a few V72 questions

[dripelectronics]

been pouring over tall the thread info .

and couldnt find a few things.

im rebuilding my v72(not 'a') , unfortunatly it had a lot of the shorting problems
that occure in the older ones.

what are the transformer ratios. ?

im looking at using sowters , i know they have the choke.


the rectifier (i think) could be replaced with a more modern one ,
does any one know what type ?

is it a 'full wave' . (what type ?)

im also using ef804s they appear to be a bit costly ,
are there any alternatives to these tubes.

im also intrested in a volume control ,
from what ive gathered R18(80k)
is the resistor that sets the volume,
any concepts on this ,

as ive read through the pages , ive seen 'stepped attentuators'
as the solution here.


i would be greatful for any help.

g.

 

[gary o]

Hiya I DIYed a V72 used the Sowter inductor and V74 OP tran from Sowter {Brian Sowters advice} it woks well ..but I have no original to compare.




Cheers ....................Gary O.

 

[jensenmann]

what are the transformer ratios. ?
input is 1+1:20


is it a 'full wave' . (what type ?)
yes, it´s selenium. If you replace it with silicon check the B+ voltage. The voltage drop in the selenium thingy is way higher than in the silicon bridge rectifier

im also using ef804s they appear to be a bit costly ,
are there any alternatives to these tubes.
Early versions used the EF40 which is supposed to be noisier than the Ef804s (s means less noise)

im also intrested in a volume control ,
from what ive gathered R18(80k)
is the resistor that sets the volume,
any concepts on this

input and/or output pad are commonly used. If you change R18 in the feedbackloop the sound of the thing will change depending on the volume

 

[CJ]

Nothing sounds like those chokes.
If you want, I can spin some 47 on there.

 

[dripelectronics]

ah , many thanks ,

i have the whole thing torn apart and am soldering new wires
and connexions.

the aeg selenium rectifier looks a little frazzeled .
so ill give replacing it a shot ,
it doesnt say on any schematics what range
of voltage or amps this would be .

did find something strange with c33 (2uf@ 250v)
on the schematics it shows this cap as being 'between'
the output transformer and pin 'AP' on the output.

but on my unit (siemens v72)

this cap is actually 'between' the Choke and pin 7 of one of the ef804's.
pin 7 on the ef804 goes directly to output xformer pin AP and also to
r18 80k
c39 50 pf
c40 40pf (shows an adjustable but on my unit is fixed)
perhaps this is where an arco from the la2a
would come in play.

gonna try to order some sowters and perhaps do a comparision.

so the input is a 1+1:20 (schemo says 2x1:20)

not showing what the output is : just says 2/2 on the schemos
does that mean '2:2' ?

thanks .

g,

 

[dripelectronics]

ill double check that ,
many thanks.

gregory

 

[dripelectronics]

ah so i see , this is like C5 (10uf) on the la-2a
the audio passes through this on it's way out
into the output x former.

*rubs hand togeather*

excellent.

g.