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leadbelly 28th July 2006 12:16 AM

Are there techniques for triode-strapping on the fly?
With all the talk about guitar overdrive pedals I am thinking of finally sitting down and developing my own through some good old trial & error.

The high end rack mount stuff has selectable triode/pentode distortion modes. I've never used one but I imagine they are refined so that you can just flip any of the mode switches hot or cold without breaking something or even hearing a big pop.

So, are there traditional techniques to accomplish triode-strapping/unstrapping of a pentode with simple, passive devices to safely do it on the fly with worrying about arcing, pops, etc?

Giaime 28th July 2006 11:03 PM

Simple, to simulate pentode mode, add NFB. ;)

*Giaime runs away hiding to avoid beating :smash:

kevinkr 29th July 2006 05:44 PM

I originally posted with some ideas that I though on reflection might confuse the issue so I deleted it.

The more I think about this the less trivial it becomes, but drastic shifts in gain/operating points plus the transient that would occur upon switching convince me that the best thing to do might be to have two parallel stages, one configured as a triode and the other as a pentode and select the one you want with a switch. This of course only works if you have the room, but if you are building it in a rack mount case this should not be a problem.

The best pentode for this might be the EF86 or similar, readily obtainable, and inexpensive it is also very quiet. I would use a pair and triode connect one and pentode connect the other. A variety of topologies are possible and you can experiment to find the one you like best.

You could match output levels with careful design and some judicious output padding on the pentode if required and obviously use a smaller load resistance to reduce the gain is another possibility particularly if you can get the distortion you want.

A triode with feedback around it sounds like, well... a triode with feedback around, doesn't imho sound at all like a pentode.

leadbelly 31st July 2006 05:05 PM

Thanks for the response. I had not even thought of the parallel stage idea, I was thinking along the lines of just trying to build a McTube with two EF184's, and letting each tube have a strapping switch, and seeing how that would work on the bench. The parallel stage is definately something to try. :)

DigitalJunkie 1st August 2006 09:11 AM

Maybe put a pot between the output of each stage,so you can "fade/blend/mix" between them.. 50/50 triode/pentode,or any combination inbetween! ;)

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