Balanced OD Tube...Advantages?

[Dr d]

Another one from me! Does anyone find using a balanced tube in the OD advantageous? Hopeing this isnt a stupid question!!

[talbany]

Another one from me! Does anyone find using a balanced tube in the OD advantageous? Hopeing this isnt a stupid question!!

Doc
Actually that's a very good question and FWIW here is my take.. Getting tubes matched and tested for gain and transconductance is always a good thing if you are looking for a more consistant performance and tone amp to amp as well as insures your getting a good healthy tube..This is where a tube tester that checks for transconductance shines since this lets you go through and document which tubes sound best to you in that particular amp..Maybe you like them matched?
So with that being said yes a matched tested tube is a good idea!!

The flip side can be looked at as being the two sides of an AX that are not matched will clip differently generating slightly different order/amounts harmonics which in some cases is not necessarily a bad thing..If your lucky and find the right offset of breakup/ harmonic content generated can be a beautiful thing..This is what I call part of the so called magic tube theory and is sometimes difficult but not impossible to find..

Hope this helps..

Tony

[Dr d]

Tony, thanks for the reply. What prompted me on this one was the discussions on the PI tube. Even with this you can fine tune with the PI adjustment. It then got me thinking about the OD tube.....unless the tube is balanced, cascading one half into the other with given plate values can be a bit hit and miss. Im going to experiment on this one!!!

[Structo]

And to continue this further, what advantage or tonal attribute does using two different plate and cathode resistors do on a tube.

Say on the OD I have 220K and 150K plate load resistors.
On the cathodes there is 3K3 and 2K2 respectively.

The ratio remains the same but the tube stages are biased differently, right?

It probably can be deciphered using a load line but try as I might, I can't figure out those things.
I mean if it is explained with numbers on it I sort of get it but not enough to draw my own.

I keep seeing that 66:1 ration on plates to cathodes, so it seems to be a good formula.

[talbany]

The ratio remains the same but the tube stages are biased differently, right?

Tom

Yes..however you should change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing, or to the place it was originally biased.... If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs... The two go hand-in-hand.

what advantage or tonal attribute does using two different plate and cathode resistors do on a tube.

I don't know if there is an advantage just different tone,feel..

Tony

[Chris333]

I always assumed (maybe wrongly) that part of the appeal of the high-plate preamp sound was asymetrical clipping... Not necessarily different amplitudes, but different shapes to the top and bottom of the wave. But maybe you get that with 100K/1.5 on all stages too...

[talbany]

Chris
Both Biasing and Plate resistor values effects harmonic content

Looking at how biasing effects clipping symmetry

By manipulating the bias point you can arrange for the signal to clip asymmetrically (on one side more than the other), or symmetrically (on both sides simultaneously). Asymmetric clipping tends to introduce even-order harmonics (particularly 2nd), although as the clipping becomes harder it also introduces odd order harmonics. Symmetrical clipping produces mainly odd order harmonics and a more traditional ‘driven’ tone, although with valves some even order harmonics will always be present...

I always assumed (maybe wrongly) that part of the appeal of the high-plate preamp sound was asymetrical clipping

The rate at which the signal is clipped depends on the source resistance (Plate resistor) since the input voltage is dropped across this. If the source resistance is very high (150k/220k) then the voltage will drop more suddenly, causing harder clipping and introducing more odd-order harmonics or symmetrical clipping

Valve Wizard explains this in great detail and is a wonderful read..

http://www.freewebs.com/valvewizard1/Co ... _Stage.pdf

Tony

[Dr d]

Thanks for your input, guys. Ive got to the stage in my amp building where I am really pleased with sounds my amp makes, but I realise that with these designs it is the attention to detail that makes the magic happen!! I have been trying out different valves in my Bluesmaster OD (funnily enough as a result of excessive bass issues...splitting hairs really!) and it occured to me that a balanced valve might have have a positive effect. There isnt much choice unfortunately but I have a JJ and a Harma on order to try in the PI and now the OD!!! Anyway, thanks again guys for the discussion. Happy New Year!!

[Chris333]

Valve Wizard explains this in great detail and is a wonderful read..

http://www.freewebs.com/valvewizard1/Co ... _Stage.pdf

Tony

Tony,

Thanks for the reply and the link. The PDF looks familiar -- Is "Valve Wizard" Merlin from MEF? I have his preamp book. I should re-read the common gain stage chapter!

[mlp-mx6]

Yep, one and the same. All the info in that pdf is expanded in the chapter you said you need to read.