G7 & Blue Bottle Transformer Question

[Jazzy_Pidjay]

hi!!

Visibly the Blue Bottle have a design
very close to the G7 :

" The Bottle vacuum tube amplifier consists
of a classic common cathode circuit using a current
source for the plate load "

"The amplified signal is taken from the plate and
fet through a high-quality Hovland polypropylene
capacitor bypassed by a smaller value polystyrene capacitor"

Why they bypass the blocking cap ? in the Auricap
web site they really against the bypassing ?!

"The Bottle Microphone employs an EF86 pentode
vacuul tube connected in a triode mode. The third grid of
the EF86 is connected with a cathode and is grounded"

"The amplifier input is separated from the microphone capsule
with a capacitance consisting of two styroflex caps (one ten times
the value of the other) mounted on special Teflon Isolators"

what is the goal of the Teflon Isolators ?

"The BLUE hand-built Bottle microphone transformer is balanced
using a symmetrical two-bobbin design, with a transforming ratio
of 13:1. With this ratio, the microphone achieves a low output impedance
typically 110 ohms. The primary transformer windings are connected in
series. The transformer's secondary windings are connected in parallel
and connect directly to the XLR output pins.

So my real question is, I don't know how to calculate
the transformer impedance in this case ?
if we have 15.000 Ohm like the G7, how many ohms we can obtain ?
what are the consequence to the output gain ?

thanks

 

[rafafredd]

The higher the turns ratio, the lower the output.

Let´s say you use a 6:1

Zsource/(turns^2)=Zout

15000/(6^2)=Zout

15000/36=416.666 output impedance.

got it?

 

[Jazzy_Pidjay]

thanks, for my OEP it's ok

but what i don't understand
is how to calculate with the primary
in serie and secondary in parallel :?

 

[rafafredd]

let´s say you have a 6+6:1+1

primary in series and secondary in parallel, you get 12:1

primary in parallel, secondary in series, you get 3:1

primary in parallel, secondary in parallel, you get 6:1

primary in series, secondary in series, also 6:1, but higher impedance/inductance.

got it?

 

[Jazzy_Pidjay]

So the Blue Bottle have a very little output gain ??
to have this 110 Ohms output impedance ?

thanks rafafredd !

 

[Gus]

Look at a u67 or brauner VM1 schematic. I would think the bottle will want to "SEE' a microphone preamp with a 2K input.

You are thinking matching and maxpower (a name you would like to touch) transfer. Most of the time useing the 10 to 1 rule works. ten time your output R as your input R this is more voltage drive.

 

[Jazzy_Pidjay]

my G7 have approximatively an output impedance of 360 Ohms
and my ISA220 have an input impedance of 1,2kOhms

so about 3 times only !

what are the consequence of this ?

thanks

 

[Gus]

Alot depends on the transformer and how it works at different loads. You are "working" the tube a little harder because the plate "sees" a lower Z load.

This can be good or bad this is why certain microphones sound different with different pres. This is why preamps like the GT viper have a select switch for input Z.

Think of it as a tone control that you don't have alot of control over.

 

[Jazzy_Pidjay]

:?

i don't understand

You are "working" the tube a little harder because the plate "sees" a lower Z load

can u explain much ? thanks

 

[Jazzy_Pidjay]

with the lundahl 1538 we have 600 Ohms so it's worst ? no ?

 

[alk509]

[quote author="Jazzy_Pidjay"]i don't understand

You are "working" the tube a little harder because the plate "sees" a lower Z load[/quote]

Current is proportional to voltage and inversely proportional to resistance (or in this case impedance). The relationship is called "Ohm's Law" and can be expressed mathematically as I=V/R

There's tons of helpful info on the Newbie Meta. Check it out! :thumb:

Peace,
Al.

 

[rafafredd]

[quote author="Jazzy_Pidjay"]:?
i don't understand
[/quote]

Transformers has no "impedance" of its own. When you say a winding is 600Z, it means that it was meant to work with this source impedance and the specs are measured using this impedance.

Transformers only reflect what is on the other windings.

Let´s say you have a mic with a plate-transformer coupled output. The plate Z alone is 15000Z. The transformer is a 12:1. So the outputZ after the transformer will be 104Z, nominal 100ohms.

Let´s say you are going to plug this mic into a tube grid throught an input transformer.

Tube grid has very high impedance. And the noise is also high, for mic level purposes. So, you use an step-up transformer. You don´t need much current to drive a high impedance tube grid, but you want lots of voltage, so that the noise level is OK.

If you use a mic transformer, you want to terminate the secondary before the tube grid. So that the impedance reflected at the mic is not TOO high. That´s not always the case. Sometimes you can load a mic transformer directly on a tube grid, if it´s very high turns ratio, like 1:12 or more. But we are talking about a 1:8 here.

You have a 1:8 turns ratio (Vpri/Vsec), so you have a 1:64 impedance ratio (Zpri/Zsec). So, if you are feeding the transformer with a 100Zout mic, and you want the mic to see a 10x (ten times) impedance (for optimal signal transference), then your mic wants to see a 1kZ at the mic pre transformer primary.

Just terminate the secondary of the mic pre transformer with a 64k resistor and you are done. The mic will see a 1k on the primary side of the transformer.

Note that 1k is in fact a somewhat lower impedance for mic inputs. It generally likes to see 2k or more. Most modern inputs are more than 5k, but the vintage units are generally 1k-2k Z.

Also, you should go higher, because if you plug a 350Z mic in there, it wants to see a 3k5 load. And a 3k5 load won´t hurt a 100Z mic.

So, for a 3k5 load, you have 3500 x impedance ratio, or 3k5 x 64 = 224000. So, I would terminate this 1:8 mic input transformer with a 220k resistor, for a 3k5 input impedance.

You should also check the datasheets of the transformers used. It generally has terminating info on paper. Sometimes you need to kill some frequencies with a RC network in parallel with the load resistor.

Yes, the input impedance the mic sees change the sound like an EQ that you have no control over. So, in the end, a rotary switch with multiple resistors for diferent mics, is a good idea!

I hope my instructions are right. If not, let´s wait and see if any of our professors will be correcting any bullshit I said... Or adding anything to what I´ve writen. This is pretty incomplete and there´s always something to be added.

 

[Jazzy_Pidjay]

great thanks rafafredd !!!

:roll: :roll: :roll:
for now, i read, read, read and still read
for understand your explanation !

 

[rafafredd]

[quote author="rafafredd"]The plate Z alone is 15000Z. The transformer is a 12:1. So the outputZ after the transformer will be 104Z, nominal 100ohms[/quote]

Just to clarify,

12:1 turns ratio = 144 impedance ratio.

15000/144=104Z

 

[Jazzy_Pidjay]

let´s say you have a 6+6:1+1

primary in series and secondary in parallel, you get 12:1

primary in parallel, secondary in series, you get 3:1

primary in parallel, secondary in parallel, you get 6:1

primary in series, secondary in series, also 6:1, but higher impedance/inductance.

got it?

So !

I have with primary & secondary in parallel
6,45²=41,6
15000/41,6=360 Ohms

But u say that in series i have an higher impedance ?????
how to calculate it ??
if the impedance is higher the gain is lower ??

sorry for this newbie question, but i don't find any element
of answer :?