#183 Preamp only in a 1u rack case? Ideas?

[martin manning]

40V diodes would be fine. Yes running 12AX7's on 12V DC filament is what I'm suggesting. Starting from 12VAC makes it much easier to get enough headroom for diode junction drops and regulator drop-out.

[rogb]

Finally got this finished, working great too!
I have run out of time for now, so didn't even fit the LED pilot light or tidy the spaghetti wiring, apologies in advance but it is running nice and sweet, quiet on both channels.
Preamp volts around 190-195v. I had it at the layout 205-207 but it sounded a little harsh, mind I am just running some old tubes that I tested and looked fine on the uTracer (which works great courtesy Martin Manning).

The flat pack transformers are really good but are designed to be mounted onto a PCB so if you bend the terminals too much they fall off.... there is enough metal to solder to if that happens but not ideal.If you have more height I would mount them to an eyelet board but with 1U it is very tight.
I ran the heaters on 12vDC (they don't glow much for some reason, compared to when I ran the tubes on the uTracer) and the relays on 5vDC and they worked just fine with my footswitch.

Of course the OD relay didn't work, wired up wrong, so again Manning helped out when I searched out a relay drawing he did for Taylor back in his fledgling days.
I really hope that TAG members new and old realise the massive contribution this man has made, the help he has given in all matters technical, and it doesn't get more technical than the uTracer, which is such an important device for me.
If you were British, no doubt you would be made Lord Manning of Manningtree

OK enough smoke, when I get back from my travels to Finland and find anew place to live, I will get back on this project, get it tidied and running through my stereo 2x100w Class D box. I ran it through the 4x 6L6 of my 100w and it sounded really cool, the clean was also sweet, but benefited from the bright switch. This was with a Strat, might try the 339 tomorrow if I get a chance.
It really mellowed out through the Dlator/Minimix2/G Major 2 with delay and verb.
So, a lot of hard work but I'm happy with my latest addition. Might get her some nice tubes sometime too.
Thanks to all who helped out, this really is an awesome place, especially after the refurb !

183 preamp1.jpg

183 preamp2.jpg

183 preamp3.jpg

[martin manning]

Looks great Rog! You're using a 5V regulator to knock down the 12V filament voltage, I guess? Any chance you drew a schematic that you could share?

[rogb]

Nothing complicated, I just used your Relay board design (yep, indebted for that, too!) ran to the 12v regulator and to the 2x1K junction then to the 5v.
Solid 5v to the relays and 12v to the filaments as far as I can tell.
Any reason why the filaments don't glow much when being fed 12vDC?
Do you think the 5v reg/relay supply is loading the 12v too much?
I suppose there is some maths to work this out
Or I could disconnect the 5v and see if they light up some more (McGyver solution)

Relay supply_MM.jpg

[martin manning]

You shouldn't see any difference in the filament brightness running 12V in series vs. 6V in parallel as it's the same power input, but you are running 12.0V instead of 12.6 (~5% low). You have the 12V connected across pins 4 and 5, and nothing on pin 9 so the filaments are in series, correct?

[rogb]

You shouldn't see any difference in the filament brightness running 12V in series vs. 6V in parallel as it's the same power input, but you are running 12.0V instead of 12.6 (~5% low). You have the 12V connected across pins 4 and 5, and nothing on pin 9 so the filaments are in series, correct?

Of course yes, it's 12 (reads 11.98 on my meter) not 6.3 x2! Yes they are in series.
I've got a spare DC buck transformer board that I could dial exactly 12.6vDC on instead of the regulator. Maybe i will give a whirl when I do the clean up on the preamp?

s-l500.jpg

[martin manning]

I wouldn't worry about it as you are well within the nominal +/- 10% spec range.

[M Fowler]

Looks really good enjoy.

Mark

[JazzGuitarGimp]

That is one beautiful build, RogB! And I agree, Martin is a prince among men around these parts - always willing to help out!

[rogb]

Thanks Mark, it really is just an experiment using up parts I had to hand and a few bits from Mouser.

Of course feeding this voltage-correct preamp into a D style PI and 100w power section will give a true to Dumble tone.
Without the crucial PI interaction (an important part of the D sound) and power tubes and into a SS Class D power amp doesn't give the same tonal and harmonic output.

Think of it as a tube-powered D style pedal, they get the basics of the tone but not the full "magic" of touch sensitivity and gain manipulation that Dumble topographies excel at. Of course, running the tubes at full voltage will give decent headroom at least.

I think many people who look for a D sound in a pedal have never played at decent volume on a properly configured D rig (though RF does well enough with Zendrives and Twins...), relying on YouTube videos to form opinions is not the best way forward, as most of us have found when acquiring sound gear.
Some thoughts on the regulated DC section:
Using Martin's relay/LED supply circuit spliced into his suggested Schottky FWB design gives 17.7vDC into the LM7812 and 7805. While fine for 7812, it is a little to close to the recommended 18v spec sheet range limit (although safe to near 35v apparently), which is why I heatsinked them both. No problem with dropout, though.

So, 17.7v > 7812> 11.98v > V1 12AX7 pins 4 to 5 >V2 12AX7 pin4 to 5 > ground. Pin 9 floating
17.7v > 7805 > 5v > 2x 5v Omron relays
17.7v > 7805 > 5v > 2x1K footpedal / LED supply

[rogb]

That is one beautiful build, RogB! And I agree, Martin is a prince among men around these parts - always willing to help out!

Lou, thanks, it is a mess but when I get time I will tidy up and rewire where necessary, twist up the AC side. I just wanted to see if it worked OK first. It does and it's quiet too. No point in having the prettiest girl on the block if she sings like a mating moose .

[norburybrook]

well done Roger I have to agree about Martin as well

looks a great little project.


what next?



M

[rogb]

well done Roger I have to agree about Martin as well

looks a great little project.


what next?



M

Not sure but thank you, the plan is to get this preamp working well into the Class D. The passive stereo>mono summing circuit is not that great so far, a lot of level loss as you predicted. It works well in stereo though! Be nice to have a single speaker option too, but all in good time!

[martin manning]

Aw, shucks. Just living vicariously, watching some cool projects come together. Thanks for the kind words.

[rogb]

So perhaps
229A12 6VA 12.6V C.T. @ 450ma 6.3V @ 900ma
229A230 6VA 230V C.T. @ 25ma 115V @ 50ma

Best to go up to 12VA for the filaments, 229B12. Use Schottky diodes in a FWB, at least 4700uF for the reservoir, and definitely use a low drop-out 12V regulator.
For the HT, 229A230 will be fine with a FWB and a CRC reservoir: 220uF, 1k, 220uF; 350V caps. That will give very low ripple. You can adjust the 1k to dial the voltage in.

I ended up with 8.6K and 120K on the FET sub to get the volts to 195-197 range.
I get 244vAC on my mains here