Need a little insight about a fender prosonic

[daradaramike3]

Hello,

This is only my second post on this forum, the last time was because I had a problem with a new Gibson GA40 RVT amp. Although nobody answered my question directly, the breadth of knowledge on this forum was awesome and eventually allowed me to find and fix the problem I was having. Thank you very much for all of the contributions and indirect help!

Today I have a question about the Fender Prosonic. I was looking at the schematic the other day and couldn't quite figure out what the gain stage before the effects loop is for. Specifically: Is this stage meant to act as recovery for the tone stack or is it meant to help push the signal through the loop? Maybe both?

I am not an expert by any means, but would like to look at maybe making an amp that is based on the clean channel of the Prosonic but at the wattage of a Deluxe Reverb. I know it will never be exact but I like the lack of nfb, the switchable rectification and that fact that the tonestack comes after the second gain stage like a Vibro-King but isn't a cathode follower like a tweed bassman.

Any insight into the purpose of the gain stage before the effects loop and insight into why this may or may not be such a great idea would be greatly appreciated.

Thanks in advance for the help.

Mike

[tubeswell]

Its a cathode follower and its there to drive the loop with a low- impedance signal (so you get a good clear signal to the FX). It doesn't have any effect on gain, because it doesn't have any gain.

[Chuck H]

Like he said.

I've noticed that if I run a high level effects output that impedance becomes more of an issue than with low level effects outputs. The Prosonic effects loop has a lot of gain so Bruce Zinky used a cathode follower to lower the impedance.

FWIW I think the Prosonic has a great clean tone AND a great dirty tone. For some reason the amp never really caught on but it does have a following, especially among builders and designers.

I was really surprised that the amp has such a "Fender" clean tone (like a BF) using such non Fender topography and part values. This allowed for the OD to be placed BEHIND the tonestack making for a distortion channel that's more versitile and contemporary. But Bruce is brilliant.

Chuck

[daradaramike3]

Hey guys,

Thanks for the help. I noticed it was a cathode follower but don't have enough knowledge to determine exactly what it does. Now I know! But now I also have the following questions:

Tubeswell-do cathode followers not contribute gain to the signal at all? What about about in the case of a 5f6-a bassman where both halves of a 12ax7 are connected before the signal is sent out the cathode to the tonestack? How does that work?

Also, if I were to take the effects loop out of this amp, could I just run the signal from the treble pot of the tonestack straight to the phase inverter like the vibro-king? Or would I require something in between?

Chuck H- you are right about the prosonic having a great clean AND dirty channel, but I would like to try simplifying the thing being that I am still a novice and have a maxon rod-881 overdrive that sounds almost exactly like the dirty channel when I run it through the clean channel of my own prosonic. If I could build something similar that could get to the sweet spot at lower volumes and them use that pedal for overdrive tones, I'm set.

Thanks for the help guys, I really appreciate it and hope you don't mind answering these other questions.

[tubeswell]

Tubeswell-do cathode followers not contribute gain to the signal at all?

No. The plate is at a fixed voltage (because there is no plate resistor), therefore any signal at the grid can only have the effect of causing the cathode voltage to swing (because of the change in current through the cathode resistor). The cathode is only a few (5-8) volts higher than the grid, so you can't get any gain out of that. What's more, when the grid signal voltage swings 'up', it 'pulls' the tube current up by a certain amount, and the cathode voltage responds by swinging 'up' by more or less the same amount as the signal grid (and vice versa when the signal grid voltage dips), but the cathode voltage can't swing up any further than the current in the tube will allow it to be pulled up by, and the current in the tube is controlled by the signal grid. At least that's my understanding. Someone will be bound to want to correct me if I'm wrong )

What about about in the case of a 5f6-a bassman where both halves of a 12ax7 are connected before the signal is sent out the cathode to the tonestack? How does that work?

The tone stack in a 5F6A is driven by what is termed as a 'DC coupled pair'. It is not a cathode follower on its own, but is comprised of a cathode follower (as the second stage in the DC pair) and a conventional gain stage (as the first stage in the DC pair), which are coupled together directly. The gain in a DC pair comes from the first stage. The cathode follower stage merely provides a low-impedance signal for driving the TMB tonestack, which is quite current-hungry.

That is to say that the TMB tonestack attenuates the signal quite a lot, so driving it with a low-impedance stage maximises the signal coming out of the tone stack.

The sum total of impedance in a tone stack at any one point in time is analogous to the lower 'leg' of a voltage divider, whereas the output impedance of a CF stage is analogous to the upper 'leg' of a voltage divider.

If you make the upper leg proportionally higher in value compared to what is in the lower leg, you will knock down more voltage at the 'knee' of the divider. Conversely if you can get the impedance of the upper leg (numerator) so small that it is a fraction of what is on the lower leg (denominator), you will preserve more voltage at the knee. In this regard, a cathode follower provides about the lowest output impedance you can get.

DC pairs are explained more eloquently here:

The Valve Wizard

Also, if I were to take the effects loop out of this amp, could I just run the signal from the treble pot of the tonestack straight to the phase inverter like the vibro-king? Or would I require something in between?

You could try that - but there is that relay network you need to think about.

[daradaramike3]

Ah yes! Now it is all very clear. Thanks Tubeswell! Your explanation was quite clear but I must say the valve wizard site is totally awesome and really drove your point home. I will have to buy his book so I can become an expert like many here on the forum.

One more question though: You said on your last post that I would still have to worry about the relay system. However, if my goal were to only copy the clean channel, what should be done about that particular relay section between the tone stack and effects loop? Sorry for the stupid questions, I was under the assumption that if it wasn't necessary I could do something like the Vibro-king, but being that I lack the experience to make this call with confidence, I thought it best to ask those in the know.

Thanks again for the help, this thread has been a very educational experience for me.