Preamp conversion question - can't find answer!

[wildebelor]

So I've been toying with this idea for a while - converting the model T into a non-tube preamp pedal.
I have most of my ideas down on paper / eagle CAD...
BUT, I'm a little confused as to where I go with the Bias test point ( -55/ E in the schematic) and where I'm assuming it becomes powered (C on the schematic)
I have seen a similar pedal run at 12v with two tubes in it, but I'm not wanting to run tubes at such a low voltage as I feel it kind of defeats the purpose.

I was originally looking at the old orange schems and comparing them with the citrus graphic, which is really straight forward... but when I compared the model T to an older marshall or fender - they didn't seem as complicated for a conversion or adaptation.

Any input would be greatly appreciated!

[anchovie]

What are you planning to use in place of tubes? All of that power supply stuff is only relevant if you're running tubes at hundreds of volts. The negative bias voltage is for the power tubes. What you actually need is a supply that's best suited to the active components that you intend to use.

[thelonious]

What Anchovie said. Also, if you are planning it to be a preamp pedal, you probably don't need to care about anything east of the Volume pot. If you're going to use FETs, keep in mind that you should probably scale the pot values and tone stack pots/caps to be better suited to solid state.

[wildebelor]

I planned on using FETS, 201s specifically, as they are pretty readily available and I haven't seen a better alternative that is.
Which is where scaling and power supply stuff gets a little shady for me.
I've isolated just the preamp section and I'm able to understand all of that, the problem I have is:

a) scaling; is it necessary and how do I implement?
b) if the above is necessary - which power supply is best and how do I decide?
c) with the last j201 - if I remove the everything south of it's gate, where does it go / what does it connect to?

[J0K3RX]

I planned on using FETS, 201s specifically, as they are pretty readily available and I haven't seen a better alternative that is.
Which is where scaling and power supply stuff gets a little shady for me.
I've isolated just the preamp section and I'm able to understand all of that, the problem I have is:

a) scaling; is it necessary and how do I implement?
b) if the above is necessary - which power supply is best and how do I decide?
c) with the last j201 - if I remove the everything south of it's gate, where does it go / what does it connect to?

Looks fairly simple... Good choice! You're wanting Channel 2 I am guessing?  By the way, that is one kick @ss amp!  

Note: I like the re-issue much better!

[wildebelor]

I planned on using FETS, 201s specifically, as they are pretty readily available and I haven't seen a better alternative that is.
Which is where scaling and power supply stuff gets a little shady for me.
I've isolated just the preamp section and I'm able to understand all of that, the problem I have is:

a) scaling; is it necessary and how do I implement?
b) if the above is necessary - which power supply is best and how do I decide?
c) with the last j201 - if I remove the everything south of it's gate, where does it go / what does it connect to?

Looks fairly simple... Good choice! You're wanting Channel 2 I am guessing?  By the way, that is one kick @ss amp!  

Note: I like the re-issue much better!

You like the reissue better?!
The original 70s version is very tasty!

Also, regarding the channels - I've actually only kept 'both' as the input because whenever I've used one I've never plugged into brite or normal... so I figured using the 'both' input will allow me to dial in a very specific gain / tone with either channel (gain) pots.

[thelonious]

a) scaling; is it necessary and how do I implement?

Try it as it is first. If you use the same load (drain/"plate") resistors for the FETs as are in the schematic it will probably be fine. If you end up lowering the load resistors on the FETs to get them to bias properly, etc., you might need to scale the tone stack resistors down similarly (and the tone stack capacitor values up).

b) if the above is necessary - which power supply is best and how do I decide?

Voltage is going to be the real question. 9V, 18V, 24V? As for filtering and polarity protection, it's going to be pretty standard unless you want to use a charge pump to get higher voltages.

c) with the last j201 - if I remove the everything south of it's gate, where does it go / what does it connect to?

I'm not sure which one you mean by the last one, but you have a lot of options. Here are a couple: 1) You can send the output of the dual 1M Vol Pot to an output buffer (another JFET?) so the output impedance of your preamp isn't too high. 2) You can keep just the top half of the phase inverter (the triode by test points 8 and 18) as your last stage. Replace C12 with a fat output cap like 10u and attach R29 to ground.

[J0K3RX]

You like the reissue better?!
The original 70s version is very tasty!

Also, regarding the channels - I've actually only kept 'both' as the input because whenever I've used one I've never plugged into brite or normal... so I figured using the 'both' input will allow me to dial in a very specific gain / tone with either channel (gain) pots.

Well, for me there is not much going on with the vintage T, could probably sit down and fiddle it out on a bread board... Not to say that the vintage isn't a cool amp but for me it's not gonna satisfy my insatiable hunger for flesh & blood!   Obviously the two are totally different, in almost every way...

This video pretty much sums it up.. The vintage is nice but, the re-issue is a slobbering beast!

If I was going to make the vintage version the bias test point would not come into play. For the baxandall tone controls you could probably just slap the "Bax in a Box" circuit in there and be done with it or modify it a little bit to get the mid control in there... after that there isn't much left to do.

[wildebelor]

Ok, So all I really need to do is remove R56 (22K) and call it a day on it!
I think I over complicated the whole thing for myself and freaked myself out. 

Buffer is a good idea though 

Well, for me there is not much going on with the vintage T, could probably sit down and fiddle it out on a bread board... Not to say that the vintage isn't a cool amp but for me it's not gonna satisfy my insatiable hunger for flesh & blood!  Obviously the two are totally different, in almost every way...

This video pretty much sums it up.. The vintage is nice but, the re-issue is a slobbering beast!

If I was going to make the vintage version the bias test point would not come into play. For the baxandall tone controls you could probably just slap the "Bax in a Box" circuit in there and be done with it or modify it a little bit to get the mid control in there... after that there isn't much left to do.

The reason for choosing vintage over reissue is that it is lower gain, so that at higher volumes it has that huge rumbling and ripping quality to it that I feel the reissues lack.
But the preamp, when cranked should sound pretty cool too