*Update* "Ultimate Studio Preamp" up and running. pic heavy thread

[nandrewjackson]

First off, another big thanks to the el34world site and admins and everyone here for their help.  I'm so glad I found this site a couple years ago.

Chassis
Front plate 18 ga. steel.
"U" shaped chassis 22 ga.
back panel 22 ga.
top removable plate 22 ga.

Color= Rust O Leum Key Lime Green

This thing came out looking like a "ZVex" product, IMO. But they probably weld a lot better than me.

Board, some components, power switch, standoffs, power plug, choke, from Hoffman parts store

Other components from Mouser, Radio Shack (knobs), and a Weber power Xformer.

I'm going to upload my first initial schematic to this thread, but the circuit is a bit different than the scan.
I've added "low cut 1, 2, and 3" which are 0.0047uF caps in series in each interstage coupling.( spdt switch in/out.)
Also, 1K Ohm in series with each Cathode "bias" pot (parallel to the switchable cap) and not the 100 Ohm in series as the drawing shows.


I'm VERY amazed at how high gain I can get with all of that wiring! and when I hit a spot where it squeals, I can change one of the cathode bias pots a bit to get the squeal out.

It's pretty cool to change from a JCM800 style (first cathode with cap, 2nd cathode no cap high R) to a Soldano SLO type
(1st and 2nd cathode with cap, 3rd cathode no cap high R) with just a few knob turns and switches.

Also, It seems to me, I get a good VHT pitbull sound with Soldano "biasing" and using the U.S.A. EQ. (fender EQ) instead of the U.K. (Marshall) EQ.

The "pregain EQ" which is between triode 1 and 2 (if selected) is Really awesome for clean sounds.
That EQ, combined with the output USA EQ with each of the 3 gain knobs at about 9 O'clock is a great clean sound.
Very pick-responsive.

HOWEVER,
The "pregain EQ" drops the signal down quite a bit, so it is practically useless for HI GAIN sounds.
I was wondering, and I'm sure someone here will know, if I can "raise it up" by putting some OHMS underneath the mid control (to ground) of the pregain EQ, without changing the EQ curve?

Here's the pics, feel free to respond with questions, answers, criticisms, whatever you got....

[nandrewjackson]

OH, I knew I would forget something, my Voltages!!!

at the choke is 398 VDC to ground
B+ to V1 is 342
B+ to V2/V3 is 349
B+ to V4/V5 is 366

V1 is 1/2 of first 12ax7
V2 and V3 is 12ax7 #2
V4 and V5 is 12ax7 #3

Anode Volts are:
V1 = 165
V2 = 171
V3 = 180
V4 = 166

Cathode Volts read with each bias pot at minimum (1K)
1.1 for V1,2,3
1.3 for V4

[kagliostro]

I was wondering, and I'm sure someone here will know, if I can "raise it up" by putting some OHMS underneath the mid control (to ground) of the pregain EQ, without changing the EQ curve?

Have you considered a raw control ?

https://taweber.powweb.com/store/pprc_schem.jpg

---

I've added "low cut 1, 2, and 3" which are 0.0047uF caps in series in each interstage coupling.( spdt switch in/out.)

Here is 04.52AM and my brain is a bit suffering, can you explain better or better post a little pencil schematic ?

Also, 1K Ohm in series with each Cathode "bias" pot (parallel to the switchable cap) and not the 100 Ohm in series as the drawing shows.

the 1K resistors are in parallel or in series with the bypass capacitors ?

---

What about V3b, unused ?

Why don't use it as a CF as to have a better impedance in exit from the preamp ?

K

p.s.: Which are the PT & Choke data ?

[nandrewjackson]

I sure need that Raw Control.  I was unsure if that would change the EQ curve, but I guess not!

The Choke is 125C3A Hoffman "Small Fender style choke"

PT is tedweber.com WRVBPTEU for standalone fender reverb replacement. (pic below)

What I'm calling "low cut" is a cap with a slightly higher R for the low freq's, it REALLY helps tighten up the HI GAIN settings, for fast palm muting on the low strings.  but I have them off for good clean tones.

[nandrewjackson]

the circuit does have CF.

the 1st 12ax7 is only using one triode, let's call that Stage 1

The 2nd 12ax7 is using both triodes, let's call them Stage 2 and stage 3

The 3rd 12ax7 is using both triodes, let's call them stage 4 and CF.

I forgot to get a cathode Voltage reading on CF, I will do it later.
The B+ for stage 4 is same as CF anode.

It gets a bit confusing when one of the triodes is not used, do we call it V1, V2, etc,, or Triode 1, Triode 2, etc,,, 

[kagliostro]

Thanks for the added info Nandrewjackson

When one triode is unused I find useful to call the tubes as V1a - V1b - V2a - V2b and so on ......

so you now have: V1a(gain) - V2a(gain) - V2b(gain) - V3a(gain) - V3b(CF ToneStack Driver)

As you have one unused tube I had like to have an arrangement like

V1a(gain) - V1b(gain) - V2a(gain) - V2b(gain) - V3a(CF ToneStack Driver) - V3b(CF exit to Power Amp)

this way you would have had a low impedance output that better suit the signal on the cable and with a bit of arrangement on the exit CF you would be able to drive pedals and line units

Alternatively, if you want to have a correct drive for the first ToneStack without gain loss

you can use V1b as a CF to drive the TS between V1a and V2a

However your's is a brave project  , what about buzz and humm ?

EDIT: I tried to follow your schematic and explanation, I'm sure I didn't understand well the voltage in relation with nodes, anode and cathode, and may be I introduced errors, please correct my draw

K

p.s.: if you are interested I can post the .fcd editable file - to draw the schematic I used FidocadJ a free multi platform version of Fidocad

you can download the program and the manual here

http://sourceforge.net/projects/fidocadj/files/?source=navbar

[tubenit]

That's a very innovative and interesting build!  THANKS for sharing it!

As an FYI,  I would have one more 22uf filter cap for that preamp.

with respect,Tubenit

[HotBluePlates]

I was wondering, and I'm sure someone here will know, if I can "raise it up" by putting some OHMS underneath the mid control (to ground) of the pregain EQ, without changing the EQ curve?

Have you considered a raw control ?

You can't really reduce the amount of insertion loss to the tone circuit without using something like a cathode follower or MOSFET source follower.

The Raw control itself will reduce the amount of mid-scoop and give the impression of boost but it does change the EQ shape to accomplish the effect. Try it yourself, but wire in a temporary pot to see for yourself what it does and whether you'll like it.

[nandrewjackson]

K, Thanks for the drawing and link to program. I have never used one of those I will download and check it out.  your drawing looks good aside from having the first TS switchable with DPDT in/out of circuit.
    buzz and hum?  it doesn't seem too bad right now, HOWEVER, I am running into a small bedroom practice amp
     (Marshall MG 10) into it's "clean" channel.  I can have lots of volume with low buzz/hum, but again, it's only a ten watt amp with 6" speaker. (solid state)

I have some better equipment on loan to friends, I will have to retrieve that stuff to hook up this preamp better.

Tubenit, thanks,
Where would another 22uF go?  There is one before the choke, after the choke, then one for each 12ax7, for a total of 5.


HBP,
I'll check out the raw control, *temporarily* like you say, I thought it would change, at least, some aspect of the curve, maybe i'll find a different TS to put in that position that has less loss.  I chose the Fender style specifically for the mid scoop, so I'll go from there,

Thanks everybody,
Noah
Nandrewjackson

[kagliostro]

Oh, yes, I completely forgot the switch, sorry

Please, can you specify the voltage looking to the schematic I've draw, I would like to add the correct and complete voltage to the schematic

EDIT: Here attached the editable file of the schematic in the .zip file

and here

some idea for an alternative Tone Conrol





look also to the arrangement on the attached image, in this amp, the tone control is splitted in two different stages

K

[kagliostro]

This is only an idea

. Mosfet Source Follower (as to drive correctly the first TS)

. Resistor in parallel with switch (anti pop)

. CF at the output (using the unused triode) with settings for line and pedal output levels

. Added 40uF filter node after the bridge, before choke

K

[nandrewjackson]

I have added a 47uF at the choke as per many suggestions here.

I have also added 500pF across the first 4 anode resistors, making the hi-gain much more controllable, with little to no effect on clean tones.

I've never used transistors, so I would need some practice with that before adding that to the circuit.

I can get quite an array of sounds from this preamp.  I will make a video/audio clip soon to show how the controls change the sounds.  I have been running it into my DAW (preSonus audiobox USB with StudioLive software) and listening with headphones.

Before I installed the Anode R caps, the distortion breakup was very pronounced on the low notes, and not as pronounced on high notes (12th fret high E and B strings, and higher).  After installing those caps, the breakup is much more even over the entire fretboard. 

I'm not going to change the "pre gain EQ".  I only use it for clean tones anyway.  If I use the USA (Fender) TS with hi-gain, I basically get the sound I was shooting for.

It's really neat, I can bias it like a JCM 800, a Soldano HI-gain, and even tread on Dimebag Darrell (Pantera) sound without any boost pedal in front of the amp.

I don't really get any "pop" with the cathode caps the way it's set up, HOWEVER
I am having some issues with the "low cut" caps in the interstage circuits.
The 3rd one (between the 3rd and 4th triode) will have no signal at all when it is off.  When the switch is off, the cap is bypassed and NO SOUND is getting through.  Could this have something to do with the cap?  When the switch is on, and the cap is in the circuit, it is fine.  Maybe a faulty switch? I dunno, I haven't dug into it yet.

Noah
nandrewjackson

[nandrewjackson]

homemade preamp demo

[kagliostro]

There is something strange on your demo

at some points sound seems to came independently if you are playing or not  ??

K

[nandrewjackson]

The sound is coming from the preamp being ran into a Presonus Audiobox USB PC interface recorded onto StudioLive software.  The video is a small Kodak digicam.  I deleted the audio from the Kodak video and added in the track from the DAW.  They are about 1 second out of sync.  My video software is very basic so there wasn't much I could do about that.

I'll try to get all of the voltages for you, K, soon.

[kagliostro]

Oh, now I understand the reason of the strange synchronization effect

K