Motown Direct Amplifier-inspired Preamp?

[DaveP]

So the circuit now has two 15k grid stoppers and a 20pF by-pass cap?
Can I update the circuit yet?
best
DaveP

 

[heatwalk]

Just chiming in to say this has been a great thread to read thus far - shouts out to Dave and CJ and everyone else! Love this forum to see and learn from projects like this.

 

[CJ]

Dave there us a 68K on both grids of the 6N1P and a 15K on the 6BX7 and a 20 pf dogbone cap on the f/b resistor but a ceramic would work just as well.

A Jensen 4:1 output would probably sound awesome in this circuit.

 

[DaveP]

This is the schematic as it stands today:-

The 6N1P was a Russian tube introduced in 1954, I'm not sure if it was imported into the US before the Motown amp was made.
The ECC88/6DJ8 was introduced into the US in 1957. We cannot be sure if we have duplicated the original circuit but it looks like we have matched the spec. Thanks CJ.
best
DaveP

 

[capacitorless]

Not to derail, but I'm just (thanks, Dave & CJ!) finding out about the 6N1P, as a possible replacement for the ecc88 type. Is this a superior device in this particular circuit?

I know there's more filament draw, just wondered about the other parameters.

 

[My3gger]

Dave there us a 68K on both grids of the 6N1P and a 15K on the 6BX7 and a 20 pf dogbone cap on the f/b resistor but a ceramic would work just as well.

A Jensen 4:1 output would probably sound awesome in this circuit.

Do nickel core outputs sound better to you in general, despite higher price for Ni, or just in particular applications like this mic amp/DI where majority would go for steel core? I like them a lot, despite slightly higher distortion if Ni core isn't quite a bit bigger than the same steel core.
If Davep, Ian, you and others think it is appropriate, we could have OEP also make some typical tube output ratios. One OT the offered to me as a sample had 80Ni core with 2:1 ratio, i will certainly call them soon and post the test (electronic and listening).

 

[My3gger]

This is the schematic as it stands today:-

The 6N1P was a Russian tube introduced in 1954, I'm not sure if it was imported into the US before the Motown amp was made.
The ECC88/6DJ8 was introduced into the US in 1957. We cannot be sure if we have duplicated the original circuit but it looks like we have matched the spec. Thanks CJ.
best
DaveP

Strange, schematic only opens if i quote that post.

 

[DaveP]

Not to derail, but I'm just (thanks, Dave & CJ!) finding out about the 6N1P, as a possible replacement for the ecc88 type. Is this a superior device in this particular circuit?

The circuit was designed for the ECC88, but as CJ has shown, it also works very well for the 6N1P with slightly different voltages.
best
DaveP

 

[DaveP]

Strange, schematic only opens if i quote that post.

I had trouble with my usual image site and had to use another.
best
DaveP

 

[Disco Volante]

Works fine here. Great work!

 

[My3gger]

The 6N1P was a Russian tube introduced in 1954, I'm not sure if it was imported into the US before the Motown amp was made.
The ECC88/6DJ8 was introduced into the US in 1957. We cannot be sure if we have duplicated the original circuit but it looks like we have matched the spec. Thanks CJ.
best
DaveP

Good question, what looks like typical Soviet datasheets with Cyrillic on the left and Latin on the right are not dated, Svetlana datasheet came later. Search of US made gear with 6N1P doesn't show much gear older than ~24 years, same is with gear from Western and Central Europe, at least i could not find anything much older, or remember older schematics by known Western companies. EI Nis from Yugoslavia didn't make this model, i'm pretty sure Yugoslav army didn't use it either, despite importing lots of Western tubes like Siemens, RCA, Telefunken, etc.
I will check the other place with more members from ex Soviet countries and post here.

 

[CJ]

you can support either Ukraine or Russia, depending on who you buy from.

the 6N1P was a leftover from a Reddi DI build.

We don't have the Motown schematic and don't want to upset ACME by reversing their box, which isn't the same anyway (6V6 instead of a 6BX7), so this is just an effort to make a nice sounding Guitar/Bass DI with hopefully the same output tube as the original. It is not an overdrive or chorus or delay pedal, so no drastic change to the tone of the instrument will occur. However, it does provide a nice tone that will transmit over a long cable to a mixing board, so good for the studio or a live situation. The difference between plugging straight into the board or using the DI can be heard quite well, at least on my cheap Yamaha mixer.

And i do not think it will make you into Joe Messina, Chank , Robert White or James Jamerson, but it might get you closer to maybe laying down some tracks in the Motown vein than say putting a 57 on a Twin Reverb,

It's nice that Dave's circuit has that high end response as it will maybe help things stand out in the mix, or compensate for a lo-fi reel to reel machine. Who knows what the engineers did to the signal after it went into the control room, that is another part of the equation for sure.

 

[CJ]

Dave one thing you might add to the schemo is another 1/4" out so that the musician can use his amp and the DI at the same time.

 

[ruffrecords]

While we are looking at alternative output tubes, the Pultech MB-1 mic pre, which must be of a similar era to the Motown, used a 12AU7 cathode follower with the two triodes in parallel.

Cheers

Ian

 

[DaveP]

Dave one thing you might add to the schemo is another 1/4" out so that the musician can use his amp and the DI at the same time.

CJ, what was the feedback resistor value you used for R4?
best
DaveP

 

[heatwalk]

While we are looking at alternative output tubes, the Pultech MB-1 mic pre, which must be of a similar era to the Motown, used a 12AU7 cathode follower with the two triodes in parallel.

And the Pultec MEQ-5 amp circuit is nearly identical to the MB-1 but uses 6AQ5 which is similar to 6V6

 

[Wavelength]

Just my 2 pennies...
I am pretty sure even though the data sheet for the 6N1P says 1M grid resistance max that several people over on DIYAUDIO saw some grid current when used that high. That coupled with the 68K grid stop (which seems a little excessive) might cause grid current which will lead to microphonics.
In general 6DJ8 is an ok tube by any means, 6922 and some of the EURO ECC88 are much better. Sonically the 6N1P is a bit sterile in the circuits I built.
That output is crazy, parallel 6BX7 (I have a bunch) could drive probably close to a watt of power.

 

[CJ]

56 k on the f/b resistor,

Thanks wavelength, I will swap that out for 560 k .

 

[DaveP]

Unless we hear from CJ otherwise, this is the final schematic:-

best
DaveP

 

[CJ]

added a 5.6 K resistor to the secondary of the output xfmr and was able to remove the 15K stopper on the 6BX7 , full props to 2112 for the tip!

stopper going into stage 2 has also been dropped to 2.7K instead of 68K,

took out 1 M grid resistor on stage 2 and replaced it with a 270K and a 250KA pot on top of that which does nothing to control volume due to the NFB but does now act as a variable hi freq atten at 200 KC,

it does a little wave shaping on the 10KC sq wave, shown here first with no stopper resistance and then 250K of stopper resistance due to the pot, now i know why vol controls suck the tone out of an amplifier,