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#1 2012-12-06 18:49:10

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

68K Plate Resistor in V Series Stage 2

So, from a gain/theory perspective - the 2nd tube triode of each channel is fed by the same 68k plate resistor.

I am working on experimenting and building a preamp clone, but without this 'shared plate resistor' configuration.

Effectively, what plate resistor should I use to duplicate this using a single triode, single plate resistor? 
Is it as if each triode is seeing a (2x68k) 134k resistor?   
Is it more like each triode is seeing a (68k/2) 34k resistor?
Is it effectively the same as each triode having an individual 68k plate resistor in terms of gain?

Also, is there any major technical factors that come into play (even if negligable) via the two triodes sharing a plate resistor (plates in parallel?), but with their own separate grid references and cathodes?

This is at least somewhat different than parallel triodes (where each of the 3 points of the triode are joined), two triodes with distinct plate and grid connections but share a common cathode point (see fender, marshall), and two triodes with plate and grids in parallel but separate cathode resistors (I've seen/heard about this approach being taken to create two bias points for 'unique' clipping characteristics in a preamp), and other various shared/separate configurations.

Thanks! 
Any info, links, theory, etc is appreciated.


Matthew

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#2 2012-12-11 05:40:16

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 68K Plate Resistor in V Series Stage 2

You would want the resisors to see half the current,  so you would double the value when going to a single plate resistor.   

It's a good summing amplifier,  hiwatt used this circuit too in some of their amps.

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#3 2012-12-11 15:46:42

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: 68K Plate Resistor in V Series Stage 2

Since I posted this, I did some LTspice simulation for more info, thought the tube models from duncan amps are not something I trust.

By comparison of 'actual' circuit configuration to a triode with it's own plate resistor, the closest value I found that duplicated the gain of the 'actual' shared plate resistor circuit was 56k (albiet, I now know my preamp tube B+ voltage was about 40v too low...which may have had an impact.

I found that strange but not certain why.  I will say this - from a design perspective, it looks like that stage had a gain of 11, which more or less equals the gain LOST by the baxandall tone stack when set flat.

Does this help?  56k?  From what you said-which was my previous assumption - with just one tube stage more like a 120k or more 'exactly' like 2x68k resistors in series. 

(PS I am not planning on using any of the V-series 'spare parts' chassis I have for my scratch-build, which should make most people happy...I'll post more about that in the thread I started when I was considering utilizing parts from that amp in conjunction with a fender chassis or a blank chassis, which  caused a mini-riot.)


Matthew

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#4 2012-12-13 00:07:03

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 68K Plate Resistor in V Series Stage 2

Ha ha...I'm glad my snobby disgust helped convinced you to change your mind about the chassis.

I don't know why LTspice would lead you to believe that was the case.  The lower plate resistor value being used for both probably plays a role in the output impedance... I wonder if that has anything to do with it.

Would be interesting to build both and see...

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