MILA-1

[pstamler]

[quote author="NewYorkDave"]Now, Paul, why would you say that? Just because of that li'l subsonic bump in the response plot? :wink: [/quote]

Actually, I hadn't even noticed that. I was thinking more of the articles Norman Koren published in Glass Audio using a simulator to sleuth out the infrasonic problems in the Dynaco PAS circuit (also a feedback pair). The interaction between the coupling cap connecting stages 1 & 2 and the coupling cap connecting the plate of stage 2 to the output cap and to the feedback network produced a big infrasonic peak, not noticeable in the output response as long as the load was exactly 50k total, but manifesting itself as susceptibility to overload on the plate of stage 1. He blamed this for the notoriously loose bass of the stock PAS design.

Peace,
Paul

 

[Kingston]

I wish I would've been able to see these images well before I started and now "finished" my two-bottle project. What a beatifully clear layout, and educational as such.

And those noise figures! :shock:

Oh well, I guess I can file the two bottle project as a first, and do the next one clean, with all the things I've learned in my mind before I even begin.

Now I'm kind of scared to show pictures of my two-bottle. They look pretty atrocious in comparison.

At least it sounds beautiful. I'll file MILA-1 as a "to do" project, when I need more tube pre swiss knives.

 

[alk509]

[quote author="Kingston"]Now I'm kind of scared to show pictures of my two-bottle. They look pretty atrocious in comparison.[/quote]

Seriously, Dave, that's some neat ******* layout! Certainly puts my stuff to shame...

A beautiful design, too! :thumb:

Peace,
Al.

 

[hejsan]

Speaking of clean and fuzzboxes.. Does anyone know of a schemo for a tube preamp that behaves relatively clean on normal song, but bursts slowly into nice overdiven sound when screamed into?

A girl I know whos in a band wants a pre like that, and I actually also see some usefull studio aspects.

Neat circuit by the way! How does it compear complexity-to-build-vise to the one bottle?

Cheers n' beers :sam:
hejsan

 

[Kingston]

[quote author="hejsan"]Speaking of clean and fuzzboxes.. Does anyone know of a schemo for a tube preamp that behaves relatively clean on normal song, but bursts slowly into nice overdiven sound when screamed into?[/quote]

I'd be interested in a similar gradual overdrive that you describe. There's a great alternative though: the two-bottle and MILA are very clean on the whole amplifying range, but if you build an output attenuator into one of these (like I did) you can nicely saturate the output transformer. Transformers like this aren't hard clippers, and for example on vocals the saturation seems to work very well indeed. and on bass.. and horns.. etc. :grin:

 

[Sleeper]

That looks cool Dave! I'm looking forward to seeing the schemo. I've said this before, but I've really learned a ton from all of your projects.

It's a sappy reference, but you know that twilight zone where the teacher wonders if he's done anything worthwhile and all these ghosts come back and tell tales of their heroic deeds inspired by his teaching.
well, there you have it.
sometimes the obvious choice is the best choice.
thanks as always
kelly

 

[BR]

Hey Dave,
the box looks great.
Maybe I'm stupid, but I can't find the pdf anywhere. can you point me to it??
Thanks
Gil

 

[emrr]

[quote author="Kingston"][quote author="hejsan"]Speaking of clean and fuzzboxes.. Does anyone know of a schemo for a tube preamp that behaves relatively clean on normal song, but bursts slowly into nice overdiven sound when screamed into?[/quote]

I'd be interested in a similar gradual overdrive that you describe. There's a great alternative though: the two-bottle and MILA are very clean on the whole amplifying range, but if you build an output attenuator into one of these (like I did) you can nicely saturate the output transformer. Transformers like this aren't hard clippers, and for example on vocals the saturation seems to work very well indeed. and on bass.. and horns.. etc. :grin:[/quote]

The best example of this I've heard in a quality vintage circuit is the Collins 6Q-1. It's schematic is floating around here somewhere. It's pretty clean and rich at lower levels and has a long knee into the type of fuzz distortion that begs the question "is it distorting, or does their voice really sound like that?". It needs an interstage gain control added to be truly useful.

If you lose the negative feedback loops in your design plans you will get more of this smooth distortion knee, and the more negative feedback you apply, the more obnoxious the clipping will become. ...of course IMO.....

 

[Kingston]

[quote author="emrr"]The best example of this I've heard in a quality vintage circuit is the Collins 6Q-1. It's schematic is floating around here somewhere. It's pretty clean and rich at lower levels and has a long knee into the type of fuzz distortion that begs the question "is it distorting, or does their voice really sound like that?". It needs an interstage gain control added to be truly useful. [/quote]

Thanks for the suggestion. I'll have a browse.

 

[Sleeper]

You might also try some kind of a hybrid.
I took the fender showman/twin preamp and tone control circuit and added NewYorkDave's 12bh7 line driver stage to the output...
A pretty tried and true distortion here.
I use it for instrument inputs, I suppose all it really needs is a transformer and possibly phantom to work with a mic.
Kelly

 

[skipwave]

I just realized that Dave's PS is in one of those RatShack project boxes. I picked up a few of those for $1 ea. from an RS store that was closing. It looks so much better painted, I guess I should do that too.

Why you gotta go raising standards, Dave, why?! :wink:

 

[NewYorkDave]

RE: the PDF, it's gone for now. It was posted prematurely. I've since made a mod to the circuit (at Paul Stamler's suggestion) which has wrought a significant improvement in the noise level, especially at the +54dB gain setting. The new 'n improved PDF will be up very soon, maybe even tomorrow.

Brian: yes, the PSU is in a Rat Shack project box. The amplifier is in an LMB "jiffy box." It's true, a nice coat of paint really helps.

 

[NewYorkDave]

The updated PDF is now available. If you downloaded the old version, you'll definitely want to delete it and replace it with the new one. All of the measurement plots in the photo album have been replaced with updated versions as well.

http://groupdiy.twin-x.com/thumbnails.php?album=138

 

[skipwave]

After a quick look over, I don't see the mod to the circuit. Can you divulge what was changed?

I'm definitely building this one, instead of a fifth One-Bottle. :grin:

 

[NewYorkDave]

The resistive "center tap" in the heater circuit is no longer returned to ground, but to a DC bias of about +30V. This reduced hum to some extent at all Range settings, but made a substantial difference at the +54dB range. At that setting, the cathode of V1A is unbypassed (reducing the gain of the stage by 6dB) and hum voltage from the heater circuit was coupling to the cathode. Elevating the heater to a higher DC potential than the cathode fixed that.

Although I've been aware of this trick for years and have used it in the past, I'd forgotten how effective it can be until Paul S. reminded me of it.

 

[Consul]

Well, now I have to make a decision. Two channels of the One-Bottle, or one channel of MILA-1 (or is it MILA-2 now?)... :wink:

Well, MILA definitely looks more versatile, and to tell the truth, I don't think I'll really need more than one channel.

Thank you for sharing this with us, Dave.

 

[skipwave]

[quote author="NewYorkDave"]The resistive "center tap" in the heater circuit is no longer returned to ground, but to a DC bias of about +30V.[/quote]

Ah ha! I didn't compare at the PS pages, and I hadn't finished the "Specifying Noise" thread. I didn't realize you could apply a DC bias to an AC heater supply. Thanks for the explanation.

 

[hodad]

Any problem with applying the heater bias trick to the one bottle? It looks pretty dang simple, & if it'll bring the noise floor down, it's certainly worth a little time & effort.

Tom

 

[NewYorkDave]

It can help in the case of hum coupling from heater to cathode. It won't hurt to try it--go ahead.

 

[Sleeper]

I was just going to post in the new books uploaded or specifying noise thread about where to learn more about this raised cathode bias thing.
I'm sure one of those design texts will explain.
I could use a point in the right direction:?:

I this might help on the showman circuit I mentioned above.

Now I have a better example to look at than the one mentioned in 12bh7 line amp. nice drawing Dave.
Tube voltages scare me a little so I figure I better to ask.

How do I come up with resistor values if I have a different supply voltage.
The bias raising part is r5,r6 and c3 correct?
It looks like a plain old voltage divider is this right?
It's only the ground voltage reference so my divider is only dropping a very small amount of current? or am I wrong and I need big honkin resistors.

Why 30 volts? Is this related to some specification of the tubes you are using?

Thanks
Kelly