Help me find the dropping resistor in Silvertone 1484

[IzzyB]

I've recapped my 1484 and put in new output tubes. I want to do a minor mod and change the amp from fixed to adjustable bias with a bias pot. Problem is I can't find the dropping resistor in the schematic: http://www.schematicheaven.com/barga...ertone1484.pdf

The load resistor (R60 100K) is easy enough to find. Is this amp designed without a dropping resistor?

and a followup question: do you know what R61 (2.2K) does? Can I ignore it and just take out that 100K load, replace it with a pot and lower value resistor to complete my mod?

Thanks

[Old Tele man]

...in the voltage-divider circuit of R60 & R61, R60 is the "series" dropping resistor and R61 is the "load" resistor (to ground).

...I'd tweek R60 and leave R61 as it is (2.2K-ohms) because making R61 smaller will (a) draw more current and (b) that'd cause it (R61) to become much hotter (not good or useful).

...if you're gonna "replace" R60 (100K-ohms) with a pot, suggest you use a combination of separate 'series' resistor in series with a "small" range potentiometer. The 'series' resistor serves as a "minimum" R to ensure some resistance is always present if the pot is "zero'd."

...since R60 is now 100K, and typically the pot is 1/5-1/10 of the total series resistance, that means a series resistor of 80K-90K and a pot value of 40K-20K (remember you want the "normal" setting of the pot to be in the "middle" of its range, hence its resistance would be: 1/2-of-R(pot)).

[soundmasterg]

To get any useful voltage adjustment on the bias supply on that amp, you have to set up a voltage doubler. The stock setup has too low of a voltage and runs the tubes pretty hot. I did this on my brother's and his works fine now and has a typical adjustment range like most Fenders. There isn't much room in there and you have to know what you'redoing to incorporate a votlage doubler in the bias supply, but it can be done and is worth it if you plan to keep the amp.

greg

[greg73]

Hi. I didn't do this but I have a link for you.
http://members.tripod.com/untweb/abswitch.htm

Greg long live Silvertone!!!