Post Phase Inverter Cathode Follower?

[FuzzFreak]

I played a super twin the other day and want to modify my fender to have 12bh7 post phase inverter tube to get that big clean sound. I can't seem to find a layout or schematic for this type of mod. can anyone point me in the right direction?

[Roe]

look at an ampeg svt

[topbrent]

It isn't too hard to figure out, as it is a pretty simple connection scheme.

Here is the schematic for the Fender Super Twin.

http://www.el34world.com/charts/Schemat ... _schem.pdf

http://www.el34world.com/charts/Schemat ... _schem.pdf

There has been some great discussion on how to implement this idea in some of the Steel String Singer threads in the Dumble discussion area.

[FuzzFreak]

From more digging I found this. can I just replace my regular fender LTP PI with this? I assume I will have to change the drop string to compensate for voltage? any help is appreciated!


[img:700:520]http://www.angelfire.com/electronic/fun ... ir-6-B.gif[/img]

[diagrammatiks]

you could do that.

you realize that you'll have to actually add a tube socket to the amp right.

[FuzzFreak]

Yes, I realize I will need to use one of the tube sockets and remove that part of the circuit. Probably the tremolo.

Has anyone ever done this?

you could do that.

you realize that you'll have to actually add a tube socket to the amp right.

[Firestorm]

can I just replace my regular fender LTP PI with this?

You could, but it's not at all what you started talking about. We've got apples, oranges and kumquats here.

This one is a common cathode gain stage feeding a Schmitt inverter; the Super Twin is a Schmitt feeding two independent CFs; the SVT is a split-load inverter feeding two independent common cathode gain stages feeding two independent direct coupled CFs (stay away from this one since it's unlikely your bias supply has the voltage to pull it off); there's also the Marshall Major with a split-load inverter feeding another type of Schmitt.

These are all perfectly usable topologies, but what are you trying to accomplish? Are you determined to use a 12BH7? Or looking for a big, clean sound? And what amp is it you want to modify?

[bluescaster]

Hi.

I have the place to had one valve in my AB763 40w clone amp and want to try cathose follower post PI. Fender SuperTwin reverb use 12AT7 with plate at +395V for this. JJ Telsa 12AT7 Ua limiting values is 300V !!! What to do ?

Thank you.

[roberto]

I don't like so much this kind of PI with guitar amps.
I had one Fender Silverface with this configuration (it was UL too, if I remember well), and it was too sterile.
For my tastes PI has to add a bit of harmonics and tone to the sound.

However you can try a VHT PI, so a 400V split load (concertina) PI with two CF after it, this should be easier to be implemented without chassis drilling.

[Firestorm]

Fender SuperTwin reverb use 12AT7 with plate at +395V for this. JJ Telsa 12AT7 Ua limiting values is 300V !!!

Yes, but the 12AT7 cathode is up at around 75V so the tube only "sees" the difference. Still a little high, but Fender often was cruel to 12AT7s.

But why add this driver stage to a 40W amp? The reason these are used in high power amps (with 6 or more output tubes) is to prevent overloading the PI. You don't have that problem so there is no need to solve it.

[David Root]

If you do it direct coupled like an old SVT, you can whang on the amp real hard and it won't get blocking distortion (no coupling cap) and the bass stays clean and clear too. Was used on big bass amps. But mainly audio circuits from the '50s and '60s.

I just finished a 100W amp with direct coupled cathode drivers using a 12BZ7 (two 12AX7 in parallel in a single envelope). It is unbelievably loud and clean too. Did I say really really LOUD? I will try a 12BH7 but will have to reduce the B+ I think.

[bluescaster]

I wanted to use SuperReverb cathode follower to prevent blocking distortion on 40W amp to maximize clean headroom. SVT is DC but need -150V that not easy to build.

So good idea or lost of time ?

My PI use 12AT7 and 47K.

Perhaps i'll try 5K1 grid resistor instead of 1K5 on power tube in first time .

[Cliff Schecht]

How is -150V hard to build? You can half way rectify one of the HV taps with the cathode pointing at the winding to get -150V. All you need to do is adjust the resistor before the diode to drop less voltage. Duncan's power supply simulator tool can help you do this. Look at any fixed bias amp where the PT doesn't have a bias tap for an example (the Express)..

[Firestorm]

You don't have to direct couple the CF driver the way the SVT does. Look at the circuit for the Fender Bassman 200. It has a 12AX7-based PI set up for higher than normal gain, with a 12AT7-based driver with the cathodes capacitively coupled to the output grids. Bias is conventional. If you sacrifice your vibrato tube, you could do that.

[bluescaster]

I wanted to use SuperTwinReverb cathode follower to prevent blocking distortion on 40W amp to maximize clean headroom. SVT is DC but need -150V that not easy to build.

So good idea or lost of time ?

My PI use 12AT7 and 47K.

Perhaps i'll try 5K1 grid resistor instead of 1K5 on power tube in first time .