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Help w/ Fender Concert Amp w/ high plate voltage


 
5/31/2000 3:07 PM
Tim P
email
Help w/ Fender Concert Amp w/ high plate voltage
I have an '83 Fender Concert Amp (Rivera era) that I just acquired that has 490 volts on the plates of the power tubes. I removed the Russian 5881 power tubes that it arrived with and put in a matched set of NOS JAN Phillips 6L6GB power tubes. I converted the bias supply from the CBS "balance" control to a standard adjustable bias.  
 
My problem is I am reading around 490 volts on the plates of the power tubes and I am having a hard time getting the bias down to the 25v range for that much plate voltage. The bias pot is the standard Fender style 10K pot and the resistor on the pot started out as 33K (33K is what the balance control wiring used). I now have a 56K resistor on it and I still can't get the bias down to 25v (I am using a bias chart and a bias probe to set the bias).  
 
Should I continue increasing the value of the resistor on the bias pot until I can bias that high 490 plate voltage or should I try and lower the plate voltage?  
 
I am concerned about the sound and tube life with 490 volts on the power tube plates. I'd like to get it down to around the 450-460 range at the most. What is the best way to lower the plate voltage?  
 
Thanks,  
Tim P.
 
5/31/2000 4:09 PM
John Stokes
Tim, it sounds to me like you're attacking the problem completely wrong. You don't adjust toe bias on the output tubes to any given voltage, you're adjusting for a certain idle plate current which is usually in the 30 to 35 ma range for most Fenders. Whatever you do, do NOT try to use idle plate current to induce a lot of pre-sag into the power supply!! Short tube life will surely result from high dissipation. With close to 500 volts on the plates (and screens), you're going to need spomething around -50 volts or so on the control grids!!  
 
If you don't know what you're doing, please take the amp to a qualified tech before disaster strikes.
 
5/31/2000 5:17 PM
Tim P
email

Thanks for the warning John but I think maybe you misunderstood my situation.  
All I really want is to bias the amp properly. I am using the cathode resistor method to measure the bias. Pin 3 to ground is measuring around 490 volts. I am measuring bias by putting my meter on opposite sides of a 1 ohm resistor that is between the cathode and ground. The chart that I have says for around 490 volts I need to set the bias at around 24-25ma. My problem is I am having a hard time getting the bias pot to that range. With the 33K resistor on the bias pot I could only get the bias down to around 40ma I think. With a 56k resistor I can get it down to around 30ma. My concern is that since I am having to increase the resistor on the bias pot so much that maybe the plate voltage is not supposed to be that high in the first place. The schematic I have says the plates should be around 460 volts.  
 
Should I keep increasing the resistor on the bias pot until I can get the bias to 25ma or is there a potential problem I am overlooking?  
 
Thanks,  
Tim
 
5/31/2000 8:36 PM
Gil Ayan
email

quote:
"Should I keep increasing the resistor on the bias pot until I can get the bias to 25ma or is there a potential problem I am overlooking?  
 
Thanks,  
Tim  
"
 
 
Sure, continue to incrase the "tail" of the bias pot. Actually, you could lift the gorund all together (equivalent to having a HUGE "tail" resistor). It it possible that you will NOT be able to get the proper bias voltage FOR YOUR SET OT TUBES that way, which means that you will have to redesign the bias supply some.  
 
There is no problem whatsoever associated with increasing the tail resistor. Your RANGE will decrease, but that's about that. If you find that a bias supply redesign is needed, you can derive a bias supply from the B+ tap (a la Marshall, look at any Marshall schematic), OR if you have a dedicated WINDING (as opposed to just a bias tap) for the bias supply, try putting a full bridge rectifier to it, as this will increase the voltage headroom and allow for more negative voltages.  
 
Gil
 
5/31/2000 8:45 PM
Tim P
email
Anyone know the max plate voltage for JAN Phillips 6L6GB/5881
Thanks Gil.  
I guess I'll continue increasing the bias pot "tail" resistor. Does 490 plate volts seem a little high? Can a 6L6 take that much for very long?  
 
Tim
 
5/31/2000 9:31 PM
Gil Ayan
email

quote:
"Thanks Gil.  
I guess I'll continue increasing the bias pot "tail" resistor. Does 490 plate volts seem a little high? Can a 6L6 take that much for very long?  
 
Tim  
"
 
 
Svetlana 6L6s are rated at 500V plate voltage, so they should be OK at 490 VDC.  
 
Gil
 
5/31/2000 9:44 PM
Richie {~}==:::
Most like to go in the bias circut and change the resistor on the rectifier board to get the ballpark figure of what voltage you may need . Then the resistor of the pot to fine tune.What i mean is, ,you can make the pot only 10 volts different for fine tuning. or use a different resistor to make it have alot more range. But then it changes fast with just a small amount of turning. I hope i'm not confusing you. Just trying to let you know another way of doing this. Richie{~}==:::
 
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