Why 3K3 value chosen for bias supply resistor?

[Charlie Wilson]

Hello, I should probably know this but I don't. Why did Dumble choose a 3K3 resistor for the AC side of the bias supply? I know Blackface Fenders use a 470 ohm that went up to 1k in the seventies and I think 2K2 was used in 1969. I always assumed that resistor was a voltage dropping resistor but is it there for other reasons? The practical reason for my question is that I have a quad of TAD 6L6GCs in my amp (Low Plate Classic) and I am up to 56k on the safety tail with the 10k pot turned all the way and I still want to go cooler with the tubes. They are at 35ma per tube right now. I can put a higher value safety tail resistor but I am wondering at what point should I maybe adjust the 3K3?

[Charlie Wilson]

Please excuse me for talking to myself a little. I found a 70s Twin schematic that has the 3k3 resistor. However the plate voltage on those is more like 405v and #124 is documented as having the 440v which is blackface. The only thing I can think of is that Dumble was using tubes that had fairly low plate current and that he biased the amp on the really cool side. I think I will try lowering the resistor to 2k2 so I don't have to use to large a safety tail. I know, the first sign of going crazy is talking to yourself.

[PaisleyTube]

because only with that handpicked value all electtrons flow in a truly magic way

mo' serious: slightly more filtering

[groovtubin]

Hello, I should probably know this but I don't. Why did Dumble choose a 3K3 resistor for the AC side of the bias supply? I know Blackface Fenders use a 470 ohm that went up to 1k in the seventies and I think 2K2 was used in 1969. I always assumed that resistor was a voltage dropping resistor but is it there for other reasons? The practical reason for my question is that I have a quad of TAD 6L6GCs in my amp (Low Plate Classic) and I am up to 56k on the safety tail with the 10k pot turned all the way and I still want to go cooler with the tubes. They are at 35ma per tube right now. I can put a higher value safety tail resistor but I am wondering at what point should I maybe adjust the 3K3?

Going HIGHER with that resistor means LESS AC voltage... meaning.... LESS NEGATIVE bias voltage to work with..meaning a MUCH HOTTER TUBE!!!!! Maybe HAD was up to something eh?

[Charlie Wilson]

OK now I am a little confused again. If you are using a blackface power transformer with a blackface spec bias winding why would you want less bias voltage if (so I have heard) you bias your tubes way on the cool side 50%-60%. Unless the plate current on the tubes he was using then were really low or maybe he did not necessarily bias all his amps on the cold side. I get it that a higher value resistor drops more of the bias voltage and I guess less bias voltage could mean less voltage to filter. Oh well I like the sound of my amp biased cooler so I will try the 2k2 resistor.

[PaisleyTube]

stock bf biasing is cool/cold
besides the resistor before the diode the bias-pot and resistors over there do also have an impact on the biasvoltage