Building a Tube Mic Pre...

[themdla]

i've got a question...

is there anyway to modify a 59 bassman 5F6-A Circuit to work with a microphone as a preamp to a DAW?

1) I'm thinking you would have to change the input impedance somehow as mics need low impedance inputs.
2) Remove the power section since your not going to be using speakers
3) Use a solid state rectifier circuit

but after that i'm unsure 

what voltage/current level is coming from the output of the preamp section after the tone stack?
(i'd like to keep the EQ as well )

is this voltage/current level enough to drive a line in on a mixer or DAW?

if not,
what circuit/components do i need to convert it to that level?

if any of this is even possible lol

-mike-

 

[themdla]

also to clarify...

i am trying to build a stand alone tube mic pre w/ EQ, from a 5F6-A preamp circuit (minus the power amp section) if possible.

inputs would inculde:

-Guitar/Bass Instrument Level (Hi-Z)
-Mic Input (Low-Z)
-and a line level input (if possible)

Outputs would include:

-Balanced and Unbalanced (for input into a line level on a mixing board or DAW)

Features include:

-Phantom Power
-Phase Reversal
-20db Pad
-80Hz low cut (if possible)

can this be done and if it could, would i want to, i.e. would it sound good?

 

[themdla]

edited with reference points....

"5f6a_Micschem.jpg"

and without (for those of you who care to elaborate)

"5f6a_Micschemraw.jpg"

A) Do i need an 1:1 input transformer here (Per flocentblack) or will something else work here?
(i'd like to have variable impedance to use for Hi-Z and Lo-Z)

B) Can i connect an output transformer directly to these two output leads or do i need other circuitry? what kind of transformer do i use?
(i'd like to have balanced and unbalanced outs, XLR and 1/4")

C) I'm guessing i'll need a different PT... what specs should i look for?

D) is it best to use a diode based full-wave rectifier here?

E) Previously there was a 56k feedback resistor running from the open presence pot to the speakers...do i need this anymore or can i delete it as i have in this diagram?

F)what type of OT should i use here? and how do i connect it to XLR's and 1/4" outputs?

sorry if my questions are noob... but as i have stated many times before that is exactly what i am lol.

or better yet if i just need to do more reading to understand this stuff just point me in the right direction.

thx
MD

 

[themdla]

for some reason this thing won't let me post my .jpg's  ???

both the ones i wanna post are under 70kb  :-\

 

[rodabod]

In my opinion, this is probably a bad idea for many reasons. I personally wouldn't use any of the compoents in your current Bassman circuit for a mic pre except for maybe some of the valves.

Are you basing this on part of an actual Bassman, or are you trying to build something from scratch?

Host your jpegs at imageshack.us and then post the links here.

 

[themdla]

<a href="http://s901.photobucket.com/albums/ac211/themdla/?action=view&current=5f6a_Micschemraw.jpg" target="_blank"><img src="http://i901.photobucket.com/albums/ac211/themdla/5f6a_Micschemraw.jpg" border="0" alt="Bassman Raw"></a>

<a href="http://s901.photobucket.com/albums/ac211/themdla/?action=view&current=5f6a_Micschem.jpg" target="_blank"><img src="http://i901.photobucket.com/albums/ac211/themdla/5f6a_Micschem.jpg" border="0" alt="Bassman"></a>

 

[themdla]

Doh!!!!

 

[themdla]

and the original minus an few things...

 

[themdla]

hopefully this is enough lol

one last schematic... this is the unaltered 5F6-A schematic...

 

[abbey road d enfer]

You could build some kind of mic pre using the a transformer (1:10 ratio) before the first triode and using the output before the tone stack. The fender tone stack is far from linear, in fact it's some kind of notch filter. You would also need an output tranny to bring the impedance and level down to standard line.
Using the Bassman schematic is an excellent basis for a DI box, but for a mic, you should look elsewhere.

 

[themdla]

awesome thx man!

i am a bit of a beginner so pardon my lack experience and whatever stupid questions may follow...

i have built this circuit although as a guitar amp (which btw sounds amazing!!).

what would you recommend as a simple tube mic pre schematic?
(similar in simplictiy, low in parts count, available parts, etc...)

i'd like to have eq also... what is a good linear eq circuit?

thanx abbey road d enfer!

 

[abbey road d enfer]

You may want to look at the REDD47 or the mic pre in the Ampex 351 (very close to the input section of the Bassman). It's relatively easy to build a Baxendall type EQ in it.

 

[themdla]

i found this Ampex 351 mockup... is this what you mean?

couldn't find a REDD.47 though... any ideas?

 

[themdla]

ok redd47 in hand (i think)

what should i delete from this thing?
what would be good substitute xfmr's for this?
can i use more common tubes without changing passive componet values?

sorry i've got about 1 1/2 months of electronics exp under my belt

thx again!

 

[themdla]

as for Baxendall here is what i found....

 

[abbey road d enfer]

i found this Ampex 351 mockup... is this what you mean?

This is a mic pre made using an actual 351 chassis and components. You could make something like the first part of it (from the input XLR to the "record board") and attach a 10:1 xfmr at the output.

 

[abbey road d enfer]

ok redd47 in hand (i think)
what should i delete from this thing?

There's nothing you can delete; all is necessary

what would be good substitute xfmr's for this?

xfmr's are available from Sowter

can i use more common tubes without changing passive componet values?

EF86 is pretty common. E88CC can be replaced with ECC82.

sorry i've got about 1 1/2 months of electronics exp under my belt
thx again!

REDD47 may be too complex for someone unexperienced. I think the "351" variation is easier.

 

[abbey road d enfer]

as for Baxendall here is what i found....

This one is solid-state. I thought you wanted all-tube?

 

[themdla]

@abbey road

so in place of the "T1" xmfr i'd put in a 10:1 and run the secondaries to the "repro board" and finish to the output as shown?

or did you mean something different?

yeah i'd prefer a tube EQ i think...

also as a beginner do you have any books you'd recommend.

 

[abbey road d enfer]

The output section is too complicated for a newbie job. Just take the upper portion of the schematic, take off the CAL. pot, instead connect the primary of the output tranny, the secondary to the output XLR. You will need a PSU; the one on this schematic is too complicated too.
Books/litt: check the internet for Radiotron pages; everything is there, from the basics to the minute details. Check also schematics for Dynaco amps and preamps