Tube preamp questions

[awazleon]

Hello guys,

I'm working on the tube preamp and couple of questions raised by studying it :

What is the output impedance ? I want to add a balanced output by using the LL1527 Lundahl transformer and I want to be sure that I can use that.

I saw a difference between the schematic and the layout. There is an additional capacitor 0.027µF and 470K resistor on the schematic (grid to plate V2-A tube) compared to the layout. Is there any reason for that ?

I’m going to use a power transformer able to deliver 206 Volts on secondary. Is that enough to power it ? In that case, do I have to change the 4.7K resistor by another value ?
Is transformer with a 230 Volts secondary is mandatory ?

I know, this is a lor of questions, but I don't want to make any mistake.  ;)

Many thx in advance for your help
Have a nice day

Pascal

[tubesornothing]

Hard to answer your questions without a schematic and layout.  Can you post these?

[awazleon]

Ooops, sorry, I'm speaking about the Hoffman Tube Preamp pedal.

shcematics are here :

Pascal

[PRR]

> What is the output impedance ?

Looking in the out jack, I see a 1Meg, a 250K, another 1Meg, a 100K.....

They don't all add up. And there's other parts. And half that stuff is variable. But output impedance will generally be over 100K.

It will work fine through a short wire to a 470K+ input.

Since it is hard to get a transformer above 50K, this is NOT suitable for driving transformers.

Also: 12AX7 is -not- a good transformer driver. In Real Studio Gear you use at least both halves of 6SN7, and I have used two 6V6.

> transformer with a 230 Volts secondary is mandatory ?

They can't arrest you for using 206V. You probably could not hear the difference. I would not change resistors.

> additional capacitor 0.027µF and 470K

Huh. Interesting. Should make quite a difference. One way may be better for some users, the other way for other users. Put the parts in, then try with one lead cut, see which way works best for you.

> I don't want to make any mistake.

Make mistakes. That is the best way to learn. 230V/206V is not a mistake. LL1527 hanging on 1Meg pot "is a mistake" if you want the very excellent response a LL1527 is capable of... but the trimmed bass and strong midrange may be a good guitar sound. Or maybe not. Depends a lot on what kind of sound you like. Many "Classic Guitar Amp Sounds" -were- "mistakes" which sounded good. If your mistake sounds bad, make more mistakes. Just keep track so you don't make the same mistake more than 3 or 4 times.

[ampgeek]

Welcome Pascal!

The cap/resistor network that shows up in the schematic but not on the layout is a local feed back loop that is purported to smooth out the tone and prevent oscillations at high gain settings.  It is similar to that which I saw on a Dumble'esque build that I did recently with one major difference.  That being the resistor value.  On that one it is something like 40M.

I am nearing completion of a tube overdrive pedal which is a melding of a Matchless Hotbox and the schematic that you show.  I put in a 1M pot on some turrets that I included for testing purposes and have found that it reduces the output volume tremendously even at 1M.  I guess that makes sense as the post plate load resistor signal and the grid signal are directly out of phase so there must be significant canceling.  No change in tone as far as I can tell.  Haven't got around to increasing the value on my pedal as of yet.  But, at this point, I can say that it is un-useable at 1M.

BTW: the cap in that network on both the pedal and Dumble'esque builds are 0.047uf as opposed to the 0.027uf that you mention.  Not sure how much of a difference that makes though.

Cheers,
Dave O.

[awazleon]

Guys,

Many thx for all that answers.

You're totaly right "mistake" is the best way to progress and this is what I'm going to do, surely  ;)
Because I'll build that on kind of turrets board, it will be easy to modify/adapt/test various setup and try to find the best sound according to my ears.

I'll keep you posted of my progress (pictures, reports...)

Have all a nice day and week-end

Pascal

[ampgeek]

My pleasure!

I did mine with turrets and have a scaled go-by that I will share if you (and anyone else) are interested.  I tossed in a few extra turrets to allow for experimentation and have found them to be very handy.

I do all of my designs in Corel Draw but it can export vectors in numerous flavors.
Do you use any programs that work with .dxf (autocad, others?)?

Dave O.

[awazleon]

Hi Dave,

I'm interested by that. I also use Corel Draw, so that's ok for this format.

Have a nice day

Pascal

[ampgeek]

You got it Pascal!

Email with CorelDraw layout attached sent to you moments ago.
Give me a scream if you have any questions.

Dave O.

[awazleon]

Hello Dave,

I started to look on your stuff you sent. Nice.
Don't know yet if I'll use a Clean and an Overdrive input. Could be only the Overdrive one. I'm also thinking about a more versatile tone stack.
One question : What about a "loop effect insert" ? It can be interesting to add that, but where ?

Have a nice day

Pascal

PS : Sorry for my English, I'm French... Nobody is perfect  ;)

[ampgeek]

Thanks Pascal!
Glad that you are finding it useful.  I didn't draw in the wires as I am fairly accomplished at laying them down with just a schematic in front of me.  Don't hesitate to ask me if you have any questions.

Take a look at this thread on my tube overdrive unit:

http://www.el34world.com/Forum/index.php?topic=5055.0

I have wired the foot switches to provide 4 modes as described.

So....it is a very flexible device and, if you do the same, you won't have to make the decision to have only a Clean or only a Overdrive option.

I would be most interested in learning your thoughts on a more versatile tone stack.

Not sure about an effect loop insert but it sounds interesting.  My guess is that the question of "where to add it?" might depend on the type(s) of effect you wan't to put in the loop.  What are your thoughts on that one?

No worries.  Your english is just fine!

Happy Holidays!!
Dave O.

[awazleon]

Hello Dave,

Concerning the tone stack I'm going to test that.

As you can see, it is referenced to the +Vcc because basically it used Fet and not tube (bouhhh)

Of course, I'll modify it to be referenced to ground.
There is a switch to modify the Mid pot response and obatain something a bit higher or lower in the midrange control.

Regarding the effect loop, you're right, it will be easier to add some effect pedals before the preamp or any reverb or whatever after the preamp before to inject in the amp or the mixer.
You know, I'm really exicting with that kind of project because it is very easy to modify and test various configurations to try to find and obtain the sound you are looking for.
I already experienced that by building some High-End HiFi amplifiers.

I understand your switchs configuration but how did you wired it ? I see for the following modes
1: Complete bypass
2: Clean boost (one gain stage)
4: Overdrive (4 gain stages)
but how and where to wire to have the 3: Pushed boost (3 gain stages) mode activated ?

Have a nice day
Merry Christmas

Pascal

[ampgeek]

Very nice Pascal!
I love adding tone options to my pedals.  I find them most useful when switching between my Les Paul and my Stratocaster.

Here is a quick sketch of how I wired the switches to obtain the various modes of operation.

Have a look and give me a yell if you have any questions.

Merry Christmas,
Dave O.

[bluesman1218]

I've been away too long, and finally got around to completing this build (Doug's Tube Preamp Pedal) which I started 2 years ago. I also haven't done any tube work since then, so thanks for bearing with me and my newbie questions. 

I corrected a few mistakes I made and still have a few things to work out. I can hear some hum and the volume varies with the Master Vol pot, but nothing from the volume pot. No signal is getting to/passing through the circuit.

First of all, my voltages are nowhere near the readings on the wiring diagram. The power supply has been wired according to the schem and wiring diagrams, which agree with each other.

These are ok
13 vac wall wart = 14.09 vac
14.9 vdc raw = 15.38 vdc
11.9 vdc regulated = 11.86 vdc

B+ Doug - me vdc
284 - 2.57
264 - 2.48
257 - 2.39

Guessing this should be fixed first and will check solder joints. Component values are OK.

Second, these component values differ between the schematic, the wiring diagram and the photos.
             The schem shows V2a Pin 2 with a 470k resistor --> .047 cap -->V2a Pin 1 & 7. The wiring diagram shows no cap at all. The photo shows a .022 cap going directly to Pin 2.

I just need a little help getting started on the right path.   

[sluckey]

Gotta fix those B+ voltages first. I would suspect the toro PT is wired incorrectly first. With dual primaries and dual secondaries, phasing is very important.

The schem shows V2a Pin 2 with a 470k resistor --> .047 cap -->V2a Pin 1 & 7. The wiring diagram shows no cap at all. The photo shows a .022 cap going directly to Pin 2.

Just to the left of the socket I see a resistor going straight to pin 2. That .047 cap and 470k resistor are mounted directly on the socket. Schematic and photo are in agreement. The layout omits the .047/470K between pins 2 and 1/7.

[bluesman1218]

Thanks so much for the quick reply, Sluckey. I'll work on it on Monday and post status with pics.