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Old 23rd November 2009, 03:22 PM   #1
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Default Optimizing a 807 PP amplifiers input stage and phase splitter...

Hello everybody!

I'm currently in the loop for 2 projects: a new SE amp. (working on that..)
and secondly optimizing my 807 PP amp. I could use some help/insights with the latter. I'm just learning how to use curves to set the tubes, and i'm not to confident that I do it the right way. The amplifier is a rebuild of an EL34 amplifier wich I build 20 years ago. Re-using the iron and stuff, but with different output tubes.(807) The original design is from Menno van der Veen, who also designed the amplimo (was ILP)/plitron OPT's.
Problem is, I think he used a ECC81 in a DC coupled gain stage-> concertina because of the relative high mu/ low output impedance of that tube. Now after 20 years, i finally measured some voltages, and must conclude that the bias of the first stage is much to low. I can understand why he does it like this, (because of the low V- anode, and the possibility to use dc coupling) but maybe an extra capacitor would be preferable to setting the tube in its least linear region.

I know there are better phase splitters, but I don't want to change to much on the amplifier, it sounds already pretty good like this.

What do I want to try:

Set the driver tube in a more linear region, and keep the dc coupling and the concertina splitter. I see no way of doing this with the ECC81, so that has to go. A friend pointed me to the ECC88, because it has decent low voltage capabilities. After checking this out, I came to the solution found in the second picture. Using a CCS, and choosing a cathode resistor, so that the V anode is about the same as with the previous schematic, should do the trick of maintaining dc coupling and not having to change the concertina.
I have included some curves, and ask al the guru's out there: Am I on track, or making a horrible mistake...
Attached Images
File Type: jpg TUBEAMP before.jpg (119.5 KB, 499 views)
File Type: jpg TUBEAMP after.jpg (125.7 KB, 479 views)
File Type: jpg 807pp.jpg (164.8 KB, 445 views)
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Old 23rd November 2009, 06:00 PM   #2
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Hi!

I can't speak for myself, but diyaudio member SY actually showed that the ECC81 can work quite well in a concertina splitter fed from a voltage between 350V and 400V (in his opinion even better than the E88CC/ECC88). So you may have a look at this thread to see what he is doing.
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Old 23rd November 2009, 06:41 PM   #3
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I looked at it; and SY said: "Linearity, pure and simple. You have to look at the split load and the input stage together as a system. If you want to get a reasonable linearity from an ECC81 voltage amplifier at the full swing needed to drive the output stage, it needs 110-120V at least on its plate (lower and you hit the bunching of the curves and distortion rises)"
Well, as you can see in the curves I included, its now set up right IN the bunching of the curves. Maybe SY can shed some light...
Still reading though... trying to find the part where he states that ecc81 would be better than ecc88 in concertina....(and why) Also wondering how many bias he set the ecc81 to...
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Old 23rd November 2009, 07:45 PM   #4
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Hmm, about the ecc81 problem: SY told us in the red light district amp story somewere that he put a ecc81 on a CCS and it was not very linear; lots of distortion.
If I look at the load line at very low plate current, and look at the amplification i'm getting from that; with a CCS the lower part of an 2.4 v pp input sinus is amplified about 38x, and the upper 53x. This is probably the distortion SY measured.
With a resistive load its a lot better; because of the angle the load line hits the curve, it actually compensates for the non linearity...

Last edited by pauldune; 23rd November 2009 at 07:48 PM.
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Old 26th November 2009, 10:24 PM   #5
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Today I got the 10M45's. Before changing the amplifier, I measured distortion in every stage. on the ecc81, on the input of the 807´s and on the speaker terminals.
After that I changed the amplifier; put in the ccs and the ecc88.After that I did the same measurements.
I´m going to post the results,but not today, i'm too tired. (I have some spectrum analyzer shots too) But I can say this already:
Distortion at 2,83v on speakerterminals (actualy 6 ohm dummyload so this is about 1,3 W) dropped from 0,7 to 0,3%. Also the output of the phase-splitter dropped from 0,54/0,53% to 0,07/0,05% !!!! A fine upgrade, but I still need to listen; the amplifier isnt connected to the speakers yet... Maybe tomorrow, I upgrade the second monoblock, and am able to listen.

Last edited by pauldune; 26th November 2009 at 10:27 PM. Reason: typo's
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