Parallel Front End

[TIMBO]

Hi guys, Got some sound out of my new build  but it comes with the usual unwanted noises. First i will address the front end,as per the preamp schem i have a switch that gives a normal plexi input or a parallel input.The difference between the two is that when parallel is selected it does give a very nice fat sound as expected and i think i will remove the switch a just have a parallel input. At one point of the test i did get some radio noise.Also when in parallel the second 1M resistor is also in the circuit, so would i need to increase the 1M to a 2.2m and do i need the 68k resistor (i'm not sure what you call that resistor). I could not find much on this type of parallel input, more info please. Thanks

[tubenit]

Yes, with the multiple gain stages I definitely think you should add the input resistor. Use 68k if parallel inputs or 33k if single inputs.

With a simple SPDT switch, you can get THREE different tones out of your paralleled 12A_7. A very cheap and useful mod to add in my opinion. I've done this on a bunch of amps.

With respect, Tubenit

[jeff]

The two 1M are in parallel. If you wanted to use only one resistor you'd need a 500K not a 2M.

With the 68K, I often wonerded. Two 68K in parallel is 34K resistance, but the point of the 68K is to attunuate the radio frequencies because of the miller effect does one 34K have the same effect as two 68K?

Just as an example to illustrate the point and ask the question another way:
Say you had one hundred 3M4s in parallel. The resistance would be 34K but would the miller effect be the same thing as using one 34K?The resistance goes down because there is 100 paths but each path is 3M4. Using two 68K also gives 34K but each path is 68K.

 I've been using one single 68K. The resistance is bigger but won't the input voltage be the same because the grids impedance? Some reason I'd think one 68K would give you more attenuation without a reduction in input signal vs one 34K.

Been meaning to ask this. In the end it probally doesn't matter, just another "how does it work" question.

[sluckey]

but the point of the 68K is to attunuate the radio frequencies

That's just a secondary benefit of those resistors. The real purpose was isolation between the input jacks. Back in the day when those resistors first appeared, it was very common to have 2 or more instruments plugged into one amp. Those resistors isolated the jacks from each other and prevented the volume control of one guitar from controlling the other guitar.

[tubeswell]

FWIW I'm intrigued by the simple versatility of this circuit from Merlin

[TIMBO]

Thanks for the info, and like a good little boy i listen to both of you and added both ideas and it was a good result. It has add depth to the sound but i am unable to get a real feel for it due to........... WAIT FOR IT........ THE BUZZ IS BACK  I don't know why i do these things but i like the challenge 

[tubeswell]

WAIT FOR IT........ THE BUZZ IS BACK 

grounding? How is your grounding done?

[tubenit]

Timbo,

Buzz is probably a grounding issue or a problem with an input jack.  I strongly recommend using a 33k input resistor and using a 1M resistor to ground.

Is there some reason to use two input jacks?

with respect, Tubenit

[TIMBO]

Is there some reason to use two input jacks?

Tubenit i'm not understanding your question. Thanks

[jojokeo]

Timbo,
Wasn't it the Silver Jubilee amp prior that had the buzzing issue? And now you have a different amp and another buzzing issue?

I think tubenit was saying why do you feel the need to have two inputs instead of only one? Personally I don't think there's anything wrong w/ having Hi and Lo inputs if that's what you have? Sorry, not on home computer so can't look at your schematics right now.

[TIMBO]

Hey jojokeo,I was only wanting one input as i think there is other ways to achieve the highs and lows.And yes the jubilee did have a buzz problem but it was the relay circuit that was causing it a 50ohm resistor to ground was added to the - side and thanks to Sluckey, gone.

I even have thoughts that the DC circuit could be doing the same thing  Thanks