Supro 1690T vs S-6424 differences

[dbishopbliss]

I'm in the very early stages of planning my next build. There have been a couple of 6424 builds based on Sluckey's layout but I'm itching to take a stab at doing my own layout (but heavily borrowing from Steve). Looking at the faceplate controls of te 1690 and the 6424 these two amps it seems like they might be fairly similar except for the single tone control. However, it looks to me like there are quite a number of differences including:

• Channel two of the 1690 is using two halves of V1B and V2B whereas the 6424 only uses V1B.
• The tremolo of the 1690 uses only one half of a V2A whereas the 6424 uses both halves of V3.
• The phase inverters seems to be different. I think they are both paraphase but the 6424 seems to have more parts.
• Power tubes 6L6 vs 6973
• The OT for the 1690 seems to want 2 ohm secondary whereas the 6424 has a typical 8 ohm.

Based on all those differences, it seems like they should sound quite different from each other. Can anyone explain (in simple terms) what is happing with Channel two? Is that an extra gain stage or something else?


For my build I will want to use a modern transformer that has 8 ohm secondary (and maybe 4 and/or 16 as well). What changes would I need to make to do this?


Schematics can be found here and here.

[sluckey]

Can anyone explain (in simple terms) what is happing with Channel two? Is that an extra gain stage or something else?

The first triode is the input preamp. A tremolo signal is applied to the cathode to wobble the gain. The second triode is just another gain stage, probably to compensate for tremolo filter losses. The extra stuff between the two triodes is the tremolo filter. It's job is to pass the guitar signal and remove the low frequency tremolo signal. Think of it as a high pass filter

For my build I will want to use a modern transformer that has 8 ohm secondary (and maybe 4 and/or 16 as well). What changes would I need to make to do this?

Nothing special to do. Just use an appropriate transformer for the type tubes you use ie, 6L6 or 6973.

[dbishopbliss]

I started laying out my board using Sluckey's stencils and I noticed that the schematic calls for a 10K 10W resistor  after the first filter cap. There are no voltages listed on the schematic but based on Sluckey's 6424 schematic and other ampliers (e.g., Fender) it seems like 10W is overkill (his shows 2W). Do you think this was a typo or do I need to figure out how to fit a 10W resistor on the board?

[acheld]

I think 10 Watts is likely overkill in that position.   But it does depend on the current (and voltage drop), and I don't know the current draw of 6973s.   

Note that sluckey's schematic actually has a second resistor @ 100R@3 watts right after the 1K resistor.   

[sluckey]

Look here...

     https://el34world.com/Forum/index.php?topic=19661.msg204690#msg204690

Fourth pic down clearly shows a 10K 10W resistor. Is it original? I can't say, but the caps are certainly replacements.

EDIT... Send a PM to TubeGeek. He still active and was last on the forum on 03/28/2021. Ask him about the resistor. He said he did a cap job. Maybe he can remember more details. He used the schematic you're using.

[dbishopbliss]

I think 10 Watts is likely overkill in that position.   But it does depend on the current (and voltage drop), and I don't know the current draw of 6973s.   

The 1690T uses 6L6 tubes. The photo that Sluckey referenced clearly shows 10K. I'm going to reach out as he suggests.

[sluckey]

I noticed TubeGeek was on the forum yesterday afternoon. Did he have any info for you?

[dbishopbliss]

I noticed TubeGeek was on the forum yesterday afternoon. Did he have any info for you?

No replies yet, I'm just going with the big resistor for now.

[dbishopbliss]

Unfortunately Classictone is no longer making transformers. Looking at their specs for the transformer was 5K, which seems unusual for two 6L6 tubes but not completely wrong. Perhaps that contributes to "the sound".


The transformer I got from Trinity Amps for my AC15 had a 5K impedance tap (and 8K) but I'm assuming was rated for 15 watts and I believe two 6L6 tubes will produce more power than that. Any other suggestions or do you think that would be ok?

[sluckey]

That OT in TubeGeek's pictures probably ain't more than 15W. I'd rather have a 30W OT. Look at Hammond's stuff. They have plenty of suitable OTs for 2 x 6L6 amps.

     http://www.hammondmfg.com/

[dbishopbliss]

Looking more closely at the schematic it says the following about the Output transformer:

Output transformer is undersized and has a VERY high turns ratio to accomodate the nominal 1.6 ohm secondary load. Primary impedance is approximately 9K to 10K.

So while the Classictone transformer for the Thunderbolt 6420 was 5K, and the 6424 was 7.5K, the schematic indicates an even higher value. Since, I am chasing "that tone" and from what I hear on Leon C's clones (look them up on youtube), that is the sound I want so I'm going to look for something in the 9K to 10K range.


Does "undersizing" the transformer have an impact on tone or just reliability?

[shooter]

didn't read it, i'm taking rusty bolts out of a Dodge so....




Output Transformers Explained (aikenamps.com)

[sluckey]

Does "undersizing" the transformer have an impact on tone or just reliability?

Using cheap undersized iron was a big part of the Danelectro and Valco sound. I doubt that was in their de$ign criteria though.   

[dbishopbliss]

This is my first attempt at creating an original layout using Sluckey's stencils. I don't have Visio and had to use a tool called OmniGraffle because I'm on a Mac. I might be able to get it, but its not cheap. This seems to work pretty well and is free for the first two weeks.


Anyway, my goal is to build the 1690T in a Stout chassis.  The layout is to scale with the holes of the Stout chassis showing through. Obviously, I haven't included the rear panel connections in the diagram but they should be pretty straight forward.


I have a couple of questions:

• I put the voltage dropping resistors along the back of the turret board and plan to run wires similar to what Doug did in his 6V6 Plexi build. One difference is Doug had the ground buss running on the outside of the resistors. I put mine inside because this design requires the larger 10 watt resistors and it would be hard to route the component wires over the big resistor. Is there a reason that this is a bad idea?
• Another option is to run the voltage dropping resistors parallel to the cathode resistor along the left side. This could require a jumper under the board to go from the big 10K to the small 10K, or a wire from the filter cap running along the output tubes. Then I could move the turrets for the ground bus out so they align with the others. Is this a better option?
• I added grid stopper and screen grid resistors to the octal sockets for the 6L6 tubes. Any reason I should not do this?
• The holes for the output transformer can be seen below where the phase inverter and the tremolo sections of the circuit occur. Should I be concerned about the location of these wires adding noise? Asking because of the distortion that I heard when I had poor routing out OT wires in my Vox.
• I omitted the "treble" inputs. Looking at the schematic, they seem to be different than the Fender Hi and Lo inputs. I think they are designed to filter out bass. Is this correct? Should I drill extra holes and include them?
• I tried to indicate how I would run the wires. I've tried to route them away from the power tubes, but I'm not sure if this is necessary.

Recommendations on how to improve the layout are welcome. Of course, if this layout is problematic because of being sort of crowded, I could always design for a larger chassis.


Thanks for looking.

[dbishopbliss]

Here is the second option for the power dropping resistors. I think I like this better. (Note: I forgot to include the jumper between the two 10K resistors in this screen capture).