measured by measuring the voltage across that 1 ohms resistor (current=voltage/resistance), use <1% resistors.  
 
Hope this helps,  
 
Chris during saturdaynight........
 
9/25/2005 1:50 PM
Carlo Pipitone
email

Chris, buddy,  
I have no tech background and my questions may sound dumb sometimes... but I do know how to bias a fixed bias amp (1-ohm cathode to ground method, OT shunt method) :)  
 
My main concern here is about the lower-than-specs B+ (about 418V), and especially about the fact that it used to be much higher (and on specs) before.  
Is there something in particular to troubleshoot when trying to cure an abnormally low B+ in your experience?  
 
Thanks a lot,  
Carlo
 
9/26/2005 2:17 AM
Chris/CMW amps
email

Hi Carlo  
 
It was saturdaynight and couldn't remember if you were able to bias :) (and using the shunt-method is way more accurate!).  
 
I would check for shorts 1st, try another pair of 6L6s and another rectifier. Check if the 1st e-caps (for the plates/OT) are good. Check the biascircuit. Is it a stock sf VR?  
 
Are there any other symptons besides a lower B+?  
 
I did experience large rises in B+ with broken/faulty tuberectifiers in the past so there must be something else going on...  
 
Good luck/let us know,  
 
Chris
 
9/26/2005 8:32 AM
Carlo Pipitone
email

Hi Chris  
 
quote:
"couldn't remember if you were able to bias"
 
No matter the day of the week or time of the day, you cannot remember the skills of all the people you meet in Ampage! :D  
 
quote:
"try another pair of 6L6s and another rectifier"
 
I did it: two 6L6 pairs and three rectifiers: the B+ remains the same.  
 
quote:
"Check if the 1st e-caps (for the plates/OT) are good"
 
I did a complete cap job about three years ago. Should I check the filter caps for leakage? Shall I use the same technique as with coupling caps?  
 
quote:
"Check the bias circuit. Is it a stock sf VR?"
 
No, it isn't. I modified it to have a bias adjust + balance circuit: I balance the bias with the original bias balance pot, while I adjust the bias with an added multiturn trimpot. It's always worked, and still works.  
 
quote:
"Are there any other symptons besides a lower B+?"
 
No, the amp works fine. Maybe (but it could be me) a little less output volume than before.  
 
My next move will be to start from the beginning: I'll set the grid voltage to specs without the output tubes, then install a pair of tubes and chck/adjust the bias.  
 
Any other thought is highly welcome! :)  
 
Carlo
 
9/26/2005 10:30 AM
Peter
I would try adding another cap in parallel with the first filter cap, to test if the voltage goes up. The symptoms (B+ is low but goes up if you remove tubes etc) could be a weak electrolytic.
 
9/27/2005 1:57 AM
Carlo Pipitone
email
Update
quote:
"I would try adding another cap in parallel with the first filter cap, to test if the voltage goes up. The symptoms (B+ is low but goes up if you remove tubes etc) could be a weak electrolytic"
 
Peter,  
should I use a parallel cap the same value as the first filter cap? No alternatives? (I don't have one right now).  
Is there any other way to test a filter cap, other than replacing with a new one?  
 
UPDATE of voltage readings:  
I put the bias negative voltage to specs (-48V) with the bias adjust pot, and measured the B+ with different power tubes. The B+ is now a little higher (around 430V instead of 415V) but the tubes are biased very cold (around 25 mA). If I adjust the bias properly, the B+ goes down again to around 415V, as expected.  
 
With a pair of Philips 6L6WGB (26W max. dissipation) and the bias voltage set at -48V, the B+ is low (417V) but the tubes are biased almost right (35mA).  
 
Any thought?  
 
Thanks  
Carlo
 
9/27/2005 4:50 AM
Chris/CMW amps
email

Hi Carlo  
 
Here's a way to test leaky e-caps:  
use a well known sturdy B+  
use a resistor that can handle that voltage and wattage  
put the resistor in series with the e-cap and measure the voltage across the resistor after a few moments; 0 Volt is ok, anything way larger means the e-cap is leaking  
 
Hope this helps/good luck!  
 
Chris
 
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