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PostPosted: 01 Sep 2008, 10:07 
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Location: Atlanta
Between tube cad and SE cad I came up with this:

a 6c4 cathode follower to get the voltage up to the needed level and the SE circuit:

Question, the cathode follower uses a 2uf output cap and the SE amp needs a .33uf input cap...can these be combined and just use a middle value or drop the 2uf all together?

This is by no means the final iteration, but I am curious as to whether I am on the right track.


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PostPosted: 01 Sep 2008, 11:43 
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Hi Bill,

You only need one cap and resistor between the two. So pick a load resistor (Rl) for the preamp and use the same value on the amplifier section.

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PostPosted: 01 Sep 2008, 12:53 
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Gotcha, so those two would combine. I need to do some back and forth then to figure out the right combination.

Am I on the right track in thinking that I want the driver tube of the KT88 to basically just provide voltage and not gain to the circuit?


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PostPosted: 01 Sep 2008, 17:09 
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Yeah, go back and forth to find what works best. The driver stage will likely be the more sensitive, so start with that one. Yes, the driver stage will provide the voltage swing and the amp the current.

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PostPosted: 01 Sep 2008, 17:43 
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Here's what I am looking at so far:


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PostPosted: 01 Sep 2008, 23:20 
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The KT88 makes for a nice amplifier. Being single ended, you will not want to use poor output transformers. Since you are using software methods to do the math, try a higher load (470k) and look at the results. I suspect that may have better results.


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PostPosted: 02 Sep 2008, 06:51 
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Does the guy who built the one on the project page still hang out here because I have a question.

I ran the input stage through tube-cad and to my best guess it looks like an I to V stage more than anything else. But with the values provided I got some really wild numbers...kilovolt swings on the output and a crazy gain on the order of 46dB!! This seems counter-intuitive because your input voltage and gain is likely to be around +10 to +20dB depending on the preamp used....seems like it is a recipe for some serious clipping. Yet people are bowled over by the design...I guess that is what confounds me about all this...I know there is more than one way to skin a cat and all, but it seems this stage is severe overkill....any input on that?

Which leads me to, if that stage is not overkill, is mine underkill? I designed it with a -30v to +25v swing and the way the output stage is modelled it needs a -25v to +25v swing to make the power as advertised (about 7.5w...anything over 8 seemed to bring up the distortion numbers quite a bit). My reasoning is that the input tube will never over-drive the output tube in this case or am I setting myself up for a lack of dynamics?

I have to admit this is a ton of fun running sims and anticipating results, but I don't want to make a $800 mistake!!


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PostPosted: 04 Sep 2008, 20:34 
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The cathode follower will have a gain of less than 1. In the case of cathode follower 6c4, around 0.8 gain. With a 2 volt input from a CD player source, it will not drive the KT88 to full power. So this stage will theoretically can only provide a maximum of 0.8X2V=1.6V of swing. The KT88 needs around 50V.

You cannot use a cathode follower to drive the KT88.

Even if the 6c4 is used as an SE gain stage, quick estimate maximum swing with a 2V source (such as a CD player) will only give you a theoretical maximum 34V swing (2Vx17mu at 250V at the plate). It will drive the KT88, but the 6c4 will clip way ahead of the KT88, which means it will not drive the KT88 into full power.

Choose a driver with about 30 or a bit more mu (amplification factor), 6n1p, 6dj8, 6922, 6sn7 (have higher voltage swing than 6c4, but might not be optimal), EF86, etc. Or you might go for a two stage driver and introduce feedback.


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PostPosted: 05 Sep 2008, 00:20 
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Thanks Alex. I did not think it was gain you were after, but voltage swing and with the CF you can supposedly get the voltage without the gain...the modelling showed -30v to +25v swings possible as designed.

I guess I will stick with the original design...it's tried and tested and you say it sounds great anyways! :up:


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PostPosted: 06 Sep 2008, 10:02 
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You need gain so that the voltage swing of the source (2V for a CD player) can be amplified enough by the driver to drive the output tube to full power.

One of my mentors, taught me that in a single stage driver SE amp, the quickest way to gauge if the driver (grounded cathode) can drive the power tube to full power is to multiply the voltage of the source (around 2V for a CD player) by the mu (amplification/gain of the tube) of the driver stage. The result should be more than the cathode bias of the power tube. So in the case of the 6n1p driven SE KT88, mu of 6n1p is around 33, so 2 x33 = 66. The KT88 is biased at around 40V on the cathode, so the 6n1p can drive the KT88 to full power.

There are more precise way of calculating voltage swings, which should give you more accurate results.

I have not used the software you are using, but I don't know why it is giving you a -30/+25V swing for the cathode follower. A cathode follower will give you less than unity (1) amplification (this is true even if you use a high mu tube such as the 12ax7), so if you input 2V into it, the amplified voltage will be less than 2V.


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