V72a inductors
« on: September 26, 2019, 07:03:13 AM »
I am willing to try to make my own replica of TAB V72a.
Schematics do not give the values for the 6AK5 anode load inductor,
nor for the E180F cathode inductor.
Can anyone give advice?
I guess the E180F cathode inductor is the most critical, since it should also have the proper DCr to polarize the tube.

Any practical suggestion about building of this classic preamp is very much welcome :)
have fun,

Ciro Marzio


moamps

Re: V72a inductors
« Reply #1 on: September 26, 2019, 09:43:02 AM »
I am willing to try to make my own replica of TAB V72a.
Schematics do not give the values for the 6AK5 anode load inductor,
nor for the E180F cathode inductor.
IIRC, in V74 the same inductors are used, please check out the schematic of V74 for numbers.

Re: V72a inductors
« Reply #2 on: September 26, 2019, 10:27:08 AM »
IIRC, in V74 the same inductors are used, please check out the schematic of V74 for numbers.
So it seems that BV2367 - loading E180F cathode - has an inductance of 45H. This makes sense.
Still it would be necessary to know at least the approx dc resistance of the winding to make a suitable substitute.

6GK5/5654 anode load inductor is ADR2/V74. According to V74 schematic this one has only 40H inductance.
This does not seem to make sense.
In V74 5654 anode is loaded by a series of two inductors for a total inductance of 340H, that is more or less what I expected to find.

Anybody out there has the real thing on hand and can give measures of actual inductance and Dc resistance of the V72a inductors?
have fun,

Ciro Marzio

EmRR

Re: V72a inductors
« Reply #3 on: September 26, 2019, 11:03:06 AM »

Anybody out there has the real thing on hand and can give measures of actual inductance ad Dc resistance of the V72a inductors?

This has been thoroughly explored and defined in other threads here.  Have a look around. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: V72a inductors
« Reply #4 on: September 26, 2019, 11:52:23 AM »
This has been thoroughly explored and defined in other threads here.  Have a look around.
Thanks Doug,
I was not using the forum search in the proper way  :-[

I have found a very interesting and complete dissection report from CJ.
So now I know that V72a anode loade inductor has around 500H inductance and 15Kohm DCR
If I have to make a new one I think I will keep DCR lower  ::)
Most difficult thing will be to shield it properly

Now I just need to find some info on cathode inductor DCR
« Last Edit: September 26, 2019, 11:55:42 AM by Ciro Marzio »
have fun,

Ciro Marzio

Re: V72a inductors
« Reply #5 on: October 03, 2019, 05:44:17 AM »
the cathode chokes DCR is important for the bias of the E180F..., 2 kΩ are OK.
That you achieve with 11000 windings of 0,1 mm wire on M42-core (0,5 mm gap).

shabtek

Re: V72a inductors
« Reply #6 on: October 03, 2019, 09:35:57 AM »
Isn’t the challenge to keep capacitances low?
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

Re: V72a inductors
« Reply #7 on: October 03, 2019, 12:20:37 PM »
Isn’t the challenge to keep capacitances low?

When a choke is used for loading a gain stage yes, it's generally desirable to keep capacitance, as well as DCR low.  This is all the more challenging if we want high inductance for loading, say, a pentode's anode.
I don't think the capacitance would be as critical in the cathode circuit of the V72a's E180F since its gain is less than 1.   
Besides the DCR of the choke for biasing up the cathode, I also see a plate to grid R of several M ohm which will be pulling up the DC on the grid a little.  I would guess that this could be fiddled with (adjusted on test)  if the DCR alone isn't putting the cathode at the right point.
« Last Edit: October 03, 2019, 12:24:34 PM by Winston O'Boogie »
Note to self:  Don't let c**ts wind you up


 

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