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previous: Alex Here's a question; Please ... -- 1/12/1999 9:21 PM

Re: I have a few really dumb questions....sorry

1/12/1999 10:18 PM
Bruce	Re: I have a few really dumb questions....sorry Not dumb questions and they are important considerations. It's probably better to ground only one end of the coax shield, if it's used as a signal feed wire. Ground the end closest to where the signal is coming from (input jack in your example) rather then both ends or the terminating end. Well, you can always tie both grids, both cathodes and both plates together in pairs and run both triodes in parallel for a few more dB gain with no increase in noise.. Or you can tie the plates together, split the grids and cathodes and use an SPDT toggle switch to steer the input signal to either grid on the dual -triode tube, but, with each triode having a different set of cathode Rk and Ck combination so each will sound a little different. Or, tie the plates together, split the grids and cathodes (as above with their different RC components) but hold one cathode's RC group above ground with an SPST switch and anti thumping resistor. Use the switch as an "extra umph-fat" switch when you want it the two triodes to be in parallel, but with each having a slightly different sound.. Using the last method with one triode section having different Rk/Ck combinations then the other stage, will make the amp sound more beefy or edgy depending on your taste when you punch in the extra parallel stage, especially if one or the other has a lot of gain in the lower freqs. What is a pre-volume knob? Do you mean "Marshalleque", as, in between two different halves of the same 12AX7 but way before the tone stack?? You will have to kill about 75% of the signal before or after the volume control to make it sounds any good. In a standard volume control pot layout the high side is the lug where the signal goes in, the wiper is the center lug where the desired signal level comes out and the bottom, or low, is usually to ground or the lowest resistance point to chassis ground. The "decoupling" cap? I can think of only one spot to decouple the 12AX7 and that would be the power supply filter cap the proceeds the plate load resistor to the plate of the 12AX7. If you ment "coupling" cap, then, that's the cap connected to the plate and plate load resistor, that allows the wanted AC signal flowing from the tube to pass (couple) to the next stage, rather then have to fight that very same plate load resistor and end up being decoupled to ground. BTW, You have the possibility of using 4 to 5 stages of gain with this setup. You really don't need more then 2 triode sections,.... (maybe 3 if you loose 50dB of signal from the tone stack and you won't if you build it right),..... to drive the socks off a pair of 6V6s... so,... what are you trying to build, even if I assume you are using one complete 12AX7 as a long tail pair phase inverter driver tube? Keep us posted. Bruce Mission Amps

1/12/1999 10:18 PM

Bruce

Re: I have a few really dumb questions....sorry

Not dumb questions and they are important considerations.

It's probably better to ground only one end of the coax shield, if it's used as a signal feed wire.
Ground the end closest to where the signal is coming from (input jack in your example) rather then both ends or the terminating end.

Well, you can always tie both grids, both cathodes and both plates together in pairs and run both triodes in parallel for a few more dB gain with no increase in noise..
Or you can tie the plates together, split the grids and cathodes and use an SPDT toggle switch to steer the input signal to either grid on the dual -triode tube, but, with each triode having a different set of cathode Rk and Ck combination so each will sound a little different.
Or, tie the plates together, split the grids and cathodes (as above with their different RC components) but hold one cathode's RC group above ground with an SPST switch and anti thumping resistor.
Use the switch as an "extra umph-fat" switch when you want it the two triodes to be in parallel, but with each having a slightly different sound..

Using the last method with one triode section having different Rk/Ck combinations then the other stage, will make the amp sound more beefy or edgy depending on your taste when you punch in the extra parallel stage, especially if one or the other has a lot of gain in the lower freqs.

What is a pre-volume knob?
Do you mean "Marshalleque", as, in between two different halves of the same 12AX7 but way before the tone stack??
You will have to kill about 75% of the signal before or after the volume control to make it sounds any good.

In a standard volume control pot layout the high side is the lug where the signal goes in, the wiper is the center lug where the desired signal level comes out and the bottom, or low, is usually to ground or the lowest resistance point to chassis ground.

The "decoupling" cap? I can think of only one spot to decouple the 12AX7 and that would be the power supply filter cap the proceeds the plate load resistor to the plate of the 12AX7.
If you ment "coupling" cap, then, that's the cap connected to the plate and plate load resistor, that allows the wanted AC signal flowing from the tube to pass (couple) to the next stage, rather then have to fight that very same plate load resistor and end up being decoupled to ground.

BTW, You have the possibility of using 4 to 5 stages of gain with this setup.
You really don't need more then 2 triode sections,....
(maybe 3 if you loose 50dB of signal from the tone stack and you won't if you build it right),.....
to drive the socks off a pair of 6V6s... so,... what are you trying to build, even if I assume you are using one complete 12AX7 as a long tail pair phase inverter driver tube?
Keep us posted.

Bruce
Mission Amps

Replies:

Alex Bruce -- 1/13/1999 8:57 AM