Help with this preamp schematic

[hesamadman]

I have a yamaha T100 in my shop right now. I've had it for some time because I wanted to study it and try a similar preamp in my 2204. I've been trying to learn more about hi gain amps for some time now. Higher gain than a 2004 that is. So I have the schematics to the T100. It has some stuff going on that I have not seen on a schematic and I can only assume they are channel switching relays. So right now I am trying to trace down the overdrive section of the schematic. I was hoping for some help here.


Here is the preamp schematic.

This is how I think signal flow goes. Hoping to get confirmation here.


Input. to V1 pin 2 via 68k resistor
Out of V1 pin 1. (here is tricky because it goes to clean preamp pot and to pin 2 of V2 BUT the channel switching is push pull knobs so im not so sure the signal goes through the clean pot?
So I am assuming signal exits pin 1 V1 and goes to V2 pin 2.
Exits pin 1 V2 an enters pin 7V2.
Exits pin 6 and enters pin 2 V3
Exits pin 1 V3 and enters pin 7 V3
Cathode follower to effects loop


Im not really understanding V4 but it looks like V5 is shared. Each half is used by a separate channel.
Through tone stack to PI.




I have a 2204 power amp setup right now with a PI and 3 empty 12ax7s ready for use. I don't believe 3 12ax7 is enough to mimmic this exactly. Theres a lot going on. I figured I would cut out the buffered FX loop.


Im really trying to achieve, as close as I can, a van halen style of high gain.


Any help would be appreciated.


Here is rest of schematic

[PRR]

> assume they are channel switching relays

They are LEDs and LDRs in a tube. An opto-coupler.

When the LED is ON, the LDR is low-resistance; otherwise the LDR is nearly infinite resistance.

The LEDs and LDRs are shown on two sheets. The power-supply also has the switching for the LEDs.

The LEDs are in two sets, Clean and Overdrive.

Foot-switch or local switch throws a relay to power one set or the other.

Clean: 51 53 35
Overdrive: 52 54 36

Ah--- the LDR ends of the optos are labeled "CL" or OD". Each is ON in that mode.

Now go to the preamp sheet.

V1A feeds V1B and V2A to V2B.

V1B output may be clean.
V2B output is sure to be nasty because of the extra stage.

V1B output is padded-down by 5.6Meg and 330K.

In Clean, V2B is muted two places. Opto 51-CL turns on and shorts its grid signal, and 52-OD is off and won't pass any stray sneak.

In Overdrive, 51-CL turns off, allowing signal to V2B, and 52-OD turns on, passing signal direct to the 330K and V3A. 

But clean signal sneaks into the overdrive tone! So? Anyway when 52-OD turns on and connects V2B's plate signal to the 330K, the ~~50K impedance of V2B plate swamps the clean signal coming through the 5.6Meg.

V4 passes signal to EF loop.

Post-loop, V4 takes the clean/overdive and FXed signal at one grid, the Reverb return at the other grid, and delivers a mix of dry and wet at its plate.

Again we have switching. There's only one reverb system but you may want different wetness with your clean/overdrive choice. 53-CL and 54-OD select one of two pots controlling the amount of boing coming out of V6B.

V5 takes this clean/overdive+FXed+reverb signal at both grids, and delivers two outputs at its two cathodes. Next find two tone-stacks and final volume controls, selected by two optos. From there a plain if hefty power amplifier jams it to the loudspeaker.

[VMS]

With fx-loop overdrive section uses 3 and a half tubes and without 2 and a half tubes.

So you could keep the clean channel but with shared tonestack. To keep things simple use a DPDT panel switch to switch between clean and overdrive.

I would do it like this:

[hesamadman]

This has been fun and very informative. I've been studying this since my last post. Wanting to get a good grasp. I am getting the opto circuit. pretty cool. But had a couple questions.

But clean signal sneaks into the overdrive tone! So? Anyway when 52-OD turns on and connects V2B's plate signal to the 330K, the ~~50K impedance of V2B plate swamps the clean signal coming through the 5.6Meg.

Can you explain this statement a little more. Maybe show me where the 50k impedance of v2b came from?
Thanks for your help.

With fx-loop overdrive section uses 3 and a half tubes and without 2 and a half tubes.

So you could keep the clean channel but with shared tonestack. To keep things simple use a DPDT panel switch to switch between clean and overdrive.

I would do it like this:

I havent tried it yet, but I will make these modifications and see what happens. With my setup now, I keep getting insane squeals when I turn the volume up at a certain point. When I pad it down to lose the squeal, the amp has less of a gain tone than it had before adding the extra half triode. I havent even got to add the full triode yet.

[PRR]

> show me where the 50k impedance of v2b came from?

All signal sources have some internal impedance.

For small amplifier tubes, 50K is a good ballpark guess.

> insane squeals ... adding the extra half triode. I havent even got to add the full triode yet.

Any amplifier WILL oscillate if the gain is high enough.

Layout is critical. The output stages must not have any sneakage back to the input stages.

Think of your house. Water comes in, sewage goes out. If the sewage sneaks-back to the water supply, any germs in the output (sewage) go back into the house and multiply in your bodies before going to sewage. Again and again. Pretty soon you are pretty sick. Keeping sewer and water well apart, possibly in well sealed pipes, greatly reduces the problem.

Eyeballing the plan, I suspect the large signals at tone controls are routed too close to the small signals at the first gain controls or the input jacks.

[hesamadman]

Think of your house. Water comes in, sewage goes out. If the sewage sneaks-back to the water supply, any germs in the output (sewage) go back into the house and multiply in your bodies before going to sewage. Again and again. Pretty soon you are pretty sick. Keeping sewer and water well apart, possibly in well sealed pipes, greatly reduces the problem.

Physically in an amp, which wires are we to keep apart? I have read that sensitive input wires should be kept from the output? I understand that implies to OT and whatnot, but my take in input and output (based upon signal flow) is pin 2 (grid) of a triode is the input and pin 1 (plate) is the output, but it is recommended to actually keep these close together if not twisted together. (within the shared half of triode anyway) So it has me a bit confused in the wording sometimes. Obviously keeping v1a wires from v1b and so forth. Is this what is meant?

[VMS]

Physically in an amp, which wires are we to keep apart?

http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm

[hesamadman]

Physically in an amp, which wires are we to keep apart?

http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm

Ive read that many times before. But after reading Merlins chapter on grounding...and then reading it again....a lot more clicks.


From Geofex Article

keep the grid wires away from the succeeding plate wires"

I guess the key word here is SUCCEEDING. When I read....and I read A LOT....it seems like little things like this don't seem to click right away and I have to end up reading the same thing a hundred times.

"Putting an input wire near the output stage is begging for intractible oscillation."

The term "output stage" is referring to the output transformer I guess right? And not the output of whatever gain stage in question...

"Shield sensitive wires if they simply have to travel through high signal level territory."

In this scenario I have only shielded the very first input wire. If I were to shield grid wires on another stage, would it be acceptable to do so as long as I do it AFTER the CC succeeding the plate from previous stage. Simply because any shielded wire would not be rated for the high voltage before the CC at the plate.

[VMS]

I think this ax84 SEL project is great example of where to use shielded wire:

http://ax84.com/static/sel/AX84_SEL_101004.pdf

[PRR]

> referring to the output transformer

ANY output.

Every wire is an antenna. It can transmit. It can receive. Both at once.

Most "stages" have high gain. Any two stages makes HIGH-HIGH gain. We often have 4-stage and 5-stage amps.

Sneakage happens about everywhere.

R.G.'s guide should be good.