Supro 1696TN Design Questions

[scstill]

A few things that I don't understand on this amp design.
(see attached well drawn schematic from the only known hand sketches available of this amp)

1) Split Plate resistor often has the coupling cap between the split resistors, but Supro kept the Coupling cap at the plate and used a grounded cap between the split resistors. Was this for tone? and notice that Channel 1 has a .005uf and Ch2 has a .05uf.

2) Treble/Bass switch has two .001uf caps in parallel - why? maybe Supro had an excess of .001 and no .002 laying around?? it shouldn't matter should it?

3) Reverb Driver has two Cathode bypass caps 35uf and .05 - why? seems that the .05 is negligible in this approach.

4) PI has .01uf and .02uf coupling caps - why this mismatch?

5) Tremelo speed pot is connected different that the original drawings - why? Orig drawings had terminal 1- NC, 2- to 100k grid, 3 - ground

6) The Supro had a strange main switch On-Off-Standby, how would this be used as going from stby to on requires going through off? it seems more logical to have Off-Stby-On

7) Can/should the reverb, tremolo, PI and preamp share the same ground since they are all coming from the same power supply node?

Attached Schematic from reply #45 in Topic...
https://el34world.com/Forum/index.php?topic=28606.msg315117#msg315117

[PRR]

> Split Plate resistor often has the coupling cap between the split resistors, but Supro kept the Coupling cap at the plate and used a grounded cap between the split resistors. Was this for tone? and notice that Channel 1 has a .005uf and Ch2 has a .05uf.

That's power filtering, not signal reduction. And I think someone made a boo-boo, 0.005 is hardly worth the effort, even 0.05 looks like a lame attempt.
https://el34world.com/Forum/index.php?action=dlattach;topic=28606.0;attach=97141

(Yes, the 0.05 side is the thumpy trem side, but I still have doubts.)

[sluckey]

1) Split Plate resistor often has the coupling cap between the split resistors, but Supro kept the Coupling cap at the plate and used a grounded cap between the split resistors. Was this for tone? and notice that Channel 1 has a .005uf and Ch2 has a .05uf.

Crazy ain't it? Valco did that on some other amps as well.

2) Treble/Bass switch has two .001uf caps in parallel - why? maybe Supro had an excess of .001 and no .002 laying around?? it shouldn't matter should it?

Who can say why? Pure speculation. Doesn't matter.

3) Reverb Driver has two Cathode bypass caps 35uf and .05 - why? seems that the .05 is negligible in this approach.

Smaller caps have better response to high frequencies than electrolytics.

4) PI has .01uf and .02uf coupling caps - why this mismatch?

Only the shadow knows.

5) Tremelo speed pot is connected different that the original drawings - why? Orig drawings had terminal 1- NC, 2- to 100k grid, 3 - ground

Since there is no original drawing I decided to draw the speed pot my way. I like to connect an unused pot lug to the wiper. Anyhow, doesn't matter. The 100K and the pot are still in series and function the same.

6) The Supro had a strange main switch On-Off-Standby, how would this be used as going from stby to on requires going through off? it seems more logical to have Off-Stby-On

Just flip the switch quickly. No need to pause in off position. Yes, an off/stby/on switch makes more sense but requires a special switch. Hoffman sells that switch.

7) Can/should the reverb, tremolo, PI and preamp share the same ground since they are all coming from the same power supply node?

Ideally, they should not share the same node, but since they do, then all the grounds should connect together and also to that filter cap ground. Don't forget, this amp never had reverb. I simply drew in the reverb circuit of another valco amp to show another option for adding reverb to this amp. I concentrated on the reverb circuit and didn't put any thought into power supply nodes. It could be improved.   

[scstill]

I wanted to understand how you decide to mount a component on the jack or switch or on the board?
Notice in the Sluckey 1696TN layout, channel one switch has its associated components mounted on the board but channel 2 has the components mounted on the switch and jacks.

Since this build's chassis is so narrow and deep, I am doing my first component mounted terminal board (rather than point to point). My in progress layout and chassis pic attached. Each of the term boards holds 12+ components and there is a board for each of the 5 stages (Pre1 Pre2 PI Trem Rev). So much spaghetti, hopefully it makes the build a little easier having the parts on boards and the extra wires. The top chassis is done the bottom chassis has chassis parts mounted and a few wire lengths from the Filter caps. Nothing close to being dressed.

[sluckey]

I wanted to understand how you decide to mount a component on the jack or switch or on the board?
Notice in the Sluckey 1696TN layout, channel one switch has its associated components mounted on the board but channel 2 has the components mounted on the switch and jacks.

I take into consideration, the chassis size, major component and tube positions and the front panel. Then the board is sized, populated, and positioned to provide short, logical interconnecting wires that work with the rest of the chassis layout. I do this for all my layout drawings. Of course, I may slightly reposition things or even lengthen the chassis as the layout drawing progresses. For this particular project, I chose a CHASSIS SIZE 20" X 6.5" X 2.5", so I had plenty of real estate to work with. I will not design for a tweed (narrow) chassis ever again.   

Now, to answer your specific question... I let the schematic speak to me. It said, "Look at that kludge of components around Channel 2 inputs. They would look much better mounted directly to the input jacks and switch. And you will only have to run ONE wire to the board. Ain't that neat?" And that's how I decide. 

[Bash]

Nice project.

Just out of curiosity, the reverb and tremolo always seem to be on the same channel. Did anyone ever try them on separate channels?

By the looks of, making the channel for the reverb switchabke wouldn't be too hard.

[scstill]

Just out of curiosity, the reverb and tremolo always seem to be on the same channel. Did anyone ever try them on separate channels?

Why? This design (and others) includes a dual footswitch that allows selection of either, both or none.
BTW - this one was honed from a 2x4 to match the Valco/Supro/Silvertone style of the period.

[jjasilli]

Parallel caps.  This may be intentional.  If so it's called Stacking.  The point usually would be to use 2 different types of caps.  E.g.: in the signal path paper-in-oil caps are thought to pass bass better; plastic caps being better for higher frequencies.  This was a fad about 10 years ago.  Haven't tried it in the signal path.

It's also done for filtering in the PS.  Large caps, like those used for PS filtering have hi ESR (unwanted internal resistance).  Smaller value caps have less ESR.  Filtering is improved if a small value plastic cap bypasses -- is placed in parallel with -- a filter cap.  This is more common in hi-fi tube amps.  I use it in guitar amps, having first read about it from Dan Torres.

[scstill]

I let the schematic speak to me. It said, "Look at that kludge of components around Channel 2 inputs. They would look much better mounted directly to the input jacks and switch. And you will only have to run ONE wire to the board.

Parallel caps.  This may be intentional.  If so it's called Stacking.  The point usually would be to use 2 different types of caps.  E.g.: in the signal path paper-in-oil caps are thought to pass bass better; plastic caps being better for higher frequencies.

I took your lead and moved those components to the channel 2 input jacks/switch which cleaned up better.
Also added 2 different type parallel .001uf for the suggested tonal effect (we'll see but my ears likely won't get it :-))

But I wanted to understand the value of the ground buss on the pots. Looks good on the Sluckey layout, but seems somewhat challenging to build, especially in my limited space. Is there any advantage over just a simple wire connection? Since it must be elevated above chassis (and hence the back of the pots to reduce ground loops) how physically would you mount it? or does it just float?

BTW - my plan is for all Pre, PI, and Reverb grounds to be connected to chassis at a single point at the filter cap.
BTW2 - my layout shows chassis grounded jacks, but what I have installed is cliff jacks so all the jacks are isolated from chassis (remove ground loop possibility)

[tdvt]

But I wanted to understand the value of the ground buss on the pots. Looks good on the Sluckey layout, but seems somewhat challenging to build, especially in my limited space. Is there any advantage over just a simple wire connection? Since it must be elevated above chassis (and hence the back of the pots to reduce ground loops) how physically would you mount it? or does it just float?

BTW - my plan is for all Pre, PI, and Reverb grounds to be connected to chassis at a single point at the filter cap.

I have adopted/adapted Sluckey's buss method & have had good results using twisted (w/a drill) 20ga buss wire for some extra rigidity,

I run it 3/8'" or so behind the pots curving in on the ends to attach to the input jack on one end & to the last pot ground point on the other.  Any pot grounds between the ends get short lengths of buss wire as well, which also act as support arms. 

I then run jumpers from all the various ground points on the board to the buss & finally one wire from the jack-end of the buss to the power supply ground (not chassis ground).

Easier to see in the attached pic.

[scstill]

Great idea, your buss looks very neat.
Grounded at the "Power Supply" is what I meant when I said at the "Filter Caps".
It is at that single point the ground connects to the chassis, no possibility for loop.

For my layout, everything in the diagram is the same ground (Pre, PI, Tremolo, Reverb).
Is there any suggested reason to split the grounds (easy to do at this point).
The black wire throughout the drawing is ground (or silver to be seen on the black board).
While the buss is a neat looking, it seems that to use a single wire might be easier here.
Unless there is some other buss advantage I'm not seeing.
Plus the wire is insulated preventing any inadvertent chassis (or worse) touches.

[tdvt]

I am not expert & grounding schemes seem to be one of the never-ending topics of discussion in any case, but I can share what I would do, looking at your layout a few posts back.

First, the way I am interpreting your drawing, your final ground point for all your pre-amp boards (& headed for the (PA) power amp ground) is from the reverb pot. That is backwards of common practice which has your input jack as the last thing before the run back to the power amp ground.

That said, the fact there are 2 sets of input jacks is a wrinkle which I addressed on a few 2-channel Fenders by each channel having it's own buss.

The pic I attached earlier is the Normal channel on a Pro Reverb, which is why it is so short. The Vibrato channel also had it's own buss, & each buss had a home-run ground wire to the power amp ground. All internal circuit grounds were kept to their respective channel.

There are varying opinions as to whether the PI is pre or power but since your sections are already nicely separated, I would run the PI board ground directly to the power amp ground.

Then I would just group things by channel;

Pre1 board ground to Ch1 pots/jack to PA ground

Pre2 board, Reverb board, Trem board grounds to Ch2 pots/jack to PA ground



Don't know if you have this chapter on grounding by Merlin, but I have found it a very helpful reference.