Using a 12A (112A)

If they glow, this is the place to be
Post Reply
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#1 Using a 12A (112A)

Post by pre65 »

I now have a couple of 12A to play with,having done a swap with Will.

There is a little idea in my bonce which I want to try but not sure of the method.

I have an anode HT of 150V (after a 6K8 resistor) and I need to work out a cathode resistor.

The grid volts need to be about -10 and the current about 7ma so how does one get the cathode volts ?

Is it then cathode volts @ 7ma to work out cathode resistor ? (using Mr Ohms law)

Also, would it need a grid stopper, and what value grid leak ?

Any ideas or suggestions welcome. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
cressy
Shed dweller
Posts: 2906
Joined: Fri Aug 03, 2007 7:07 pm
Location: the great white space
Contact:

#2

Post by cressy »

i think this is right, please correct me if i get it wrong, get the curves, draw a straight diagonal line between 150v on the bottom line, and 7ma on the vertical line, pick a point on that line where the curves are about the same width apart so its at its reasonably linear. read to the top of the curve for the grid volts and there will be a voltage figure (eg -6v) the calculate v divided by ma.

for example, when i did the calcs for the 6h6n i got -6v at the top of the curve and had picked 15ma on the vertical line so it gave 6 divided by 15 so 0.4 this is 0.4k, so gave a 400r cathode resistor

i think this is right, so as i say please correct if im wrong.

using your figures phil i get 10 divided by 7 so 1.42 so 1k4 if you can get those figs off the curves
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#3

Post by pre65 »

Thanks Ant !

I can make up 1.57K from the "box" so that will give me 6.3ma which will get me going.

Now to do the cutting,drilling,filing and bleeding to get the buggers fitted and working.

Soldering iron is now plugged in :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#4

Post by pre65 »

In order to keep the anode voltage at 150v (ish) I've re calculated the load resistor at 15K (to drop 100V @ 7 ish ma).

I think I've got enough bits to make this work now.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#5

Post by Nick »

It was all in the long thread Dave worked through.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#6

Post by pre65 »

Nick wrote:It was all in the long thread Dave worked through.
Yes-I read through that again this morning,and my head went "funny" after a couple of pages. Some of it went in though. :oops:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#7

Post by Mike H »

cressy wrote:i think this is right, please correct me if i get it wrong, get the curves, draw a straight diagonal line between 150v on the bottom line, <snip>
Yes I concur. Image Usually the starting point.

If the chosen V is the B+, then the anode resistor value will be that Voltage figure divided by the left hand current figure. In other words you have to imagine your line is the resistor stretched between the V anf the mA points.

If you then manage to get the valve biased exactly halfway then it's Va and Ia will be those figures divided by 2, respectively. In other words read off the grid bias from exactly halfway along the line.

As an aside, if you can keep the Ia up, then as a general rule mu is increased and anode resistance decreased, thereby increasing gain. The limit being what's the minimum cathode bias Volts that can be tolerated. At which point I start looking for more B+ Volts, if it's an issue.


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#8

Post by Mike H »

Appendix
Oh yes the cathode resistor value is then the grid Volts / the Ia current that you end up with.


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Nick
Site Admin
Posts: 15706
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#9

Post by Nick »

Though increasing Ia for a given B+ will decrease the load resistor and so potentially reduce actual gain as opposed to the µ of the valve and increase distortion.

Its all fun :-)
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#10

Post by Mike H »

Quite right, neat in theory but you can't go raving mad with it.


I believe this is where choke loads have an advantage, as you are able to maximise Ia from a small B+. :?:


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#11

Post by pre65 »

I might resurrect the idea of a 112A preamp.

I found an interesting variation on filament supplies from Rod Coleman on diyaudio.

http://www.diyaudio.com/forums/attachme ... _250ma.png

http://www.diyaudio.com/forums/tubes-va ... ost2683766
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8863
Joined: Mon May 21, 2007 9:42 pm

#12

Post by Paul Barker »

Having looked at flow diagram your above discussion Abbott cathode r is no longer appropriate because Rod is passing current round his reg to reduce size of cathode resistor. Follow his flow chart and buy his kit.

I used 4 112a's two stages with two channels and one common cathode resistor for all four valves. AC heating and hum dingers.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#13

Post by pre65 »

What power output might I expect if 112a was used as an output valve ?

I could drive it with a 26. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Post Reply