Studio .22 Pre-amp Question (Attention Techies)

[mr.parker]

Hello there,


I've been looking at the schematic for the studio .22 and i'm a bit puzzled.
Usually (as far as i can tell) amps have a 1M pull down resistor in front of the
first valve stage as this sets the amp's input impedance. But in the studio.22
i've noticed in addition to the 1M pull down resistor (R103) there is a parallell
cap/resistor array before the pull down resistor: 47pf (C6) / 3.3M (R104).

Can anybody tell me what these do/are for?

Also i'm struggling to understand how the lead function works. Can anybody
explain what happens when the lead function is engaged as i've never come
across a cascading system like this.

YES i am a bit of an electronics noob as you may have gathered, but I find
it very interesting and would like to learn more about it.
Any help you can give would be fantastic.

Cheers.
Chris P[/img]

 

[StephenF]

The resister+cap in parallel are a type of high pass filter, cutting out some of the low end of the signal path. This might be for 60hz hum, not sure. When the lead channel is kicked in, it is also part of the V1 input filter. (which looks like it may boost what it cuts in normal mode, my tube theory is almost non-existent.)

The lead channel on the caliber series (i have a 50 cal) inject an additional gain stage at the front end of the "clean" pre-amp. The LDR's that connect V1 to the input line, will engage V1 into the circuit, which adds your over driven tone. I also think due to looking at this circuit this closely, explains to me why my lead channel has a completely different tone.

That's my take on it, hope it helps...or at least didn't confuse, or provide misinformation.

Stephen

 

[StephenF]

Just got off the phone with George, at Mesa.

I am 100% correct (self pat on the back). The RC tank on the input of these amps is just a high pass filter to set the initial tone of the signal coming in to the amp. And the LDR's do exactly what I deduced.

He also answered my questions regarding my channel switching problems, defined the resistor that is burned, and the capacitor that is bad in my amp.

15 minute phone call, will cost me like, $5 in parts, and have my amp 100% functional again.

Sweet...

Stephen

PS: Did I mention he forwarded me the "official" schematic while on the phone, which is the same one i have seen floating around everywhere already.

 

[mr.parker]

Stephen,

Well done *accompanies with the patting*. You mentioned that the R/C filter on the input stage acting as a boost
when the lead channel is kicked in, have I understood you correctly? If so how does this cut turn into a boost?

I know the 0.001 cap (C5) is the coupling/blocking cap for V1, so would I be right in assuming that the 680K/1M
resistors in series, in parallell with the 470K resistor forms anothe R/C filter that is active in the lead mode?

If my rough calculations are correct, that would roll off all the frequencies below about 100Hz after V1.
Has this been done to prevent muddiness in the Lead mode?

If not, what are the 680/470K resistors for?

Cheers again.
Chris P.

 

[StephenF]

Oooh, I was too excited I was actually right when reading a tube schematic. Years out of practice (used to be a consumer electronic repair tech, where only tube circuits i saw were CRT variety). I was only 100% right, on 2 of my 3 theories.

I was right in that the high pass filter is only an input filter for the normal channel. It appears that it has little to no effect when in lead channel. George didn't want to guess as to it's function in lead mode, due to his lack of expertise in tube circuit theory (just a break fix tech).

Stephen