Grid Leak Bias Input Question

[gtrbryan]

I am looking at the Gretsch 6150 as a possible project (aka Supro 1606).  Schematic is here http://www.valcopages.com/Schematics/Gretsch6150.jpg

This would be my first adventure into grid leak bias.  The amp I'm using to convert only has room for a single input.  The inputs on the Gretsch schematic are confusing to me.  It looks like if you plug into the 47k input, you then have parallel 100k resistors going to ground (50k) which I would think would split your signal. If you plug into one of the 100k inputs, you then get a 100k to ground, again splitting your signal.  Does anyone see a problem in just omitting the two 100k inputs, leaving a 47k into a .005 cap with the 6.8M to ground?  Any better ideas for a single input jack?

[sluckey]

Does anyone see a problem in just omitting the two 100k inputs, leaving a 47k into a .005 cap with the 6.8M to ground?

That's fine, but use a switching jack so you can ground the input jack when nothing is plugged in.

[HotBluePlates]

I'd also move the 47k resistor from the input jack itself to the grid of the input tube.

It is too small a value to upset the grid leak biasing, given the 6.8M resistor, but would now provide some grid stopper action to reduce the chance of interference.

[gtrbryan]

Thanks for the suggestions.  So I'm sure I'm on the right page, is the following schematic consistent with what you are suggesting?

[HotBluePlates]

Yep, that's what I was thinking.

[TIMBO]

Hi guy's, What does the cap at the input do?? I've seen it on the marshall silver jubilee.Thanks

[Tiny_Daddy]

The cap at the input prevents your guitar pickup from shorting out the bias.

[TIMBO]

ARHHHHHHHHHHH,Very interesting.I was trying some different components values on a V1 cathode and when i tried a direct connection to ground i got alot of scretchyness on the guitar pot.Thanks

[jjasilli]

The cap at the input prevents your guitar pickup from shorting out the bias.

Yes, but more importantly for grid leak bias:  that cap, together with the grid leak resistor, causes the (+)input signal to develop a bias voltage.  http://electriciantraining.tpub.com/14178/css/14178_45.htm 

A cap in that position also protects against DC voltage (possibly fatal doses) getting into guitar chord > guitar.  But these days this is remedied by better grounding schemes, and 3-prong power chords.

[HotBluePlates]

Hi guy's, What does the cap at the input do??

On a microscope level:

The grid is a wire helix; in general, electrons traveling from cathode to plate pass through the extremely wide gaps (as judged by an electron) between the wire. But occasionally, electrons will run into the grid wire itself.

If you have a big resistance from grid to ground, it's hard for these electrons to be bled away from the grid. They tend to accumulate, and if you add a cap between the input and the grid reference resistor, the electrons charge the cap. The plate of the cap closest to the grid now looks negative due to the accumulated electron charge, and as TD says, the cap keeps the guitar from bleeding away this charge.

This effect is why tube data sheets prescribe a maximum grid resistance, especially for power tubes (whose prescribed resistance is much lower than small-signal tubes). A small resistor keeps occasional normal grid current from building up a charge and turning the tube off.

I've seen it on the marshall silver jubilee.

Again, what TD said. The 1M grid reference is too small for there to be any grid-leak biasing action, and the -3dB point for 0.047uF against 1M is ~3Hz so there's no real bass roll-off.