Motown Direct Amplifier-inspired Preamp?

[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,

 

[CJ]

there is no free lunch with those stoppers, here is a 100 K Hz sine wave input,

attenuation is not that bad with the stopper, Jeff Beck might have been able to generate 100 K Hz but i do not have that note on my guitar or bass,

last night's listening tests confirmed what i see on the scope, Dave's preamp had a bit more articulation on the high end, overall the two circuits were very close to each other, Dave's circuit was unforgiving of guitar technique, you have to play perfect or you will hear it, the smearing from my circuit made it a little less stressful when trying to play clean. i will give the circuit with the stoppers another side by side listening test,

both circuits sound wonderful compared to plugging directly into the board.

 

[CJ]

spun a choke last night, 625 EI core (5/8" tongue)

got 6000 turns of #34 on a square bobbin, about 450 ohms DCR but 80 Henries

so at 120 Hz that will be 6.28 x 120 Hz x 80 Henrys = 60 K ohms of inductive reactance to squelch out that sawtooth rectification noise.

this circuit is not push pull so you do not get any free cancellation of pwr supply hum,

ripple on my circuit was 5 volts on the first cap and 600 mv P-P after the 200 ohm dropping resistor

now it is 5 or 10 mv after the choke,

there was a slight buzz with the dropping resistor, now it should be gone.

Dave's circuit has another advantage over mine, it only uses two tubes,

green choke on chassis, those end bells might cut down on noise coming in or going out, i do not know,>

 

[samic]

Hello everyone
unfortunately in Europe the 6bx7 tube is very expensive and it is almost only available in the USA!
what would be a good alternative according to you?

 

[DaveP]

It looks like you have got it sorted CJ, those square waves at 10KHz are remarkable. The choke was a good idea.
best
DaveP

 

[DaveP]

Hello everyone
unfortunately in Europe the 6bx7 tube is very expensive and it is almost only available in the USA!
what would be a good alternative according to you?

It depends what you call expensive, the 6BX7-GT is listed at only $12 at Vacuum Tubes in Florida. All the tube sellers on ebay in europe are making their money by charging you around £20 for shipping.
https://vacuumtubesinc.com/index.php/6bx7gt.html Have a look through Franks data sheets for large TV triodes or try a 6V6 triode wired as mentioned earlier in the thread.
best
DaveP

 

[CJ]

Tried it out on acoustic guitar and jazz bass and it sounds great!

I might have gone overboard on the grid stopper values, I will try lower values and play with a zobel on the xfmr or a cap across the NFB resistor.

Surprisingly the choke did not kill all the 120 hzstuff so it might not be needed, I will try an extra pi section in front of the 6BX7,

Got a little heater him I believe, will try swapping the green wires on one or two tubes.

 

[Disco Volante]

the 6080 springs to mind as an alternative, lower mu though... Or triode strap a few 6L6's, the metal-can version can be had cheap..

 

[DaveP]

The 6BL7 is $10 against $12 for the 6BX7, looks like a possible sub.
best
DaveP

 

[ruffrecords]

The 6BL7 is $10 against $12 for the 6BX7, looks like a possible sub.
best
DaveP

If memory serves, we are only running about 5mA through each triode so an ECC88/6922 would do equally well.

Cheers

Ian

 

[DaveP]

Surprisingly the choke did not kill all the 120 hzstuff so it might not be needed, I will try an extra pi section in front of the 6BX7,

I noticed that the commercial model has put the choke on a bracket.......

Got a little heater hum I believe, will try swapping the green wires on one or two tubes.

Would a couple of 50 ohm resistors to make a centre-tap help?
best
DaveP

 

[abbey road d enfer]

Or triode strap a few 6L6's, the metal-can version can be had cheap..

6L6 for a line output? Isnt'it like the proverbial sledgehammer to crack a nut? 5W of heating power...

 

[Disco Volante]

I agree, that was a bit of a tongue-in-cheek remark

 

[CJ]

sq wave taming-

reduced stopper resistor on 6BX7 from 68 K to 15 K,

added 20 pf cap across feed back resistor, this caused a tiny bit of phase shift at 10 KC,

tried a 0.003 uf - 62k Zobel on the OPT but it did not do much,

so the first shot is the original 10 KC sq wave, the second is with the grid stoppers, and the third is with the stoppers and the 20 pf cap across the f/b resistor>

 

[map2112]

What is the load impedance / resistance that the secondary of the output transformer terminated into during those scope measurements?

 

[CJ]

Good point! I will stick a 620 on there and see what happens...

 

[map2112]

It would also be good to see it with a 10k to emulate a modern audio interface input impedance

 

[CJ]

Got rid of some more 60 and 120 him with an extra pi stage of filtering, so the AC heater wiring is fine.

With the two grid stoppers added and the 20 of cap on the feedback the high end is back and the circuit sounds better than the one I did

If you only used 1 section of the 6BX7 you could build two channels with only three tubes.

Time to fab a lid and hook up a VU meter. Tweaking a circuit takes more time than building the whole thing.

 

[CJ]

Got rid of some more 60 and 120 him with an extra pi stage of filtering, so the AC heater wiring is fine.

With the two grid stoppers added and the 20 of cap on the feedback the high end is back and the circuit sounds better than the one I did

If you only used 1 section of the 6BX7 you could build two channels with only three tubes.

Time to fab a lid and hook up a VU meter. Tweaking a circuit takes more time than building the whole thing.