Gentlemen:
I will copy some comments sent to me by a very good friend concerning this design.
djk: Insert a coupling cap on the wiper of the 100K pot, 0.1 uf or larger. Change the grid stopper resistor to 1K on the 12AY7. Add a 470K grid leak from the grid to ground. Decide on how much current you want in the first stage and vary the cathode resistor to get it (ohms Law, 4V/1.5K=2.7ma which sounds about right).
Add 100 ohm grid resistor to each of the 6SN7 grids. Adjust the 470 ohm resistor to get the current you want in the 6SN7 (use Ohms Law with the voltage across the resistor, 8v/470R=17ma, which sounds about right). Change the 2.7K load resistor to 470K.
The mistake made by the designer here was to assume a plate impedance of 0R.
Since you have 2V across the 1.5K that is 1.33mA, with a 250V supply that would be an RP of 87.5k (87.5K RP+100K plate load resistor= 187K,250V/187.5K=1.33mA, 1.5K * 1.33mA=2V).
Any comments?
Joe
I will copy some comments sent to me by a very good friend concerning this design.
djk: Insert a coupling cap on the wiper of the 100K pot, 0.1 uf or larger. Change the grid stopper resistor to 1K on the 12AY7. Add a 470K grid leak from the grid to ground. Decide on how much current you want in the first stage and vary the cathode resistor to get it (ohms Law, 4V/1.5K=2.7ma which sounds about right).
Add 100 ohm grid resistor to each of the 6SN7 grids. Adjust the 470 ohm resistor to get the current you want in the 6SN7 (use Ohms Law with the voltage across the resistor, 8v/470R=17ma, which sounds about right). Change the 2.7K load resistor to 470K.
The mistake made by the designer here was to assume a plate impedance of 0R.
Since you have 2V across the 1.5K that is 1.33mA, with a 250V supply that would be an RP of 87.5k (87.5K RP+100K plate load resistor= 187K,250V/187.5K=1.33mA, 1.5K * 1.33mA=2V).
Any comments?
Joe
RE:6SN7 redone.
Hi,
I very much doubt that this is correct, Rp is roughly 22K8.
Do you have 250V on the B+?
Measure the voltage prior to the 100K anode R with respect to ground and use that in the formula.
I redid the calcs once again and I arrive at almost 4V which happens to be the textbook value for a 12AY7.
The rest of the advise your friend gives you is open to debate but it does not make a significant change to the workings of the circuit.
I suppose Joel used the 10K gridstopper on 12AY7 to slightly reduce the gain at the input.
Adding a cap at the input is not going to improve sound in any way.
Cheers,
Hi,
I very much doubt that this is correct, Rp is roughly 22K8.
Do you have 250V on the B+?
Measure the voltage prior to the 100K anode R with respect to ground and use that in the formula.
I redid the calcs once again and I arrive at almost 4V which happens to be the textbook value for a 12AY7.
The rest of the advise your friend gives you is open to debate but it does not make a significant change to the workings of the circuit.
I suppose Joel used the 10K gridstopper on 12AY7 to slightly reduce the gain at the input.
Adding a cap at the input is not going to improve sound in any way.
Cheers,
Frank
I have made the modifications that were suggested by djk with the exception of the 470K grid stopper resistor. I have changed the cathode resistor to achieve the bias of -4volts per section on the 12Ay7. I added the grid stoppers on the 6sn7 sections and rebiased the 6SN7's at -8volts. The hum has gone down considerably and after changing the heaters to DC is almost gone.
I also changed the output cap to a smaller value and changed the 2.7K resistor to 1meg. The bass seemed to improve.
Concerning the coupling cap... I don't see a sonic benefit but is considered by some to be a preventative measure.
I am thinking now of scrapping the 12AY7 and replacing it with a 6SN7.
Any thoughts?
Joe
I have made the modifications that were suggested by djk with the exception of the 470K grid stopper resistor. I have changed the cathode resistor to achieve the bias of -4volts per section on the 12Ay7. I added the grid stoppers on the 6sn7 sections and rebiased the 6SN7's at -8volts. The hum has gone down considerably and after changing the heaters to DC is almost gone.
I also changed the output cap to a smaller value and changed the 2.7K resistor to 1meg. The bass seemed to improve.
Concerning the coupling cap... I don't see a sonic benefit but is considered by some to be a preventative measure.
I am thinking now of scrapping the 12AY7 and replacing it with a 6SN7.
Any thoughts?
Joe
RE:6SN7 REDONE.
Hi,
Joe:
That is not a gridstopper Joe.It is a grid leak resistor with a total value of 477K7.
It should be totally inauble with DC on the heaters, I still think the hum gets picked up somewhere else.
Sure, but in real life you will not need it though.
Just put a 100 uF/25V (a bit more won't hurt in case you don't have the exact value) across the cathode Rs of the 12AY7 and you'll have -4V on the grids...
By doing so you may end up with too much gain to your liking and a bit more distortion (most will be nulled by the CF) but at least two other advantages:
The bias will come up correctly and the Zout to the 6SN7s will be lower, so you'll have better drive.
Also, IME cascading the same tubes is not my thing and the 12AY7s are fine sounding tubes...the 6072 even better.
Just waiting to be flamed by our friend Joel,
Hi,
Joe:
That is not a gridstopper Joe.It is a grid leak resistor with a total value of 477K7.
It should be totally inauble with DC on the heaters, I still think the hum gets picked up somewhere else.
Sure, but in real life you will not need it though.
Just put a 100 uF/25V (a bit more won't hurt in case you don't have the exact value) across the cathode Rs of the 12AY7 and you'll have -4V on the grids...
By doing so you may end up with too much gain to your liking and a bit more distortion (most will be nulled by the CF) but at least two other advantages:
The bias will come up correctly and the Zout to the 6SN7s will be lower, so you'll have better drive.
Also, IME cascading the same tubes is not my thing and the 12AY7s are fine sounding tubes...the 6072 even better.
Just waiting to be flamed by our friend Joel,
There's nothing to flame about Frank.
Nothing to argue about here. Yes, those are the bias points I stated months ago. And the fact that you have the hum diminishing by switching to DC heaters says that it had nothing to do with the circuit. It was a layout/wiring issue.
That is definitely psychological. Lowering the value of the coupling cap can only diminish bass response. Also, changing the resistor to 1 meg is risking a very nasty pop on your headphones/speakers - it's way too high to bleed off the charge stored on that big old cap quickly. Plus, you're not adding any gain, or power by changing that value.
As Frank said, your friend's suggestions are fine - follow them for added "insurance" against oscillation, etc. But they don't change the operation or performance of the circuit at all.
burnedfingers said:
Nothing to argue about here. Yes, those are the bias points I stated months ago. And the fact that you have the hum diminishing by switching to DC heaters says that it had nothing to do with the circuit. It was a layout/wiring issue.
That is definitely psychological. Lowering the value of the coupling cap can only diminish bass response. Also, changing the resistor to 1 meg is risking a very nasty pop on your headphones/speakers - it's way too high to bleed off the charge stored on that big old cap quickly. Plus, you're not adding any gain, or power by changing that value.
As Frank said, your friend's suggestions are fine - follow them for added "insurance" against oscillation, etc. But they don't change the operation or performance of the circuit at all.
Joel
What? No comment on the fact that you made a miscalculation?
You assumed a 0 grid resistance and it has been proven wrong.
Your bias figures are unobtainable with the resistance values stated. No comment on the fact that my wiring/components were correctly done? The fact that you thought I put 5volts on the heaters instead of 6.3 volts as if that was going to be the tie breaker.
With respect to your comments concerning changing the 2.7K from output to ground to a higher value..... Ever hear of an AE3 lineamp? Check the schematic of this particular piece, it has a one meg to ground in place of the 2.7K it has a smaller output cap as I don't believe the 220/100 is needed if this is used to run a poweramp.
What you deem as psychological....
Actually Joel other people actually have speakers capable of full response instead of the point source speakers you listen to. if I were to plot the response of your Altec you would find out in a hurry that they are severely lacking in full response. In other words Joel, we have speakers capable of hearing the very small changes that you cannot hear.
You claim not to be able to hear the small changes as a result of different types of capacitors, therefore they don't exhist in your frame of mind. I on the other hand have tried several different types of caps and resistors and have heard the difference as others have.
There is a difference between us Joel, I have stated my lack of knowledge concerning tubes only to be met with your sarcasm and ridicule.
Joe
What? No comment on the fact that you made a miscalculation?
You assumed a 0 grid resistance and it has been proven wrong.
Your bias figures are unobtainable with the resistance values stated. No comment on the fact that my wiring/components were correctly done? The fact that you thought I put 5volts on the heaters instead of 6.3 volts as if that was going to be the tie breaker.
With respect to your comments concerning changing the 2.7K from output to ground to a higher value..... Ever hear of an AE3 lineamp? Check the schematic of this particular piece, it has a one meg to ground in place of the 2.7K it has a smaller output cap as I don't believe the 220/100 is needed if this is used to run a poweramp.
What you deem as psychological....
Actually Joel other people actually have speakers capable of full response instead of the point source speakers you listen to. if I were to plot the response of your Altec you would find out in a hurry that they are severely lacking in full response. In other words Joel, we have speakers capable of hearing the very small changes that you cannot hear.
You claim not to be able to hear the small changes as a result of different types of capacitors, therefore they don't exhist in your frame of mind. I on the other hand have tried several different types of caps and resistors and have heard the difference as others have.
There is a difference between us Joel, I have stated my lack of knowledge concerning tubes only to be met with your sarcasm and ridicule.
Joe
burnedfingers said:
I'm supposedly rude and sarcastic, and yet you're the one calling me a penis? Go back and actually read my posts to you Joe - they are neither rude, nor sarcastic - and I certainly have never resorted to name calling - even though you've attempted to publicly humiliate me somehow by "exposing" my circuit as non-working. I'm sorry it isn't working out for you, and I would have enjoyed further debugging - but, for whatever reason you've desired to make this into a personal battle between the two of us, and I believe I'll opt out of that. Good luck.
Joel
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