C3g->2A3

Lee S · Post by **Lee S** » Sat Dec 01, 2007 9:23 pm

Thanks Nick. That is interesting. Is it because it is cold cathode rather than heated cathode?

On another note, let's see if I can get this right. Here goes...

On the C3g, the g3 connection (suppressor grid) is connected to the top of Rk. The data-sheet says this should be at 0V. Is that relative to the anode? If it is, why doesn't this grid just go to ground if it needs to be at 0v rather than the top of the cathode?

The g2 connection (screen grid) is held at 150v by the 0A2. Is this the grid that needs the 150-500R grid stopper on it?

g1 (control grid) is connected directly to the wiper of the pot with a 1M grid-earth resistor in between. Does this need a grid stopper on it too or is it OK as is?

Cheers all

Post by **Nick** » Sat Dec 01, 2007 9:31 pm

Lee S wrote:Thanks Nick. That is interesting. Is it because it is cold cathode rather than heated cathode?

On another note, let's see if I can get this right. Here goes...

On the C3g, the g3 connection (suppressor grid) is connected to the top of Rk. The data-sheet says this should be at 0V. Is that relative to the anode? If it is, why doesn't this grid just go to ground if it needs to be at 0v rather than the top of the cathode?

The g2 connection (screen grid) is held at 150v by the 0A2. Is this the grid that needs the 150-500R grid stopper on it?

g1 (control grid) is connected directly to the wiper of the pot with a 1M grid-earth resistor in between. Does this need a grid stopper on it too or is it OK as is?

Cheers all

Ok,

The 0v is relative to the cathode, the valve knows nothing about any other voltages than those at it pins. If the grid went to what you are calling ground, then the fact that the cathode was at a +ve voltage because of the cathode resistor, would put the grid at a -ve voltage with respect to the cathode.

I would put stoppers on both g1 and g2.

Lee S · Post by **Lee S** » Sat Dec 01, 2007 9:37 pm

Excellent. Cheers Nick. I understand that now. I thought it may be something to do with relative voltages that the valve is seeing. I thought it may have been relative to the anode though. Thanks for the clarification.

Same value grid-stoppers for g1 and g2 ? About 1K, say, or lower?

Post by **Nick** » Sat Dec 01, 2007 9:39 pm

Ik carbon should be a good place to start.

BTW, to answer the other question, yes, the VR conducts by ionic movement, not electrons, as you say, there is no hot emmiter of electrons.

Lee S · Post by **Lee S** » Sat Dec 01, 2007 9:57 pm

Cheers!!

Lee S · Post by **Lee S** » Fri Dec 21, 2007 1:07 pm

In the Doddington C3g circuit, is the 220k resistor on the grid of the 2A3 a grid leak or a grid-to-ground resistor? I thought a grid leak resistor was used to bias the valve, as it produces a potential divider with the anode resistor of the previous stage. I know it also produces a filter along with the coupling capacitor too, but what is the purpose of the 220k in this particular circuit?

How would I calculate the correct value for a grid-stopper resistor too? I wonder why James didn't fit one to the control grid of the C3g? Most other amps use one on a pentode, so why not here? Also, why do I not need to fit a grid stopper to the 2A3?

Cheers

Post by **Nick** » Fri Dec 21, 2007 2:02 pm

Ypu can call the rsistor either, it does the same job in both cases. If the valve is biased via a rased cathode, or by the end of the grid resistor going to a bias source.

In the case of cathode or fixed bias, the resistor does a couple of things, it provides a resistance for the grid signal to develop a voltage across. It also references the grid to ground or a bias voltage, and by allowing any grid current on signal peaks from raising the grid voltage.

In cathode or fixed bias, from the valves perspective the lower this value the better, but as its also in parrallel with the driving valve anode load, and forms a low pass filter with any coupling cap, then the higher the better from the driving valves perspective.

The valve specs will state the minimum resistance in the grid circuit, and it will differ depending if its cathode or fixed bias.

Grid leak bias (or contact bias) also uses the same resistor, but in this case a hight value resistor is used to allow the valve to bas its self by a combination of grid current across the resistor, and charging the grid (via its capacitance) -ve.

I think (though its only a guess) that James didn't show that circuit as a complete set of instructions as to how to build an amp, I think it was more of a shape, that then got some values added, and little things like grid resistors were left as "An excercise for the reader"

simon · Post by **simon** » Fri Dec 21, 2007 2:10 pm

Nick wrote:I think (though its only a guess) that James didn't show that circuit as a complete set of instructions as to how to build an amp, I think it was more of a shape, that then got some values added, and little things like grid resistors were left as "An excercise for the reader"

Yes, I think that's correct. James didn't show any snubbing caps either...

Lee S · Post by **Lee S** » Fri Dec 21, 2007 2:19 pm

OK. Cheers guys. I will read, re-read and inwardly digest that lot Nick. I will just have to have a play with it when it's up and running. Still waiting for chokes. Bloody Xmas post !!!

Richard Higgins · Post by **Richard Higgins** » Sun Dec 23, 2007 4:13 pm

Well my amps been running for two years without grid resistors and snubbers, now you're making me nervous.
I built mine using a C3m, and changed the anode and cathode resistors slightly.
I also used choke inut. The only initial problem I had was instability in the power supply which I cured by adding a 100uf cap.
But having modelled the supply on PSU Designer I've added a .75R resistor after the takeoff to the output transformer and changed the 100uf cap to 22uf before choke 2 to ground.
The secondary aim was to use only Black Gates after the initial voltage setting cap.
So after 5ar4 I have, 1.66uf - choke - 100uf - 10k resistor (to ground) -takeoff to Output Trans - 0.75R - 22uf to ground - 22uf to 100uf - choke 2 - 22 uf -output to OA2/C3m.

Regards Richard

simon · Post by **simon** » Sun Dec 23, 2007 4:49 pm

Hi Richard,

I don't think there's any need to be nervous

. I guess you don't need gridstoppers - the oscillation would probably be 'orrible if you did. Snubbers are worth trying - I found they made a nice difference in cleaning the sound up, lifting a veil if you like.

Richard Higgins · Post by **Richard Higgins** » Sun Dec 23, 2007 5:39 pm

Hi Simon
Thanks for quick response, I'll give snubbers a try once I've got the blackgates in the second amp.
I've been given a LG DVD/CD/VCR thing to mend, it's beyond me, but I'm learning. So far I've learnt that the problem of it cutting out after a couple of minutes use is related to one of the power amp modules, the heatsink burnt me, while the others were just warm.

Cheers Richard

al newall · Post by **al newall** » Mon Dec 24, 2007 11:48 am

I've been given a LG DVD/CD/VCR thing to mend

Best of luck Richard.

I've just had a similar thing with a Panasonic unit. Unless you can get a circuit diagram , which i couldn't, i don't think there's much chance of sorting these things.
As someone on the other side said..... they're not meant to be fixed: you're supposed to rush out and buy another.

Lee S · Post by **Lee S** » Thu Jan 17, 2008 11:26 am

OK.... HERE WE GO!!! The soldering iron is warming up and the Loftin White is about to GET IT !!!! Hopefully in a couple of hours I will emerge triumphant with a new amplifier to best the old one!! I am going in.... Nurse.. forceps....

Dave the bass · Post by **Dave the bass** » Thu Jan 17, 2008 1:25 pm

Nearly 2 hours under the knife/iron.

DTB

audio-talk

C3g->2A3

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