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previous: Chris B I have a Silvertone 1482 and have b... -- 8/11/2000 3:55 PM View Thread

Re: Silvertone 1482 Schematic, especialy the tremolo

8/11/2000 8:10 PM
Doc
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Re: Silvertone 1482 Schematic, especialy the tremolo
Until you are able to locate a schematic, here's some suggestions to get you going:  
 
You can slow down the oscillator speed range by enlarging the value of the phase shift network capacitors. They should be connected between the signal grid and plate of the 6AU6. There should be a network of 3 or 4 capacitors with resistors connected at the capacitor lead junctions. Replace them with capacitors of twice the value. Ceramics are ok, but film types are more accurate and more stable over time. The replacements should have a 400 volt (or higher) rating.  
 
You're basically working with the RC "time constant" of the phase shift oscillator. The product of the resistance and capacitance relates to the oscillation frequency. The larger the R x C, the lower the frequency, hence slower the tremelo speed.  
 
If the amp's tremelo is extremely fast, I just go ahead and replace all 3 phase shift capacitors with larger ones. If the resultant isn't slow enough, I tack on another cap in parallel at one station at a time until the speed range is down where I prefer it. All three caps don't have to end up with the same capacitance value. It's ok to have 3 different, or 2 same 1 different. Nothing sacred here, just don't make one extremely large or small. Since the value of the speed control pot may very well be figured into the speed range equation, a pot whose resistance value has drifted upward can cause a higher than designed tremelo speed. So check the resistance of that pot.  
 
A single triode can be configured as a tremelo oscillator, such as the circuit in a BF/SF Princeton. The 6AU6 heater current is the same (0.3A) as a 12AX7 wired for 6.3v. I would insert the extra gain stage between the existing preamp and the phase inverter. You may find you need to waste some gain using signal voltage divider networks between stages, or configure the extra gain stage with a low voltage gain using local feedback (a resistor connected from plate to grid). Or you could use a 12DW7 instead of a 12AX7. (It's not currently manufactured, but can be found.) The section that matches 12AX7 characteristics would be your tremelo oscillator, and the section that matches the lower gain 12AU7 would be your extra gain stage.  
 
Doc