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SF Super Reverb Plate Current?

12/11/1997 12:26 AM
SF Super Reverb Plate Current?
I've recaped and retubed my SF super reverb, the plate current is around 508 the bias is around 57 is this the norm for these mid 70's amps, if not whats the best way to remedy the situation? i'm using 5881's it seems to be working fine, no tubes glowing red anyways.
12/11/1997 6:44 AM
J Fletcher
Do you mean the plate voltage is 508 volts?The plate current should probably be in the 20 to 30 ma range.You really should check with a scope and signal generator as well.
12/11/1997 8:47 AM

With these voltage readings, assuming your power line voltage is no greater than 125vac, also assuming you are taking the measurements you mentioned with the standby switch in the play mode, it would seem that your output tubes are biased for a very low current draw. This would allow the power supply voltage to charge up pretty high.  
Do you have an actual bias voltage level adjustment capability, or does your amp still have the bias balance control (easily recognized by a center tap lug, 4 lugs total, on the adjust pot)?  
With new caps, higher line voltages, and highly variable characteristics among the various currently available 6L6 types, you need the adjustable bias voltage level feature.  
12/11/1997 11:25 AM
Re: SF Super Reverb Plate volts?
Doc, its the one with hum balance pot and output tube matching pot. What would be the best way to go about fixing the problem. should i change it to adj. bias or could i leave it alone and fix the problem else where?
12/11/1997 9:14 AM
Jim S.
Re: SF Super Reverb Plate Current?
508 volts is very high for a mid-70's SF Super Reverb. What type rectifier tube are you using? The tube chart probably calls for a 5U4GB. If you are using a 5AR4/GZ34 or (especially) a solid-state plug-in replacement, then that could be one reason why the voltage is higher than normal.
12/11/1997 11:15 AM
Re: SF Super Reverb Plate volts?
Jim, this is one of SF models with out a rec. tube, it has the rec's arranged in a square type pattern with disk caps across each one, i checked the rec's they seem to be in within tolerance.Is there something i'm missing here or should i change something in the amp?
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