ECC88/5687/2A3 amp voltages - diyAudio
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Old 4th March 2005, 11:48 PM   #1
Taffi is offline Taffi  Norway
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Default ECC88/5687/2A3 amp voltages

Hello!
I have an amp where I have just change the input stage from 12AT7 to ECC88 to get lower gain (which I did).
Problem is that I do not find the amp dynamic anymore.
Before I convert the amp back to 12AT7 input again I have been adviced to try to run some more current throw both the input (ECC88) and the driver/phase splitter (5687).
Problem is that I am not sure on how to do it.
Can anyone give me some clues where to start and which operating points (and voltages) I should get ideally if I would like to drive the input- and driver tubes hard?
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Old 5th March 2005, 12:40 AM   #2
SY is offline SY  United States
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The current through the ECC88 looks OK, around 4-5 ma. It could go higher, but no need. The 5687 is running a bit light. If it were my amp, I'd go for an active current source in the 5687 cathode, try to raise the B+ on it, equalize the plate resistors, and up the current to more like 3-4 ma per section.

Understand that the ECC88 will give you significantly less gain than a 12AT7/ECC81, so that lack of dynamics may simply be a function of where you've set the system levels.
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Old 5th March 2005, 01:17 AM   #3
SY is offline SY  United States
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Looking at the 5687 data sheet, I have second thoughts- get that current up to more like 10mA per section. That means you'll either have to have a higher B+ for it (much higher!) or drop the plate resistor values or both. With a 450V B+, you could drop the plate resistors to something like 30K. And a current source for the cathodes will be almost mandatory; lowering the cathode resistor value will increase the phase splitter unbalance.
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Old 5th March 2005, 01:17 AM   #4
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Hi,

You could also try to lose the global feedback loop too.
It's eating most of the gain and I really doubt it's needed for a PP 2A3.
The unbypassed cathode resistor of the ECC88 should provide some local degenerative feedback, linearizing that stage.

Phase splitter as per SY's suggestion, get those plate resistors equal at the very least.....

You could have done the lot with a single ECC88, one half for input voltage gain, the other half as a cathodyne splitter, for instance.
Input sensitivity would probably sink considerably compared to the ECC81 set up but as you are not complaining about that, I assume you're using a preamp of the "gain to burn" type anyway.

Or else:
Courtesy of Dutch Triode Dick. I tried it, nice and simple....After all making things complicated is easy..........

Cheers,
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Old 5th March 2005, 01:27 AM   #5
SY is offline SY  United States
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Frank, is that schematic correct? The tap for the other side of the phase splitter looks funny.

The problem I would anticipate with ECC88 voltage amp to a split-load is the ability of the first section to linearly swing enough voltage to drive the 2A3. Or do I worry too much?
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Old 5th March 2005, 01:45 AM   #6
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Hi,

Quote:
The tap for the other side of the phase splitter looks funny.
The bottom part I suppose you mean?

According to the fellow who designed it, it is correct...
It had me scratching my head too but then the entire phase splitting looks off with only one of the cathodes bypassed too.
Dixit the same chap, he has that trick from a Philips engineering book.
I once modded a PP 2A3 of mine that way and it seemed to work just fine.

Quote:
Or do I worry too much?
No, you're quite right.
It was just a cheap and cheerful suggestion, one I think would still work better than what Taffi has now IMO.

If it were me facing that amp I'd leave the ECC81 and just optimize the splitter, perhaps fiddle with the feedback loop or throw it out altogether.
I don't see why the input sensitivity/gain of the amp would be a problem if it's working fine otherwise.

Cheers,
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Old 5th March 2005, 02:22 AM   #7
SY is offline SY  United States
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Well, I can see how it works, I just had problems seeing how it could work well. I need to play around and see if indeed the source impedances are balanced because at first glance, it looks like they're not.
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Old 5th March 2005, 12:37 PM   #8
Yvesm is offline Yvesm  France
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Quote:
Originally posted by SY
Well, I can see how it works, I just had problems seeing how it could work well. I need to play around and see if indeed the source impedances are balanced because at first glance, it looks like they're not.
Yes, it looks like if that was done deliberatly, and I can't understand why

What about having a common unbypassed cathode resistor ?
Mmmmh, may be worst because it introduces some positive feedback on the first triode

Yves.
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Old 5th March 2005, 12:48 PM   #9
SY is offline SY  United States
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Well, it looks like he's deliberately increasing the gain of that tube to bring the circuit into voltage balance; calculations will show if the designer is devilishly clever and managed to balance source Z, but I doubt it.
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Old 5th March 2005, 04:38 PM   #10
Taffi is offline Taffi  Norway
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Well, first of all:
Thanks to all that are willing to help me on this

Let me first tell you a little bit more about the amp.
The amp by itself are not a completly DIY amp, but a kit that I have bought and built.
Because of that it is not so easy to make bigger changes such as getting a much higher B+ and so on.
For now I have to stick with small changes such as changing resistors.

The reason for changing from 12AT7 to ECC88 was to get less gain since I use speakers with very high sensivety( Duo's).
So I was adviced to use ECC88 from the man who sells the amp and he also told me that he thought it sounded better too.
Well IMO it don't (even with a good Siemens ECC88).
So I just want to try the ECC88 with higher current before I eventually have to rebuilt it again with 12AT7.

Regarding the current source I am not enough into that to understand it at once, but I am sure that if I seek this forum I will find the info needed.
(IF it is very easy though maybe someone could just tell me).

I do not either understand why the amp is built with different values plate-resistors on the 5687's.
Has it something to do with the 1M restistor the signal passes throw before it goes to grid on the one halt of the tube?
If I understood it correctly your advice is to use the same values plate resistors (which I has done), but I still wonder why it originally are made with different values.

And the same goes for the negative feedback.
Why is it there in the first place if I just can take it away?
And should I just take away the 20K feedback resistor or should I instead try another value (higher or lower?)?

In the schematics below I have done my best to get the currents up just by changing resistors, but can not see how I can get the current throw the 5687's as high as 10mA.
It all depends on how low cathode resistor on the 5687's I can use.
More advice needed!
Now I am down to 10K, but even then it's less then 7mA throw each section of the 5687's.
There is one way that I can get this current higher and that is by reducing the plate resistor on the ECC88 from 10K to about 5K.
This will make the grid- and cathode voltages up by 20V and then also the current throw the 5687's (both halfs together) up to 15,mA), but at the same time it will reduce the voltages over the ECC88 plate resistor from 70V to 50V which I don't know is wise.
Or is it just no way around using the current source (which I first have to find out what is)

Is it safe to try the amp as it is right now or have overdone it and need to higher some resistors again?
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