vibro king circuit

[Alex R]

A guy asked me to rebuild the normal channel of his much-messed about sf Bassman into a vibroking preamp - leaving the bass inst channel as it was (he plays bass through it sometimes) but with the deep switch on permanently.

Different power supply, but not too different. No nfb in the VK - so put it on a switch - feasible, I thought, and so I did the job and it worked. Only compromise was a couple of 220k mixer resistors and a simplified switchable nfb circuit. Bright switch became fat switch.

I'm very struck by the tonal differences of these preamps running through the same power amp. The VK sounds much more aggressive and 'present', and at around 4 it is very touch-sensitive. A satisfying amp to play.

So what's good about the VK? Three stages before the tone controls is one unusual thing. And the first stage, which holds some mysteries for me, is fairly unusual too - cathode-follower to amplify current, ok, and also what I guess is grid-leak bias? Why is this stage built this way? Anyone like to further my education?

[Angelo Skouras]

Dear Alex.

A valve (tube) is a capacitive load. Anode (plate) and grid are two electrodes insulated by a dielectric (vacum) so they form a capacitor "Cgk". This capacitor is multiplied with the gain of the vave ( I don'd know the mechanism that causes this, I can tell you only that this is called "Miller effect"). If we tried to drive the input of an amplifier valve with the hight resistance output of a passive guitar (maybe 50kOhm) this would form a lowpass filter, that atenuates the hight frequencies of our guitar and makes the entire amplifier have a poor trebble response.
So if we use a kathode follower, wich has a voltage gain about 1, the input seems a far smaller capacitor (multiplied by 1 and not 60), giveing a much better frequency responce.
Kathode followers also tend to have much smaller output resistance (kathode resistance of a valve equals to the anode resistance divided dy the gain of the valve plus one Rk=Ra/(mu+1) (mu is the aplification factor of a valve). This way they are very powerfull for driving the input capacitance of a voltage amplifier valve creating a very good HF responce of the stages.
Now what about the strange bias?......
A valve's grid is biased with a voltage negative with respect to the kathode, and -5V is a very possible value. For a common kathode amplifying valve, we can create this voltage at the cathode of the valve using a series resistor and then just connect the grid at the groud poetential with a large valued grid leak resistor.
Now for a kathode follower that has its kathode at a hight poetential, about 150V, we must bias the grid at 145V. Such a voltage can be made using a poetential divider, a leg of two series resistors from the HT suply to ground poetential. The appropriate analogy of resistor values gives the apropriate voltage at the node between the two resistors, and finaly, we connect the grid there and the valve is biased.

I hope you understand some of what I wrote..... I know, my english are terrible..........


I advice you to by a good book of analog electronics. This would make such questions a nightmare of your past.... Cheers!

[Alex R]

Thanks Angelo! Your English is fine. Hadn't thought of those 2M2 resistors as a voltage divider, that made it all clear.

Cathode followers I'm familiar with from Marshall/5F6 type preamps as low impedance drivers for tone stacks. But not as simply a good way of driving the next valve with good HF response. This amp crunches so nicely, and has a really sweet sustained distorted note when you turn it up - I'm wondering about using cathode follower/plate coupled pairs in a multi-stage high-distortion type circuit, separated by the usual parallel cap/resistor filters.

I do look at books, but my training in reading is literary and it distracts me. I tend to find myself having specific questions to fill in gaps in my knowledge, and I find the answers here. Thanks again for helping with that.

[Angelo Skouras]

Thanks Angelo! Your English is fine. Hadn't thought of those 2M2 resistors as a voltage divider, that made it all clear.

Cathode followers I'm familiar with from Marshall/5F6 type preamps as low impedance drivers for tone stacks. But not as simply a good way of driving the next valve with good HF response. This amp crunches so nicely, and has a really sweet sustained distorted note when you turn it up - I'm wondering about using cathode follower/plate coupled pairs in a multi-stage high-distortion type circuit, separated by the usual parallel cap/resistor filters.

I do look at books, but my training in reading is literary and it distracts me. I tend to find myself having specific questions to fill in gaps in my knowledge, and I find the answers here. Thanks again for helping with that.

Alex my friend you are realy wellcome!

I am proud if my mail helped you. Whatever you want to ask, we are here to answer if we know.

You can also find me at o_glykolemis@yahoo.gr

A public question: I have problem with registering here!!!

The page apears my mails only when I reply to someone. I cannot put a question of mine in that page.

Can anybody help?

[Alex R]

The New Thread button doesn't appear in many places, maybe you're missing it? If you click on 'Music Electronics Forum' at the top of the page you get a list of threads - down at the bottom of the list is a 'New Thread' button.

[roknroll]

Look at it with a different view.
The Vibroking is Fender basics with a Fender 6G15 Stand alone reverb unit in series to it.

[Alex R]

The first stage doesn't look at all like Fender basics to me.

[roknroll]

Check this schem as well.

[roknroll]

vibroking

[roknroll]

reverb

[loudthud]

The main things acheived by the cathode follower in the reverb unit are to isolate the reverb signal from the input so that the reverb signal does not feedback on itself which would be a horrible sounding feedback and to provide more elimination of the reverb signal at the dry end of the blend control.

Other variations from the more common Fender circuits are the lack of a grid stopper on the first voltage gain stage, (looks like) V4A, the extra RC network C6/R15 at the output of the V4A. This would give a slight LF rolloff. Changes to the caps in the tone stack. Lastly, changes in the phase inverter, the 39K tail resistor, the 91K load and the lack of feedback around the power amp. I don't remember ever seeing a 91K resistor in a Fender amp. The one weird thing is the dual volume pot that reduces the reverb as the volume is turned up. Most of these things are voicing changes that you might find in a boutique amp.

Without the cathode follower, I would expect it to sound slightly different but not a day and night difference.

[Alex R]

But the cathode follower isn't in the reverb circuit. Are we talking about the same thing - V2a?

[Alex R]

...and isn't V4a the second stage? V2a being the first stage (and the one I was asking about)?

[loudthud]

The signal does go through V2a but that stage doesn't provide any Voltage Gain. If you have access to a Vibro King, compare how it sounds through Input 1 against how it sounds when you plug into the FX Return. You could use an A/B box to rapidly switch between the two. I wouldn't expect much difference but I've heard of people who like to play with an old reverb unit ahead of their amp. Study the schematics and you'll see that everything ahead of the FX Out in the Vibro King is very close to the old Reverb Unit.