I have joined the Dark Side (well not really)

[Psycho Bass Guy]

I did. What I see is output from the cathode. Which is not what I see in the Bass 400, nor what you described above, when you said this:

It's still just a modified CF. The plates of the "driver" feed the plates and cathodes of the the "buffer." I checked both versions, the Bass 400 and 400+.

I'm quite familiar with the PS series, I have a 160. Regardless of the terminology the schematic uses, the transformer IS the inverter, there is no tube inversion happening.

Evidently you need to take another look at the schematic, specifically the part labelled "DRIVER TRANS. 010491" and its location relative to the output section and the PI. And in the 400PS, which I have, it's "DRIVER TRANS 024108."

http://www.schematicheaven.com/fenderamps/160ps.pdf

Link Removed

To get back to the original topic...the reason the 12AU7 cleans the amp up is because it's simply not hitting the power tubes with as much signal swing. Current capacity/impedances don't really apply, since the tubes are plate-driven.

Not so, and it's easy to prove. Substitute a 5751, and the amp is quieter, but no cleaner. I don't have to play word games with you; the results are obvious and easy enough for anyone to check for themselves.

 

[Chef]

"Easy enough for anyone to check for themselves"

Which is what I did.
I used to have a brand new 400+ some years ago.
I didn't like it as it was gritty, brittle, and went to overdrive way to easy for me.
I tried ditching the Mesa tubes, going to all JJ's...didn't help.
I tried both 12au7's, and then 12at7's in the preamp slots...with the biggest effect being losing headroom.
So, I sold it, very disappointed.

I heard from several guys here who's sonic opinions seem to be close to mine that the 400 was warmer, softer and cleaner.

So, when I had the chance to buy a nice one, I did. Had it serviced up with new JJ's all around, biased, etc. If there was a difference, it was not major, and I did not like it better.

I tried PSG's 12au7 driver swap as a last measure before I sold this one.
It's now very much cleaner, warm and responsive, no discernable loss of headroom, and what I had hoped a Mesa 400, or 400+, would be. I have yet to try his "Chinese 12ax7's are better yet in the preamp spots," but I will.

I don't understand any of the how's or why's, but I can tell you from first hand experience that it does work, very well.

 

[AdmiralB]

It's still just a modified CF. The plates of the "driver" feed the plates and cathodes of the the "buffer." I checked both versions, the Bass 400 and 400+.

Then you don't understand what a cathode follower is. It's where the CATHODES of a tube drive the following circuitry. PLEASE show me where, in the 400's PI section, that that occurs. It is COMPLETELY plate-driven. Yes, the grids of the second tube are referenced to the cathodes, but that is NOT cathode following. Please show me where the cathodes of the PI tube(s) drive a successive stage.

Evidently you need to take another look at the schematic, specifically the part labelled "DRIVER TRANS. 010491" and its location relative to the output section and the PI. And in the 400PS, which I have, it's "DRIVER TRANS 024108."

And apparently you need to read what I wrote...especially the part about "terminology notwithstanding".

If the transformer isn't performing phase inversion...tell me what is.

Not so, and it's easy to prove. Substitute a 5751, and the amp is quieter, but no cleaner. I don't have to play word games with you; the results are obvious and easy enough for anyone to check for themselves.

Um...DUH. Of course it's quieter, there's less voltage gain. It's no cleaner because the PI clips before the power tubes do. I presume that's what the 12AU7 is accomplishing, accepting a larger input signal without clipping itself.

 

[timv]

on the ps fenders. That driver transformer acts as the driver and the inverter. Each secondary has the same signal just out of phase and it isn't a 1-1 transformer so it does some driving also......

The 400 and 400+ have almost identical schematics and the d-180 doesn't have weird transistor fed driver. That solid state device is pretty much just a diode in the cathode circuit which keeps a specific voltage drop there, when the master control is changed, there is resistance added/subtracted in series so it increases/decreases the voltage at the cathode changing the gain, response, of that stage. It creates nice sounding distortion at certain levels.

Anyway, my thoughts....

 

[Psycho Bass Guy]

Then you don't understand what a cathode follower is. It's where the CATHODES of a tube drive the following circuitry. PLEASE show me where, in the 400's PI section, that that occurs. It is COMPLETELY plate-driven. Yes, the grids of the second tube are referenced to the cathodes, but that is NOT cathode following. Please show me where the cathodes of the PI tube(s) drive a successive stage. etc... etc...

...wonder what's driving those plates in that stage; wouldn't be the respective cathode now would it?

Also, isn't it odd that my suggestion works and yours doesn't?

 

[Foxworthy925]

Wow, this discussion is getting heated. Very informative though- by all means, go on.

 

[AdmiralB]

...wonder what's driving those plates in that stage; wouldn't be the respective cathode now would it?

Well, no...it wouldn't be.

Nothing is driving 'plates'....grids get driven (and occasionally cathodes).

The grids of the PI tube - V3 - are driven, through the loop and the master, by the cathode of V2B - which *is* a CF.

The grids of V4 are driven by the plates of V3. And the power tube grids are driven by the plates of V4. The grids of V4 are referenced to their own cathodes, not unlike a traditional LTP inverter, but that ain't cathode-following.

Also, isn't it odd that my suggestion works and yours doesn't?

I don't know what you're talking about. Your suggestion limits the drive to the 6L6s (at a given preamp level), and also probably itself accepts larger inputs from 'upstream' before clipping. I propose that using a 12AU7 for V3 would be another step in that direction, although I'm sure the V4 swap is more pronounced.

What "doesn't work" about that?

 

[AdmiralB]

Wow, this discussion is getting heated.

Nah. You want heated, try the Gear Page!

 

[AdmiralB]

on the ps fenders. That driver transformer acts as the driver and the inverter. Each secondary has the same signal just out of phase and it isn't a 1-1 transformer so it does some driving also......

I think technically it's not a driver, since it's a passive device. The driver, in the absolute sense, is the 6L6 or 6V6 - the transformer performs phase inversion and also knocks the output impedance down to a more appropriate level.

 

[Tim1]

This is a bit too technical for me. All I know is that Psycho's advice has totally turned around my 400+. The improvement is significant and huge, and has saved me from a possible amp change. Others who have heard the change have made the same comment and I continue to recommend Psycho's mod to others without reservation.

 

[pedro]

PBG is might generous with his advice and has answered more than a few of my idiotic questions. He's a nice guy.

 

[Kindness]

PBG is might generous with his advice and has answered more than a few of my idiotic questions. He's a nice guy.

Not that it is on topic at all, but +100!

 

[Psycho Bass Guy]

I don't know what you're talking about. Your suggestion limits the drive to the 6L6s (at a given preamp level), and also probably itself accepts larger inputs from 'upstream' before clipping. I propose that using a 12AU7 for V3 would be another step in that direction, although I'm sure the V4 swap is more pronounced.

Nice turnaround, flip-flop, or whatever there, especially after the majority of your posts to this thread are arguments to the contrary. I also like how EVERYONE, including Fender who drew the schematic and designed another amp you claim to be an "expert" on, knows less about it than you.

The grids of V4 are driven by the plates of V3. And the power tube grids are driven by the plates of V4. The grids of V4 are referenced to their own cathodes, not unlike a traditional LTP inverter, but that ain't cathode-following.

...until you happen to notice that the plate lines of V3 are what is DRIVING the cathodes of V4! You're just playing word games.

The driver, in the absolute sense, is the 6L6 or 6V6 - the transformer performs phase inversion and also knocks the output impedance down to a more appropriate level.

I guess "absolute sense" really means 'according to you.' A cathode follower does that, too, so I suppose that's why you refuse to consider a CF as a driver as well.

On that note, if tube output sections were pure voltage amplifiers as you claim, then tone controls wouldn't work at all, nor would filter cap banks require more than minimal capacitance. Current output AND current drive are very important, whether you believe they are or not.

 

[AdmiralB]

...until you happen to notice that the plate lines of V3 are what is DRIVING the cathodes of V4! You're just playing word games.

And you're displaying a complete lack of understanding of what a cathode follower is. The word "follower" is important, you see...it means "follows". Go back to the helpful link you provided and read the definition of a CF...you'll see that involves the CATHODES driving something...NOT the cathodes being driven by something.

Nice turnaround, flip-flop, or whatever there, especially after the majority of your posts to this thread are arguments to the contrary. I also like how EVERYONE, including Fender who drew the schematic and designed another amp you claim to be an "expert" on, knows less about it than you.

Where did I claim to be an expert on anything? Flip-flop, whatever. I suspect it's a semantics issue. You interpret it as 'a transformer that drives', I think it's really a 'transformer of the driver'. Seems to me a 'driver' has to be able to 'drive' and a transformer isn't, without something on the other side.

On that note, if tube output sections were pure voltage amplifiers as you claim, then tone controls wouldn't work at all, nor would filter cap banks require more than minimal capacitance. Current output AND current drive are very important, whether you believe they are or not.

Output sections are certainly voltage amplifiers. Are you claiming that the input grids of the power tubes - or preamp tubes, for that matter - draw current? Not in anything above Class B, unless the outputs are heavily clipping.

 

[AdmiralB]

He's a nice guy.

Unless you disagree with him, apparently.

 

[bigbajo60]

Ok... Psycho Bass Guy and AdmiralB... I need to know...



Which one of you is Hillary and which one Obama?



PEACE, Y'ALL!!!

 

[Psycho Bass Guy]

And you're displaying a complete lack of understanding of what a cathode follower is. The word "follower" is important, you see...it means "follows". Go back to the helpful link you provided and read the definition of a CF...you'll see that involves the CATHODES driving something...NOT the cathodes being driven by something.

Again, you're playing word games. In this case, the driver circuit in question is a modified cathode follower with plate current drive to the "buffer" section, sort of a minature power section before the power section, except in this case it provides current drive.

Where did I claim to be an expert on anything?

When you make claims like 'no matter what the schematic may say, I know better' you set yourself up an expert. I've been known to disagree with schematics, too, but that's only because I have the amp in front of me and can say that parts listed on it are not what are in the amp, not that the parts listed on a skit don't do what they say they do.

Flip-flop, whatever. I suspect it's a semantics issue. You interpret it as 'a transformer that drives', I think it's really a 'transformer of the driver'.

So how exactly do you define "is?"

Output sections are certainly voltage amplifiers.

I didn't say they weren't. Output sections are POWER amplifiers, which includes current as well as voltage.( ...unless of course, you're trying to say that the output transformer somehow GENERATES artificial current to move the speakers rather than transforming the existing current flow in the output section.)

Are you claiming that the input grids of the power tubes - or preamp tubes, for that matter - draw current? Not in anything above Class B, unless the outputs are heavily clipping.

It's called Class AB1 and AB2.

 

[Psycho Bass Guy]

Unless you disagree with him, apparently.

...or perhaps when you attack his credibility for no reason.

I published an easy, noninvasive mod for an amp that has been around for 25 years that makes it much more user friendly as well as sound better. Several here have tried it to their great satisfaction, yet you somehow felt the need to post and say that it doesn't do what I say it does, even though I, as well as ALL of the others whom have tried it, can easily observe that it does. That'd aggravate anybody in my shoes.

 

[AdmiralB]

Again, you're playing word games. In this case, the driver circuit in question is a modified cathode follower with plate current drive to the "buffer" section, sort of a minature power section before the power section, except in this case it provides current drive.

Not at all. What it is, is a pair of differential amplifiers in series. The 220K resistors on the grids of V4 are the DC ground reference, it has nothing to do with 'driving' the cathodes. There is no cathode following, and I think by now you know that. In fact, V4 is really nothing more than a traditional Schmitt/LTP inverter with both sides driven.

I'm not playing word games, you're either ignorant (which I doubt), or you're simply can't admit that your assessment isn't correct.

If I'm wrong, please show me ANY reference that indicates that there is cathode following going on. Even the example you linked is nothing like the 400, and I'm pretty sure by now you know that.

Tell me - what is driving the grids of the 6L6s? Are they plate driven, or cathode driven? It's a simple, yes-or-no question.

And answer my other question - are you claiming that the 6L6 grids are drawing current? Either they are or they are not.

 

[AdmiralB]

...or perhaps when you attack his credibility for no reason.

I didn't attack anything or anybody. I casually mentioned that the inverter in the amp is actually V3, and you got all, well, psycho. You started off with your current-drive and cathode-follower stuff, and in this case, you're simply wrong. You've put words in my mouth on several occasions. So it appears that you're either - as I said above - ignorant (which I do not believe to be the case), or you're too stubborn to admit that in this case, you're incorrect.

you somehow felt the need to post and say that it doesn't do what I say it does

Again, you're putting words in my mouth. I NEVER said it doesn't work, or make the amp better for a lot of applications. I DID say the reason it does what it does is different than the reason you've stated. And I also suggested that maybe a 12AU7 in V3 would be a further improvement.

So, for the rest of the folks reading this....yes, the 12AU7 swap will clean the amp up, at higher volumes. If you like it, you might try one in V3 in addition to, or instead of V4. You might like it, or you might not. For practical application, that's really all you need, I think.