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Nordic 9th November 2006 09:56 AM

Some cathode follower questions

I would like to use 2 halves of 1 tube as 1 channel to try to get a bit more gain out of the setup.

As it stands now it seems to give less than unity gain using only the first half of the schematic, I.e. 1 valve shared between 2 channels.

My questions are, do I need to seperate the anodes with matched resistors like you would do with opamps...

Would a resistor between the 2 stages be enough to ensure the second stage does net get too much voltage on the input? Or is the less than unity gain of the first stage enough protection?
I'm not sure but it looks like the datasheet says inputs should be less than 1.3V.
Would it maybe be better to use the two halves in parallel?

Any advice, other than use diffirent tubes?
I understand I'll have to set up some gain balanceing arrangement on the output to help match the channels, but that seems easy enough, not to bother you with.

SY 9th November 2006 11:12 AM

I'm not sure what you're doing here. A cathode follower always is less than unity gain. In the case of an ECC88 loaded with a 10k resistor, the gain will be something like 0.9-0.95.

If you cascade two cathode followers (or any other two stages), the total gain is the product of the individual gains. The product of two 0.9 gain stages will be 0.81, lower than a single one. So what's the point of doing that? If you want gain, don't use a cathode follower.

ashok 9th November 2006 11:12 AM

Both stages are cathode followers ( almost unity gain ).
How do you expect any "gain" from this configuration ? In fact if each stage had 0.95 gain , then cascaded it would be even lower at 0.9 !

Ha...SY , you are on line too !

Nordic 9th November 2006 03:30 PM

Thanks gentleman, I see, I thought it would be added together...

I have it on good authority that voltage gain in a preamp is not a bad idea... so I would like to have some :) BUT I also like the output impendance of the cathode follower.

So there is no benifit from paralelling em even? Probably more current yes?

SY 9th November 2006 03:40 PM

Yes, but if you define your load and use the right tube in the first place, that's not necessary.

ashok 9th November 2006 03:44 PM

You can have gain in the first stage and use a cathode follower after that. So if the input stage has a gain of 10 ( just an example ) and the cathode follower has a gain of 0.95 , the overall gain would be 10* 0.95 = 9.5 .

What's your output voltage requirement and what is the load ?
Lets say a max of 5 volts output into a 10K ohm load .
Then the max current would be 5/10K = 0.5mA .
You need more than that ?

In any case for the configuration mentioned in the first paragraph , do a " tube preamp " search and you should come up with many circuits to try out !

Whew........Sy this is uncanny ! Look at the time we have posted twice !

Nordic 9th November 2006 03:54 PM

I have looked at a number of preamp ciruits, sadly all use only a positive and ground rail.

The only valve circuit I build that works runs off +, - and oV rails, which also keeps grounding schemes easy when combining it with bipolar chipamps.

Sadly I think it is a birth defect, but I have no instinctive intrinsic knowledege of valves and their operation...

Peter M. 9th November 2006 04:06 PM

ashok 9th November 2006 04:12 PM

Are you new to tube circuitry ? I think MOST tube circuits run of a +V supply. So why are you sad ? It's easier to have a single polarity supply.
Your input can be capacitor less even with a single supply but in both cases the output will need a decoupling capacitor.
Grounding should not be a problem if you go about it the right way.
No rocket science here , just common sense.
Just keep at it ( tube circuitry ) and you will get the hang of it pretty soon.

Nordic 9th November 2006 04:25 PM

Its been about a year since I built the valve buffer, but I am now actively pursuing the learning phase.

But I have been heavily tainted by opamps and the way one would go around using them, and this is influenceing my tinking.. therefor the bipolar design requirements etc. Lets assume I have a bipolar PSU allready, I can't just ignore the ground rail can I ?

My current setup is built around a bipolar greinacher voltage doubler with voltage regulated heater.

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