ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Re: Help...Bassman Bias Supply Question


 
6/2/2004 12:56 PM
Wild Bill
email
Re: Help...Bassman Bias Supply Question
Bill, you will get a higher voltage after the bias circuitry.  
 
A diode by itself will only pass half the cycle. This means you get a negative pulse, only half the time. Your meter will interpret this as an average of less than half of the original voltage. It's less than half 'cuz the pulse starts out from zero, goes up to max (half of the original AC voltage) and then back to zero. The average is then less than the peak value.  
 
When it gets to the rest of the bias supply the bias filter cap will charge up on each negative pulse. Since the current draw from a bias supply is "mice nuts" the cap will charge up to the the peak value of the negative pulse and stay there.  
 
At that point your meter will read the maximum available negative voltage.  
 
---Wild Bill
 
6/2/2004 1:12 PM
Bill Harbour
email

Thanks Wild Bill. That explaination makes sense.  
 
I appreciate the help.  
 
Bill
 
6/2/2004 11:51 PM
Bruce /Mission Amps
email

Well, I have not worked with half wave rectifiers in quite a while but I don't agree with Bill on that point....regardless.... put a small resistance load (33K-68K) on the diode's anode end to the center tap (ground if it is grounded) and measure the DC votlage on that diode again.  
I think it should be DC and within a couple volts or so of the AC input voltage unless you have utter crap for a voltmeter.  
If it's still half voltage... you got problems, or I'm going back to school... won't be the first time either! ha ha.  
Bruce
 
6/2/2004 11:57 PM
Bruce /Mission Amps
email

This tranny doesn't have a seperate, dual tap bias supply leads does it?  
In other words there IS NOT two bias supply wires from the PT.  
Yours has one bias tap wire and the center tap is the "other" lead.  
How in the world did you use a full wave rectifier in that setup?  
That grounded center tap and one bias wire makes it a halfwave supply, where you only need one diode and it's DC voltage, BEFORE any filter or smoothing cap, should be very close to the same as the AC voltage from the center tap to the bias lead wire out!!  
Bruce
 
6/3/2004 8:59 AM
Bill Harbour
email

Bruce..I tried it with a single diode and a full wave rectifier with only the + and - hooked up...same result...23vdc. I hooked up the rectifier to the board last night but didn't get a chance to try it. Will do so tonight if I can and see what the voltage is after the bias circuitry.  
 
Thanks,  
 
Bill
 
6/3/2004 6:46 PM
Bruce /Mission Amps
email

OK that's wrong ... but I think your going to find I'm right... you need a little current to flow through that diode in order to get a proper reading.  
It's not really "biased" on with out it.  
Bruce
 
6/3/2004 7:27 PM
Bill Harbour
email

k...I'll give it a try with the board hooked up.  
 
As always...thanks for the help Bruce!!!!!!  
 
Bill
 
<<First Page<Prev Page 2 of 2