A nice D3A---2A3 Monkey circuit

Mike H · Post by **Mike H** » Tue Oct 05, 2010 1:04 am

pre65 wrote:G2 & G3 of the D3a go to 2A3 grid.

Oh yay-yas.

Perhaps Mike is right ! Would it work otherwise ?

D3a anode comes from 2A3 cathode.

Oh yay-yas.

Bu-uut, must also go to 2a3 grid else it ain't got no negative bias from across choke.

Consider also, the two filament balance resistors, one looks like it just goes down to the 5k, actually they're both joined at the top, choke end.

Not to pick holes in Nic's drawing skills but confusion abounds

Apparently

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pre65 · Post by **pre65** » Tue Oct 05, 2010 10:49 am

Having a think ! Hmm.

So, looking through the box I find 12GN7 which has a lower RA (50K) than the D3a (100K).

And with 80H (2 X 40H) as the anode load ? DCR is 550R each.

That would mean only having to buy a few resistors.

Thing is, would it still work ? Only one way to see I suppose.

Post by **Nick** » Tue Oct 05, 2010 11:23 am

So, looking through the box I find 12GN7 which has a lower Ra (50K) than the D3a (100K).

Both those Ra are as pentode, they will be far less as a triode, more like 1k for a 12gn7 and 2k for a d3a.

pre65 · Post by **pre65** » Tue Oct 05, 2010 11:51 am

Anyone know the Ra and gain for a trioded C3g ?

Also, looking at Pete Milletts triode data (for Pentodes) it seems the gain with a CCS is about double.

So, a 12GN7 (trioded) with a CCS will have similar gain to a D3a (trioded) without.

Have I got that right ? If so how would that be implemented ?

Just that I do have a couple of the 12GN7/12HG7 in my box.

http://www.pmillett.com/pentodes.htm

Post by **Nick** » Tue Oct 05, 2010 1:08 pm

Anyone know the Ra and gain for a trioded C3g

Dont know for certain, Andrew L may know, but I would guess about 50 and 35ma/v. For your needs I don't think it matters that much.

Have I got that right ? If so how would that be implemented ?

Not sure what you mean there? the gain of a triode stage is always dependent on the mu of the valve and the ratio of load resistor to ra. Higher load resistor closer to mu. CCS gives a high load resistor, as do a 1M resistor and 10kv power supply. I would implement it with either a load resistor or CCS, what else are you asking about the implementation?

pre65 · Post by **pre65** » Tue Oct 05, 2010 1:19 pm

What I am trying to do is see if what I have (ie C3g or 12GN7) would do instead of D3a for the 2A3 monkey circuit.

If I use a CCS on either in that circuit (instead of the choke load) will that be a good idea ?

Or, should I just bite the bullet and get some D3a's ?

Post by **Nick** » Tue Oct 05, 2010 1:24 pm

I think you would need too much voltage drop to use a CCS in a monkey/LW bias arrangement, You could never get the grid near 0v which you can with a choke.

I wasn't paying attention when I said close enough, I had not noticed you were talking about a monkey. A different valve will need redesigning. Buying some d3a would be the simple solution.

You would need triode curves for the different valve to work out how it hangs together, or do it by trial and error.

pre65 · Post by **pre65** » Tue Oct 05, 2010 3:53 pm

OK dokey, I have sourced a couple of D3a now, so that is another project waiting in the wings.

Mike H · Post by **Mike H** » Tue Oct 05, 2010 4:03 pm

I got D3A as triode LTspice model so have something to work with.

Your 2 x 40H chokes are going to make 160 Henries in series, DCR 1.11k

Post by **Nick** » Tue Oct 05, 2010 4:15 pm

Your 2 x 40H chokes are going to make 160 Henries in series, DCR 1.11k

Is that right for two separate chokes? Are you sure its not 80H?

pre65 · Post by **pre65** » Tue Oct 05, 2010 4:32 pm

Nick wrote:
Your 2 x 40H chokes are going to make 160 Henries in series, DCR 1.11k
Is that right for two separate chokes? Are you sure its not 80H?

From Wikipedia

The current through inductors in series stays the same, but the voltage across each inductor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total inductance:

L1 + L2 etc = Lt

These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual inductors.

Mike H · Post by **Mike H** » Tue Oct 05, 2010 4:32 pm

2 x the number of turns = impedance/inductance x 2 squared? Or does that only work if on the same core

My bwain 'urts ....

pre65 · Post by **pre65** » Tue Oct 05, 2010 4:49 pm

All my anode chokes (two pairs) are in use on other amps.

Mike H · Post by **Mike H** » Tue Oct 05, 2010 4:54 pm

pre65 wrote:These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual inductors.

A-hah!! Hadn't read this bit yet. 80H then.

pre65 · Post by **pre65** » Tue Oct 05, 2010 4:56 pm

Mike H wrote:
pre65 wrote:These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual inductors.
A-hah!! Hadn't read this bit yet. 80H then.

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Will 80H be enuff (sic) ?

audio-talk

A nice D3A---2A3 Monkey circuit

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