Motown Direct Amplifier-inspired Preamp?

[DaveP]

Followers of this thread should also check-out CJ's build here:-https://groupdiy.com/threads/motown-direct-box-diy.88733/page-2
best
DaveP

 

[MidnightArrakis]

Here's a 3D CAD-model "Chassis Mounting Clamp" I have created for the -- 47,000uF @ 25VDC -- tall electrolytic capacitor:




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[Brian Roth]

This is what I should have done the first time!

best
daveP

Dave, what app are you using to draw your schematics? I like the look of them!

Bri

 

[chrispbass]

Dave, what app are you using to draw your schematics? I like the look of them!

Bri

I was thinking the same thing!

 

[DaveP]

Thanks Brian!
There is no app as such, you might notice the similarity to the original LA-2 schematic?
When I made an LA-2 back in 2008, I copied the schematic into MS Paint and after tidying it up, I have been using it ever since.

I just copy and paste, its quite therapeutic!
best
DaveP

 

[untune]

There is no app as such, you might notice the similarity to the original LA-2 schematic?
When I made an LA-2 back in 2008, I copied the schematic into MS Paint and after tidying it up, I have been using it ever since.

Same technique I was using for the schematics at the start of the thread (albeit with Photoshop and a Pultec diagram) and I agree, there's just something relaxing about it!

 

[DaveP]

CJ has made the first practical version of this circuit with the tubes he had "in his sock drawer", being a 6N1P.
I made this combined chart to show the difference in the working point when using the 6N1P instead of the ECC88.

The green traces are the 6N1P @ OV, -2V, -4V, -6V and -8V, the orange traces are the same voltages for the ECC88.
The red line is the 47k load line.
The 6N1P has very curved lines down there compared to the ECC88. I think it is also more linear around the -4V working point.
This may make it more transparent, but maybe the 6N1P may have more "tone"?
best
DaveP

 

[CJ]

Dave do you think I should change that 10 K resistor on the cathode to get a voltage closer to your schematic?

 

[DaveP]

​

CDave do you think I should change that 10 K resistor on the cathode to get a voltage closer to your schematic?

I think around 150V is a safe cathode voltage for long life, but I guess its the plate voltage of the triode feeding it that is controlling the cathode voltage, that needs to be about 135V to give 150V at the cathode. Maybe lower the B+ for the final triode?
best
DaveP

 

[CJ]

here are some new voltages after the 6N1P swap for the 12AX7a and some adjustments to the pwr supply,
input level was jacked up to 800 mx P-P, starts to clip at 1.5 V-RMS input>

 

[CJ]

did some side by side tests,

showing the attenuation and shift of my circuit compared to Dave's for a 800 mv P-P sine wave at various frequencies>

 

[CJ]

square wave tests, this is on the primary side of the output xfmr, notice the crisp corners on Dave's circuit at 1 KC , no doubt due to the NFB being applied to the first stage,

 

[CJ]

here's where things get interesting, to me at least,

sq wave shots on the secondary of the output xfmr,

Dave's circuit has that transformer ringing a lot, is this because of the better sq wave response producing more harmonics or something? i doubt the transformers are different, they look like they came from the same lot,

remember the "sharper corners" comment up one post? i bet that is the reason for the ringing.

so a preamp that is lo-fi, with no NFB might be better when it comes to ringing in the transformer?

 

[CJ]

clip test, sine wave in at 1 KC, 1,2,3 and 5 v rms into the input jacks, out of sequence,

going to take it home for another listening test,

 

[ruffrecords]

Ringing is indicative of a resonance. A good square wave will have more energy at the resonance than a poor one so will create more ringing. Perhap you need a secondary Zobel network to tame it.

Cheers

Ian

 

[DaveP]

Thank you CJ for all the work you have put into this project and especially for all the scope shots.
The V72 (and all the V series) had a feedback resistor bypass cap, 50pF plus 40pF variable, to tame the top end. They were trying to limit the pick-up of the tape head bias in the studios, so they limited the top end to 15kHz. Now that we don't use tape recorders so much, we can allow the response to 50kHz but I don't think we need 500kHz do we? I suppose grid stoppers would also do the trick?
best
DaveP

 

[DaveP]

here are some new voltages after the 6N1P swap for the 12AX7a and some adjustments to the pwr supply,
input level was jacked up to 800 mx P-P, starts to clip at 1.5 V-RMS input>

Attachments​

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The voltages are looking much better now, the 6BX7 cathode voltage has come down, and the other cathode voltages have gone up, all much closer to the original working points.
best
DaveP

 

[MidnightArrakis]

The voltages are looking much better now, the 6BX7 cathode voltage has come down, and the other cathode voltages have gone up, all much closer to the original working points.
best
DaveP

>> I just received the 2 - 6BX7's and the capacitor mounting-clamps I had ordered a few days ago in the mail yesterday. COOL!!!.....

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[CJ]

playing around with grid stoppers in order to tame the ringing,

here is a before and after of a stopper on the 2nd stage, there is already a 68 k input resistor on stage 1,

seems to help,

 

[CJ]

adding an 82 k stopper to the grids of the 6BX7 output tube helps even more,

now the ringing looks manageable

interesting that you can not see the ringing on the input of the transformer, it takes the transformer to make all that ringing happen from the hi freq response of the preamp,

theses shots are all 10 k hz,