Why does this amp sound so good?

[wizard333]

Looking at the layout, I am drawn to your NFB circuit. How does that compare with other stuff?

The negative feedback circuit is switchable on/off; it has almost no audible effect when switched in other than to dull things up just a touch. Even with it switched in, it doesn't appear much nfb is applied given the resistor value involved.

[wizard333]

I've heard complaints about someone using slotted head screws instead of Philips type

There's a case to be made there as flat heads are far more likely to slip. With the amp off, you could potentially slip and the screwdriver could damage something; with the amp on...well....

A bigger case can be made on guitars. Nothing makes me more nervous than when I'm working on someone's $8k-$10k+ historic LP and I'm adjusting pickup heights with slotted screw heads......

[darkbluemurder]

The channels aren't run in parallel, and the channels are quite effectively isolated in that configuration, so that isn't it.

Of course they are not in parallel - they are fed into each side of the PI.

Check out this article: Vox AC30 MYTHS ? Tone Lizard
Towards the end he is describing that part of the AC-30 tone is the feeding of the tremolo channel into the PI (instead of having the second PI input grounded as in most amps). I was thinking that some similar stuff could happen here.

Looking at the preamp nothing jumps at me that could impart a special tone (that is, more special than other preamps out there).

[frus]

it has almost no audible effect

I thought the NFB should be fed to the (normally) unused PI input
Here it feeds only into the tail, no wonder it does nothing

[wizard333]

I see what you're saying, but note, also from that article, this point:

" As long as the Volume control is set to ‘0’, the grid is at AC ground. "

And so it is on this amp as well. The volume control on the EF86 channel has no audible effect on the 12AX7 channel, and is generally kept off when running the other channel as the way they have the inputs set up necessitates this for noise purposes.

[frus]

OK, but having the grid at AC ground doesn't make the NFB more effective (and Vox doesn't even have one)

For instance, look at a typical fender schematic (with NFB values similar to yours), like

http://www.thevintagesound.com/ffg/s...b763_schem.gif

The signal from the speaker is attenuated by 820/100 divider and then fed directly to the lower PI tube grid (as well as the tail), where it's amplified by like 25 times and mixed with the signal from the preamp. If I remember correctly, it's already been argued that it doesn't have to be fed to the tail at all, but it helps the balance somehow (don't remember exactly).

Both of your PI inputs are used up by channels, so there's no place to feed the NFB signal.

Someone correct me if I'm stupid

[wizard333]

OK, but having the grid at AC ground doesn't make the NFB more effective (and Vox doesn't even have one)

For instance, look at a typical fender schematic (with NFB values similar to yours), like

http://www.thevintagesound.com/ffg/s...b763_schem.gif

The signal from the speaker is attenuated by 820/100 divider and then fed directly to the lower PI tube grid (as well as the tail), where it's amplified by like 25 times and mixed with the signal from the preamp. If I remember correctly, it's already been argued that it doesn't have to be fed to the tail at all, but it helps the balance somehow (don't remember exactly).

Both of your PI inputs are used up by channels, so there's no place to feed the NFB signal.

Someone correct me if I'm stupid

My previous response was to Dark Blue Murder RE him suggesting the difference in tone vs. standard Plexi or Vox bright channel tones is due to the EF86 channel feeding the PI, the replies are shown out of sequence.

As noted, the NFB circuit is irrelevant because it is switchable and does almost nothing, so I just leave it off.

[Richard]

Their market is Musicians who mostly don´t have much use for a schematic either because they plain can´t *read* them or best case can read but not analyze them (as in : "hmmmm, so the tone stack is driven by a cathode follower, that means it will follow more closely the curve that TSC shows" but can follow a "paint by the numbers" layout, which is exactly that, they show not only coloured wires and their path, but even resistors identified by their colour bands.
And drawn jacks instead of showing the internal leaf switch connections and so on.
Not a bad idea, I guess it expands potential market base by, say, 10X ... yet they *should* offer a schematic anyway, doesn´t hurt and worst case can be ignored by those not interested.
Oh well.

I am not a tech but I still prefer a schematic. I have trouble connecting a wire from A to B if I don't know what A and B are. The worst is some exotic pickup switching on a guitar. I need to know what is going on but most of those are layout diagrams.

[GainFreak]

There's a case to be made there as flat heads are far more likely to slip.

I agree however a regular user shouldn't have any business with most of the screws in the first place.

[loudthud]

Gain:

The 1M/500pF doesn't have the usual 470K to ground so you pickup 6dB of gain that is normally thrown away on treble peaking. That and the EL84s have significantly more gain (6 or 8dB) than what you see in a Plexi. The 56K on the cathode follower lowers the headroom by rounding off the top of the waveform and that will give more distortion as the Master is turned down. The feedback switch only feeds the tail resistor, I'm surprised it does anything at all.

[Mike Sulzer]

Gain:

The 1M/500pF doesn't have the usual 470K to ground so you pickup 6dB of gain that is normally thrown away on treble peaking. That and the EL84s have significantly more gain (6 or 8dB) than what you see in a Plexi. The 56K on the cathode follower lowers the headroom by rounding off the top of the waveform and that will give more distortion as the Master is turned down. The feedback switch only feeds the tail resistor, I'm surprised it does anything at all.

If the feedback feeds only the tail, then its purpose is to make the cathode supply look more like an ideal current source. So it affects the balance slightly, but not the linearity.

[nickb]

We'd be more clever if we had a schematic to look at. Can you link one in?

I had a stab at it. I didn't bother with all three taps on the OPT the PT or the GZ34 but I think all the important stuff is there.

EDIT: The first version had errors. I replaced it with this corrected one.

[uneumann]

The channels aren't run in parallel, and the channels are quite effectively isolated in that configuration, so that isn't it.

Have little experience with a Plexi - but on a Bassman, changing the cap and/or adding a resistor to ground on the unused PI input has a significant impact. Just wonder if you tried twiddling the EF86 vol control to see if there is any change. The impedance to ground changes from 100's of K to zero, which could have some impact - especially when the PI is pushed hard.

[66 Kicks]

I penciled this one out while I was supposed to be working earlier today. I got the following differences:

C5 .0022uf A significant difference.

R21 56K

R22 47K Another significant difference.

C15 330pf

C20 22uf/63V with the positive end hooked to ground.

R30 100

RV4 Pin 1 not connected, pin 2 (wiper) connected to RV3 pin 3. Reverses the action of the knob.

The Master Volume is missing between the tone stack and the PI.

[J M Fahey]

I had a stab at it. I didn't bother with all three taps on the OPT the PT or the GZ34 but I think all the important stuff is there.

Thanks NickB

Only slight difference I see is that R22 (bass slope) is 47k and R21 (cathode ) is 56k
OF COURSE, looking at an actual schematic shows what parts DO relative to others and the circuit makes sense, instead of just showing what goes where.
Thanks again.

PS: and might very well miss some other small difference