Tube bass overdrive

Bernardduur · Post by **Bernardduur** » 05 Oct 2008, 12:06

Hey all!

I found this picture a while ago on the internet by searching on "bass overdrive" in google. Right now I can't seem to find it anymore.

Can anybody tell me something about it? I can't find if it is an idea, an already exsisting pedal or something completely different. The 6E5 tube is awesome (magic Eye)......... although I don't know it's function there then to determine the overdrive.

I'm asking as I am designing a new bass amp for myself (ultra lin 6550 amp) and I know it'll lack OD. This circuit is maybe quite nice in it

Dirk_Hendrik · Post by **Dirk_Hendrik** » 05 Oct 2008, 15:36

right. DC coupling to the first tube's grid.. that's going to work...

Use your volume control to bias your tube...

Bernardduur · Post by **Bernardduur** » 05 Oct 2008, 16:07

Dirk_Hendrik wrote:right. DC coupling to the first tube's grid.. that's going to work... Use your volume control to bias your tube...

????

Looks like a normal tube input without any grid resistors.

Dirk_Hendrik · Post by **Dirk_Hendrik** » 05 Oct 2008, 16:11

your grid is negative biased toward the cathode through a 1M resistor. Of you turn down your bass volumecontrol to 0 what's the voltage on the grid? Yep. Zero volts as well. What you miss here is an input cap.

Bernardduur · Post by **Bernardduur** » 05 Oct 2008, 16:19

Ehm, usually I am not that quick on this, but all amps I build have the same input stage (usually w/ grid resistor).

We're talking about the input, right?

Dirk_Hendrik · Post by **Dirk_Hendrik** » 05 Oct 2008, 17:17

jep

admittedly, unbuffered non-ac decopled stages seem normal in fx count. Especially outputs. brrr

analogguru · Post by **analogguru** » 05 Oct 2008, 17:27

Dirk_Hendrik wrote:your grid is negative biased toward the cathode through a 1M resistor.

tube technology...... thats an own matter.....
The grid is biased at 0 volts with the 1M resistor.
The cathode is biased between + 1,2V and + 1,8 V with the cathode resistor. Therefore the grid appears negative to the cathode.

Of you turn down your bass volumecontrol to 0 what's the voltage on the grid? Yep. Zero volts as well. What you miss here is an input cap.

It stays at zero volt and the cathode between + 1,2 V and + 1,8 V...... BUT:
There is a (small) DC-current flowing from the grid to ground. If the 1M resistor is replaced by the pot (or a pot in parallel) then this DC-current will flow over the pot. DC current flowing over a pot causes: CRACKLE !!!
Therefore the wiper of the potentiometer should never be connected directly to the grid. Better use a fixed 1M to ground and a cap between it.

Now lets assume you have an old pedal in front of the amp and the output cap has become leaky and there is a DC of lets say 2 volts at the output of the pedal. What will happen ?

analogguru

Stompmodder · Post by **Stompmodder** » 21 Nov 2008, 21:47

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