Princeton Chorus DSL Conversion

[2deaf]

I have been seeing these things dirt cheap for awhile, so I bought one to see what I could do with it.  I took a Skilsaw to the chassis, beefed it up a little, and turned it around backwards.  Since the chassis was originally slanted, it is a little too deep at level.  I decided against flattening the aluminum and re-bending it, so the reverb plugs stick out too far.

Everything worked as expected, except the reverb fed back.  I had screwed a MOD tank to the bottom thinking that the soft feet, MDF cabinet and low wattage would allow it.  So I put the tank in a bag with some cardboard and it now works fine.

Despite the aluminum chassis and lightweight transformers, the sucker weighs in at 46 lbs.  All in all, I wouldn't do this again.     

[2deaf]

She's a little rough compared to the work that you guys do, but here are a couple of pictures:

[DummyLoad]

nice work. 

--pete

[SILVERGUN]

Yes, very nice. Would love to see her with the top down.
 
- How did you handle covering up the pins on that PT?  (I have used the 185F230 but converted to laydown)
 
- It appears that the IN4007 diode in the bias supply might be drawn wrong (?)
 
- What's going on at V1-B where the grid appears to be grounded?

[Ed_Chambley]

What you talking about?  That is nice work and a great idea.  I will "borrow it".

[tubenit]

I'm impressed! Job well done! 

Any chance that you have an ExpressSCH version of the schematic available, please? 

I am very intrigued by the tone controls you have in that preamp and also how the reverb is set up.

With respect, Tubenit

[2deaf]

Yes, very nice. Would love to see her with the top down.

It's a classic rat's nest inside.  It's pretty embarrassing compared to the stellar work that you guys do.

- How did you handle covering up the pins on that PT?

I placed the transformers with the pins facing each other and about 1cm apart.  I cut a section out of an isopropyl alcohol bottle and installed it with silicon.  It sounds kinda hokey, but it is actually structurally sound and plenty sufficient for inquisitive fingers.  I was originally going to bend up a brass cover, but I lost my enthusiasm as the project dragged on.

It appears that the IN4007 diode in the bias supply might be drawn wrong (?)

It's the Vox approach as opposed to the Ampeg approach that is more familiar.  With the Vox setup, the entire signal is negative on the bias side of the 0.1/630V cap. while the Ampeg setup only has part of the signal in the negative.  The Vox setup will charge up faster than the Ampeg setup, but it is not without some quirks just as the Ampeg has some quirks.  The Vox setup has to have a load on the B+ or the bias will go to zero and the B+ turns into a voltage doubler.  The 100K/3W resistor in the B+ supply is the minimum load necessary to to keep both supplies functioning properly in the event someone removes the tubes.  A 220K resistor is too large for this function.

- What's going on at V1-B where the grid appears to be grounded?

This goes back to the '70's when I was primarily modding Fenders and Marshalls and I noticed that the high gain preamps always sounded better with the Fenders than the Marshalls (to me).  The only difference was that the Fenders typically had that 220K/220K mixing thing and when I put that in a Marshall, then it was identical to the Fender.  It didn't seem to matter what the triode was doing that was being mixed into the main signal chain by those 220K/220K mixing resistors, so just sitting there with the grid grounded is just as good as anything.  Then I started replacing the 220K mixing resistor on the null tube with a 100K resistor in series with a pot. and calling it a "harsh" control.  Now to me, this sounds different than various set ups going to ground instead of to a plate, but I haven't found any objective evidence with a 'scope.

The one I am working on right now has a two-stage reverb recovery that mixes into the main signal chain with 220K/220K resistors, so this time the triode is actually doing something.     

[2deaf]

Any chance that you have an ExpressSCH version of the schematic available, please? 

I don't have ExpressSCH, I only have the free JSchem and I'm struggling with that.

I am very intrigued by the tone controls you have in that preamp and also how the reverb is set up.

The full version of that particular tone setup has a pull on the treble, also.  The 250pF treble cap. is replaced by two 500pF cap.'s in series and one of them is shorted when the knob is pulled. 

[SILVERGUN]

Thanks for those explanations.
I love that "it didn't seem to matter what that triode was doing"
 
That mixing part of the circuit stood out to me because I'm working on something similar that was borrowed from Dumble's SSS topology of mixing the reverb signal with the dry.
 
I was able to find an example of the bias diode setup that way (like yours) on a VOX AC-50 (didn't look very hard)
It wasn't obvious to me how the AC gets rectified when it's set up like that.